Stormwater Master Planning: An Important Tool for Climate Adaptation and Resilience

Sussette Irizarry, Gabriel Milian, Marlon Medina, and Christine Wartman

The Miami-Dade Water and Sewer Department (WASD) currently operates three wastewater treatment plants that serve one of the largest metropolitan areas in the United States. Its largest facility is the Central District Wastewater Treatment Plant (CDWWTP), which was constructed in 1956 and is the oldest treatment facility in Miami. The CDWWTP is also adjacent to the Biscayne Bay, which is considered an aquatic preserve. The plant is currently located in Virginia Key, a low-lying barrier island that is vulnerable to flooding and designated as a flood hazard area.

Over time, increased episodes of flooding have increased stormwater runoff, and sea level rise (SLR) has become an integral consideration in planning and adaptation strategies for WASD, focusing on protecting essential facilities within the wastewater treatment plants. The objective of this project is to develop a stormwater master plan (SWMP) that identifies improvements to reduce stormwater discharges and considers groundwater table increases due to SLR.

A SWMP was developed in 2017 for the CDWWTP focusing on a long-term stormwater management strategy for the site. The report evaluated limitations of the existing stormwater system, additional impervious areas anticipated by 2027, potential disposal options, and stormwater management alternatives. A comparative model analysis of alternatives was conducted and final recommendations were provided. The selected alternative proposed a stormwater drainage system collecting and pumping stormwater from seven drainage subbasins into a centralized dry retention area.

The SWMP update will consider gradual implementation of stormwater system improvements based on the selected alternative within the 2030 time horizon. An interconnected pond routing (ICPR) model will be used to conduct modeling analysis, refine the selected alternative, and develop new asset recommendations. The SWMP will then be used to update the existing plantwide environmental resource permit (ERP).

This project provides an example of the design challenges faced when transforming an aging stormwater drainage system and identifying conveyance, pumping, and storage options at a location with unique site constraints. In addition, coordination with multiple interested parties is critical to understanding the needs of the plant and defining a stormwater design criteria incorporating SLR. Stormwater master planning in vulnerable areas such as southeast Florida, which are susceptible to SLR and flooding, is an important tool used to apply climate change adaptation strategies for longterm stormwater control.

Background

There are several drivers leading to infrastructure improvements at WASD’s three wastewater treatment plants, including the consent decree (CD), ocean outfall legislation (OOL), repair and replacement projects (R&R), and capital improvement projects (CIP). There are many challenges facing WASD in replacing aged equipment, protecting its existing infrastructure, and prioritizing critical assets vulnerable to flooding. As a result, WASD incorporated SLR as an important design criterion in hardening efforts for all treatment plants. The Central District, North District, and South District wastewater treatment plants are currently operated by WASD, and CDWWTP, located in Virginia Key, is its oldest existing WWTP (Figure 1).

Sussette Irizarry, PSM, is an associate project manager with an environmental engineering background, and Marlon Medina, P.E., is a senior associate project manager, with Stantec in Coral Gables. Gabriel Milian, P.E., is a senior project engineer with Milian, Swain & Associates in Miami. Christine Wartman, P.E., is a senior professional engineer with Miami-Dade Water and Sewer Department in Miami.

The plant has undergone numerous expansions and upgrades from its original permitted capacity of 47 mil gal per day (mgd) as a modified activated sludge process to its current configuration as a 143-mgd, highpurity oxygen activated sludge facility. The raw wastewater that is pumped to the CDWWTP is hydraulically split into two treatment plants: Plant 1 and Plant 2. Plant 1 has a treatment capacity of 60 mgd based on annual average daily flow (AADF), and Plant 2 has a treatment capacity of 83 mgd based on AADF. Although the treatment capacities are different, the treatment processes used are identical. The treatment process used at CDWWTP consists of pretreatment (grit removal), high-purity oxygen activated sludge process, secondary clarification, and basic disinfection using chlorine.

The CDWWTP stormwater collection system consists of a network of drainage inlets and approximately 6,926 lin ft of stormwater

Continued from page piping that convey the runoff to pump stations and infiltration trenches. The system is over 40 years old and collects and pumps stormwater from seven drainage basins. The pump stations are located throughout the site and discharge to various process trains within the plant. Excess stormwater is collected and sent to infiltration trenches, which percolate into the groundwater. Figure 2 illustrates the seven drainage basins. Various stormwater management options were evaluated as part of the 2017 SMWP, including exfiltration trenches, dry retention, and drainage wells. The selected alternative proposed a hybrid approach of incorporating both dry retention and drainage wells, in addition to conveying all stormwater via force main to a centralized location. An existing solids drying bed will be converted to a dry retention area with drainage wells within the retention area. This hybrid option allows water to drain through the drainage wells, but also percolate in the dry retention area.

Stormwater must also be contained within the site. The Florida Department of Environmental Protection (FDEP) water quality standards will be followed for dry retention swales that will provide water quality enhancements. Mechanical components that will handle water quantity, such as the pump stations and wet wells, will be sized for the 100-year storm event. The existing stormwater pump stations will be upsized to convey the stormwater via force main to the centralized dry retention area.

The proposed stormwater collection system under the SWMP update also incorporates existing infrastructure where possible to reduce cost and underground utility conflicts that may be encountered in congested areas of the plant. An existing conditions assessment will be completed to identify sections of the existing system that may require replacement. The system will include an interconnection of stormwater manholes and the collection system will be piped by gravity to the proposed wet wells or interconnected basins and then pumped via force main to the dry retention area.

Modeling Analysis: Interconnected Pond Routing Model

The SWMP update considers gradual implementation of the selected alternative within the 2030 planning horizon. An ICPR model was developed as part of this project to evaluate the current operation of the CDWWTP’s drainage system under three storm-event conditions: 10-year, 24-hour; 25-year, 72-hour; and 100-year, 72-hour storm events, and the impact of proposed improvements on the stormwater drainage system. The model also incorporated pervious and impervious areas based on existing topographic survey data and projected impervious areas estimated for future projects.

Stormwater runoff collected prior to the 2030 time horizon will depend on in situ measures. Stormwater runoff collected during the 2030 time horizon will depend on the stormwater pump stations and associated wet wells, drainage wells within the retention pond, and new force mains. The ICPR model considered an estimated SLR projection of 54 in. for the year 2070 in accordance with the 2019 Southeast Florida Regional Climate Change Compact Unified Sea Level Rise Projections. This approach was based on WASD guidance documents; specifically, the “Design Guide for Hardening Wastewater Treatment Facilities Against Flooding from Surge, Sea Level Rise, and Extreme Rainfall” developed by the OOL program. Due to the proximity to the ocean and transmissivity of soil in the area, increases in the groundwater table were factored as a result of SLR.

The new stormwater system will also include a level of redundancy to mitigate future mechanical failures. Redundant pumps will be provided for the 10-year, 24-hour storm event, and larger storm events will require all stormwater pumps to be in service. Table 1 shows the rainfall depths based on the National

Oceanic and Atmospheric Administration (NOAA) Atlas 14, and an additional rainfall volume was incorporated based on the 2016 report, “South Florida Water Management District Determination of Future IntensityDuration-Frequency Curves for Level of Service Planning Projects.”

Stormwater Pump Stations and Wet Wells

The CDWWTP currently has six stormwater pump stations; each pump station contains two pumps and the redundant pump turns on for larger storms events. As part of the new stormwater improvements, one pump will run for the 10-year, 24-hour storm event and two pumps will be required for the 25-year, 72-hour and 100-year, 72hour storm event. Table 2 lists the number of pumps and horsepower needed based on the pumping capacity required by the ICPR model for the 2030 time horizon. Existing stormwater pump stations will be upsized to accommodate the additional pumping capacity. A wet well storage volume was also dimensions were determined based on that volume. Existing wet wells will be modified to meet the new design criteria.

Drainage Wells

Based on the ICPR model, approximately 10 drainage wells are needed within the retention pond. Drainage well capacity computations were developed considering a groundwater elevation of 7.1 National Geodetic Vertical Datum (NGVD) and a 3.0 safety factor. The groundwater elevation considers increases due to SLR. Figure 3 shows the detention box dimensions calculated for the drainage well. The detention box is comprised of a pollutant-retardant baffle that provides a second level of water quality, as well as a well head pipe that is sized according to a calculated amount of discharge required. This design incorporates multiple drainage wells to provide redundancy and maintain stormwater runoff onsite.

Dry Retention Pond

On the north side of the CDWWTP an existing solids drying bed will be converted to a dry retention area. The retention area will be excavated to an elevation of 7.6 NGVD, removing the existing concrete lining and excavating additional soil below the drying bed. The 7.6 NGVD elevation will also provide a safety factor above the SLR groundwater table elevation of 5.6 NGVD. The top of the berm will be placed at an elevation of 16.5 NGVD, which will also provide a 6-in. freeboard for the 100-year, 72-hour storm event. The SWMP update proposes infrastructure improvements based on the 2030 time horizon of development and the NOAA 2070 SLR. Beyond 2030 it’s recommended that additional pumps be used to aid the drainage wells. An existing berm borders the CDWWTP property to maintain stormwater onsite and all stormwater is planned to be contained within the site. The 100-year, 72-hour storm will be used to determine the minimum boundary berm elevation.

As the future scenarios are modeled, additional inlets, pump stations, and drainage wells may be needed to offset future areas of pavement, building, and impervious areas.

Design Approach and Challenges

The CDWWTP is undergoing several infrastructure improvements in various stages of design and construction driven by the CD, OOL, R&R, and CIP programs. An overall site plan (Figure 4) was created

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