Enhanced water-quality treatment must be balanced against potential hydrologic and hydraulic changes.

By Thomas R. Decker and Qizhong "George" Guo

Stormwater runoff from urban developments is a major source of water pollution and is of increasing concern in the United States. Runoff from rain and melting snow traverses impermeable surfaces and lawns, picking up oil, grease, airborne fallout, chemicals, nutrients, fertilizers, bacteria, sediments, trash, and other contaminants. This nonpoint-source (NPS) pollution affects the aesthetic value of waterways and raises questions about such public health issues as contamination of fish and drinking water.

Over the last several decades, beginning with the 1948 Federal Water Pollution Control Act, progressively tighter regulations have been developed to improve and preserve the quality of our water resources. For the last decade or so, most regulations focused principally on municipal and industrial (point-source) discharges as opposed to NPS discharges.

USEPA and state regulatory agencies are implementing Phase II of the National Pollutant Discharge Elimination System to control stormwater runoff from small urban storm sewer systems and small construction sites (USEPA, 1999). Monitoring and retrofitting of existing stormwater management facilities, such as detention basins, might be one key component and best management practice for complying with Phase II requirements.

A single-family residential development with two existing dry detention basins was selected to perform drainage evaluations and final design studies as part of retrofitting one of the basins with a subsurface flow gravel-bed wetland system to enhance water-quality treatment. The original demonstration project, started in 1996 by Rutgers University, was designed to provide solid data on the cost and effectiveness of retrofitting existing stormwater detention basins for water-quality improvement. Two temporary retrofits (3-in. orifice and floating riser) were previously installed at the existing upper basin and have been monitored (Guo et al., 1999, 2000).

The anticipated third retrofit includes installation of a subsurface flow gravel-bed wetland (SSW) system, which consists of a shallow basin with an underground seepage bed with wetland plants on the surface. The bed is filled with a porous media (gravel), and the vegetation is planted so that the roots grow into the porous media. Pollutants are removed through biological activity on the surface of the rocks, as well as by pollutant uptake of the plants and vegetation. Total suspended solid (TSS) removal is typically accomplished by settling prior to the stormwater flowing to the SSW. The SSW was selected in lieu of the more conventional aboveground surface wetland/wet pond because of concern that a surface wetland might result in mosquito infestation.

This project included site assessment, detailed hydrologic and hydraulic evaluations, and final design studies. Retrofits of existing basins must consider the impacts on existing stormwater flows (hydrologic impacts) and impacts to existing normal water surface and flood elevations (hydraulic impacts) to avoid adverse results, which could include increased flooding, additional stream erosion, water temperature change, excess tree removal, and construction impacts. Each site assessment must be sensitive to these secondary impacts.          

Site Description

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The existing detention basins (designated as the upper and lower basins) are located in Morris Township, NJ. Drainage area to the upper basin is approximately 22.5 ac. and to the lower basin is approximately 29 ac. Figure 1 shows the layout of the drainage system and detention basins.

Data Collection and Preparation of Base Plans

An important first step in any study is the collection of available data, dissemination of available information, and identification of required additional data. Available records for the basins, from the original residential development completed in 1980, were obtained from the township’s Municipal Engineering Office. Electronic copies of topographic mapping and tax maps in an AutoCAD drawing were obtained from its engineering office. The mapping showed existing planimetric features and contours and was supplemented by a field survey to create base plans. Information regarding the existing drainage system (pipes, manholes, and inlets) and utilities was obtained from record drawings and supplemented by a field survey.

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The composite base plans were used to perform the site assessment and the hydrologic and hydraulic studies. Figure 2 shows the existing plan of the upper and lower basin.

Site Assessments

The site assessments and evaluations took into consideration the hydrologic, hydraulic, environmental, geotechnical, cost, and socioeconomic impacts. The major site considerations are outlined below:

• Required size and grading impacts for the installation of the SSW
• Required size and hydraulic impacts as a result of installation of a forebay system
• Hydraulic impacts of the proposed fill and installation of the SSW
• Impacts to wetlands, existing groundwater, trees, and utilities
• Required permits and local approvals, and costs

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The original goal, based on previous conceptual evaluations (Agnoli, 1999), was to place the SSW inside the existing upper basin to minimize site impacts. The conceptual plan was reassessed after base mapping, resulting in a designated initial preferred retrofit plan (IPRP), which formed the basis for the hydrologic and hydraulic analysis of the proposed improvements at the upper basin. Figure 3 shows the plans.

Hydrology

The existing flows and basin routings were developed with the US Army Corps of Engineers (USACE) HEC-1 program using the Natural Resources Conservation Service (NRCS) methodology (USACE, 1999). An overall HEC-1 model of the project’s watershed was prepared to facilitate calculation of peak flows and hydrographs, routing of the detention basins, and the combination of hydrographs. Drainage areas, times of concentrations, and land uses were developed from topographical mapping and field reconnaissance. The Morris County soil survey provided information on soil types.

Peak runoff for various return frequencies, including two-, five-, 10-, 25-, 50-, and 100-year recurrence intervals, was computed based on the NRCS methodology with a Type III rainfall distribution. Times of concentration were determined in accordance with the procedures in the NRCS Technical Release No. 55 (NRCS, 1986).

Calibration and Verification of the HEC-1 Model

The peak flow to the upper basin was calibrated and then verified using previous measured data from storm events. Calibration of the model was performed by comparing computed and measured results for an independent storm event based on previous data. Verification was performed by using the calibrated model and inputting measured rainfall from another storm event into the model.

The final model parameters are summarized below:

Area to Upper Basin
Drainage Area = 22.5 ac. = 0.0352 mi.²
Pervious Runoff Curve Number = 67
Percent Impervious = 21
Time of Concentration = 29.1 min.
Area to Lower Basin
Drainage Area = 29.0 ac. = 0.0453 mi.²
Pervious Runoff Curve Number = 67
Percent Impervious = 21
Time of Concentration = 31.8 min.

First-Flush Water-Quality Storm

The first-flush event is typically defined as the elevated pollutant level associated with the initial runoff from a watershed. This elevated level remains for various lengths of time but is generally the highest in the early part of the storm. The SSW system will detain and treat the first flush and direct the rest of the stormwater volume to the dry detention basin. The Rutgers University study allowed an analysis of the pollutant load for the upper basin. Analysis of sampled storm events determined that the highest pollutant levels generally occurred within the first 30 minutes of runoff (Agnoli, 1999).

Therefore, the volume of runoff from the first 30 minutes of the water-quality storm was used to estimate the volume required for the first-flush treatment. This previous information allowed a cost-effective design that minimized the size of the SSW. Without such information, a typical approach would be to design the SSW to store the entire runoff from the designated water-quality storm. The one-year New Jersey Department of Environmental Protection water-quality storm with a total rainfall of 1.25 in. falling uniformly over two hours was used to generate the water-quality storm rainfall distribution. The NRCS methodology with the HEC-1 program was used to generate the water-quality storm hydrograph and runoff volumes for design. The 1.25 in. per two-hour storm was used for this study in lieu of the NRCS one-year, 24-hour Type III storm event since it was believed to better represent the short intense rainfall event, or first flush, that would carry most of the runoff pollutants. It also better represented the previously sampled storm events. The results are summarized below.

Water-Quality Design Storm to the Upper Basin
Water-quality first-flush design storm: P = 1.25 in. (0.625 in./hr. for 2 hours)
Peak Q = 3.34 cubic feet per second (cfs)
Total runoff volume of 1,850 cubic feet for the first 30 min.
Q = 2.33 cfs at hydrograph time = 30 min.
A peak flow of Q = 2.5 cfs was used for hydraulic design of the SSW that must be distributed to the underground perforated pipe system.

The surface area of the gravel wetland was based on providing a surface area of 0.25% of the drainage area (Center for Watershed Protection, 2001), which resulted in a surface area of the SSW of approximately 2,500 ft.² A 3-ft. stone bed and a void ratio of 0.35 provided storage volume of 2,625 ft.³, an adequate factor of safety according to the design goal.

Because of the need for the SSW to capture the sediment-laden first flush of the storm, pretreatment with a baffle box or other type of forebay trap device is strongly advised. The runoff should be pretreated to remove TSS to avoid clogging of the SSW system.

Detention Basin Modeling

The elevation, or stage-versus-basin-storage relationship, was based on the areas in the basins as determined by topographic mapping and field survey. The elevation-versus-area relationship for existing conditions and the proposed alternatives were inputted into the HEC-1 model. The basin routings used the "level pool" method in the HEC-1 model.

Review of site topography and initial basin routings revealed a low area (approximately 287.5-ft. elevation) in Manette Road just east of the upper basin that is actually lower than the berm elevation of approximately 288.5 ft. This area was lower than the initial basin routing peak water surface elevations during high flows. Runoff ponding in the upper basin above 287.5 would flow to the roadway and sidewalk area and not drain to the upper basin. This overflow was modeled as a broad-crested weir, and a diversion analysis was prepared using an option available within the HEC-1 model.

The analysis included evaluating the proposed fill and regrading that would be required to install the SSW. The grading was based on setting the top of the SSW at approximately the two-year peak routed water surface elevation. This would prevent water from backing up from the lower part of the basin and submerging the SSW and plants during frequent storms. Regrading was required to provide cover over the relocated 30-in. overflow pipe that would direct peak flows around the SSW to the remaining basin.

The hydrologic information and existing and proposed basin data were used in the HEC-1 model. Table 1 summarizes the hydrologic and hydraulic results for the upper and lower basins for existing and proposed conditions. The total peak outflow at the upper basin includes both the flow that would overtop the roadway low point and be diverted around the basin as previously described and the flow that outlets the existing 15-in. outlet pipe and drains to the lower basin. The table also includes the separate flow from the diverted overflow and the flow out of the 15-in. outlet pipe. The total inflow to the lower basin is the combination of the upper basin 15-in. outflow and the flow of the tributary 29 ac. to the lower basin as combined with the HEC-1 program. Full details on the modeling and its results are available in Decker (2002).

The results from Table 1 reveal that the proposed SSW would result in a significant increase in peak outflows for the flow over the roadway. The magnitude of peak overflow increases is unacceptable because of the potential downstream increase in flooding and erosion. The results also show that the proposed SSW would increase peak outflows from the lower basin.

The reasons for these increases are the proposed regrading and fill inside the upper basin, which would reduce flood storage. The additional volume that would normally be available under existing conditions would no longer be available because of the proposed fill. The proposed modification and loss of storage also cause the basin outflow to peak sooner, resulting in the outflow hydrograph peak occurring closer to the peak of the hydrograph coming into the lower basin, contributing to an increase in total flow to the lower basin and the associated outflow. The peak flow and water surface elevation effects of larger, less frequent storm events are less than those of smaller, more frequent storm events because the existing basin has less of an effect on runoff as the flow increases.

As mentioned previously, runoff ponding in the upper basin and in Manette Road above 287.5 ft. would overflow the roadway and sidewalk area and bypass the upper basin. This runoff would also not flow into the lower basin, based on the topography of the area. The runoff would flow south to southeast and drain into an existing ditch that flows in front of residences. The ditch has flooded in the past, and an important goal of the project would be to prevent exacerbation of the flooding problem.

The original conceptual plan and the IPRP alternative were rejected for the following reasons:

• The loss of flood storage due to the filling inside the basin would result in an increase peak flows downstream.
• The flat existing basins slope prevented providing a positive slope from the inlet to the outlet or installation of any peninsulas to increase particle flow distance.
• The cost of an underground concrete forebay system was excessive.
• Introduction of the forebay would result in additional friction and head loss that would cause flooding upstream of the basin in Manette Road.
• The proposed 30-in.-diameter overflow pipe from the flow splitter would have to be raised to provide a positive slope to the outlet. Raising the pipe would increase the upstream hydraulic grade line and would cause flooding upstream of the basin.

Additional alternatives to minimize hydrologic/hydraulic and the environmental impacts for retrofitting at the upper basin in combination of enlarging/modifying the lower basin were evaluated and ultimately rejected because of excessive costs, site constraints, or adverse environmental impacts.

Recommended Improvement Plan

The high cost of an underground concrete-type forebay system, the negative hydrologic/hydraulic impacts, and the site constraints at the upper basin made it necessary to investigate alternative locations and designs for the SSW. Review of the project area revealed one other possible location. A flat, grassy area between the upper and lower basins provided a suitable area that would leave the basins unfilled and result in minimum tree removal and provide easy access for construction. This location also would facilitate redirection of flow from the upper basin to the SSW with minimal new pipe.

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The recommended improvement plan is shown in Figure 4.

Placing the SSW adjacent to the lower basin allowed the upper basin to be used as a natural sediment forebay, resulting in a significant cost savings and minimizing hydraulic impacts. The rate of sediment accumulation in the SSW and the need for maintenance should be studied and monitored in the future.

Details of the SSW at the alternative location remain to be finalized with considerations of both hydraulics and water-quality performance.

Conclusions and Future Considerations

This study demonstrated the importance of performing a site assessment and detailed hydrologic and hydraulic analysis of the anticipated retrofit. Incorporation of retrofits into existing basins will enhance water-quality treatment but might have negative hydrologic/hydraulic effects, such as increasing existing peak stormwater flows downstream or floodwater surface elevations upstream. Siting SSWs outside existing stormwater management facilities might be necessary to minimize hydrologic and hydraulic impacts.

The following steps should be taken when evaluating retrofits of detention basins:

• Establish a budget based on available funds and anticipated goals, including the costs of permits, application fees, soft costs (engineering and survey), and hard costs (construction).
• Assemble an interdisciplinary team of licensed hydraulic, geotechnical, environmental, survey, and construction specialists.
• Assess the available site data and plans and identify base mapping and survey needs.
• Evaluate the suitability of the site, particularly for access for construction and future maintenance. A site that is difficult to access might not be adequately maintained in the future.
• Determine if the proposed retrofit can be accommodated inside an existing basin and if the site area allows for increasing the size of the basin.
• Determine if any suitable alternative location is available, if necessary, outside of the basin but near existing drainage inflow and outlet points. Confirm that the proposed retrofit is contained within the existing property limits, taking into account any setbacks or local zoning requirements. Check locations of utility easements or property deed restrictions.
• Perform a preliminary wetland delineation if an approved delineation is not available. Determine if the basin retrofit will impact any environmentally sensitive areas.
• Identify permit requirements. The design should try to satisfy the regulatory general permits and the local requirements and design standards.
• Identify impacts to any existing utilities.
• Once an initial preliminary location is determined, hydrologic and hydraulic analysis should be performed to determine the upstream and downstream impacts.
• If the hydrologic/hydraulic impacts are not acceptable, evaluate modifications to the preliminary retrofit design or the existing stormwater management facility to achieve compliance.
• Determine location of the seasonal high groundwater level and perform geotechnical assessments.
• Reassess the initial budget and project goals and obtain approval of the preliminary design from all stakeholders before proceeding to the final design. Identify and obtain approval from the entity that will be responsible for maintenance of the proposed retrofit.
• Perform final design and obtain regulatory approvals.

As the need for point- and nonpoint-source pollution control increases, public and private agencies will be looking for ways to enhance water-quality control. Retrofits of existing stormwater management facilities might improve water-quality control at the cost of increased flooding, additional stream erosion, water temperature change, excess tree removal, and construction impacts. Each site assessment must be sensitive to these secondary impacts.          

References

Agnoli, N. Modifying Existing Dry Detention Basins to Improve Non-Point Source Pollutant Removal. Master’s thesis submitted to Rutgers, State University of New Jersey, New Brunswick, NJ. January 1999.

Center for Watershed Protection. The Vermont Stormwater Management Manual, public review draft. 2001.

Decker, T. Morris Township Detention Basin Retrofit with a Subsurface Gravel-Bed Wetland System: Drainage Evaluations and Final Design Studies. Special project report submitted to graduate program in civil and environmental engineering, Rutgers, State University of New Jersey, New Brunswick, NJ, September 2002.

Guo, Q., N. Agnoli, N. Zhang, and B. Hayes. Retrofitting Stormwater Detention Basins: Water Quality Performance Before and After. Final Report, Department of Civil and Environmental Engineering, Rutgers, State University of New Jersey, Piscataway, NJ. September 1999.

Guo, Q., N. Agnoli, N. Zhang, and B. Hayes. "Hydraulic and Water Quality Performance of Urban Storm Water Detention Basin Before and After Outlet Modification," Proceedings of 2000 Joint Conference on Water Resources Engineering and Water Resources Planning & Management. R.H. Hotchkiss and M. Glade, Editors. Minneapolis, MN. July 30–August 2, 2000.

Natural Resources Soil Conservation Service (NRCS). Technical Release No. 55. US Department of Agriculture. 1986.

US Environmental Protection Agency (USEPA). Reducing Polluted Runoff: The Storm Water Phase II Rule, EPA-833-F-99-020. Office of Water, Washington, DC. 1999.

US Army Corps of Engineers (USACE), Hydrologic Engineering Center. Flood Hydrograph Package (HEC-1), Version 4.0.1E, Lahey F77L-EM/32 Version 5.01. Dodson & Associates Inc., 1999.

Thomas R. Decker, P.E., is a graduate student at Rutgers University, is a senior associate with Edwards and Kelcey Inc. in Morristown, NJ, and specializes in the design of water resource–related projects. Qizhong "George" Guo, Ph.D., P.E., is an associate professor of civil and environmental engineering at Rutgers University and has directed a number of stormwater-related research projects.

SW - May/June 2003

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