Table of Contents 1.0
INTRODUCTION............................................................................................................... 1 1.1 Previous Studies ...................................................................................................1 1.2 Scope of Update....................................................................................................1
2.0
DRAINAGE PUBLIC SURVEY ......................................................................................... 2 2.1 Drainage Survey Form ..........................................................................................2 2.2 Summary of Responses ........................................................................................2 2.3 Geocoding of Responses ......................................................................................2
3.0
REVIEW OF DRAINAGE CIP PROJECTS....................................................................... 3 3.1 Completed CIP Projects ........................................................................................3 3.2 Review of Remaining Drainage CIP Projects ........................................................3 3.3 Effective Hydrologic and Hydraulic Models ...........................................................3 3.4 Ultimate Hydrology ................................................................................................4 3.5 Extents of Waters of the U.S. ..............................................................................10 3.6 Additional Storm Drain Improvements.................................................................10 3.7 Regional Detention Opportunities .......................................................................10
4.0
FINALIZATION OF DRAINAGE CIP .............................................................................. 17 4.1 Prioritization Criteria ............................................................................................17 4.2 Phasing of Drainage CIP Projects .......................................................................17 4.3 Sequential Drainage CIP List ..............................................................................17
5.0
DRAINAGE UTILITY RATE STUDY .............................................................................. 18 5.1 Existing Drainage Utility Fee ...............................................................................18 5.2 Cost of Service Analysis......................................................................................18 5.3 Rate Structure Modifications ...............................................................................19 5.4 Rate Scenarios ....................................................................................................19 5.5 Financial Model ...................................................................................................19 5.6 Recommendations ..............................................................................................20
6.0
DRAINAGE POLICY REVIEW ....................................................................................... 21 6.1 NFIP Program Administration..............................................................................21 6.2 Drainage Criteria Recommendations ..................................................................22 6.3 Streambank Erosion Policy .................................................................................24 6.4 Localized Flooding Policy ....................................................................................27
7.0
REFERENCES................................................................................................................ 29 APPENDICES 1 2 3 4 5 6 7 8 9 10
Drainage Capital Improvement Projects Regional Detention Opportunities Drainage CIP Projects and Responses – City Council District # 1 Drainage CIP Projects and Responses – City Council District # 2 Drainage CIP Projects and Responses – City Council District # 3 Drainage CIP Projects and Responses – City Council District # 4 Drainage CIP Financial Model – Scenario 1 Drainage CIP Financial Model – Scenario 2 Drainage CIP Financial Model – Scenario 3 Drainage CIP Financial Model – Scenario 4 i
1.0
INTRODUCTION The City of Temple was founded in 1881, and in 1951 the City enacted the first subdivision ordinance, which did not mention drainage requirements. In 1954 the subdivision ordinance was amended to say that “storm sewers are required”, and in 1958 further amendments added “to adequately serve the subdivision”, but detailed design criteria was not provided. The first drainage criteria was implemented in 1985, establishing the design storm as the 10-year event. In 1997 the Drainage Criteria Manual added additional requirements to keep the 100-year event within the right-ofway. Since Temple grew under an evolving set of drainage design criteria, various parts of the city continue to experience flooding, and drainage capital improvements are required to safely convey the floodwaters out of Temple.
1.1
Previous Studies The City of Temple’s Drainage Master Plan was developed by HDR Engineering, Inc. in 1997. This study attributed the flooding in Temple to two chief causes – a lack of adequate capacity in storm water conveyance in streets and storm sewers and the inability of existing major streams to convey storm water within their banks through the city. The study included detailed hydrologic and hydraulic analyses of the watersheds in Temple, focusing on inundation of structures by the 100-year flood event and inundation of roadway crossings. Recommendations were developed for improvements to Major Storm Drainage Systems Impacting Buildings totaling $6,765,000, improvements to Major Storm Drainage Systems Not Impacting Buildings totaling $3,913,000 and 7 regional detention basins in the Pepper Creek basin totaling $4,231,000, for a grand total of $14,909,000. As funding becomes available, the City has been constructing projects on this list of Drainage CIP projects for the last 10 years. Currently the remaining list stands at $12,275,000 of projects to be constructed.
1.2
Scope of Update For the 2008 update, the City chose to build upon the foundation laid by the 1997 Drainage Basin Study and update the list of projects to address current flooding problems throughout Temple. As such, the City retained Jacobs Carter Burgess, Inc. to update the Drainage Master Plan. Major tasks included: • • • • • • •
Updating the cost of service analysis for the Drainage Utility Fee to include the estimated Phase II TPDES implementation costs. Conduct a public survey on drainage and geocode the responses Review and prioritize the 1997 Drainage CIP projects Determine the extents of the waters of the U.S. along each creek Evaluate additional regional detention basins in other watersheds Review and provide recommendations on the existing drainage criteria, NFIP program administration, streambank erosion policy and localized flooding policy. Develop a Drainage Utility Financial Model, including rate structure modifications
Page 1
2.0
DRAINAGE PUBLIC SURVEY
2.1
Drainage Survey Form Although the City could have spent a lot of effort updating the hydrology and hydraulic models from the 1997 study, the most reliable indicator of the location of drainage problem areas in Temple is the experience of the residents themselves. To capture this experience, in March of 2008 the City sent out more than 23,000 public survey forms in one complete round of their water bills using the form shown below.
2.2
Summary of Responses The City received over 1,400 responses, representing a 6% return rate, as summarized on the following page. The average number of years the respondents have lived at that address was 18.1 years, so most of them include the August 2001 flood event, leading to a high level of confidence that they accurately represent the problem areas in Temple. Almost half of the responses (46%) did not have any of the 10 boxes checked, indicating no drainage issues at that location. Of the flooding responses, the majority were back yard (creek) flooding (38%), followed by front yard (street) flooding (21%), with house flooding at 11%. Fewer responses were received for “creek bank erosion�, although 3% reported that their houses were being threatened by erosion. The survey response spreadsheet will continue to be updated as citizens call the City with drainage concerns.
2.3
Geocoding of Responses Since the address of each response was included on the form, the responses could be geocoded to estimate their spatial location. Geocoding is based on the address ranges of each block, similar to the system that 9-1-1 operators use to locate callers. By geocoding the responses and using different symbols to represent the type of flooding or erosion, maps were prepared to show the areas where the problems are concentrated. As expected, the majority of the responses were not located within the FEMA 100-year floodplain, but were further up in the watershed. To address these issues, additional storm drain improvements will need to be constructed in the street right-of-way to intercept the water and convey it safely to the channel. Page 2
City of Temple 2008 Drainage Capital Improvement Implementation Plan Summary of Public Survey Responses Not Included in Identified Project List Surveys mailed out =
23000
Surveys received =
1416
6.16%
1 or more boxes no boxes
753 663
53.18% 46.82%
Average number of years
18.1
years
Total Responses 722
Not in Project List 175
24.2%
House
164
18
11.0%
Garage
96
19
19.8%
Front Yard
191
58
30.4%
Back Yard
261
76
29.1%
Car
10
4
40.0%
Total Responses 137
Not in Project List 40
29.2%
House
45
15
33.3%
Garage
10
3
30.0%
Storage Building
14
4
28.6%
Fence
50
13
26.0%
Tree
18
5
27.8%
I've had water in my:
Creek bank erosion is threatening my:
3.0
REVIEW OF DRAINAGE CIP PROJECTS The 1997 Drainage Basin Study developed a list of improvements to Major Storm Drainage Systems Impacting Buildings totaling $6,765,000, improvements to Major Storm Drainage Systems Not Impacting Buildings totaling $3,913,000 and 7 regional detention basins in the Pepper Creek basin totaling $4,231,000, for a grand total of $14,909,000.
3.1
Completed CIP Projects The City of Temple has constructed several of the drainage CIP projects on the 1997 list using funds from several bond issues in 1999 and 2000. In addition, several drainage improvement projects have been contracted as part of developments in various areas. The list of completed Drainage CIP projects throughout the City of Temple is provided in Appendix 1.
3.2
Review of Remaining Drainage CIP Projects Polygons were developed in GIS to show the location and extent of the remaining Drainage CIP projects in the 1997 study, based on the figures in the report. Maps showing these GIS polygons, along with the geocoded survey responses overlaid on the FIRM maps and aerials are shown in Appendix 3 for Council District #1, Appendix 4 for Council District #2, Appendix 5 for Council District #3 and Appendix 6 for Council District #4. In GIS each polygon is associated with an entry in the database, and information concerning the drainage criteria violation and proposed improvements were added to the database, as well as the 1997 conceptual project cost. The Engineering News Record (ENR) maintains a construction cost index, which was used to develop a ratio of 1.3872 to convert the 1997 estimated costs to 2008 dollars.
3.3
Effective Hydrologic and Hydraulic Models At this point in time, it does not appear that any of the hydrology or hydraulic models from the 1997 Drainage Basin Study have been submitted to FEMA as Letters of Map Revision (LOMRs) to be reflected on the FIRM maps. As such, most of the effective models for Temple are still the original WSP2 models developed by the NRCS. Since the Bell County DFIRM Update consisted mainly of redelineation, most of the current effective models are still the original FIS models. A data request was submitted to the FEMA library, and scanned copies of the TR-20 and WSP2 models were obtained. Directories were developed for each watershed, containing the NRCS effective models and the HEC-1 and HEC-2 models from the 1997 Drainage Basin Study. As LOMRs are prepared, these directories can be used to by the City to store and provide the latest models for a watershed.
Page 3
3.4
Ultimate Hydrology In the City of Temple’s 1997 Drainage Basin Study, hydrology was modeled using the HEC-1 software package. As part of the Temple Drainage Master Plan Update, Jacobs was tasked to convert these existing-conditions HEC-1 models into HEC-HMS models, and then to update the curve numbers in the future-conditions models to reflect zoning changes and development that have occurred since the models were originally built. The reasons for doing this are twofold. First, the HEC-HMS program has officially replaced HEC-1 as the standard hydrologic modeling software package in most cases, as HEC-1 is no longer being developed, updated, or supported by the Hydrologic Engineering Center. Most of the more advanced functions have been incorporated into HEC-HMS in recent years, and the computation routines have been refined, resulting in slightly different and more accurate results. Secondly, HEC-HMS features a graphical user interface (GUI), which allows features to be added, modified, copied, or deleted with the click of the mouse. The GUI has the added benefit of some basic geographic information system (GIS) capabilities, which allow it to display georeferenced images or other spatial data as background images.
3.4.1
Rainfall Data The basin features of the HEC-1 models were imported into the new basin elements, but the rainfall data did not import, so it had to be dealt with separately. HEC-HMS has the capability to develop rainfall distribution curves using the SCS mass curves and a usersupplied depth, or by converting a user supplied depth-duration frequency curve to an incremental rainfall curve using the alternating block method. However, using one of these methods will provide different results than the HEC-1 model. The HEC-1 models in the 1997 Drainage Basin Study provided a depth for each return event to be modeled, and the HEC-1 software computed the actual incremental rainfall values for each storm. In order to replicate this process in HEC-HMS, the normalized rainfall distribution curve and storm depths were entered into a spreadsheet and the depths multiplied by the normalized curve to produce rainfall curves for each storm. These storms were entered into HEC-HMS as rain gage elements, one for each subwatershed, per storm event. Due to the large amount of data entry, only the 100-year storm was modeled this way. The results of the HEC-HMS model containing the imported HEC-1 data aligned well with the results from the older HEC-1 version. In addition, discharges were computed using the same 100-year rainfall depth and the SCS Type 1 distribution curve that is built into HEC-HMS, as well as a frequency storm using the 100-year depth-duration rainfall data for Bell County, as published by the USGS. The results of original HEC-1 models, the HEC-HMS models with imported HEC-1 parameters, and HEC-HMS models with imported HEC-1 parameters and alternate rainfall distributions are shown in Table 3.4.1 on the next page. The discharges from the current effective Flood Insurance Study for Bell County, which were computed using different models that were not available for inspection, are also included for reference. As can be seen from Table 3.4.1, the results from the alternating block method (Frequency Storm) are significantly different from the results using the SCS Type 1 rainfall distribution.
Page 4
Table 3.4.1: Comparison of Discharges – Various Models & Rainfall Distributions Hog Pen Creek Drainage Basin Summary of Peak Runoffs HEC-1
100-Year Peak Runoff Rates (cfs) 1997 Hydrology Data
Current
1997 Hydrology Data
Future
Effective
Existing
Future
Frequency
SCS Ty. I
HEC-1
HEC-1
FIS
HEC-HMS
HEC-HMS
Storm
Storm
M1 (J)
980
1,360
967
1,341
2,045
1,467
1.18
M2 (J)
1,480
1,990
1,485
2,017
3,025
2,098
1.50
M3 (J)
1,700
2,170
1,782
2,362
3,582
2,482
1,992
2,601
3,890
2,674
3,670
4,775
7,185
4,960
Drainage
(HMS)
Channel
Area
HEC-1
Existing
Class
(sq mi.)
Node
Trib 2 Confluence
Main
0.76
FM 2305 (Adams Ave)
Main
Pea Ridge
Main
Location
1.64
**650' U/S of Hogan Ln Main
Hogan Rd
1,903 UM4 (J)
1,900
2,400
1.87
**1050' D/S of Hogan Ln. Main
Trib 1 Confluence
1.73
3.11
2,416 M4 (J)
3,470
4,370
3.62
**4540' D/S of Hogan Ln.
2008 Hydrology Data
4,400
Poison Oak Road
Main
3.66
M5 (J)
3,610
4,450
3,913
4,967
7,423
5,083
Leon River Confluence
Main
4.38
M8 (J)
3,790
4,580
4,104
5,047
7,514
5,195
FM 317
Trib 1
0.40
T2-1
590
830
583
818
1,219
913
FM 2305 (Adams Ave) Main Channel Confluence
Trib 1
0.64
UT2-2(J)
870
1,180
889
1,225
1,804
1,313
Trib 1
1.38
T2-4 (J)
1,560
1,970
1,691
2,174
3,295
2,286
1,659
** Discharge point from Draft Bell Co. FIS
3.4.2
Drainage Area Determination Since the original work maps from the 1997 Drainage Basin Study are not available, the first step to update the hydrology is to create a shapefile of the subwatershed boundaries for Hog Pen Creek. When the scope for the hydrology task was developed, it was assumed that the watershed and subwatershed boundaries that were used in the 1997 study would be available, to facilitate calculation of the updated curve numbers and resulting discharges. However, the work maps could not be recovered. In an effort to re-create the boundaries, each discharge location was plotted on a map based on the descriptions found in the model notes and discharge tables in the 1997 study. Subwatersheds were delineated based on these locations, and the area of each subwatershed was calculated using GIS and compared back to the 1997 study. In some cases, these new boundaries had to be modified or adjusted to try to match the previous values. Both the previous and new values for each subwatershed are shown in Table 3.4.2 on the next page. Most drainage areas matched to a tolerance of 0.01 mi2, with a maximum difference of 0.06 mi2 for the T2-1 subwatershed, an average absolute difference for all subwatersheds of 0.02 mi2 and a total difference of 0.13 mi2. It is important to recognize that this maximum difference, 0.06 mi2, causes the new area to be approximately 15 percent larger than the HEC-1 drainage area for the same
Page 5
subwatershed, which would make a significant difference in the results of the hydrologic calculations.
Table 3.4.2- Subwatershed Area Comparison for Hog Pen Creek
Hog Pen Creek Subwatershed Area Comparison
3.4.3
Subwatershed
1997 HEC-1
2008 HEC-HMS
Difference
Name
(sq. mi.)
(sq. mi.)
(sq. mi.)
IM1
0.49
0.48
0.01
IM2
0.42
0.42
0.00
IM3
0.32
0.30
0.02
IM4
0.23
0.23
0.00
IM5
0.55
0.57
-0.02
IM6
0.29
0.31
-0.02
IM7
0.12
0.14
-0.02
IM8
0.12
0.10
0.02
IT2-2
0.24
0.29
-0.05
IT2-3
0.26
0.27
-0.01
IT2-4
0.23
0.24
-0.01
IT2A-1
0.25
0.24
0.01
T1-1
0.27
0.25
0.02
T2-1
0.40
0.46
-0.06
T3-1
0.19
0.20
-0.01
Total
4.38
4.51
-0.13
Curve Number Determination Once the subbasin boundaries shapefile was finalized, revised composite curve numbers for Hog Pen Creek were calculated using GIS. An ultimate conditions land-use shapefile was created using aerial photographs to determine hydrologic land uses for developed areas, supplemented with zoning data provided by the City to assign hydrologic land use classifications for all undeveloped areas. This ultimate land use shapefile was then intersected with a shapefile of the hydrologic soil types and the subwatersheds shapefile. A large table was produced that was sorted by subwatershed name and contained the composition of each subwatershed with respect to future land use and soil type, which are used to determine curve number. The percentage of each curve number in a given subwatershed was then computed and used to calculate the composite curve number value for each subwatershed. This calculation is represented in the formula below.
Composite CN = ÎŁ
CN i • Areai Areatotal Page 6
Values for individual curve numbers were taken from Table 2-4 in the City’s Drainage Criteria and Design Manual; which references the SCS (NRCS) TR-55 manual, and upon inspection appears to be reproduced without modification. These revised future conditions curve numbers can be compared to both the existing and future curve numbers in Table 3.4.3 below. The 1997 and 2008 drainage area values are also included again for reference. Table 3.4.3 – Comparison of Hydrologic Parameters for Hog Pen Creek 1997
1997 Existing
1997 Future
2008 Future*
Basin IM1
DA (mi2) 0.49
CN 80
TL (min) 44.4
CN 87
TL (min) 27.0
DA (mi2) 0.48
CN 86
IM2
0.42
82
34.8
87
22.2
0.42
86
IM3
0.32
76
42.0
85
25.8
0.30
87
IM4
0.23
76
41.4
85
25.8
0.23
87
IM5
0.55
74
41.4
83
26.4
0.57
84
IM6
0.29
66
46.2
78
25.8
0.31
77
IM7
0.12
70
28.2
81
16.2
0.14
81
IM8
0.12
70
25.8
80
15.0
0.10
82
IT2-2
0.24
80
40.2
87
25.8
0.29
89
IT2-3
0.26
80
28.8
87
16.8
0.27
89
IT2-4
0.23
81
31.8
87
18.6
0.24
87
T1-1
0.27
81
46.8
87
25.8
0.25
83
T2-1
0.40
80
33.6
87
18.6
0.46
87
T2A-1
0.25
80
43.2
87
27.0
0.24
92
T3-1
0.19
71
51.6
81
34.8
0.20
80
* Future conditions lag time from 1997 study was used for 2008 study
3.4.4
Time of Concentration The original times of concentration were used for this study. In an effort to determine if the original data was reasonable to use, the future conditions values were evaluated against the existing conditions values, with aerial photographs taken in the mid-1990’s used to provide context. If a time of concentration changed significantly from existing to future conditions, that subwatershed was checked in the aerial photograph to make sure it was indeed undeveloped at the time the 1997 study was done. The future conditions value was evaluated for reasonableness against the future land use map and the values of developed subwatersheds. It was determined that the future conditions time of concentration values were reasonable based on this evaluation of available data. The existing and future conditions times of concentration can be seen in Table 3.4.3 above.
Page 7
3.4.5
Calculation of Revised Discharges After evaluating the available data and the significant effort that was expended trying to match the previous drainage areas, the new curve numbers and subwatershed areas were input into a HEC-HMS model. First, the 1997 HEC-1 models for Hog Pen Creek, both the existing and future conditions, were imported into a new HEC-HMS model. These imported models were adjusted as required to account for differences in data format requirements between HEC-1 and HEC-HMS, such as the rainfall data as described above. Many differences exist between the two programs with respect to input data requirements for features such as basins and outlet structures, but none of these exist in the Hog Pen Creek models and therefore did not present a problem. The imported future conditions basin element was duplicated using the basin manager tool in HEC-HMS. The new drainage area and curve number values were then populated into this basin, and discharges computed for comparison with the original HEC-1 computations and imported HEC-1 data computed in HEC-HMS. The results are shown below in Table 3.4.4. Table 3.4.4: Comparison of Ultimate 100-Year Discharges 100-Year Peak Runoff Rates (cfs)
HEC-1 Drainage
Current
(HMS)
1997
1997
1997
2008
Channel
Area
Effective
HEC-1
Existing
Future
Future
SCS Ty. I
Location
Class
(sq mi.)
FIS
Node
HEC-1
HEC-1
HEC-HMS
Storm
Trib 2 Confluence
Main
0.76
M1 (J)
980
1,360
1,341
1,467
FM 2305 (Adams Ave)
Main
1.18
M2 (J)
1,480
1,990
2,017
2,098
Pea Ridge
Main
1.50
M3 (J)
1,700
2,170
2,362
2,482
UM4 (J)
1,900
2,400
2,601
2,674
M4 (J)
3,470
4,370
4,775
4,960
1.64
**650' U/S of Hogan Ln Hogan Rd
Main
Trib 1 Confluence
1.73 1.87
**1050' D/S of Hogan Ln. Main
2,416
3.11 3.62
**4540' D/S of Hogan Ln.
1,903
4,400
Poison Oak Road
Main
3.66
M5 (J)
3,610
4,450
4,967
5,083
Leon River Confluence
Main
4.38
M8 (J)
3,790
4,580
5,047
5,195
FM 317
Trib 1
0.40
T2-1
590
830
818
913
FM 2305 (Adams Ave)
Trib 1
0.64
UT2-2 (J)
870
1,180
1,225
1,313
Main Channel Confluence
Trib 1
1.38
T2-4 (J)
1,560
1,970
2,174
2,286
1,659
** Discharge point from Draft Bell Co. FIS
Page 8
3.4.6
Conclusions These findings were presented to the City in a project update meeting on March 14, 2008. As can be seen from Table 3.4.4, the discharges in the effective FIS do not match either the 1997 Existing or Proposed HEC-1. In fact, at Hogan Road and Poison Oak Road, the effective FIS discharges are much closer to the 1997 Future HEC-1. The conversion from HEC-1 to HEC-HMS can affect the results by up to 10% alone. As such, it would be difficult for the City of Temple to provide the updated HEC-HMS models to developers to use as the basis for their LOMRs. Although the conversion from HEC-2 to HEC-RAS is more straightforward, it is unknown how well the current effective WSP2 models compare to the 1997 Drainage Basin Study HEC-2 models. In order to provide maximum value to the City of Temple, it was agreed that instead of updating the curve numbers and subsequently revising hydrologic models, Jacobs would continue to import the HEC-1 models into HEC-HMS and reconcile any errors that were present due to the nature of importing HEC-1 files. Although the Hog Pen watershed was straightforward inasmuch as there were no basins present, many of the other watersheds have basins present. This requires a certain amount of manual effort to modify the format of the data to meet the HEC-HMS requirements. Once these differences have been resolved, the HEC-HMS models will be useful for planning purposes. For example, Section 3.7 below describes the effort to investigate additional sites for possible regional detention. The original intent of the hydrology update task of the Master Drainage Plan update was to use readily available data to account for any changes that may have occurred to the land use assumptions that were made to compute ultimate discharges in the 1997 study. It was expected that all of the data used or generated in the 1997 study would be readily available, which would make the task of recomputing the curve numbers a fairly straightforward procedure. However, having to determine the subwatershed boundaries that matched the original study essentially from scratch proved to be a time-consuming task, and the effort required far exceeded that which was originally estimated. Additionally, despite the effort and care taken to match drainage area values as closely as possible, some differences in computed area were unavoidable, and no assurance could be made that the boundaries were actually the same as those determined in the original study. Without this guarantee, the comparison of the revised curve numbers to those from the original study is suspect, and limited confidence can be placed in any conclusions drawn from such comparison. In the case of Hog Pen Creek, the curve numbers seemed to match fairly well. From this, the conclusion was drawn that the effort required to update the curve numbers far outweighed the value that was gained, and that the City of Temple could get more value by revising the remaining scope to apply towards other tasks.
Page 9
3.5
Extents of Waters of the U.S. A major change since the development of the 1997 projects was the elimination of Nationwide Permit 26, which allowed up to 1 acre of impacts to waters of the U.S. As such, obtaining 404 permits for these projects may require an Individual Permit, which has been taking between 1.5 and 2 years to obtain from the U.S. Army Corps of Engineers. To estimate the impact of this regulatory change on the 1997 list, a wetland scientist performed a field visit to Temple to estimate the point at which the waters of the U.S. stopped for each stream, and a modified stream centerline shapefile was developed to show these extents for planning purposes.
3.6
Additional Storm Drain Improvements As can be seen on the exhibits in Appendix 3 through Appendix 6, most of the public survey responses were outside of the FEMA floodplain, further up in the watershed. To address these issues, the City would need to construct additional storm drain lines, most likely within the existing street right-of-way. The conceptual alignments of storm drain systems were added to address clusters of survey responses, represented by the white and pink dashed lines on these exhibits. The proposed storm drain alignments address the majority of the public survey responses, including 91% of the reports of flooding in houses, 75% of the total flooding responses and 70% of the total erosion responses.
3.7
Regional Detention Opportunities In addition to the regional detention basins identified in the 1997 Drainage Basin Study for Pepper Creek and modified in the 2005 Pepper Creek Master Plan, additional regional detention opportunities were identified in five additional watersheds; Hogpen Creek, Bird Creek, Williamson Creek, Fryers Creek, and Little Elm Creek as shown in Exhibit 1 of Appendix 2. The main goal for constructing a regional detention basin is to decrease the peak runoff rates generated downstream of the basin. Generally, the impact of a detention basin, as measured by the reduction in peak runoff, decreases as the distance between the basin and the design point increases. The methodology employed while selecting candidate sites to construct regional detention basins was based on the following criteria: •
Availability of undeveloped land – The undeveloped properties in the five watersheds that are in close proximity to the creeks were identified.
•
Synergy with prioritized storm water CIP projects – The number of prioritized stormwater CIP projects that would benefit from having a regional detention basin constructed upstream of them were evaluated. The sites that would benefit more CIP projects were given a higher desirability rating.
•
Site location – The sites that are located in the upper half of their watersheds, provided that they are upstream of the more populated areas were considered more desirable since these sites would be beneficial for a higher percentage of the population in the watershed.
Page 10
3.7.1
Hog Pen Creek Regional Detention
The proposed Hog Pen Creek regional detention basin is located within a City of Temple Park and constrained by W. Adams Avenue on the south, Montpark Rd on the west, Alaska Avenue on the north, and Pea Ridge Road on the east as shown in Exhibit 2 of Appendix 2. This site presents two major advantages; the property is already owned by the City, and the existing topography of the site can already be classified as a shallow detention basin. The proposed grading plan shown in Exhibit 2 would increase the detention capacity of the site. Furthermore, the existing trail system within the park can be modified and still be used as a hiking/biking trail. With the conceptual grading plan, the proposed detention volume would be approximately 82 acre-ft. The hydrologic benefit of developing the Hog Pen regional detention basin is illustrated in Table 3.7.1 below. The existing FIS HEC-1 model for the watershed was imported into HEC-HMS; the results of the HMS model are presented under the “SCS” column. The results of the HMS model with the proposed regional detention basin are presented under the “W/Basin” column. An approximate estimate for soil excavation volume is 15,000 C.Y. A preliminary opinion of probable construction cost is presented in Appendix 2. Table 3.7.1 – Hog Pen Creek Regional Detention - Summary of Peak Runoffs
Trib 2 Confluence FM 2305 (Adams Ave) Pea Ridge **650' U/S of Hogan Ln Hogan Rd **1050' D/S of Hogan Ln. Trib 1 Confluence **4540' D/S of Hogan Ln. Poison Oak Road Leon River Confluence FM 317
Channel Class Main Main Main
Drainage Area (sq mi.) 0.76 1.18 1.50 1.64
Main
1.73
UM4 (J)
2,605
2,674
1,818
855
32%
Main
1.87 3.11
M4 (J)
4,787
4,960
4,105
855
17%
Main Main Trib 1
3.62 3.66 4.38 0.40
M5 (J) M8 (J) T2-1 UT2-2 (J) T2-4 (J)
4,986 5,077 821
5,083 5,195 913
4,300 4,469 913
784 726 0
15% 14% 0%
1,228
1,313
1,313
0
0%
2,182
2,286
2,286
0
0%
FM 2305 (Adams Ave) Trib 1 0.64 Main Channel Confluence Trib 1 1.38 ** Discharge point from Draft Bell Co. FIS
HMS HEC-1 Node M1 (J) M2 (J) M3 (J)
Peak Runoff Rates (cfs) 100YR HMS SCS Future W/Basin Diff. Diff.(%) 1,345 1,467 1,467 0 0% 2,023 2,098 1,164 934 45% 2,360 2,482 1,579 903 36%
Page 11
3.7.2
Bird Creek Regional Detention
Finding a suitable site for regional detention basin along Bird Creek has been challenging due to the densely developed nature of the watershed. One potential solution is to modify the principal spillway of Lake Thornton to increase the detention capacity of the lake. This option was further studied by analyzing the topography of the site surrounding the lake, the existing plans for the dam spillways, and a site visit. The results of the study indicate that modifying the principal spillway to increase the detention capacity of Lake Thornton is not a desirable option for reducing peak runoff discharges downstream of the lake. In order to increase the volume of runoff that can be detained in Lake Thornton, the principal spillway flow line would have to be raised; meaning the water surface elevation in the lake would also be increased. The topography of the site surrounding the lake indicates that if the water surface elevation in Lake Thornton is raised, several properties around the lake may be inundated even during minor rain events. Due to the spillway backwater effect the golf course surrounding the lake may need to be modified to stay operational. For these reasons, modifying Lake Thornton spillway was not considered to be a viable option in this study. After determining the Lake Thornton option was not viable, a suitable site for a regional detention basin in Bird Creek Watershed was identified. The proposed Bird Creek regional detention basin is located immediately downstream of the Sammons Park Golf Course and upstream of the newly developed Temple Retail Center as shown in Exhibit 3 of Appendix 2. With the conceptual grading plan, the proposed detention volume would be approximately 40 acre-ft. The hydrologic benefit of developing the Bird Creek regional detention basin is illustrated in Table 3.7.2 on the next page. An approximate estimate for soil excavation volume is 175,000 C.Y. A preliminary opinion of probable construction cost is presented in Appendix 2.
Page 12
Table 3.7.2 – Bird Creek Regional Detention - Summary of Peak Runoffs
IH-35 (Upstream) ***Cross Section AN 850 ft U/S of Osage Rd *** West Nugent Rd ***Cross Section AI 550 ft U/S of Saulsbury Rd Saulsbury Rd Hwy 36 (Airport Rd) ***Hwy 36 (Airport Rd) Lake Jim Thornton Inflow Lake Jim Thornton Outflow ***Lake Polk Jim Thornton Dam ***Cross Section X 2600 ft U/S of I 35 IH-35 (downstream) Loop 363 ***Loop 363 Gilmeister Ln ***Battle Dr Midway Dr ***Cross Section H 3200 ft U/S of Shallow Ford Rd Shallow Ford Rd ***Shallow Ford Rd Trib 2 Confluence
Channel Class Main
Drainage Area (sq mi.) 0.34
Main Main
0.46 0.67
Main Main Main Main Main Main
0.84 0.87 1.67 1.72 2.47 2.47
Main
2.53
Main Main Main Main Main Main Main
2.89 3.26 4.10 4.05 4.85 4.92 5.89
Main Main Main Main
6.29 6.60 11.32 7.94
Trib 1 Confluence Main ***Cross Section D Main Just U/S of confluence ***Cross Section E Just D/S of Shallow Ford Rd Main Leon River Confluence Main Loop 363 Trib 1 ***Cross Section J Trib 2 Loop 363 D/S side Marlandwood Drive Trib 3 ***Marlandwood Drive Trib 4 ***Cross Section A Trib 5 150 ft D/S of Stratford Rd Main Channel Confluence Trib 6 ***Discharge Point from Draft Bell Co. FIS
9.86
HMS HEC 1 Node M1
Peak Runoff Rates (cfs) 100YR HMS SCS Future W/Basin Diff. Diff(%) 671 732 732 0 0.0%
M2 M4
1,550 2,501
1,678 2,647
1,678 2,647
0 0
0.0% 0.0%
M5 RTDAM
3,601 2,763
3,793 2,863
3,793 2,863
0 0
0.0% 0.0%
M7 M8
3,217 3,838
3,278 3,876
3,143 3,619
135 257
4.1% 6.6%
RTM10
4,582
4,774
4,522
253
5.3%
RTM12
5,745
6,001
5,706
295
4.9%
RTM14
6,098
6,234
6,084
150
2.4%
RTM15
7,303
7,470
7,520
-0.7%
RTM17
8,327
8,496
8,604
-50 108
M19 RTT1-1
8,690 852
8,828 947
8,881 947
-53 0
-0.6% 0.0%
T1-2
1,467
1,592
1,592
0
0.0%
T1-3
2,355
2,488
2,488
0
0.0%
-1.3%
10.30
8.11 11.00 0.49 0.51 0.91 0.73 1.52 1.57
Page 13
3.7.3
Williamson Creek Regional Detention
The proposed Williamson Creek regional detention basin is located within City owned Miller Park and constrained by W. Shell Avenue on the south, N. 1st Street on the west, E. Victory Avenue on the north, and E. Shell Avenue on the east as shown in Exhibit 4 of Appendix 2. As can be seen in Exhibit 4, there is an existing decorative pond located within Miller Park, which can be regraded to establish a regional detention basin with a detention volume of approximately 77 acre-ft, which would decrease the downstream peak runoff rates conveyed by Williamson Creek as shown in Table 3.7.3 below. In order to attain the maximum detention capacity from this site, some segments of the existing walking trail in the park would need to be modified. An approximate estimate for soil excavation volume is 102,000 C.Y. A preliminary opinion of probable construction cost is presented in Appendix 2. Another option would be to grade around the walking trail to construct the detention basin; however this option would generate a lower detention volume, which would reduce the hydrologic benefit of the basin. Several water crossings (bridge or culvert) would be required along the hiking trail, which would increase the cost of the project. Table 3.7.3 - Williamson Creek Regional Detention - Summary of Peak Runoffs ` 15th Street ***15th Street 3rd Street ***3rd Street 1st Street ***Victory Avenue Shell Ave Trib 1 Confluence ***CS N - Trib 1 Confluence French Street ***French Street Adams Ave ***SH 53 (Adams Ave) ***CS D - 1800 ft U/S of Little Flock Rd Little Flock Rd ***Little Flock Rd ***CS A - 1800 ft D/S of Little Flock Rd 7th Street ***7th Street 1st Street ***1st Street 8th Street ***8th Street ***CS A - 500 ft U/S of Trib 1 Conf. Main Channel Confluence ***Discharge Point from Draft Bell Co. FIS
Channel Class Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main
Drainage Area (sq mi.) 0.24 0.12 0.30 0.37 0.49 0.51 1.09 2.03 2.07 2.32 2.35 2.92 2.87 3.51 3.57 3.69 3.85 0.16 0.17 0.27 0.29 0.45 0.48 0.70 0.67
HMS HEC 1 Node M1
Peak Runoff Rates (cfs) 100YR HMS SCS Future W/Basin Diff. Diff(%) 533 609 609 0 0.0%
M2
646.4
741
741
0
0.0%
M3
952.5
1,027
1,027
0
0.0%
M5 M6
1868.3 3425.5
1,952 3,616
1,082 2,841
870 775
44.6% 21.4%
M7
3735.6
3,906
3,248
658
16.8%
M9
4283
4,423
3,884
538
12.2%
RTM11
4710
4,833
4,384
448
9.3%
T1-1
305
322
322
0
0.0%
T1-2
517
548
548
0
0.0%
T1-3
857.4
897
897
0
0.0%
T1-4
1246.9
1,327
1,327
0
0.0%
Page 14
3.7.4
Fryers Creek Regional Detention
The proposed Fryers Creek regional detention basin is constrained by Canyon Creek Drive on the south, Marlandwood Road on the north, and S 5th Street on the east as shown in Exhibit 5 of Appendix 2. As can be seen in Exhibit 5, the site is fairly open and suitable for constructing a regional detention basin. The proposed grading plan would form a regional detention basin with a detention volume of approximately 272 acre-ft, which would decrease the downstream peak run off rates conveyed by Fryers Creek as shown in Table 3.7.4 below. An approximate estimate for soil excavation volume is 204,000 C.Y. A preliminary opinion of probable construction cost is presented in Appendix 2. Table 3.7.4 - Fryers Creek Regional Detention - Summary of Peak Runoffs
Discharge Location ***CS AI - 500 ft U/S of Ave P Avenue P Avenue R ***Avenue R ***CS AB - 1600 ft U/S of Fryers Cr Dr Fryers Creek Dr ***Fryers Creek Dr *** CS V - 330 ft U/S of Marlandwood Marlandwood Dr Blackland Rd ***CS T - Waters Dairy Rd Waters Dairy Rd Hartrick Bluff Rd Trib 1Confluence ***CS K - 1000 ft U/S of Trib 1 Conf. ***CS J - just D/S of Trib 1 Conf. FM 93 ***FM 93 Forrester Rd ***Forrester Rd Leon River ***CS F - 450 ft D/S of Winchester Rd 31st Street ***31st Street ***CS A - 600 ft U/S of Main Confluence Fryers Creek Confluence ***Discharge point from Draft Bell Co. FIS
Channel Class Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Trib 1
Drainage Area (sq mi.) 0.36 0.32 0.54 0.48 0.86 1.23 1.38 1.99 1.80 2.20 2.95 2.86 3.69 5.23 4.05 5.33 5.77 5.87 6.50 6.66 8.02 0.4
Trib 1 Trib 1
0.86 0.82
Trib 1 Trib 1
0.98 1.06
HMS HEC 1 Node
Peak Runoff Rates (cfs) 100YR HMS SCS Future W/Basin Diff. Diff(%)
M1 M2
561 948
623 1,044
623 1,044
0 0
0.0% 0.0%
M4
2040
2,275
2,275
0
0.0%
M5 M5-5
2708 3212
3,097 3,654
3,097 2,198
0 1,456
0.0% 39.8%
M6 M7 M9
3862 4614 5715
4,387 5,175 6,345
2,852 3,484 5,056
1,534 1,691 1,290
35.0% 32.7% 20.3%
RTM11
5893
6,603
5,523
1,080
16.4%
RTM13
6141
6,925
5,978
948
13.7%
M16
6955
7,867
7,052
816
10.4%
RTT12
1333
1,543
1,543
0
0.0%
T1-3
1624
1,903
1,903
0
0.0%
Page 15
3.7.5
Little Elm Creek Regional Detention
The proposed Little Elm Creek regional detention basin is constrained by Moores Mill Road on the south, Brewster Road on the west, and Pegasus Drive on the east as shown in Exhibit 6 of Appendix 2. The proposed grading plan would form a regional detention basin with a detention volume of approximately 206 acre-ft, which would decrease the downstream peak runoff rates conveyed by Little Elm Creek as shown in Table 3.7.5 below. An approximate estimate for soil excavation volume is 190,000 C.Y. A preliminary opinion of probable construction cost is presented in Appendix 2. Table 3.7.5 - Little Elm Creek Regional Detention - Summary of Peak Runoffs
Discharge Location Brewster Rd **Brewster Rd ***Old Hwy 81 IH-35 (Downstream) ***CS AV - IH 35 Trib 2 Confluence ***CS AT - 1200 ft U/S of Lower Troy Rd Lower Troy Rd ***Lower Troy Rd ***CS AN - 1600 ft U/S of Gun Club Rd Gun Club Rd ***Gun Club Rd FM 438 ***FM 438 ***CS AF - 2800 ft U/S of Munroe Rd Monroe Rd Dairy Rd ***Dairy Rd FM 53 ***CS W - 1250 ft U/S of Little Flock Rd ***Little Flock Rd ***Discharge point from Draft Bell Co. FIS
Channel Class Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main
Drainage Area (sq mi.) 1.80 2.10 3.70 5.20 6.96 7.27 7.34 7.73 7.79 9.17 9.51 9.45 10.52 10.55 11.60 12.08 12.47 12.58 12.81 13.19 17.45
HMS HEC 1 Node M3
Peak Runoff Rates (cfs) 100YR SCS W/Basin Diff. Diff(%) 2,166 2,166 0 0.0%
M5
6,449
5,286
1,163
18.0%
M6
10,139
9,253
886
8.7%
M7
10,606
9,686
920
8.7%
M9
12,295
11,549
746
6.1%
RTM11
13,106
12,360
746
5.7%
RTM14 RTM15
13,449 13,489
12,670 12,748
779 741
5.8% 5.5%
RTM16
13,400
12,705
696
5.2%
Page 16
4.0
FINALIZATION OF DRAINAGE CIP
4.1
Prioritization Criteria Criteria to prioritize the CIP projects were established by maximizing the use of information available for each project. The 1997 study provided the number of structures inundated by the 100-year floodplain and the frequency of overtopping of the roadway crossings, so point systems were established for each of these. The type of roadway that is overtopped is also important, since the higher traffic volumes on arterial streets provide more chances of motorists driving into flooded crossings. To factor in the public survey results, the numbers of responses were counted for each project limits and points were awarded for each survey response. Since 1997 the USACE has significantly tightened the 404 permitting regulations, so to account for this, a negative point system was established based on the length of waters of the U.S. impacted by the project. A seventh criteria was established for the project cost, to give additional weight to smaller projects that are easier to fund. The last criteria was “project sequencing�, which provides a fine tuning knob for City staff to adjust the weighting by +/- 2 points. The final criteria used to rank the Drainage CIP projects is shown on the next page.
4.2
Phasing of Drainage CIP Projects Many of the Drainage CIP projects can be constructed in phases, which are easier for the City to fund. If a regional detention basin is located upstream of a project area, the regional basin should be constructed first, since it would reduce the size and construction cost of the necessary improvements. The 1997 base project should be constructed next, followed by the highest priority storm drain system.
4.3
Sequential Drainage CIP List The survey responses within 500 feet of each storm drain were counted, and the remaining responses were attributed to the base project, resulting in specific priority scores for each storm drain, as shown in the spreadsheet in Appendix 1, both citywide and sorted for each Council District. Lastly, the list of 104 projects was sorted according to score, resulting in a blended sequential list, as shown in Appendix 1.
Page 17
9 or more 7 to 8 5 to 6 3 to 4 1 to 2 none
20
16
12
8
4
0
Criteria House Garage Storage Fence Tree none
Points
5
4
3
2
1
0
Public Survey Erosion (Each)
Criteria
Points
Number of Structures Flooded by 100-Year
0
2
4
6
8
10
none
Car
Back Yard
Front Yard
Garage
House
Criteria
> $1 million
$601 - $999 K
$301 - $600 K
$151 - $300 K
$51 - $150K
< $50K
Criteria
Project Cost (2008 Dollars) Points
0
1
2
3
4
5
Points
Public Survey Flooding (Each)
100-year
50-year
25-year
10-year
5-year
2-year
Criteria
-10
-8
-6
-4
-2
0
Points
> 1,000 feet
500-1,000
300-500
201-300
101 - 200
<100 feet
Criteria
Potential 404 Permit Requirements
0
3
6
9
12
15
Points
Roadway Overtopping Frequency
Drainage CIP Prioritization Criteria
Residential
Collector
Arterial
Criteria
1
3
5
Points
Later
Same
Earlier
Criteria
Project Sequencing
2
6
10
Points
Street Type
2008 Drainage Capital Improvement Implementation Plan
City of Temple
5.0
DRAINAGE UTILITY RATE STUDY
5.1
Existing Drainage Utility Fee In October of 1998, the City adopted an ordinance establishing the Special Drainage Revenue Fund, in accordance with Chapter 402 of the Local Government Code. The drainage utility fee was set at $2.00 for residential properties and a tiered rate structure of $5.00, $10.00, $25.00, $37.50 and $50.00 was established for commercial properties depending on their building size. These rates have not changed for the last 10 years.
5.2
Cost of Service Analysis As shown on the following pages, the current rate structure generates approximately $693,000 in revenue annually, roughly $525,000 of which supports the drainage maintenance crews. Implementation of the Phase II TPDES program is estimated to increase these expenditures to $1,061,550 by the time the SWMP is fully implemented in Year 4 (2011), requiring an additional increase in the residential of roughly $0.75 to fund. The assumed increases in both the revenue from each category and the operations and maintenance costs are also shown on the following page.
Page 18
5.3
Rate Structure Modifications Templeâ&#x20AC;&#x2122;s existing drainage utility fee was already roughly balanced, with 52% of the revenue coming from residential accounts and 48% of the revenue coming from commercial accounts. However, the largest commercial category, for 100,000 square feet and above includes a wide range of businesses, and the City may want to consider establishing another larger category with a higher rate, such as for 200,000 square feet and above. This change would only increase revenues by roughly 2%.
5.4
Rate Scenarios Four potential scenarios were developed to fund the sequential list of Drainage CIP projects, representing initial residential rates of $4.00, $5.00, $6.00 and $2.75, with the same percentage increase for the commercial category rates. In addition to the initial rate increase, additional increases are planned every 5 years as necessary, as shown on the next two pages.
5.5
Financial Model A detailed financial model was prepared for each scenario in which each of the 104 projects was assigned to a year, to be funded with either cash or debt, and bond issues are planned out every 3 years. A summary of the dates planned for each project under each of the 4 scenarios is provided on the next two pages, and the detailed financial model for Scenario 1 is provided in Appendix 7, the model for Scenario 2 is in Appendix 8, the model for Scenario 3 is in Appendix 9 and the model for Scenario 4 is in Appendix 10. These financial models are intended to be a living spreadsheet for the City to update as Drainage CIP projects are constructed, adjusting and updating the sequencing to maximize the benefit with the available design and construction dollars.
Page 19
5.6
Recommendations The table below shows the number of projects for each five year period for each of the four rate scenarios.
Project Years
Scenario 1
2009 – 2014
10
18
28
7
2015 – 2019
9
12
16
8
2020 – 2024
7
6
14
6
2025 – 2029
6
13
16
9
2030 – 2034
11
17
20
7
2035 – 2039
11
21
10
10
2040 – 2044
11
8
9
2045 – 2049
10
7
9
2050 – 2054
16
2
11
2055 – 2059
6
15
2060 – 2064
3
5
2065 – 2069
4
4
2070 – 2075
Total
Initial
$4.00
Scenario 2 Scenario 3 Initial
$5.00
Initial
$6.00
Scenario 4 Initial
$2.75
4 $125,850,457 $77,382,485 $58,675,417 $160,531,141
This table clearly shows that to construct the list of Drainage CIP projects within a 25 year period, the City would need to significantly increase the drainage utility fees, and due to inflation the longer the City waits to construct these projects, the higher the overall cost will be for the same amount of projects. The table assumes that the only source of funding is the drainage utility fee, and the City has demonstrated over the last 10 years that a number of Drainage CIP projects can be constructed using other revenue sources, such as the Tax Increment Reinvestment Zone and agreements with developers.
Page 20
6.0
DRAINAGE POLICY REVIEW
6.1
NFIP Program Administration Minimum Requirements To reduce the federal government’s exposure to flood damages, in 1968 Congress passed the National Flood Insurance Act of 1968, establishing the National Flood Insurance Program (NFIP) to ensure that our nation’s citizens could purchase affordable flood insurance for their property. Insurance is obtained from private insurance companies, but the federal government underwrites and administers the program under the Federal Emergency Management Agency (FEMA). The NFIP is a voluntary program and local governments have the ability to opt into or out of the program. If a community elects not to participate in the NFIP, citizens within their corporate boundaries will not be able to purchase flood insurance at federally controlled rates. Flood insurance might, in theory, be available from private insurers, although in practice, it usually is not available or is available only at market rates. If a community elects to participate in the program, citizens within the local government’s boundaries have the ability to purchase flood insurance at federally controlled rates provided by private insurers. In return for providing this benefit to their citizens, the local government agrees to implement a local regulatory program designed to reduce flood losses. When communities elect to participate in the NFIP, FEMA agrees to provide flood mapping (Flood Insurance Rate Maps otherwise known as FIRMs) to establish the appropriate rates to charge for NFIP flood insurance throughout the community. Communities, in turn, agree to adopt and enforce a floodplain ordinance regulating floodplain development and to establish minimum standards for structures to be constructed in and around the floodplain. FEMA’s minimum ordinance standards establish a local permit program requiring floodplain development permits for proposed fill in the floodplain. Minimum finished floor elevations must be established at least 1 foot above the 100-year floodplain water surface elevation. FEMA has the ability to suspend communities from the NFIP for continued lack of compliance with the regulatory aspects of the program. Flood mapping to support the program is normally based on a set of hydrologic and hydraulic (H&H) models, as described in Section 3.3 of this report. Depending on the work proposed around or in the floodplain, it may be necessary for floodplain development permit applicants to revise the effective H&H models to reflect the impact of their proposed development. The community acts as the gatekeeper for letters of map revision (LOMRs), conditional letters of map revision (CLOMRs), and letters of map amendment (LOMAs), which are the methods established by FEMA to adjust flood maps based on floodplain development permit submittals. The permit review, approval, and map revision gatekeeper function provided by local communities is usually tasked to the community floodplain administrator. In Temple, the duties of floodplain administrator reside with the City Engineer.
Page 21
6.2
Drainage Criteria Recommendations Although Temple’s current Drainage Criteria and Design Manual, dated August 1997, establishes a defined policy for drainage design and a solid set of design criteria for drainage improvements, there are still some areas that could benefit from additional clarification. The current manual was reviewed to identify potential modifications and the benefits associated with each change. Recommended changes include the following: CHAPTER 1 – Storm Water Management Policy 1.3.4
Clarify “serve” and require easements for both onsite and offsite structures.
1.4.3
Clarify designer’s responsibility to either detain to pre-developed rate or improve downstream conveyances.
1.4.4
Include HEC-HMS, clarify that SCS method is required and reduce drainage area for Rational Method to 100 acres.
1.4.5
Clarify that natural drainageways shall be used if possible, add reference for USACE 404 requirements, and include access dedications for drainage channels.
1.4.13 Clarify shall instead of should, delete “potential” 1.4.14 Define when and where detention shall be used, encourage regional detention, design outlets for 2-, 10- and 100-year events. 1.4.15 Update references for Phase II TPDES SWMP 1.4.16 Require minimum permissible building floor elevations on construction plans, require surface overflow routes in Drainage Plan. 1.4.17 Require maintenance agreement for sediment/debris removal biannually CHAPTER 2 – Storm Water Runoff 2.1
Limit the use of the Rational Method to less than 100 acres.
2.2
Limit the use of the Rational Method to less than 100 acres.
2.2.1
Specify ultimate development as defined by the City’s land use plan.
2.2.3
Reduce the maximum lengths for Overland Flow to 200 feet for developed areas and 400 feet for undeveloped areas.
2.3.3
For SCS Tabular Method clarify “shall” instead of “should”.
2.4
Require hydrologic models for drainage areas greater than 100 acres.
CHAPTER 3 – Street Drainage 3.1
Include requirement to account for maximum capacity of downstream facilities.
Page 22
CHAPTER 4 – Storm Inlets 4.2
Apply clogging factor to non-residential properties or private systems, and consider defining when City is responsible for inlet cleaning maintenance.
4.3.2
Consider requiring three inlets at the sag of a vertical curve on all major streets.
CHAPTER 5 – Storm Sewers 5.1
Condition G - define when private lot to lot runoff exceeds allowable limits and require public improvements and public easements.
5.2.3
Increase minimum pipe size to 24-inches.
5.2.5
Define when a field connection is allowed and when such connection shall be prefabricated.
5.2.6
Encourage straight alignments for gravity systems to eliminate joint failures.
CHAPTER 6 – Open Channels 6.2.1
Require channels to convey fully developed flows.
6.4.3
Correct minimum energy to maximum energy.
CHAPTER 7 – Culverts and Bridges 7.5.2
Strike statement allowing the use of 12” culvert pipes.
CHAPTER 8 – Detention Basins 8.2.1
Add the 2-year design storm
8.2.2
Update the reference from TNRCC to TCEQ, and incorporated the current Dam Safety requirements which became effective on January 1, 2009 at: http://www.tceq.state.tx.us/compliance/field_ops/dam_safety/damsafetyprog. html.
8.4.1
Consider reducing the applicability of the Modified Rational Method to less than 10 acres, and strike last sentence allowing it to be used for larger areas.
CHAPTER 9 – Sediment and Erosion Control All
Consider updating to reflect the new TPDES CGP requirements and recent developments in BMP technology such as erosion control logs, mulching practices and hydraulic application of tackifiers.
Page 23
6.3
Streambank Erosion Policy In addition to the riverine flooding concerns, Temple has also been experiencing localized streambank erosion as the increased urbanization results in higher and quicker peak flows. Such urban erosion has threatened private property including houses, businesses, fences, storage buildings. Since streambank erosion is a natural process that only becomes a problem when it starts threatening private property, several cities have adopted stream buffer or erosion hazard setback requirements to ensure that future structures do not get built too close to the creek. Two of these potential approaches were evaluated – the City of Arlington and the City of Austin. 6.3.1
City of Arlington Erosion Clear Zone
Section 5.03.A.1.c of the City of Arlington’s Subdivision Regulations includes the requirement for an Erosion Clear Zone (ECZ) as follows: An ECZ shall apply to any development activity. No portion of any building, pavement surface, fence, wall, swimming pool, or other structure shall be located or constructed within the ECZ. The ECZ shall be shown, labeled, and described by metes and bounds on the plat and on the site plan when the ECZ lies outside the drainage easement. 1. To establish the ECZ, a line shall be projected from the toe of the slope of the bank of the natural creek on a three horizontal to one vertical slope to the natural ground surface. a. If the resulting intersecting line is greater than 50 feet horizontally from the toe of the slope of the bank, the ECZ shall be located at the intersection point. This is illustrated in Exhibit V-1, Figure 1, Deep Depth Creeks. b. If the resulting intersecting line is at least 40 feet, but less than 50 feet horizontally from the toe of the slope of the bank, additional footage shall be added to the requirements, so that a total of 50 feet measured horizontally from the toe of the slope of the bank is in the setback. This is illustrated in Exhibit V-1, Figure 2, Medium Depth Creeks. c. If the resulting intersecting line is less than 40 feet horizontally from the toe of the slope of the bank, an additional 10 feet shall be added to the requirements. This is illustrated in Exhibit V-1, Figure 3, Shallow Depth Creeks. 2. When an ECZ is established, the following note or an amended version approved by the City Engineer shall be added to the plat/plan: “No buildings, fences or other structures/improvements are allowed to be placed within the Erosion Clear Zone.” 3. In lieu of an ECZ, the developer may submit a plan to stabilize and protect the banks of the creek. The plan shall be submitted and accepted by the City Engineer. Page 24
The figures below describe City of Arlingtonâ&#x20AC;&#x2122;s Erosion Clear Zone requirements for deep depth creeks, medium depth creeks and shallow depth creeks. ECZ
ECZ
ECZ
The City of Arlington also requires a Creek Buffer Zone, which is a riparian zone primarily for the preservation and encouragement of natural habitat, consisting of 25 feet on each side of the creek, measured from the top of bank of the creek perpendicular away from the creek. The City of Arlington implemented the ECZ requirement in 1996.
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6.3.2
City of Austin Erosion Hazard Zone
To establish a boundary inside of which placement of resources should be avoided to minimize the potential threat from streambank erosion, the City of Austin requires the calculation and delineation of an Erosion Hazard Zone. Resources including private or public housing, buildings, apartments, fences, utilities, infrastructure or any other feature of appreciable value are prohibited in this zone. In cases where the results of the preliminary procedure are challenged as being too conservative, the City allows for either an independent detailed erosion hazard analysis on a case by case basis or for the implementation of stream stabilization measures upon approval from the City. The procedure requires identification of the elevation of the top of the lowest bank within which most flows are contained (bankfull), in order to establish the existing channel depth Dex. Projecting the top of the lowest bank elevation onto the opposite bank establishes the topwidth of the existing channel, Wex. The future incision depth is then estimated as 3 times the existing channel depth Dex. From either side of the topwidth of the existing channel, Wex, a line is projected at a 4:1 slope until it reaches natural ground to define the horizontal limits of the Erosion Hazard Zone, as shown in Figure 4 below. An additional vertical offset of 3 feet is required for utility crossings. For streams with a sinuosity greater than 1.2, the procedure requires evaluation of the meander belt width.
6.3.3
Conclusions
The Arlington and Austin requirements provide the bounds between which the City of Temple can establish an erosion clear zone requirement. While Arlington requires a 3:1 slope from the existing channel flowline, Austin requires a 4:1 slope from an Ultimate Incision Depth. Whether the slope is 3:1 or 4:1 could depend on the predominant soils in the Temple area. A reasonable estimate of the future depth of incision for a channel could also be a minimum slope from the flowline of the nearest culvert crossing downstream. The City of Temple will work with their Developers Stakeholder Committee to review and finalize the details of the erosion clear zone requirements. Page 26
6.4
Localized Flooding Policy As seen in the results of the drainage public survey, often flooding of structures occurs high up in the watershed, far away from the FEMA regulatory floodplain. These cases are commonly referred to as “localized flooding” or “lot-to-lot flooding”, because they originate from the grading configuration of one or several adjacent lots. These situations are common in subdivisions constructed in hilly terrain, where one or more upslope lots may drain to the back yard of a downslope lot, requiring the runoff to flow around the house to get to the street and storm drain system. Localized flooding is also aggravated by slab on grade construction, especially when the finished floor of the slab is close to the adjacent grade. Often State law is cited as regulating lot-to-lot flooding. Section 11.086(a) of the Texas Water Code prohibits a person from diverting or impounding the natural flow of "surface water" in a manner that damages the property of another from the overflow of water diverted or impounded. On first glance, Section 11.086(a) appears to be a clear prohibition of any sort of drainage-related action that might cause harm to another. However, over the years, the courts have held that “surface water” means “diffused surface water”. As soon as surface water reaches some sort of channel or defined course, it is no longer diffused surface water and the provisions of Section 11.086(a) no longer apply. In fact, courts have often held that downstream property owners have a certain obligation to accept upstream water in existing watercourses, even if the upstream flow has been changed somewhat as a result of an action of an upstream landowner. Subsequent common law court interpretations have provided little guidance for municipalities related to many localized flooding issues involving upstream and downstream landowners. The failings of Section 11.086 in defining landowner rights related to drainage are so great that a recent Texas appeals court noted "a landowner might divert the entire Brazos River across his neighbor's property without subjecting himself to liability under Section 11.086 of the Texas Water Code." Texas Courts have slowly provided more definition related to the rights of landowners and cities related to drainage. One case that bears watching is "City of Keller v. Wilson". In this case, a downstream landowner (Wilson) sued the City of Keller on an inverse condemnation theory related to the City's approval of drainage plans for an upstream developer. Wilson argued that perceived future damages resulting from the City's approval of the upstream developer’s plans resulted in a taking of Wilson's property. The lower courts ruled in Wilson's favor, and the case is currently awaiting a hearing with the Supreme Court of Texas. 6.4.1
Suggested Definition of Public Runoff
The revisions to Section 5.1.G of the ‘Design Manual’ define the point at which runoff water becomes public. The rule is that when water crosses two lots and enters a third; upon entering the third lot the water is public and requires a dedicated drainage easement and may require a public improvement such as a concrete flume. The ‘Design Manual’ then requires improvements within a dedicated drainage easement. It is expected this policy will result in some circumstances that require extra consideration to comply with this rule.
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6.4.2
FHA Lot Grading Requirements
An alternative to the policy described above is the FHAâ&#x20AC;&#x2122;s Type A, B or C lot grading. In November of 1960 the Federal Housing Administration published their Land Planning Bulletin No. 3, which defined the following three general types of lot grading:
Type A - Lots are graded to drain all of the lot towards the street
Type B - Lots are graded to drain the front yard towards the street and the back yard towards the rear lot line.
Type C - Lots are graded to drain all of the lot towards the rear lot line. Four typical lot-to-lot grading scenarios were also defined, as shown on the following 4 pages.
The City of Arlington requires an engineered overall site grading plan to be submitted with the subdivisionâ&#x20AC;&#x2122;s paving and drainage plans, including flow arrows and Type A, B, or C drainage for each lot within the subdivision. Type 1 or 2 block grading as shown in the HUD information is preferred. Type 3 and 4 block grading is allowed only if a flume or channel is constructed at the rear of the lot to intercept runoff; or runoff from no more than 3 lots is accumulated prior to constructing an underground drainage system, flume or channel to intercept the runoff. 6.4.3
Conclusions
The City of Temple will work with their Developers Stakeholder Committee to review and finalize the details of the requirements to prevent lot-to-lot flooding.
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7.0
REFERENCES
1. HDR Engineering, Inc., “City of Temple, Texas Drainage Basin Study”, December 1997. 2. Bell County Central Appraisal District, Property Search of 2008 Certified Values. 3. U.S. Army Corps of Engineers, Hydraulic Engineering Center, HEC-HMS Hydrologic Modeling System, Davis, California, Version 3.2, April 2008. 4. HDR Engineering, Inc., “City of Temple, Drainage Criteria and Design Manual”, August 1997. 5. City of Arlington, “Design Criteria Manual”, August 2003. 6. City of Arlington, “Subdivision rules and Regulations”, June 2005. 7. City of Austin, “Guidance on Establishing the Preliminary “Erosion Hazard Zone” for Structure and Utility Locations Near Streams”, November 2005. 8. Federal Housing Administration, Land Planning Bulletin No. 3, November 1960.
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Appendix 1 Drainage CIP Projects City-wide
Appendix 2 Regional Detention Opportunities
Appendix 3 Drainage CIP Projects and Responses City Council District #1
Appendix 4 Drainage CIP Projects and Responses City Council District #2
Appendix 5 Drainage CIP Projects and Responses City Council District #3
Appendix 6 Drainage CIP Projects and Responses City Council District #4
Appendix 7 Drainage CIP Financial Model Scenario 1
Appendix 8 Drainage CIP Financial Model Scenario 2
Appendix 9 Drainage CIP Financial Model Scenario 3
Appendix 10 Drainage CIP Financial Model Scenario 4