City of Grande Prairie 1st Floor, 10205 98 Street Grande Prairie, AB T8V 2E7
Attention: Richard
Sali, C.E.T. Drainage & Environmental Supervisor
Re: City of Grande Prairie - Storm Drainage Master Plan 2023 Final Report (Revision 001)
Dear Mr Sali:
201, 17205 106A Avenue NW Edmonton, AB T5S 1M7
780-482-2557
services@sameng.com
www.sameng.com
Our File: 1427
Please find enclosed a copy of the final report (Revision 001) of the City of Grande Prairie Storm Drainage Master Plan 2023 project.
Should you have any questions or wish to discuss any component of this project, please do not hesitate to contact us.
Sincerely,
David Yue, P.Eng. Project Manager
Storm Drainage Master Plan 2023
Final Report Rev 001 City of Grande Prairie
Executive Summary
PROJECT BACKGROUND AND OBJECTIVES
Sameng was retained by the City of Grande Prairie in August of 2022 to undertake an update to its Storm Drainage Master Plan. Since the last storm drainage master plan in 2017-18, the City of Grande Prairie has completed portions of the recommended drainage system improvements, and continued its collection of rainfall and flow monitoring data to improve its understanding of stormwater flows within the City boundary.
After a decade of rapid growth, the City’s population has held relatively steady for the past 5 years at about 64,000 residents (2021 Municipal Census). According to the City’s 2008 Growth Study, population in Grande Prairie is projected to reach 80,000 by 2028 and 100,000 by 2038. As growth patterns for small cities often happens in cycles of rapid growth followed by periods of stability, this indicates a proactive approach of infrastructure that can be implemented with flexibility, while maintaining high standards for effectiveness, sustainability, and safety.
The main objectives of this Master Plan are to provide a comprehensive update of the City’s existing Mike Urban Drainage Model, a review and assessment the City’s rainfall and flow monitoring program, review drainage improvement recommendations from the previous master plan and adjust to address updated conditions, and model and assess the existing Hermit Lake Basin Study area.
REVIEW OF IDF CURVES, RAINFLOW DATA, & FLOW MONITORING DATA
The City currently owns and operates 6 rain gauges and 5 flow monitoring locations across the City. The reviewed historical rainfall data sets are generally in good agreement with the proposed IDF curves from the 2018 SDMP. The recorded flow readings contain certain errors but most of the records could be used for system evaluation.
Based on the received historical rainfall and flow monitoring data, Sameng recommends continuing with the rainfall and flow monitoring at current locations to better understand rainfall patterns and intensities throughout the City. Improved data management techniques should be utilized to unify the future flow monitoring process and decrease resources required for data processing. Potential improvements include increased frequency of data review to diagnose and rectify problematic data sets.
As the recorded rainfall data sets are generally in good agreement the proposed IDF curves from the 2018 SDMP, there are no recommendation changes to the City IDF curve at this time. Future collected rainfall and flow monitoring data should be reviewed to determine if additional adjustments are warranted.
COMPUTER HYDRAULIC MODEL REFINEMENT
The previous Mike Urban dual drainage hydraulic computer model provided in the 2018 SDMP has been updated and converted to a comprehensive hydraulic model in Mike+ by DHI. Mike+ is DHI’s successor program to Mike Urban, has achieved significant updates, and provides access to ongoing technical support and updates through DHI. The 2023 SDMP model utilizes updated LiDAR information which more accurately represents surface elevations within the City boundary and includes key drainage infrastructure improvements implemented since the 2018 SDMP. This model should be continuously updated as the City continues to improve the existing drainage system, and calibrated using future rainfall and
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City of Grande Prairie
flow data. The 2023 SDMP model is a 1D-2D integrated hydraulic model rainfall-Runoff hydrology as well as overland flows are modeled by a 2D mesh, which simulates ponding volumes, flow resistance, and infiltration on the surface.
Existing system surface ponding simulation results were prepared for all areas within the City boundary for both the 5-year and 100-year 4-hour design storm events. Existing system sewer surcharge simulation results were prepared for both the 5-year and 100-year 4-hour design storm events for urban areas of the City. Approximately 24% of manholes in the City are surcharged to 1.0m or less below grade during the 5-year (4-hour) design storm event, with 59% of manholes surcharge to 1.0m or less below grade during the 100-year (4-hour) design event. Surface ponding is variable within the City, anticipated to generally remain below 35cm during the 100-year event, however select trapped surface lows are anticipated to reach surface ponding levels of 1m.
UPDATE OF IMPROVEMENT CONCEPT PLANS FOR EXISTING DRAINAGE SYSTEM
The 2023 SDMP identifies improvement concept plans to provide 100-year levels of flood protection to the following five (5) urban areas at high risk of flooding:
1- Northridge Area (North – Central)
2- Avondale / Montrose Area (North – Central)
3- Highland Park / Swanavon Area (Central)
4- Ivy Lake / Cobblestone / Smith Area (Central–East)
5- Richmond Industrial Area (Central–West)
Portions of the recommended urban improvements have been completed by the City since the completion of the 2018 SDMP. The impacts of completed improvements were reviewed and were found to have significantly reduced surface and sewer surcharge flood risks for the affected areas during the 100-year storm event
The outstanding recommendations from the 2018 SDMP were reviewed and adjusted to reflect updated conditions. Conceptual cost estimates for the improvement concept areas were adjusted to reflect current market rates. As the original improvement recommendations were extensive, with estimated costs from $10-25 Million per area, the outstanding recommended project scopes have been split into recommended phases which more closely align with typical City drainage project budgets. These phases also provide additional guidance for the City when sequencing the proposed improvement concepts. The total estimated cost of the 2023 updated urban improvement concepts (including engineering & contingency) are 101.4M.
FUTURE SERVICE AREAS – UPDATED MODELLING
Updated surface ponding model results for the 5-year and 100-year event were created for the City’s future service areas. The future service area recommendations provided in the 2018 SDMP were reviewed as part of the 2023 SDMP scope and were confirmed to remain relevant and beneficial in the development of future service areas. As such, the 2018 SDMP future service area recommendations are upheld under the 2023 SDMP to ensure consistent development requirements for developers and the City moving forward. For specific future service development recommendations, the 2018 SDMP should continue to be referenced.
Storm Drainage Master Plan 2023
Final Report Rev 001 City of Grande Prairie
STAKEHOLDER AND PUBLIC CONSULTATION
To integrate stakeholder feedback and provide transparency regarding the 2023 Storm Drainage Master Plan, stakeholder consultations were completed throughout the project duration. A virtual stakeholder engagement session was held on November 1, 2023, to present the stormwater master plan concept to prospective developers, developer consultants, City of Grande Prairie departments, and utility providers.
A public open house was held on October 18, 2023, to present the updated stormwater master plan improvement concepts, provide an overview of stormwater improvements completed since the 2018 SWMP, and provide an opportunity for public feedback. An informational webpage regarding the 2023 Storm Drainage Master Plan was made available on the City website prior to the event, and an online survey was made available to collect feedback from stakeholders. Relevant feedback from the public open house and virtual survey were integrated into the 2023 Storm Drainage Master Plan and inquiries not related to the Plan were forwarded to the appropriate City staff for review and follow-up.
HERMIT LAKE BASIN STUDY
An additional drainage study was completed for the Hermit Lake study area, located on the western City boundary. The overall topography of the area is well graded towards a central band of wetlands and environmentally sensitive areas, which should be preserved and enhanced so as to provide an adequate stormwater outlet for future development. Key culvert crossings and roadside ditches were identified for regular inspection & maintenance by the City, to maintain conveyance capacity during large storm events. Overall, the area was noted as being at low risk for significant flooding during the 100-year storm event, however the capacity of the existing drainage system is highly dependent on proper function of the existing culvert crossings.
The internal Hockey Estates Neighbourhood was reviewed and noted as being at low risk of flooding during the 100-year storm event, however culvert maintenance and improvement recommendations were provided to increase the existing 25 to 50-year culvert capacities present in the neighbourhood.
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List of Appendices
Appendix A – November 9, 2022: Rainfall & Flow Monitoring Technical Memorandum
Appendix B – Flow Monitoring Datasets
Appendix C – Computer Model of Storm Drainage System
Appendix D – 2023 SDMP Urban Improvements Conceptual Cost Estimates
This document was prepared by Sameng Inc. for the account of the City of Grande Prairie. The material in it reflects Sameng’s best judgment considering the information available to it at the time of preparation. Any use which a third party makes of this report, or any reliance on or decisions made based on it, are the responsibilities of such parties. Sameng Inc. accepts no responsibility for damages, if any, suffered by any third party because of decisions made or actions based on this report.
PROFESSIONAL SEAL
David Yue, P.Eng.
Storm Drainage Master Plan 2023
Final Report Rev 001 City of Grande Prairie
Acknowledgements
We wish to thank those who contributed time and knowledge to the development of this study including, but not limited to, the following:
CITY OF GRANDE PRAIRIE
• Richard Sali, C.E.T.
• Mike Harvard, P.Eng , M.A.SC.,
• Corbin Bauman
SAMENG INC
• David Yue, P.Eng.
• Nathan Forsyth, P.Eng.
• Nicholas Stuhec, E.I.T.
• Jianan Cai, Ph.D.
• Jared Nicholas, C.E.T.
Storm Drainage Master Plan 2023
Final Report Rev 001 City of Grande Prairie
1.0 Introduction
1.1
Project Background
Sameng Inc. (Sameng) was retained by the City of Grande Prairie (City) in September 2022 to undertake an update to its Storm Drainage Master Plan (SDMP) The last update to the Master Plan was completed in 2018, with previous versions of the Master Plan having been completed in 2013, 2004 and 1995.
After a decade of rapid growth, the City’s population has held relatively steady for the past 5 years at about 64,000 residents (2021 Municipal Census). According to the City’s 2008 Growth Study, population in Grande Prairie is projected to reach 80,000 by 2028 and 100,000 by 2038. As growth patterns for small cities often happens in cycles of rapid growth followed by periods of stability, this indicates a proactive approach of infrastructure that can be implemented with flexibility, while maintaining high standards for effectiveness, sustainability, and safety.
1.2
Project Location
Grande Prairie is a City in northwest Alberta, Canada, in the heart of the Peace Region. It is located approximately 465 km northwest of Edmonton at the intersection of Highway 43 and Highway 40 at Latitude 55.17N and Longitude 118.79W. The City is surrounded by the County of Grande Prairie No. 1, which includes the Hamlet of Clairmont just north of the City. The City covers an area of 13,658 ha.
A location plan of the City is shown in Figure 1-1 below, while a map showing the current City neighborhoods and key areas is shown in Figure 1-2. Note that the location of the Hermit Lake rural study area and Hockey Estates Neighbourhood are located outside of the Figure 1-2 boundary but described in additional detail in Section 9.0.
1.3 Existing Land Use Plan
Figure 1-3 demonstrates the current approved land use plan in the City (Bylaw C-1260).
Most of the City is residential with some small commercial and public service/institution nodes scattered throughout the City. The main commercial areas are focused around Highway 43 north and west of the City. The main industrial areas are west of the City, with a few other industrial nodes north of the City and east of the railroad tracks east of Resources Road. The Grande Prairie Airport is located to the northwest of the City. The Bear Creek area is surrounded by a recreational open space which often acts as a buffer against surface flooding.
1.4 Future Land Use
Figure 1-4 demonstrates the current future land use development plan (Bylaw C-1237Q). Future commercial/industrial developments are anticipated to focus along the northwest and northeast of the City. Residential areas will mostly extend from the existing residential areas to the east and south of the City and will cover a large portion of land to the northwest near the Bear Creek Corridor and near Hermit Lake and Bear Lake. The area near the
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intersection of 100 Avenue and 100 Street is designated as the Central Business District where some of the higher density residential and commercial office sites are planned.
1.5
Project Objectives
The main objective of this Master Plan is to review and advance the recommendations of the previous storm drainage plans and address the following key project actions.
The key project actions are as follows:
• Review and assess recently collected rainfall and flow monitoring data and evaluate the rainfall and flow monitoring program.
• Review of the current Municipal Development Standard design storms, and determine if updates to the current IDF & design storm values require update
• Update the performance assessment of the City’s existing drainage system with refined modelling.
• Further refine conceptual drainage improvement recommendations in urban areas to mitigate flooding in at-risk areas, and phase proposed improvements to increase the feasibility of implementation.
• Update existing Mike Urban stormwater model & develop results for future development areas of the City for the 5 and 100-year design event. The updated model is to aid the City in future feasibility assessments & designs of stormwater management facilities
• Complete stakeholder consultations and open houses in support of the 2023 SDMP development, to raise awareness of the initiative and implement feedback from key stakeholders
• Provide an updated Mike+ model, existing drainage capacity review, and summary of drainage improvement recommendations for the Hermit Lake Study Area.
Drainage Master Plans prior to 2015 for the City were completed in 1995, 2004, and 2013. These previous plans were focused on reviews of the stormwater servicing standards for the City and included overall plans for future servicing.
2.2 2018 Storm Drainage Master Plan
Sameng Inc was retained by the City of Grande Prairie in November of 2017 to undertake an update to its Storm Drainage Master Plan (SDMP). The main objectives of the 2018 SDMP were to provide a comprehensive review of the City’s existing drainage design standards, a thorough inventory of the City’s existing drainage system, the development of a comprehensive computer model of the existing drainage system, examine issues with the existing drainage system and identify upgrades to mitigate these issues, and develop an effective long-term drainage plan to allow future development to proceed.
2.3 2023 SDMP Update
The primary goal of the 2023 SDMP is to further refine the advancements of the 2018 SDMP, provide a more accurate model representation of the existing drainage systems for City use, and provide recommendations to improve implementation of drainage improvements to the existing and future drainage service areas.
• Review and assess recently collected rainfall and flow monitoring data, evaluate the rainfall and flow monitoring program, and determine if recent results require updates to the existing servicing standards & design storms
• Update the existing 2018 SDMP model to include recent developments and infrastructure improvements. The 2023 SDMP model also utilizes improved LiDAR data and updates the 2018 Mike Urban model to its successor program, Mike+.
• Review and describe the flood-mitigation impacts of implemented urban drainage improvement projects recommended under the 2018 SDMP.
• Further refine conceptual drainage improvement recommendations in urban areas to mitigate flooding in at-risk areas and recommend phasing for the proposed improvements to increase the feasibility of their implementation under the existing City budget cycle
• Update the existing Mike Urban stormwater model & develop model result figures for future development areas of the City for the 5 and 100-year design event. The updated 2023 SDMP model is to aid the City in future feasibility assessments & designs of stormwater management facilities
• Complete stakeholder consultations and open houses in support of the 2023 SDMP development, to raise awareness of the initiative and implement feedback from key stakeholders
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• Provide an updated Mike+ model, existing drainage capacity review, and summary of drainage improvement recommendations for the Hermit Lake Study Area.
3.0 Existing Drainage System Updates
3.1 New Development Areas
New developments areas have been implemented since the 2018 SDMP. Table 3-1 summarizes these new developments, which have been integrated into the new 2023 SDMP Mike+ stormwater model
3-1:
3.2 Existing Urban Area Drainage Improvements
The City has also completed a variety of drainage improvements such as culvert, ditch, SWMF and storm pipe improvements which have been integrated into the 2023 SDMP Mike+ model. The locations of these improvements are demonstrated in Figure 3-1 and their flood-mitigation impacts are summarized in section 7.1.
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4.0 Review of Rainfall & Flow Monitoring Program
Sameng collected and compiled the available rainfall and flow data recorded at 5 rain gauges and 5 flow monitoring locations across the City. The original November 9, 2022, technical memorandum regarding the rain gauge and flow monitoring data review is included in Appendix A. Note that a 6th rain gauge location (and level sensor) is present near the SE edge Bear Lake, located near the northwest City boundary. Due to the limited information available at the Bear Lake rain gauge location at the time of the rainfall data analysis, due to it’s shorter monitoring duration, the Bear Lake gauge data is recommended for review during the next SDMP iteration. The analysis provided within the 2023 SDMP consisted of detailed review of rainfall and flow monitoring data from 2019 to 2022, however it should be noted that the City’s collected rainfall & flow monitoring data from 2009-2017 were previously reviewed under the 2018 SDMP.
4.1 Rainfall and Flow Monitoring Analysis
The rainfall recordings from 5 rain gauges were downloaded from the City-provided online data portal link. Table 4-1 summarizes the range of data collected.
Table 4-1: Summary of Rain Gauge Data 2019-2022 Rain Gauge Location From To No. of Records
A statistical analysis of the collected data was completed to determine the number of large storm events measured in the intervening time. A summary of this review is presented in Table 4-2 below.
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Table 4-2 Statistical Summary for Recorded Rainfall Events at 5 Rain Gauges (2019-2022)
The compiled rainfall data from 5 rain gauges are demonstrated in Figure 4-1
Figure 4-1 Compiled Rainfall Data from 5 Rain Gauges
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The observations from the rainfall data include
• In total, Sameng received 13 years of data if combining all rainfall records.
• Two outlier events were detected and filtered out at Rain Gauge 2 (June 16, 2020) and Rain Gauge 3 (June 14, 2020). Both of these events demonstrated potential of rain gauge malfunction, where the measured rainfall intensities were of consistent (within 2mm) high intensities for 6-8 5-minute timesteps.
• Other recorded events at all 5 rain gauge locations are within the expected ranges
• All historical rainfall records are in line with the defined IDF curves proposed in the 2018 SDMP.
• The most intense rainfall events were recorded in 2020, on June 24 at RG02, June 28 at RG04, and June 14 at RG03. This indicates a certain level of spatial variation of the storm cells moving through the City.
• Other less intense rainfall events were recorded on August 02, 2020, May 27, 2021, and July 30, 2011. These events were recorded by multiple rain gauges on the same date indicating bigger storm sizes for less intense events.
• An additional rainfall event from July 10, 2023, was reviewed at the request of the City. This 1 hour, 100-year event was recorded at RG02, however upon review the localized effects and low impact of the event do not warrant changes to the core IDF recommendations from the 2018 SDMP.
The locations of the Rain gauge and flow monitoring locations (with the exception of the rural Bear Lake location) are demonstrated in Figure 4-2 Figure 4-3 through Figure 4-7 present the rain gauge summary reports developed through the City’s online data portal link.
4.2 Data Quality Review – Flow Monitoring
Since 2019, the City has begun monitoring 5 locations for stormwater flow across its stormwater system. As of November 2022, Sameng has received 60 flow monitoring datasets from the City. Each dataset recorded the water level, velocity, and flow data at a frequency of every 5 to 15 minutes. Given the different formats of these datasets, we exported and compiled all records into excel worksheets and validated the records for further evaluation. Appendix B (attached separately) includes an excel overview of all 60 flow monitoring datasets.
The observations from the flow monitoring data include
• In total, we have received 60 datasets at 5 locations from the years 2019 – 2022.
• The formats of these datasets are inconsistent varying from year to year. Some are in text file format “.csv”, and others are in database formats of “.sdb” or “.mdb”.
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• The recorded flow monitoring frequency was every 5 minutes in 2019 but switched to 15 mins in 2020 and 2021.
• In general, many datasets contain negative flow rate readings which will be filtered out for further evaluation.
• Three problematic datasets were identified
o From the year 2019, “Flow Meter 2 95th Ave – isco.sdb” contains data from two locations identified as Unit 1 and Unit 3. A portion of the flow, level and velocity records under “Unit 1” overlaps with the FM2 dataset “A19-01002_1.csv”. A portion of the data under “Unit 3” overlaps with the FM1 dataset “A19-010-01 - Feb - Sept 30.csv”. This indicates the unit ID and flow monitor ID may not be consistent.
o From the year 2019, “A19-010-04-Sept 29-Nov 8.csv” contains errors as the records of the flow rate and the water depth are exactly the same.
o From the year 2022, “A19-010-01_1.csv” contains values of “0” for all flow, flow rate, velocity and level values.
The post-filtered flow monitoring datasets were utilized for the calibration of the 2023 SDMP model.
4.3 Water Quality Monitoring
The 2018 SDMP recommended two locations for water quality monitoring, so as to provide the City with an understanding of typical unbuffered runoff from commercial and industrial areas. The two recommended locations were:
1) MH80004, located along the 2100mm diameter storm pipe on 116 Avenue between 101 Street and 102 Street. As no storage or water quality facilities are present between this location and the downstream outfall, this will be a suitable location to measure typical, unbuffered highway commercial development loading to the creek.
2) MH 57100, currently the location of “Flow Monitoring Location #5” as presented in Figure 4-2. Water quality measurements at this location will provide measurements that capture a large commercial/industrial area, upstream of the Canfor ditch.
The two above locations continue to be recommended for future water quality monitoring in this 2023 SDMP, due to their relatively large contributing catchments and lack of upstream stormwater buffering. The capture and review of water quality data from these areas would be valuable in determining typical commercial & industrial stormwater runoff water quality under existing City development standards. Furthermore, the results would provide insight into the effectiveness of existing oil & sediment controls in established commercial/industrial areas.
As many of the future development areas of the City are anticipated to drain into natural watercourses via new outlets, the collected water quality data would be also valuable in determining whether adjustments to the existing development oil & sediment control requirements are warranted for future developments.
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4.4 Recommendations
The received historical rainfall data sets are generally in good agreement with the proposed IDF curves from the 2018 SDMP As such, the City design standards should maintain the design storm & IDF curves recommendations provided by the 2018 SDMP The recorded flow readings contain certain errors but most of the records could be used for system evaluation.
Based on the received historical rainfall and flow monitoring data, Sameng recommends continuing with the rainfall and flow monitoring at current locations. A better-structured data collection system or database management techniques should be utilized to unify the future flow monitoring process and decrease resources required for data processing.
The newest City rain gauge (Bear Lake location), had limited rainfall information at the time of the 2023 SDMP rainfall analysis due to its shorter installation duration (installed in November of 2021). As such, the analysis for this location is recommended for review in the next SDMP iteration.
Furthermore, increased frequency of data collection and data review for rain gauges and/or flow monitoring units can allow for earlier diagnosis of problematic data sets. By reviewing data sets more frequently for errors such as those identified in section 4.2 above, erroneous data collection units can be rectified earlier in the data collection process so as to minimize post-processing efforts later in the review stages.
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4-2: Rain Gauge and Flow Monitoring Locations
Legend Rain Gauges
Service Centre
RG02 South Fire Hall
RG03 North Fire Hall
RG04 Eastlink Centre
RG05 Woodgrove
Flow Monitoring
Flow Meter #1
Flow Meter #2
Flow Meter #3
Flow Meter #4
Flow Meter #5
Figure
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Figure 4-3: Service Center Rain Gauge Report
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Figure 4-4: South Fire Hall Rain Gauge Report
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Figure 4-5: North Fire Hall Rain Gauge Report
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Figure 4-6: Eastlink Center Rain Gauge Report
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Figure 4-7: Woodgrove Rain Gauge Report
Storm Drainage Master Plan 2023
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5.0 Hydraulic Model Refinement
5.1 Overview
The 2023 SDMP Mike+ computer model was updated from the Mike Urban model from the 2018 Storm Drainage Master Plan and the provided sub-models from the City The new model was updated using improved LiDAR data, updated utility data from the City, and converted to Mike+ by DHI as it is the successor program to Mike Urban. As the majority of technical updates and supplier support for Mike Urban are no longer active, the change to Mike+ was implemented to improve model support in the coming years. This section gives an overview of the modeling parameters from the previous models and the modifications made.
The resulting computer model, provided under Appendix C provides a detailed representation of the existing major and minor drainage systems within the City, and we trust that it provides realistic hydraulic results given the information available. This comprehensive hydraulic model was used to evaluate the existing and improved hydraulic conditions of the drainage system in the project area. Simulation results figures are presented in Section 6.0
5.2 Methodology and Parameters
The following provides a brief model construction methodology and important model parameters:
• The 2023 Storm Drainage Master Plan Model is a 1D-2D integrated hydraulic model. Within the City limits, rainfall-runoff hydrology as well as overland flows are modeled by a 2D mesh, which simulates ponding volumes, flow resistance, and infiltration on the surface.
• Outside the City boundary, hydrology is modeled by catchment shapes with direct input into the sewer system.
• Connections between the 2D and 1D portions of the model are simulated by ‘curbinlet’ algorithms.
o Inflows from surface to pipe are based on tables relating the depth above the CB grate with the inflow rate. These tables are derived from laboratory tests for each grate, assuming placement in sags.
o All CBs in the model are assigned a specific grate function. Many of these grates are obsolete types, and the grate types were not verified by Sameng.
o Some curb inlets are identified as restricted, simulating the bottom part of the grate being closed with only the side-inlet portion open. These have not been verified.
o Where the depth of ponding exceeds the range of the table, or when surcharging in the pipe reaches the surface elevation, the algorithm switches to an orifice equation based on the flow area of the grate.
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o For storm ponds throughout the City, weir equations are used to simulate the head-loss in the inlets and outlets. Control devices are modelled as requested by the City.
5.2.1 Mesh Development
In order to assess the impact of extreme rainfall events on the 2D surface, a layer of flexible mesh over the entire City area is developed using the 2D Overland module in MIKE+ and Terrain Analysis in GRASS GIS.
As the first step, feature lines were identified from LiDAR information to precisely describe the overland drainage connectivity consist of high and low points, including:
- Road crowns and edges (3 parallel lines) with gutters from catchbasin to catchbasin
- Ditches and valleys (2 to 3 parallel lines 1-2m apart on the bottom, 2 additional lines along the banks)
- Obstacles e.g. berms and dikes (2 lines on the top, 2 lines at toes)
Secondly, individual areas were created so high and low meshing densities can be assigned accordingly and tailored for different regions. High mesh densities were allocated for specific areas with concentrated flows, flooding histories, or identified problems. In general, 3 mesh densities were created within the model:
- East of Bear River (Low density)
- West of Bear River (Less than 1ha)
- Specific Areas (Less than 10m2), e.g. ditches, storm pond, and problematic areas.
5.2.2 Topographic Interpolation
The raster files we received are 1m or finer grids which take longer computer time for interpolation. Terrain Analysis tools in GRASS GIS were utilized to process the data. Samplings were taken at consistent intervals to simplify the topography.
- Every 10m for and within the high and low points
- every 5m for roads
- every 1m for ridges and valleys
- Every 5m for buildings
Topographic information was then interpolated from the nearest point
5.2.3
Catchment Characteristics
Catchment delineations were also developed using Terrain Analysis tools in GRASS GIS. Basin slope, infiltration rate and hydraulic roughness are defined based on the catchment characteristics and land use map.
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- Road and buildings (high imperviousness and smooth surface)
- Parks (low imperviousness and rough surface)
- Commercial development areas (high imperviousness and smooth surface)
- Residential areas (medium imperviousness and surface)
- Rural areas (low imperviousness and rough surface)
5.2.4 Model Coupling
When the hydrological parameters are all defined, the 2D module of the surface is ready for simulating the overland flow taking into consideration of different flow directions. Culverts and surface drainage features are included as 2D elements.
The 1D component of the model is constructed based on the City’s storm pipe network database. It represents the linear underground drainage system including manholes.
Coupling of the 1D and 2D systems were made through the catchbasin and outfall connections between 2D meshes and nodes in the 1D layer.
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6.0 Existing Drainage System Assessment
6.1 Overview
The updated model was used to determine the effectiveness of the minor system. Simulation results for the 5-year 4-hour Chicago distribution rainfall event, the 100-year 4hour Chicago distribution rainfall event and the 100-year 24-hour Chicago distribution rainfall event are included in this section.
6.1.1
Minor System Drainage
For the storm sewer system, the simulation results show the theoretical loading of the pipes (Qpeak/Qcap) as well as the surcharge depth in each pipe (identified at the manholes). From these results, it is possible to identify which sewer pipes are flowing beyond their pipefull capacity (Qpeak/Qcap > 1.0), and how surcharged the pipes are.
6.1.2
Major System Drainage
For the major drainage system, the simulation results show the ponding depth and extents along the main drainage paths, as well as the peak flow rate on the ground surface.
The maximum ponding elevations from the 2D overland model outputs were compared with a ‘dry’ reference surface elevation model to generate the maximum ponding depths. The 2D ponding maps are very useful as they estimate the extent of the ponding areas on public and private properties, and they can consequently be used to identify flood risk areas. However, as both the model results and the reference surface use interpolation between discrete nodes, these 2D ponding maps are subject to interpretation from the computer modelers. Therefore, these 2D ponding maps shall not be assumed to be 100% correct, but they are very useful to identify areas at higher risk of flooding.
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6.1.3 Simulation Results 4-hour, 5-year Chicago Design Event
Under the 4-hour, 5-year design event, 76% of the minor system manholes and CBMHs are anticipated to receive surcharge of 1.0m or more below ground level, providing a minimallow risk of sewer surcharge. The remaining 24% of manhole and catchbasin & manhole infrastructure are anticipated to surcharge from 1.0m below ground to over 0.5m above ground, leading to medium to high risks of sewer surcharge flooding and potentially contributing to surface flood risks. The existing system sewer surcharge results for the 4hour, 5-year Chicago design event are provided in Figure 6-1.
The surface ponding model results for the 5-year design event are provided in Figure 6-2. During the 5-year design event, the majority of roadways are anticipated to see no or minor (<15cm) surface ponding. However, select trapped lows in residential areas or along major roadways which convey significant surface flows may see surface ponding depths which exceed 35cm (the recommended 100-year surface ponding threshold). Select residential properties with poor surrounding surface conveyance experience surface ponding up to 0.5m during this event.
6.1.4 Simulation Results 4-hour, 100-year Chicago Design Event
Under the 4-hour, 100-year design event, 41% of the minor system manholes and CBMHs are anticipated to receive surcharge of 1.0m or more below ground level, providing a minimal-low risk of sewer surcharge. The remaining 59% of manhole and catchbasin & manhole infrastructure are anticipated to surcharge from 1.0m below ground to over 0.5m above ground, leading to medium to high risks of sewer surcharge flooding and potentially contributing to surface flood risks. The existing system sewer surcharge results for the 4hour, 100-year Chicago design event are provided in Figure 6-3
The surface ponding model results for the 100-year design event are provided in Figure 6-4. During the 100-year design event, various trapped roadway lows are anticipated to reach surface ponding depths up to 1.0m. At this level of depth, roadway flooding can become impassable for standard vehicles, and pose significant flood risks to parked vehicles and adjacent residences. The City should review the updated model results when planning upcoming minor system improvements, to determine effective placement of proposed improvements.
6.1.5 Rural Area Simulation Results
The 2023 SDMP model results include the existing system surface ponding simulations for the 5-year and 100-year 4-hour design rainfall events for rural service areas within the City boundary. These results are provided in Figure 6-5 through Figure 6-17 and are split into 6 “areas” for clarity of review and discussion. The provided figures were developed to be a high-level tool to assist the City in identifying existing rural properties at potential risk of surface flooding and to provide additional context when reviewing new service area development drainage designs.
The drainage simulations of the rural service areas suggest that the majority of rural lands surrounding the City’s urban boundary are well-graded towards existing natural waterways and drainage pathways. Under existing conditions, surface flooding risks are likely to be related to:
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1. Drainage barriers such as roadways which can concentrate stormwater flows. Inadequate roadside drainage ditches and/or culvert crossings along roadways may cause surface ponding which impacts adjacent private properties and/or roadway function following storm events.
2. Trapped low areas which may experience surface overflows during large rainfall events
3. Naturalized drainage pathways which may experience surface overflows during large rainfall events
The above elements should be considered when considering future development adjacent to surface ponding areas identified by the 2023 SDMP model surface ponding results. Future investigations may be required to determine appropriate development setbacks from naturalized areas, as well as determine whether roadway culvert improvements are warranted as development increases.
WOOD LAKE
CREEK
BEAR
HUGHESLAKE HERMIT LAKE
HUGHESLAKE
HERMIT LAKE
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7.0 Improvement Concept Plans For Existing Urban Areas
7.1 Overview of the of 2018 Storm Drainage Master Plan Recommendations for Urban Areas
The 2018 SDMP contained conceptual drainage improvement designs for five urban areas identified as being of high flood risk. The primary goal of the proposed improvements was to provide a 100-year level of stormwater protection, significantly reducing surface ponding, sewer surcharge levels, and overall flood risks. Conceptual drainage improvement designs and cost estimates for each of the five selected urban areas were assembled, with a total estimated cost of 98.5 million dollars. Table 7-1 below summarizes the 5 locations and estimated total costs of improvements proposed in the 2018 Storm Drainage Mater Plan
Ivy Lake / Cobblestone / Smith Area (Central-East) $
Richmond Industrial Area (Central-West)
M
98.5 M
Since 2018, the City of Grande Prairie has completed portions of the recommended urban drainage improvements recommendation in the 2018 SDMP. The 2023 SDMP has assessed the impacts of the completed improvements and residual flood risks in these areas through review of the updated 2023 SDMP stormwater model results. The results of this review have found the implemented improvements to have significantly reduced stormwater flood risks during the 100-year storm event in the affected areas, and the review results are summarized below.
7.1.1 Completed Ivy Lake/Cobblestone/Smith Area Urban Improvements
The 2018 SDMP identified the industrial area south of 92 Avenue between Park Road and 92nd Street as being at high risk for surface flooding during the 100-year event, with a substantial portion of the area anticipated to receive ponding depths upwards of 50cm. While less severe, these properties were anticipated to generate ponding depths under 35cm during the 5-year event, posing surface flood risks during smaller rainfall events. The area receives significant overland flows from the Smith and Morgan Meadows to the north
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and is reliant on an industrial ditch system south of 92nd Avenue to adequately drain large overland flows.
To improve overland drainage within the area, the City has completed ditch and culvert improvements south of 92 Avenue to more efficiently convey major drainage overflow southwards into the Woody channel. The completed improvements have reduced anticipated surface ponding for the majority of adjacent properties to 35cm or below during the 100-year storm event, and improved previous culvert bottlenecks which risked additional surface flooding during such events. Some residual risk areas remain directly north of the ditch improvements, such as on 93 Avenue between 92a and Juno crossing, where roadway ponding up to 50cm during the 100-year event may pose a risk to parked vehicles. However, these areas are anticipated to improve as the remainder of the Ivy Lake /Cobblestone / Smith Area improvements are gradually implemented and divert stormwater from the area. Additional details on the outstanding recommended drainage improvements for this area are included in Section 7.2.8.
7.1.2 Completed Richmond Industrial Area Urban Improvements
Three primary drainage improvement packages have been completed within the Richmond Industrial Area, contributing to significantly reduced flood risks.
1) 116 St Pond Expansion
The existing stormwater pond located southwest of the 95 Avenue & 116 Street Intersection was expanded to prevent overflow during the 100-year event. The 2018 SDMP model indicated that overflow from the facility during the 100-year storm event could contribute to surface ponding greater than 50cm to the surrounding properties and roadways.
Following the completed improvements, the surrounding area is anticipated to receive minimal ponding during the 100-year event, with the majority of surface ponding restricted to the adjacent roadways and at a level below the City’s acceptable 35cm for the 100-year event. The implemented improvements appear to have decreased surface flood risks for the surrounding area, while also providing a potential storage location for future surface runoff as the area continues to develop.
2) Richmond Pond Improvements
The 2018 SDMP identified significant surface flood risks at both intersections of 89 Avenue & 112 Street, with surface ponding anticipated to achieve up to 1m on the adjacent roadways. This area was identified as being a trapped low at the downstream end of a large catchment area (>50ha), which will receive significant overflow drainage during the 100-year event. Surface flows which are not trapped within this depression or captured by the local storm sewers were likely to flow eastwards on 89 Avenue, where a significant portion would be trapped west of 108 Street and escalate surface flood risks in the area.
To more effectively divert stormwater drainage from the above areas, the 2018 SDMP recommended a series of drainage improvements including implementation of the Richmond Pond, related stormwater piping upgrades, and implementation of a high-capacity sewer grate on 89 Avenue near 112 Street to reduce ponding depths at this location. These improvements created improved stormwater capture, storage, and conveyance for the area.
The above drainage improvements outlet to the existing 84 Avenue storm system via a newly installed 1200mm storm sewer connection.
Following implementation of the above improvements, a significant reduction of surface ponding was noted on 89 Avenue near 108 Street during the 5 and 100-year design events, which is further described in the “Canfor Ditch Improvements” subsection below. In addition, the 2023 SDMP model indicates a reduction of surface ponding of the 89 Avenue & 112 Street intersections from 1m to 35cm for the western intersection, and from 1m to 50cm for the eastern intersection during the 100-year storm event. Overall, the implemented improvements have significantly reduced the 100-year event surface ponding depths of select roadway trapped lows on 89 Avenue between 116 St and 108 Street, significantly reduced property flood risks on 89 Avenue near 108 Street, and improved conveyance of overland drainage flows to the 84 Avenue storm system.
Select trapped lows along 89 Avenue between 112 Street and 108 St are anticipated to reach surface ponding levels up to 50cm during the 100-year storm event, exceeding the City benchmark of 35cm, however the depth and extent of these ponding areas have been significantly reduced following the implemented Richmond Pond improvements. Future utility rehabilitation projects within the area should consider additional catchbasin capacity within these trapped lows to further reduce surface flood risks.
3) Canfor Ditch Improvements
The 2018 SDMP identified the intersection of 89 Avenue & 108 St, adjacent CANFOR ditch, and surrounding area at high risk of flooding during the 5-year and 100-year storm events. Due to the area’s location at the downstream end of Richmond Industrial, both in terms of sewer flows and major drainage flows, the area collects significant major and minor stormwater flows. Restricted stormwater conveyance in the area due to the high elevation of 108 Street, multiple restrictive culverts within the Canfor ditch, and low elevation of the downstream CANFOR ditch were noted as contributors to the areas high risk of properties flooding during events as minor as the 5-year event. Furthermore, as the stormwater sewer system west of the 89 Avenue & 108 Street intersection is located within a large trapped low, sewer surcharge during the 100-year storm event via manholes and catchbasins were anticipated to contribute to surface flood risks. For the 100-year event, surface ponding levels as high as 155cm were anticipated for 89 Avenue, with adjacent roadways and property edges exceeding surface ponding levels of 85cm.
Since the 2018 SDMP, the City has successfully implemented a series of drainage improvements to the CANFOR ditch including upsizing of the existing stormwater pipes, upgrading of restrictive culverts to larger box culverts with improved conveyance capacity, and regrading of the ditch to improve stormwater conveyance.
Following the above improvements, the 2023 SDMP model shows significantly reduced surface ponding risks within the area. The deepest surface ponding during the 100-year storm event is anticipated to occur on 89 Avenue immediately west of 108 Street, where surface ponding is anticipated to reach depths of approximately 50cm during the 100-year event. However, this level of surface ponding is anticipated to remain confined to the roadway surface. Surface ponding of 89 Avenue (between 108 and 109 Street) as well as 109 Street (between 89 Avenue & 92 Avenue) are anticipated to remain below 35cm during the 100-year storm event. Property edges along these portions of 89 Avenue and 109 Street are anticipated to see minor surface ponding of up to 35cm during the 100-year storm event, however damages to the industrial yards and/or buildings are anticipated to be minimal.
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Overall, the implemented changes have significantly reduced surface and surcharge flood risks for the area for events up to the 100-year storm event. The outstanding drainage improvements recommended for the Richmond Industrial area are detailed in Section 7.2.9.
7.2 2023 Update of Improvement Concept for Existing Urban Areas
Review of the 2023 SDMP model has confirmed the flood-mitigation effectiveness of the 2018 urban drainage improvements implemented by the City, as well as confirmed significant outstanding flood risks associated with the 2018 urban drainage improvements awaiting implementation.
The most at-risk areas of the City due to intense rainfall events are (in no particular order):
These eleven areas are primarily at risk of flooding due to a poor major drainage system (i.e. overland drainage). In other words, during an intense downpour, the storm sewer pipes are flowing at their maximum capacity and excess rainfall runoff is trapped on the ground surface until the sewer system regains capacity. Flooding in these areas is generally only of concern for rainfall events larger than the 1:5-year event.
The 2023 SDMP confirms the 2018 urban improvement recommendations as being effective long-term infrastructure goals to achieve an ultimate 100-year level of drainage servicing in the identified areas. The concepts were designed so as to mitigate flood risks through the significant reduction of surface ponding and sewer surcharge during large storm events. However, as the planning, funding, and implementation of close to $100 million dollars in drainage improvements is a significant effort which may require decades, the 2023 SDMP includes updated urban improvement concepts which improve upon the original 2018 SDMP urban improvement recommendations. The revision includes updated quantity & cost estimates which reflect current market conditions, minor refinements of the 2018 SDMP concept design scopes, and the creation of recommended phasing for implementation of the updated 2023 SDMP urban improvement recommendations. The 2023 SDMP urban improvements have been organized into phases so as to:
1. Provide the City with a series of drainage improvements in the identified areas which, when implemented sequentially, allow for the gradual achievement of 100year levels of drainage service in these areas
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2. Separate the required scopes of work into phases which more closely align with standard City drainage project budgets
3. In cases where implementation is possible without negatively impacting the downstream system, items with increased flood-risk benefits are prioritized through inclusion in earlier phases
It should be noted that in many cases, the phasing of improvements is sequenced such that downstream improvements are implemented first, to ensure downstream drainage systems have the required capacity to accept increased stormwater flows from upstream improvements.
Conceptual cost estimates for the 2023 SDMP Urban Improvement recommendations are summarized in Table 7-2, with detailed cost estimates and quantity breakdowns included in Appendix D. Further description of the 2023 SDMP urban improvement recommendations are described by area in the following sections. In general, unit rates have increased since the 2018 SDMP, which is reflected in the 2023 urban improvement recommendations
Ivy Lake / Cobblestone / Smith Area (Central-East)
Richmond Industrial Area (Central-West)
$ 13.9 M
$ 7.14 M
TOTAL $ 101.4 M
7.2.1 2018 SDMP Urban Improvement Concept – Northridge Area
The main improvement concepts for the Northridge Area in the 2018 SDMP consisted of capture & conveyance of major system drainage flows in the area, reducing the hydraulic grade line in the existing sewer network at strategic locations by installing large diameter sewer pipes, and by conveying increased drainage from the improved upstream system through an upgrade conveyance channel (ditch) south of 116th Avenue.
The 2018 Northridge Area Improvements can be segmented into 4 distinct scopes:
1. Deepening of the drainage ditch south of 116th Avenue to provide a flow capacity of 20,000L/s so as to better convey overland drainage during large storm events and provide capacity for upstream minor drainage system improvements. Related ditch improvements included implementation of a 2700mm diameter downstream storm pipe and new downstream 2700mm diameter outfall to receive flows from the ditch.
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2. Installation of a proposed 1800mm pipe along 116 Avenue from east of the railroad to 100th Street. At the railroad, this pipe would capture drainage from the swale north of 116 Avenue (via inlet) and prevent that drainage from overflowing 116 Avenue and flowing southwards into the Avondale/Montrose Area.
3. Implementation of large diameter storm pipes (1500mm – 2100mm) along 102nd Street from the 116th Avenue ditch northwards to 124th avenue, as well as branches eastwards on 124th Avenue and 120th Avenue. These improvements, alongside catchbasin improvements, are anticipated to reduce surface ponding during large storm events, increase local storm sewer capacity, and better convey stormwater downstream to the 116th Avenue ditch
4. (Provisional) As an alternative to the pipe improvements along 102nd Street, ditch improvements such as ditch widening, ditch lowering, and culvert upgrades were proposed for the north/south oriented ditch east of 102 Street. However, the 2018 SDMP model confirmed that the proposed ditch improvements were not adequate to replace the 100-year level of service provided by the proposed 102nd Street pipe improvements. These findings were supported by the updated 2023 SDMP model. As such, this item has been removed from the 2023 SDMP recommendations
7.2.2 2023 SDMP Urban Recommendations & Phasing – Northridge Area
The 2023 SDMP Northridge Area recommendations are illustrated in Figure 7-1 and are recommended for implementation in the following sequencing:
1) Phase 1A and 1B are the farthest downstream of the proposed Northridge area improvements and are centered on the 116th Avenue ditch improvements and proposed outfall. These improvements provide the required stormwater capacity for the proposed upstream improvements.
Phase 1A consists of the installation of a new 2700mm diameter outfall and storm pipe to function as an outlet to the proposed 116th Avenue ditch improvements, as well as a new 900mm connection to the existing outfall to increase flow capacity during low-flow events. Ideally, the new outfall, connecting 2700mm pipe, and 900mm connection to the existing outfall will be installed in parallel or in advance of the 116th Avenue ditch improvements to ensure adequate flow capacity as an outfall for the ditch and other upstream improvements
Phase 1B consists of the deepening of the 116th Avenue ditch from 106th Street to 100 Street, replacement of select culverts within the 116th Avenue ditch which currently restrict the flow of stormwater, and implementation of upgraded connections and/or overflow pathways into the ditch from 116th Avenue. Improvement of the 116 Avenue & 102 Street overflow into the 116th Avenue ditch should be implemented during this phase to reduce ponding during large storm events. As these improvements are anticipated to significantly improve the area’s overland drainage capacity, alleviate stormwater ponding, reduce the risk of overflow from the 116th Avenue ditch, and are located downstream of phases 2A & 2B, this phase should be a priority for implementation. Implementation of this item is reliant on adequate downstream capacity of the existing outfall, and therefore is likely to require implementation following the completion of the Phase 1A improvements.
2) Phase 2A consists of new storm pipe connections and catchbasin improvements along 116th Avenue between 98 Street and 100 Street to reduce surface pooling at
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local intersections and intercept additional overland flows before they flow southwards into the Avondale/Montrose Area. This improvement will also interconnect with the existing storm system adjacent to the railroad, reducing the local hydraulic gradeline and mitigating flood risks near the rail lines. While the implementation order of Phase 2A and 2B can be exchanged, the reduction of risk to both the adjacent railway, interception of overland flows from the Avondale/Montrose area, and the smaller capital cost provides incentive for the City to prioritize Phase 2A.
3) Phase 2B consists of new storm pipes ranging from 2100mm diameter to 1200mm diameter northwards from 116th Avenue to 124th Avenue, eastward storm pipe extensions to capture areas of significant surface ponding, and catchbasin improvements. The primary objective of these improvements are to convey additional surface flows southwards to the 116th Avenue ditch and reduce high hydraulic gradelines & surface ponding anticipated at 124th Avenue and 121st Avenue just east of 102nd Street during the 100-year design event. Due to the significant length of pipe required for the Phase 2B improvements, the improvements scope could be further segmented to reduce upfront capital costs in return for increased management costs. The cost/benefit analysis of additional segmentation of the Phase 2B will require additional review outside of the scope of this Master Plan.
Note that selective road regrading has been recommended for the Northridge area to prevent stormwater from flowing southwards of 116th Avenue into the neighbouring residential areas. These roadway improvements are included in the cost estimates for the above Northridge concept phases based on their proximity to the proposed drainage improvements.
The total estimated base cost for the 2023 SDMP Northridge Area improvements (including contingency and engineering) is 32 7M.
2018 SDMP Urban Improvement Concept – Avondale/Montrose Area
The main improvement concepts for the Avondale/Montrose area in the 2018 SDMP consisted of select road & surface regrading to redirect overland drainage from trapped lows, implementation of a large diameter storm sewer (1050mm diameter to 2100mm diameter) connected to the existing storm system to decrease the local hydraulic gradelines, installation of large capacity CB grates to intercept major drainage flows from floodvulnerable residential areas to the south, ditch upgrades to improve overland drainage along 103rd Street, and pipe installation to provide relief to the existing Mountview stormwater management facility.
The proposed 2018 Avondale/Montrose Upgrades can be segmented into 4 distinct scopes:
1. Roadway and surface regrading was proposed to divert overland flows from vulnerable residential neighbourhoods and trapped lows. Surface regrading of west of 104th Street between 108 & 107 Avenue, as well as intersection regrading of the 112 Avenue & 113 Avenue intersection and 113 Avenue & 102a Street intersections were suggested improvements which could feasibly be completed separately of the area storm pipe improvements. Intersection regrading along 108th Avenue from 103 Street to 98th Street were proposed in tandem with the 108th Street pipe improvements to aid in conveying overland flows into the proposed 108th Avenue sewer.
2. Regrading and improvement of the 103 Street overland drainage path to improve conveyance southward from this high flood-risk area.
3. Upgrading of the existing 1050mm diameter outfall on 103 Street to a 2400mm outfall, significantly improving outlet capacity. Installation of 2100mm diameter storm pipes northwards along 103 Street to 108th Avenue and eastwards (1500-2100mm diameter) to the intersection of 99th Street & 108th Avenue.
4. Installation of a 1050mm to 1500mm pipe from the existing Mountview stormwater pond southward and westwards to the proposed 108th Avenue storm pipe to mitigate potential overflow from the stormwater facility during large storm events. As a lower cost alternative, a 900mm diameter pond overflow pipe was proposed to convey excess stormwater from the facility westwards to 99th Street where it may drain overland southwards to improved catchbasins at the intersection of 108 Avenue & 99 Street.
7.2.4 2023 SDMP Urban Recommendations & Phasing – Avondale/Montrose Area
While the Avondale/Montrose area is anticipated to benefit from overland flows diverted due to the Northridge recommendations, additional improvements are required to achieve a 100year level of drainage service.
The 2023 phasing of the Avondale/Montrose recommendations are illustrated in Figure 7-2 and are recommended for implementation in the following sequencing:
1) Interim Item 1* (Phase 1A) consists of an opportunity to bridge three existing minor sewer system gaps to improve the existing storm sewer flow connectivity and mitigate localized high sewer surcharge during smaller storm events. The three gaps in question are located at the intersections of 108 Avenue & 100 Street, between the intersections of 98 Street and 99 Street along 108th Avenue, and at the intersection
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of 105 Avenue & 100 Street. This item is a new addition to the 2023 SDMP recommendations for the Avondale/Montrose area and is included in the 2023 improvement cost estimate.
Due to its lower capital costs and surface impacts, this improvement is a potential interim solution to reduce sewer surcharge in the area. Should the City decide to implement the ultimate large diameter sewer upgrades along 108th Street in the near future (<10 years), this interim item should be disregarded.
2) Phase 2A consists of installation of a new outfall at the south end of 103rd Street, large diameter storm sewer installation (2100mm diameter to 1500mm diameter) running north from the proposed outfall to 108th Avenue and then Eastwards to 98th Avenue, and the following improvements. The installation of large diameter storm sewers, improved intersection catchbasins, connections to the existing storm sewers, and regrading of adjacent intersections along 108th Avenue to intercept overland flows to the south are key to providing a 100-year level of drainage service to the Avondale/Montrose Area. These improvements will significantly reduce the local hydraulic gradelines in the area and address existing surface ponding during the 100-year design event along 108th avenue such as the 100th St intersection In addition, the newly installed 1500mm storm pipe at 98 Street should be connected to the existing 600mm 98 Street storm sewer to provide relief and lower the existing sewer hydraulic gradeline.
Due to the significant quantity of surface disruption anticipated along 108th Avenue for the Phase 2A improvements, Sameng strongly recommends the City coordinate these improvements to coincide with roadway rehabilitation of 108th Avenue or adjacent underground improvements.
3) Phase 2B consists of 1050-1500mm diameter stormwater pipe installation from the Mountview SWMF southwards to the proposed 108th Avenue storm pipes to mitigate the existing overland flooding risks from the Mountview SWMF. The proposed sizing is adequate to provide 100-year drainage service to the SWMF and mitigate significant existing flood risks to the adjacent railway and private residences. Furthermore, this storm sewer connection bridges an existing minor system gap from 108th Avenue northwards along 96a Street, westwards along 109 Avenue, and northwards along 97 Street to the SWMF. By connecting this proposed pipe to the existing system at the intersection of 108th Avenue and 96a Street, this line will also provide additional relief to the local storm sewer system. The Phase 2B improvements in this area are dependent on additional capacity provided by the Phase 2A improvements, and therefore should be scheduled for completion following downstream capacity upgrades.
Note: Phase 2B* is an alternative overflow outlet for the Mountview SWMF which consists of a western overflow pipe across the existing railroad, letting water overflow onto 98/99th Street roadway. This option may allow for a less costly outlet for the Mountview SWMF during the 100-year event, the at the trade-off of potential risks to properties and motorists along 99th Street during the 100-year event. While the conceptual costs for this option have been provided as part of the 2023 SDMP, additional review of this potential alternative are required to quantify associated flood risks.
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4) Phase 3A primarily aims to mitigate surface and sewer surcharge flooding north of Avondale school by improving the existing surface conveyance pathways. The existing north/south major drainage pathway between 112 Avenue and 108th Avenue (roughly in line with 102B St) is to be regraded to facilitate southward overland drainage out of the area. Furthermore, surrounding intersections such as 113 Avenue & 102 Street, as well as 112 Avenue & 104 Street, are recommended for regrading to prevent overland flows from entering the adjacent residential areas and overwhelming the existing minor storm system.
Phase 3A also includes parkland surface regrading west of 104th Street between 108th Avenue and 107th Avenue, as well as intersection regrading of the 112 Avenue & 113 Avenue intersection and 113 Avenue & 102a Street intersections to mitigate eastward overland flows into the adjacent residential areas. These changes will redirect eastward overland flows which are currently anticipated to pool in existing local surface depressions.
Note: Phase 3A* is a potential alternative to Phase 3A, where the north/south drainage pathway improvements are replaced by upsizing the existing storm sewer pipe with a new 900mm diameter storm sewer. The conceptual cost for this alternative over twice the cost of the base Phase 3A improvements, however it may provide additional ease of constructability. The decision between implementation of Phase 3A or Phase 3A* should be further reviewed as the designs for this area are advanced.
The total estimated base cost for the 2023 SDMP Avondale/Montrose Area improvements (including contingency and engineering) is 31.3M.
7.2.5 2018 SDMP Urban Improvement Concept – Highland Park / Swanavon Area
The main improvement concepts for the 2018 SDMP for the Highland Park / Swanavon Area were to capture and convey major drainage flow from two large localized depressions and reduce the hydraulic grade lines of the existing sewer to mitigate local sewer surcharge flood risks. The 2018 concepts proposed installation of large diameter storm sewers and a new outfall structure into Bear Creek so as to bridge existing gaps in the minor storm sewer and convey stormwater flows from the two identified trapped lows to Bear Creek. One such trapped low was identified near the intersection of 96 Avenue & 100 Street, while the other was identified north of the intersection of 92 Avenue and 99 Street.
The 2023 phasing of the Avondale/Montrose recommendations are illustrated in Figure 7-3 and are recommended for implementation in the following sequencing:
1) Phase 1A consists of installing a new 1800mm diameter outfall into Bear Creek, so as to provide adequate capacity for the 100-year level of service provided by the proposed upstream improvements. The Phase 1A scope of work also includes the required road structure removal and restoration.
Note: Phase 1A* is a provisional scope item consisting of pipe connections from the existing storm sewer systems on 102 Street to the proposed Phase 1A outfall, such that the existing outfall can be abandoned. Should this option be developed in further designs, the designer should review whether the proposed Phase 1A 1800mm outfall will require upsizing.
2) Phase 2A consists of installation of a new 1800mm storm pipe from the proposed Bear Creek outfall eastward to the intersection of 100 Street and 94 Avenue, northwards to 96th Avenue, and eastward to mitigate surface ponding of the first of two major surface depressions in the area. This sewer improvement will also provide relief to the existing 375mm storm sewer at 94 Avenue & 101 Street via a new sewer connection and provide improved capacity for capture and conveyance of surface stormwater on 94th Avenue.
3) Phase 3A consists of a continuation of the 1800mm diameter sewer at 94 Avenue & 100 Street, extending westward to a local low area approximately 120m west of Resource Road, with a branch extending southwards from 94th Avenue along 99 Street until 93rd Avenue to reach the second major surface depression in the area. The 99 Street branch will interconnect with the existing 99 Street sewer to reduce the hydraulic gradeline during large storm events.
Installation of the above improvement will provide staged reduction of severe surface ponding within the area by improving outlet capacity to Bear creek, increasing capture of surface flows, providing improved capture of surface flows and ponding within trapped low areas by the minor storm system, and providing additional relief of the existing storm sewer as the proposed large diameter sewers are connected to the existing sewer systems at 94 Avenue as well as on 99 Street.
The total estimated base cost for the 2023 SDMP Highland Park / Swanavon Area improvements (including contingency and engineering) is 26.4M.
7.2.7 2018 SDMP Urban Improvement Concept – Ivy Lake / Cobblestone / Smith Area
The main improvement concepts for the 2018 SDMP for the Ivy Lake / Smith / Cobblestone Area were to prevent major flows from entering the Smith neighbourhood, to reduce ponding & surcharge in the area via a relief pipe to the Ivy Lake Estates Pond, and to improve the downstream conveyance/ditches to Woody Channel.
The 2018 recommendations for this area can be separated into 3 distinct groups:
1. Regrade select roadway intersections to divert surface flows from flood vulnerable residential areas in the Smith neighbourhood. These neighbourhoods often contain trapped low areas which contribute to severe surface flooding and storm sewers.
2. Install a 2100mm diameter pipe from the intersection of 100 Avenue and 92nd Street, which is anticipated to receive significant ponding during the 100-year design event, westwards to Ivy Lake
3. Improve ditch drainage and culverts to convey overland drainage more effectively from the north of this area southwards into the Woody channel.
7.2.8 2023 SDMP Urban Recommendations & Phasing – Ivy Lake / Cobblestone / Smith Area
Since the completion of the 2018 SDMP, the recommended improvements to the southern ditch drainage, culverts, and Woody Channel have been completed. However, select road regrading, implementation of improved capacity catchbasins at the intersection of 100 Avenue and 92 Street, and implementation of a new 2100mm storm pipe from this intersection to Ivy Lake await implementation
In addition to the 2018 SDMP recommendations, Sameng has identified a significant minor storm system gap in the Smith area (700+ metres) along 93 Street from 100 Avenue to 92nd Avenue. While the existing area relies heavily on overland drainage to convey flows southwards along streets and ditches to downstream stormwater facilities, filling of this minor storm system gap with a storm pipe and catchbasins capable of conveying the 5-year design event is anticipated to improve the capacity of the local minor storm system, reduce overland flows eastwards into adjacent vulnerable residential roadways, and increase the rate at which stormwater is removed from local roadway surfaces following large storm events.
The 2023 SDMP recommended phasing of the Ivy Lake / Cobblestone / Smith improvements are illustrated in Figure 7-4 and are recommended for implementation in the following sequencing:
1) Phase 1A consists of a new 2100mm storm pipe from the intersection of 100 Avenue & 92nd Street eastwards to Ivy Lake, catchbasin improvements within the intersection of 100 Avenue & 92nd Avenue, and regrading of the 100 Avenue &92 Street intersection. These improvements have been designed to relieve the existing high surface ponding and hydraulic gradelines anticipated for this intersection during the 100-year storm event. Furthermore, the catchbasin improvements and intersection regrading are anticipated to capture additional overland flows along 100th Avenue and convey them to the Ivy Lake SWMF. As this phase contains the highest flood-risk mitigation benefit for the area, it is recommended to be completed first out of the proposed area upgrades.
2) Phase 2A consists of regrading the intersection of 96th avenue and 92nd Street so as to mitigate overland flows from 92nd Street infiltrating the adjacent residential areas. Diversion of these overland flows are anticipated to reduce the sewer surcharge and surface ponding for the Morgan Meadows and Cobblestone neighbourhoods. As no new pipe installations are recommended by Sameng for this intersection, the intersection regrading at this location should be coordinated with local roadway improvements to improve the cost-benefit ratio of this item. The sequencing of Phase 2A is considered interchangeable with Phase 2B for this area and is dependent on available coordination with other roadway and underground improvements.
3) Phase 2B consists of implementation of a 5-year capacity minor storm sewer pipe along 93 Street from 100th Avenue to east of Juno Crossing, as well as local road regrading on the east side of select 93rd Street intersections to prevent overland flows on 93rd Street from overflowing eastwards into vulnerable residential neighbourhoods. While the Phase 2B improvements are anticipated to improve surface ponding and sewer surcharge in the Morgan Meadows neighbourhood during storm events, it is anticipated that the 100-year design event overflows will continue to travel southwards via overland drainage to the improved ditch system. As such, the Phase 2B improvements to be considered as a lower priority improvement and should be implemented in coordination with local road and/or underground improvements.
It should be noted that existing private trailer parks within the area have private drainage systems which may not comply with City standard. Under existing conditions, the 2023 SDMP model suggests these areas to be at low flood risk due to their storm outlet into the newly improved Woody Channel. However, significant updates to local roadways or overland drainage pathways should be reviewed to ensure adequate capacity of the potentially altered overland drainage pathways.
The total estimated base cost for the 2023 SDMP Ivy Lake / Cobblestone / Smith Area improvements (including contingency and engineering) is 13.9M. This cost includes the newly proposed 93 Street minor storm sewer installation.
7.2.9 2018 SDMP Urban Improvement Concept – Richmond Industrial Area
The 2018 SDMP recommendations noted the difficulty and high potential costs of providing a 100-year level of drainage service to the Richmond Industrial area. In total, five potential options were recommended, all of which had some overlapping scope.
The main improvement concepts for all 5 options were to convey the 100-year runoff flows from the Richmond Industrial area to the Canfor Ditch without posing flood risks to the surrounding area during the 100-year event. For Concept Option 3, additional recommendations included a 17,000m3 capacity storm pond on the west edge of 108th Street to reduce ponding during large storm events, upgrades to the railside ditch to mitigate large eastward drainage flows travelling towards 108th Street, and larger grates & pipe connections to Richmond Pond to prevent 100-year event overflow. Due to the variety of concept recommendations provided between all five Richmond area concept designs, review section 8.3.5 of the 2018 Storm Drainage Master Plan for additional details.
7.2.10 2023 SDMP Urban Recommendations & Phasing – Richmond Area
Since the finalization of the 2018 SDMP, significant drainage improvements have been completed in the Richmond Industrial area, which has significantly reduced the risk of surface flood risks during the 100-year design event. Key improvements include expansion of the pond west of 116 St, construction of the Richmond Pond extension & related conveyance improvements, and upgrades to the Canfor ditch.
To continue to reduce the Richmond area flood risks, additional drainage improvements are required beyond the neighbourhood’s existing conditions. Due to the high elevation of 108 Street in relation to the surrounding site elevations, significant eastward overland flows are anticipated to pond near properties on the west edge of 108 Street during the 100-year storm event. To further mitigate these flood risks, the following phased improvements are recommended for implementation. The 2023 SDMP recommended phasing of the Richmond Industrial Area improvements are illustrated in Figure 7-5 and are recommended for implementation in the following sequencing:
Phase 1 aims to mitigate surface flood risks at 89 Avenue and 108 Street by improving minor system conveyance from west of the intersection to the recently upgraded Canfor ditch. By implementing catchbasin improvements north and south of the intersection, as well as upgrading the existing 1500mm storm pipe west of the intersection to 2100mm, the area will more effectively outlet stormwater to the downstream Canfor ditch and mitigate surface flood risk to adjacent properties.
Phase 2 improvements are designed to mitigate surface flooding from a local depression north of the 108 St & railway intersection, reduce hydraulic gradelines in the local minor storm sewers via new storm sewer connections to a new 17,000m3 storm surge pond, and improve the existing railway ditch overland drainage pathway east of 108 Street to improve the areas flood resilience during large storm events.
While staging of the above improvements are flexible, the City should consider completing the new storm sewer connections to the proposed dry pond prior to construction the pond itself. In this way, the SWMF outlets will be ready during pond construction, allowing for evacuation of stormwater as necessary.
The total estimated cost for the 2023 SDMP Richmond Industrial Area improvements (including contingency and engineering) is 7.1M.
One of the primary objectives of a Storm Drainage Master plan is to provide a framework for planning and implementing drainage infrastructure to allow for growth and new development. Future infrastructure should be coordinated to provide consistent servicing throughout all new development.
8.2 Findings of 2018 Storm Drainage Master Plan
The 2018 SDMP included recommendations for future development areas and identified nineteen (19) watersheds with distinct challenges with respect to hydrology, economics, and environmental protection. Individual watershed studies were recommended to determine pre-development flow rates, protect environmentally sensitive areas, and preserve critical drainage corridors for each area.
29 existing natural areas were identified for potential enhanced stormwater retention, as well as 54 artificial management facilities. Due to the increased buffering capacity and surface conveyance of existing low and natural areas, it was noted the proposed 5L/s/ha postdevelopment could be relaxed for some facilities based on the findings of future watershed studies.
Relevant environmental & regulator considerations with regards to future developments were also reviewed, with information provided regarding the Environmental Protection and Enhancement Act (EPEA), Water Act, Public Lands act, and others.
The 2018 Master Plan included consideration for the following items in the City’s future development areas:
• Standards for capturing runoff in the stormwater drainage system.
• Conveyance of captured runoff to storage facilities and outfalls to prevent flooding.
• Stormwater Management Facilities to buffer peak flows and manage water quality.
• Plan for maintenance and operational requirements.
• Monitor the system.
8.3 Findings of 2023 Storm Drainage Master Plan
The 2023 SDMP aims to build upon the previous master plan by providing updated subbasin boundaries and model simulation results to more accurately reflect existing conditions in the City’s future service areas. The improved 2023 model, upheld 2018 SDMP future service area recommendations, and future watershed studies can be used in tandem by the City to effectively pursue improved designs for the future development areas.
The future service area recommendations provided in the 2018 SDMP were reviewed as part of the 2023 SDMP scope and were confirmed to remain relevant and beneficial in the development of future service areas. As such, the 2018 SDMP future service area recommendations are upheld under the 2023 SDMP to ensure consistent development
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requirements for developers and the City moving forward. For specific future service area development recommendations, the 2018 SDMP should continue to be referenced.
As noted in section 6.0, the 2023 Storm Drainage Master Plan model provides updated surface ponding simulation results in future service areas for the 5 and 100-year 4-hour design storm events.
The 2023 SDMP surface ponding simulation results for the 100-year, 4-hour storm event, updated aerial imagery, and the 2018 SDMP future development plan recommendations (including proposed stormwater management facilities, proposed stormwater management facility characteristics, and proposed forebays), have been combined in Figure 8-1 through Figure 8-6 for ease of reference and review.
A virtual stakeholder engagement session was held on November 1, 2023, to present the stormwater master plan concept to prospective developers, developer consultants, City of Grande Prairie departments, and utility providers.
A public open house was held on October 18, 2023, to present the updated stormwater master plan improvement concepts, provide an overview of stormwater improvements completed since the 2018 Storm Drainage Master Plan, and provide an opportunity for public feedback. An informational webpage regarding the 2023 Storm Drainage Master Plan was made available on the City website prior to the event, and an online survey was made available to collect feedback from stakeholders. Relevant feedback from the public open house and virtual survey were integrated into the Storm Drainage Master Plan and Survey responses not related to the Plan were forwarded to the appropriate City staff for review and follow-up.
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9.0 Hermit Lake Study Area
9.1 Overview
The Hermit Lake Study Area, identified in Figure 9-1, is an area of 483 hectares located along the western boundary of the City lands. The area is bounded by Highway 43 to the east and south, the existing City boundary to the West, and the edge of Hermit Lake to the north. The area is bisected by a mostly continuous band of ESA/Marshland which extends from the northwest corner of the area near Hermit Lake to the southeast corner near Highway 43. This band of marshland will be referred to as the “proposed drainage parkway”. The surrounding topography is well graded towards this drainage parkway from both the East and West sides. The study area is anticipated for future commercial use and contains the existing Hockey Estates residential area. Since late 2017, various roadway and related drainage crossing improvements have been completed in the area, including a curved section of Highway 43 in the southeast, an extension of Range Road 71 to TWP RD 751, and roadway improvements to TWP RD 751.
9.2
Model Update & Results Overview
The 2023 Storm Drainage Master Plan model includes a 2D hydraulic model for the Hermit Lake Study Area. Rainfall-runoff hydrology as well as overland flows were modelled by a 2D mesh which simulates ponding volumes, flow resistance, and surface infiltration. Major roadway crossing culverts for the environmental area were simulated as part of the model, while minor culverts such as those of the Hockey Estates area were not included in the current model iteration. Due to minor culverts not being included in the surface model, the existing results identify potential extents of surface flooding should the area’s culverts become blocked or damaged. The 2023 SDMP model results for the 100-year event existing conditions surface flooding, as well as the delineated subbasin boundaries, are included in Figure 9-2. Additional details on the surface ponding model results for specific portions of the Study area are further described in the below sections.
Air photos and contour maps indicate the band of environmentally sensitive areas (ESA)/marshlands in the center of the Hermit Lake Study area is frequently flooded and could be converted to an effective drainage parkway. As such, preservation and enhancement of this area would provide significant stormwater benefits and developmental flexibility to the Hermit Lake area. Additional hydraulic and biological assessments of this area should be undertaken to aid in establishing the parkway and flood limits, and more accurately determine the potential stormwater buffering capabilities of the area. Figure 9-3 provides a plan and profile of the existing ESA area, an identifies key roadway drainage crossings.
As the potential buffering capabilities are anticipated to be significant, the primary stormwater concern for development of adjacent lands will be ensuring effective water quality and erosion control for overland runoff entering the drainage parkway. Establishment and enhancement of the riparian zone of the parkway, limiting the slopes of future drainage pathways to the parkway, and use of water quality features where drainage pathways enter the riparian zone are all methods to improve water quality and mitigate potential erosion.
Under existing conditions, the proposed drainage parkway traverses Range Road 71 via a 600mm culvert, TWP RD 715 via a 1800mm culvert, and outlets the Hermit Lake Area via a series of three large diameter culverts in proximity to Highway 43 (2200mm, 2200, and 2400mm from upstream to downstream, respectively). The 2023 Storm Drainage Master Plan Model shows little to no ponding at these culverts during the 100-year storm event, suggesting they are adequate for the 100-year event under existing conditions.
A major cost of establishing the parkway to accept future development would be the design and construction of hydraulic structures at road crossings to control flow rate and protect downstream channels, such as at Highway 43, Range Road 71, and Township Road 715 Inflow channels through the riparian zone would also need to be designed to control erosion and provide opportunities to contain potential pollution sources. The staging and design of such improvements will be dependent on the rate of future development, and therefore additional hydrological reviews are recommended to identify required drainage infrastructure improvement in advance of future developments in the Hermit Lake Study area.
Development below the historic flood limit should also be limited to reduce surface flood risk and maintain the area’s ability to buffer stormwater flows.
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Email:services@sameng.com
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9.4 Existing Drainage Conditions – East of Drainage Parkway
Under existing conditions, the land east of the drainage parkway is well graded from east to west and shows little stormwater surface ponding during the 100-year storm event with the exception of select trapped lows The above factors, combined with close proximity to the proposed drainage parkway, provides flexibility for the development of future stormwater infrastructure. The 2023 Storm Drainage Master Plan model reveals few defined drainage pathways for the eastern study area, suggesting the area primarily drains overland via sheet flows. The model results also suggest minimal surface ponding for existing conditions during the 100-year storm event. The proposed drainage parkway is anticipated to provide significant stormwater buffering capacity and is likely to remain the primary stormwater outlet for the area.
Stormwater planning with regards to this area would benefit from the following guidelines:
1) Coordinate the construction of defined continuous drainage pathways between developments to ensure eastern developments can outlet stormwater to the proposed drainage parkway.
2) Use of forebays and other water quality improvement features near the riparian boundary are likely to be required and should be reviewed for design as development progresses.
3) Where feasible, preserve existing trapped lows for potential use as private stormwater storage/detention facilities.
4) Additional studies of the proposed drainage parkway and downstream waterbodies will be required to determine an appropriate release rate from the developing lands. These individual studies are outside of the scope of the current master plan.
5) Maintain existing drainage infrastructure crossing Highway 43, Range Road 71 and Township Road 715, and stage future improvements based on future release rate determinations.
9.5 Existing Drainage Conditions – West of Environmental Area
The topography for study area lands west of the proposed drainage pathway is well graded from west to east, with surface flows anticipated to outlet into the proposed drainage parkway Surface ponding results from the 2023 Storm Drainage Master Plan model suggest the existing overland drainage in the western study area is often consolidated into defined drainage pathways due to existing roadways acting as stormwater barriers. Range Road 71, local roadways near the Hockey Estates residential area, and select private property accesses act as the primary stormwater barriers for this area due to their heightened elevation in comparison to the surrounding landscape. To ensure effective conveyance of surface flows across the western study area, preservation & maintenance of roadside ditches and roadway/access culverts are essential.
Three primary drainage locations of the western study area are identified in Figure 9-4 and described further below. Note that the drainage of the Hockey Estates neighbourhood is described separately in Section 9.7.
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Figure 9-4: Western Study Area – Key Locations
Location 1
Location 2
Location 3
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The 2023 Storm Drainage Master Plan surface ponding results suggests the following key details for the 3 identified western study area locations:
1) Location 1: Modelling results from the existing topography note a series of low areas which are at risk of extensive surface ponding during the 100-year storm event. The anticipated 100-year surface flood extents do not appear to threaten existing roadways or residential buildings but could pose a flood risk to new buildings if the area further develops.
Recommendations: Prior to future development of the area, a defined continuous drainage pathway should be established to mitigate surface flood risks and to ensure stormwater can be conveyed eastwards to the proposed drainage parkway. Furthermore, the downstream end of this drainage pathway will likely require a forebay or similar structure to improve the water quality of outlet stormwater. These improvements can be further reviewed when the Location 1 area is designated for further development and defined through planning documents such as an Area Structure Plan (ASP).
2) Location 2: Directly west of the Hockey Estates residential area, two distinct surface drainage pathways travel eastwards, converge at Range Road 71, turn northwards at the western edge of Range Road 71, and travel northwards along the western Range Road 71 ditch before crossing eastwards across Range Road 71 via culvert crossings and outletting into the proposed drainage parkway. This drainage pathway is anticipated to convey significant flows during the 100-year storm event due to the subbasin receiving stormwater from west of the Hermit Lake study area boundary.
Figure 9-5 and Figure 9-6 demonstrate plan profiles of this drainage pathway and note the sizing of all City-provided key culvert information. The local road ditches between stations 0+000 and 0+950 should be maintained to ensure effective surface flow pathways, and the City should review the existing accesses to confirm the presence, size, and condition of access culverts between these stations The 2023 SDMP 100-year surface ponding results suggest the existing structure south of station 0+450 may be at risk of surface flooding up to 0.5m, however this result is likely due to the lack of surveyed access culverts for integration into the model. The City should review this area and document the status of existing access culverts in the area.
The existing culverts located between stations 0+900 and 1+600 are essential for conveyance of stormwater northwards Range Road 71 and for outletting surface flows across Range Road 71 to the proposed drainage pathway. The City should ensure regular review and maintenance of these culverts to mitigate surface ponding and/or overtopping of Range Road 71 to the east during the 100-year storm event. Furthermore, while the 2023 SDMP model results do not indicate surface flood risks to adjacent properties during the 100-year storm event, the City should pursue expanding the existing 600mm diameter culvert at station 1+350 to twin 800mm culverts or a 1000mm culvert to match the upstream culvert capacities along Range Road 71.
Recommendations: Document the presence, size, and condition of access culverts and the roadside ditches between stations 0+000 and 0+900. Ensure these drainage items are reviewed and maintained on a regular basis to mitigate surface flood risks to adjacent properties. Maintain the western Range Road 71 ditch and culverts
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between stations 0+900 and 1+600. Upsize the existing 600mm culvert at station 1+350 to match the capacity of the upstream culverts along Range Road 71.
3) Location 3 encompasses a stretch of roadside ditch approximately 400m long along the west edge of range road 71 (north of HWY 43), which appears to collect overland surface drainage before crossing eastwards across Range Road 71, before flowing overland to the proposed drainage parkway. Provided City survey records do not indicate a culvert crossing of Range Road 71 at this location, however air photos suggest a defined drainage channel on the east side of Range Road 71 indicative of culvert infrastructure.
Recommendations: The City should review Range Road 71 approximately 400m north of the curved portion of Highway 43 for the existence and condition of a culvert crossing. If present the City should ensure the culvert is maintained in adequate condition to allow for effective conveyance of stormwater from the west to east side of Range Road 71. If not present, the City should review the area and determine the feasibility of installing a new culvert crossing. The west roadside ditch of Location 3 should be maintained such that surface flows are directed to the indicated Range Road 71 crossing point.
Overall, the Western Hermit Lake area is anticipated to receive significant flows from west of the study area during large storm events. While the area is well-graded to the east, existing local roadways and residential accesses channel surface flows into defined drainage pathways reliant on culvert crossings to effectively outlet towards the eastern proposed drainage parkway. The 2023 SDMP review indicates that the majority of the existing major drainage system will be effective in conveying surface flows to the proposed drainage parkway system. However, operational issues such as sediment buildup or culvert/ditch overgrowth may impede the existing system’s effectiveness and increase surface flood risks.
To mitigate the above risks, the City should implement regular (ideally annual) inspection of the identified key drainage ditches and culverts and include the inspected infrastructure into its regular asset maintenance programs. The suggested culvert improvements should be implemented so as to mitigate potential bottlenecks in the existing major drainage system. The City should also undertake additional inspections and survey of culvert infrastructure in the area to capture culvert end information that may have been missed due to vegetation growth, as well as to confirm the existence of access culverts along local roadways west of Range Road 71.
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9.6 Existing Culvert Infrastructure
9.6.1 Overview
Provided City survey data confirmed the presence of 43 drainage culverts within the Hermit Lake Study area, ranging from 300mm to 1800mm in diameter. A summary of existing culvert sizes, number of culverts for each size, and an approximate length for each culvert diameter is demonstrated in Table 9-1 below.
Table 9-1: Hermit Lake Study Area Culvert Sizing
*600mm Culvert E of RR71 Under old HWY 43 Frontage Road not included in estimated length, due to of downstream invert not found in field
**Culvert size was noted as 740mm from City data, adjusted to 750mm (Closest standard sizing)
City comments on culvert condition were provided alongside survey data for the above culvert infrastructure Of the 43 surveyed culverts within the Hermit Lake Study area, the following deficiencies were identified (note that select sites included multiple deficiencies) Twenty-one (21) culverts were noted as having no deficiencies.
Structural:
• 8 Culverts were identified with structural defects, ranging from bent ends to collapsed culvert ends.
Erosion/Undercut:
• 3 Culverts were identified with visible erosion.
Sedimentation
• 4 Culverts were identified with significant or obstructing sediment.
Standing Water
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• 5 Culverts were noted as having standing water at a minimum of one end.
Vegetation:
• 3 Culverts were noted as having dense vegetation near the culvert ends. Missing Marker
• 1 Culvert was noted as having a missing side marker.
Culverts within the study area provide key stormwater crossings for roadways and resident driveways, mitigating flood risks and/or infrastructure damage. As such, the regular maintenance and inspection of culvert infrastructure is key to successful drainage within the area. The study area culvert data provided for the 2023 SDMP often contained qualitative inspector comments which made review of culvert asset conditions more difficult. To ensure sustainable culvert function, Sameng recommends the following items with regards to culvert assessment and maintenance.
1) Use and development of a standard culvert condition matrix during culvert assessments. The matrix should identify key potential defects, and a quantitative scale for assessing these defects. Use of a standard assessment matrix will allow the City to more accurately assess and prioritize culvert maintenance.
2) If not already, the Hermit Lake area culverts and major overland drainage ditches should be regularly inspected and maintained, ideally on a yearly basis. Regular culvert maintenance mitigates the risk of culvert failure during large storm events.
9.7 Hockey Estates Neighbourhood
9.7.1 Location
The Hockey Estates neighbourhood occupies an area of approximately 5 hectares east of Range Road 71, located southeast of the intersection of Range Road 71 and Township Road 715. The overall topography of the neighbourhood is well-graded from west to east. Located East of the neighbourhood is the environmentally sensitive area which bisects the Hermit Lake study area Figure 9-7 provides an overview of the Hockey Estates Area, with Figure 9-8 providing a plan and profile through the area. Both of these figures are superimposed with the 2023 SDMP model surface ponding results for the 4-hour, 100-year design event.
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9.7.2 Existing Neighbourhood Drainage
The basin topography surrounding the neighbourhood is generously sloped from West to East. Range Road 71 acts as a stormwater barrier at the western edge of the neighbourhood, diverting a majority of external stormwater north or southwards away from the neighbourhood via an existing roadside ditch. As such, the neighbourhood’s major drainage system is fed primarily from stormwater falling within the neighbourhood area, as well as a catchment area of approximately 1 hectare which flows northeast against the neighbourhood roadway from the east side of Range Road 71. Under existing conditions, stormwater within the neighbourhood is conveyed from west to east via grassed roadside swales, driveway culverts, and/or overland flow due to the natural grading of the area. Stormwater outlets the neighbourhood via an 800mm CSP culvert crossing the eastern edge of the hockey estates roadway loop, before draining overland to the eastern downstream environmentally sensitive area. During large events such as the 4-hour 100-year event, blockage of culverts west of Range Road 71 could theoretically cause stormwater to overtop Range Road 71 into the Hockey Estates Neighbourhood. Under this scenario, surface ponding from this overflow is anticipated to remain primarily within the wooded area of central Hockey Estates, adjacent to Range Road 71, with little or no (<15cm) surface flooding adjacent to the existing buildings.
City provided survey data suggests that eight (8) drainage culverts are present within the neighbourhood. Seven (7) culverts with diameters ranging from 300mm to 600mm convey drainage within the neighbourhood (primarily under resident driveways) and an 800mm culvert allows stormwater within the neighbourhood to exit to the east. Recent City culvert assessments note the neighbourhood culverts to be in good condition. City records also indicate that at least one driveway in the neighbourhood has sustained culvert-related stormwater damages.
9.7.3 Culvert Capacity Review
For culvert capacity review of the Hockey Estates Neighbourhood, the 4-hour Chicago design storms were used alongside the rational method to estimate the flow requirement for 4 key culverts in the hockey estates neighbourhood. These culverts were selected due to their small diameter, relatively high contributing area, and/or being the outlet culvert for the neighbourhood. These values were then cross referenced with idealized culvert flow capacities calculated with Manning’s equation (not including entry/exit loss). The purpose of this review is to provide the City with a rough estimate of the storm return frequency that these culverts can convey, recommend select culvert improvements as necessary, and provide a recommended access culvert sizing for upcoming accesses in the Hockey Estates neighbourhood.
Figure 9-9 identifies the four culverts reviewed via the rational method, and Table 9-2 summarizes the key parameters and outcomes of the review.
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Table 9-2: Key Rational Method Parameters
Figure 9-9: Hockey Estates Key Culverts
1) Culvert 1 is 800mm diameter and acts as the sole drainage outlet for the interior Hockey Estates Neighbourhood. This culvert is currently anticipated to provide a 50year level of service under ideal conditions. Due to the importance of this culvert in evacuating stormwater from the interior of the neighbourhood during storm events, the culvert and downstream drainage pathway should be maintained regularly. Furthermore, should additional interior area of the neighbourhood be converted to more impermeable surfaces such as roofs or asphalt/gravel pads, this culvert should be reviewed for potential upsizing to maintain the existing level of service.
2) Culvert 2 is a 400mm diameter access culvert which primarily conveys surface flows from the southern interior edge of the neighbourhood. The culvert is anticipated to provide a 50-year level of service under existing conditions.
3) Culvert 3 is a 400mm diameter access culvert which primarily conveys surface flows from outside the neighbourhood and a portion of Range Road 71 towards the proposed drainage parkway. The culvert is currently anticipated to provide a 25-year level of service, and therefore should be inspected and maintained regularly to ensure large storm events do not damage the access proper.
4) Culvert 4 is a 300mm diameter culvert which conveys drainage eastwards from a relatively small interior portion of the neighbourhood (the SW corner). Due to the small diameter of this culvert, it is quite susceptible to sedimentation and/or over vegetation which may limit its conveyance capacity. As such, the City should pursue upgrading this culvert to the 450mm minimum diameter generally installed in other locations.
Overall, the Hockey Estates area is well protected from overland surface flows. The surrounding topography is well graded to the northeast, and Range Road 71 to the west of the neighbourhood is anticipated to divert the majority of surface flows northwards away from the neighbourhood during storms up to the 100-year design event. Furthermore, the topography within the neighbourhood and to the exterior are well graded to the northeast, aiding in the evacuation of surface flows from the area. The existing layout of properties within the neighbourhood and well graded topography to the northeast are anticipated to mitigate surface flood risks up to the 100-year design event.
However, to mitigate potential risks to property accesses and the neighbourhood roadway, the following improvements are recommended:
• As the 800mm culvert provides the sole stormwater outlet for the neighbourhood before overflowing the roadway, the City should regularly inspect & maintain the culvert and downstream drainage pathway to ensure flows an effective drainage outlet for the area. During these inspections, the remainder of the neighbourhood culverts and roadside ditches should be reviewed and flagged for maintenance as required.
• The neighbourhood currently contains two 300mm access culverts which are highly susceptible to sedimentation and loss of flow capacity. As such, it is recommended these culverts and any future access culverts within the neighbourhood be installed with a minimum diameter of 450mm, similar to the standard for other rural neighbourhood areas within the City.
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• The identified culvert 3 location is anticipated to receive overland flows from lands to the SW and a portion of the Range Road 71 runoff. As such the existing culvert should be regularly reviewed by the City and maintained to ensure sediment buildup or other deficiencies do not contribute to loss of flow capacity. Alternatively, the City can review options to upsize this culvert once it reaches the end of its service life.
9.8 Hermit Lake Study Area - Conclusions & Recommendations
• The overall topography of the Hermit Lake Study Area is well graded towards a central pathway of linked marshlands & environmentally sensitive areas (the proposed drainage parkway), which along with a series of roadway culverts provide a stormwater outlet for the area. This environmental area should be preserved during future development so as to ensure an adequate stormwater outlet for future developments. Additional environmental and hydrological studies should be completed for this proposed parkway to better determine its anticipated flood boundaries as well as it’s potential to buffer postdevelopment stormwater flows.
• The 2023 SDMP model results for the 4-hour 100-year design event suggests the study area to be well-draining during large storm events.
• Both the east and west study areas generally outfall into the proposed drainage parkway. Establishment of continuous overland drainage pathways to the parkway, use of existing trapped lows for stormwater management facilities, and maintenance of roadway culvert & ditch crossings will be instrumental in maintaining stormwater outlets for future developments. Existing drainage infrastructure should be reviewed for improvements or replacements as development within the study area progresses.
• Establishment of regular inspection and maintenance for the Hermit Lake drainage assets such as culverts and roadside ditches, will mitigate potential flood risks. Standard assessment matrices should be developed for ditch and culvert reviews to allow for improved prioritization of maintenance activities.
• The existing Hockey Estates Neighbourhood has adequate topographical drainage, and the internal culverts are anticipated to provide a 25 to 50-year level of service. Despite low anticipated flood risks to existing properties during the 100-year design storm, private accesses may be at risk of damage due to undersized access culverts. Recommended culvert improvements have been provided.
• The western study area is heavily dependant on roadside ditches and culvert crossings to convey stormwater, recommendations have been provided for City review for the maintenance and potential upgrade of key overland drainage infrastructure.
10.0 Conclusions and Recommendations
10.1 Overview
• After a decade of rapid growth, the City’s population has held relatively steady for the past 5 years at about 64,000 residents (2021 Municipal Census). According to the City’s 2008 Growth Study, population in Grande Prairie is projected to reach 80,000 by 2028 and 100,000 by 2038. As growth patterns for small cities often happens in cycles of rapid growth followed by periods of stability, this indicates a proactive approach of infrastructure that can be implemented with flexibility, while maintaining high standards for effectiveness, sustainability, and safety.
• The City currently has six rainfall gauges and five flow monitoring gauges. It is recommended that the City continue to maintain these gauges at the current locations to further improve its understanding of local storm events. Improved database management techniques should be utilized to identify and correct faulty data sets closer to the collection times. These efforts will increase the collection of usable data and reduce post-processing efforts. The recorded flow readings contain certain errors but most of the records can be used for system evaluation. The newest City rain gauge (Bear Lake location) had limited rainfall information at the time of the 2023 SDMP rainfall analysis due to its shorter installation duration. As such, the analysis for this location is recommended for review in the next SDMP iteration.
• The received historical rainfall data sets are generally in good agreement with the proposed IDF curves from the 2018 SDMP. As such, the City design standards should maintain the design storm & IDF curves recommendations provided by the 2018 SDMP.
• The previous Mike Urban dual drainage hydraulic computer model provided in the 2018 SDMP has been updated and converted to a comprehensive hydraulic model in Mike+ by DHI. Mike+ is DHI’s successor program to Mike Urban, has achieved significant updates, and provides access to ongoing technical support and updates through DHI. The 2023 SDMP model utilizes updated LiDAR information which more accurately represents surface elevations within the City boundary and includes key drainage infrastructure improvements implemented since the 2018 SDMP. This model should be continuously updated as the City continues improve the existing drainage system, and calibrated using future rainfall and flow data.
• Approximately 24% and 59% of manholes in the city are surcharged to 1.0m or less below surface grades during the 5-year (4-hour) and 100-year (4-hour) design storms, respectively. Surface ponding is variable within the City, with select trapped roadway lows anticipated to reach surface ponding levels of 1m during the 100-year event.
• Since the 2018 SDMP, significant drainage improvement projects have been completed in the Richmond Industrial area. The implemented improvements were included in the 2023 SDMP hydraulic model and were found to significantly reduce flood risks in the area.
• Outstanding urban drainage improvement concepts from the 2018 SDMP were reviewed and found to be relevant long-term goals to provide 100-year levels of stormwater service to identified high-risk flood areas. The 2023 SDMP updated the
Storm Drainage Master Plan 2023
Final Report Rev 001 City of Grande Prairie
previous urban drainage improvement concepts so as to provide updated cost estimates & quantities, propose phased implementation which more closely align with standard City budget cycles, and identified potential interim lower-cost improvements. The total estimated cost of the 2023 updated urban improvement concepts is 101.4M.
• Updated surface ponding model results for the 5-year and 100-year event were created for the City’s future service areas. The 2018 SDMP recommendations for future services areas were reviewed and are recommended to remain in place under the 2023 SDMP The 2023 SDMP model provides ponding simulation results for the City’s future service areas. These results are anticipated for use in identifying key future service areas for additional drainage review & planning.
• Public consultations were conducted to increase awareness of the 2023 SDMP and the City’s recent drainage improvement initiatives, as well as to collect stakeholder feedback for integration into the master plan. Two key completed consultations were a virtual stakeholder engagement session on November 1, 2023, and an in-person public open-house held on October 18, 2023.
• An additional drainage study was completed for the Hermit Lake study area, located on the western City boundary. The overall topography of the area is well graded towards a central band of wetlands and environmentally sensitive areas, which should be preserved and enhanced so as to provide an adequate stormwater outlet for future development. Key culvert crossings and roadside ditches were identified for regular inspection & maintenance by the City, to maintain conveyance capacity during large storm events. The internal Hockey Estates Neighbourhood was noted as being at low risk of flooding during the 100-year storm event, however culvert maintenance and improvement recommendations were provided to increase the existing 25 to 50-year culvert capacity present in the neighbourhood.
Preserve this section break, after this the Appendices have different footers (no page numbers)
Storm Drainage Master Plan 2023
Final Report Rev 001 City of Grande Prairie
Appendix A – November 9, 2022: Rainfall & Flow Monitoring
Sameng Inc. (“Sameng”) was retained by the City of Grande Prairie (the “City”) to update its Storm Drainage Master Plan (DMP) from the year 2017. As the first step of this project, Sameng collected and compiled the available rainfall and flow data recorded at 5 rain gauges and 5 flow monitoring locations across the City. This memorandum will provide a summary of the obtained data records and an overview of the observations from these records.
Figure 1 Monitoring Locations - Rain Gauges and Flow Meters
2 Rainfall Data
The rainfall recordings from 5 rain gauges were downloaded from the City-provided online data portal link (https://www.flowworks.com/) on October 3, 2022. Table 1 presented a summary of the rain gauge locations and their recorded ranges.
These historical rainfall data were recorded at a 5-minute interval across all 5 gauges. The raw rainfall data series were compiled and checked against their statistical extremes, as shown in
Table 1 Rain Gauges at the City of Grande Prairie
201, 17205 106A Avenue NW Edmonton, AB T5S 1M7
780-482-2557
services@sameng.com
www.sameng.com
Table 2. Outliers were further removed through the statistical screening process, and the resulting 5-min rainfall intensity plots are presented in
5min
Table 2 Statistical Summary for Recorded Rainfall Events at 5 Rain Gauges
RG01-Service Centre RG02-South Fire Hall
Fire Hall RG04-East Link Centre
2 Compiled Historical Rainfall Data from 5 Rain Gauges
The observations from the rainfall data include
• In total, we received 13 years of data if combine all rainfall records.
• Two outlier events were detected and filtered out at Rain Gauge 2 and Rain Gauge 3.
Figure
• Other recorded events at all 5 rain gauge locations are within the expected range.
• All historical rainfall records are in line with the defined IDF curves proposed for the 2017 DMP.
• The most intense rainfall events were recorded in 2020, on June 24 at RG02, June 28 at RG04, and June 14 at RG03. This indicates a certain level of spatial variation of the storm cells moving through the City.
• Other less intense rainfall events were recorded on August 02, 2020, May 27, 2021, and July 30, 2011 These events were recorded by multiple rain gauges on the same date indicating bigger storm sizes for less intense events.
3 Flow Monitoring Data
Since 2019, the City started monitoring 5 locations across its stormwater system. As of November 2022, we have received 60 flow monitoring datasets from the City. Each dataset recorded the water level, velocity, and flow data at a frequency of every 5 to15 minutes Given the different formats of these datasets, we exported and compiled all records into excel worksheets and validated the records for further evaluation. Table 3 provides an overview of the datasets.
The observations from the flow monitoring data include
• In total, we have received 60 datasets at 5 locations from the year 2019 – 2022.
• The formats of these datasets are inconsistent varying from year to year. Some are in text file format “.csv”, and others are in database formats of “.sdb” or “.mdb”.
• The recorded frequency was every 5 minutes in 2019, but switched to 15 mins in 2020 and 2021.
• In general, many datasets contain negative flow rate readings which will be filtered out for further evaluation.
• Two problematic datasets
o From the year 2019, “Flow Meter 2 95th Ave – isco.sdb” contains data from two locations identified as Unit 1 and Unit 3. A portion of the flow, level and velocity records under “Unit 1” overlaps with the FM2 dataset “A19-010-02_1.csv”. A portion of the data under “Unit 3” overlaps with the FM1 dataset “A19-010-01Feb - Sept 30.csv”. This indicates the unit ID and flow monitor ID may not be consistent.
o From the year 2019, “A19-010-04-Sept 29-Nov 8.csv” contains errors as the records of the flow and the water depth are exactly the same.
4 Recommendations
The received historical rainfall data sets are generally in good agreement with the proposed IDF curves for the 2017 Stormwater Master Plan. The recorded flow readings contain certain errors but most of the records could be used for system evaluation.
Based on the received historical rainfall and flow monitoring data, Sameng recommends continuing with the rainfall and flow monitoring at current locations. A better-structured data collection system or database management techniques should be utilized to unify the future flow monitoring process.
A – FlowWorks IDF Curves at Individual Rain Gauges
Appendix
Storm Drainage Master Plan 2023
Final Report Rev 001 City of Grande Prairie
Appendix B – Flow Monitoring Datasets
(Attached Separately)
Storm Drainage Master Plan 2023
Final Report Rev 001 City of Grande Prairie
Appendix
C – Computer Model of Storm Drainage System
(Attached Separately)
Storm Drainage Master Plan 2023
Final Report Rev 001 City of Grande Prairie
Appendix D – 2023 SDMP Urban Improvements Conceptual Cost Estimates
2B.8
Table E-1: Conceptual Cost Estimate for Northridge Area
2.7
Table E-4: Conceptual Cost Estimate for Ivy Lake/Coblestone/Smith Area
