Design Tools for Stormwater Management

The New York State Stormwater Phase II Program regulations for controlling stormwater discharges from construction activities require the design of Stormwater Management Practices (SWMPs) according to the state’s technical standards. These standards are documented in the New York State Stormwater Management Design Manual. Design tools recommended by the New York State Department of Environmental Conservation are available in the stormwater construction toolbox (www.dec.state.ny.us/website/dow/toolbox/toolbox.htm) on the DEC’s website. This article will briefly discuss the variety of tools used by the engineering community for the sizing and design of treatment and control practices.

To meet New York State design standards, two sets of criteria needs to be met:

• The unified sizing criteria for sizing the captured Stormwater volume
• The performance criteria for selection of acceptable practices

The design manual presents a unified approach to sizing that achieves several objectives. The first objective is to capture and treat the water quality volume (WQv) based on the simple method: WQv=[(P)(Rv)(A)]/12 that uses the 90 percent rainfall event (P), runoff coefficient (Rv=0.05+0.009(I)), and the area (A) of the site, where I represents the percentage of impervious cover. This computation method, which is fairly simple, along with some other considerations and design details, could be calculated in a simple spreadsheet. The 90 percent rainfall maps are made available on the stormwater interactive map (www.dec.state.ny.us/website/imsmaps/stormwater) and also as an individual geographic information system dataset.

Other objectives of the unified sizing criteria are met by the capture and detention of discharges from three rain events:

• Extended detention of the 1-year 24-hour rainfall to provide channel protection
• Detention of peak discharge to the predevelopment rate of the 10-year 24-hour event to prevent overbank flooding and of the 100-year 24-hour event to safely pass the extreme flood

Sizing of the quantity controls at minimum levels requires a hydrologic computation for runoff volume and discharge rates. Although the design manual provides instruction for the manual calculation of runoff volume, flow rate and sizing of pretreatment, treatment facility and discharge structures, manual calculation is impractical for most post-construction drainage system design. Hydrology of an urbanized area frequently involves dividing the site into smaller drainage areas, factoring in numerous manmade control structures, and accounting for multiple design points. The extent of such calculations is frequently more practical and efficient using hydrology and hydraulics modeling tools.

The design manual requires the use of an approved hydrologic model such as TR-55 (Technical Release No. 55 Urban Unit Hydrology for Small Watersheds) to determine peak discharge rates from a watershed. Hydrologic models using Natural Resource Conservation Service procedures (www.wcc.nrcs.usda.gov/hydro), such as TR-20, or Army Corps of Engineers procedures like HEC-1 or HEC-HMS (Hydrologic Engineering Center models: www.hec.usace.army.mil/software/hec-hms/hechms-hechms.html) can be used to perform hydrologic calculations and routing. Frequently, consulting engineers use commercially available software such as HydroCAD or Haestad Method’s PondPack because of the availability of technical support or customized post-processing tools. Some designers choose to use other detailed modeling tools such as the stormwater management model, in its many forms of interface (http://ccee.oregonstate.edu/swmm). Use of calculation methods, such as the rational method and lag method, which lack the level of detail for the design analysis, is discouraged.

Some design engineers also find integrated hydrology/geographic information systems or computer aided drafting (CAD) tools helpful in their design analysis. AutoDesk, Haestad Method StormCAD, or HEC-GeoHMS are some of the software that provide additional analysis tools.

The performance criteria require the selection of practices that are capable of 80 percent total suspended solid and 40 percent phosphorus removal. All practices must have acceptable longevity in the field and include a pretreatment mechanism. Chapter 6 of the design manual offers six groups of acceptable practices.

Acceptable practices fall within these general groups: ponds, wetlands, infiltration systems, filtering systems, and open channels. These practices need to be designed according to the specifications, which are either required elements or design guidance documented in the design manual. Design of a typical treatment facility needs to address stable inflow and outflow, nonerosive discharge, adequate pretreatment and treatment volume distribution; proper sizing of the orifices, weirs, and emergency spillways; micro-topology, and safety and/or aquatic benches if necessary; access roads and maintenance provisions, along with some other design details that are integral components of successful performance.

Designers normally use the same hydrology and hydraulic modeling tools mentioned above for the design of stormwater facilities. To use these tools one must incorporate reasonable assumptions and ensure that the principle treatment features in design configuration are factored into the program input process. What is important in the use of computer models for treatment practices is to understand the driving force in the treatment processes of each practice and try to mimic that function in the model setup.

Design of stormwater facilities may vary depending on the site-specific physical and natural characteristics of the area. A design engineer may have to make modifications to the structure or use commercially available design components. Deviations are allowed based on the inadequate space, head, or other physical constraints of the site. In support of design flexibility and innovative technologies, use of proprietary practices is accepted, provided there is adequate justification for such practices.

Proper justification lies within the presentation of design details. As an example, the New York State Stormwater Management Design Manual sets required specifications for the design of the filtering practices including the area of the filter media, sizing pretreatment to 25 percent, and a minimum of the specific coefficients of permeability. If an alternative filtering system is proposed, presenting the general prototype design configurations or mass loading calculation is not considered adequate justification. The designers need to configure the key elements of the system in their model setup, justify the specifications of the designed practices according to the state standards, and include schematics showing that those requirements are met.

The one unique design resource, which ranks as high as all computation methods and configuration details, is the capability of the site itself. Utilizing the natural hydrology of the site early in the process of site plan development can minimize many design obstacles. A better site design facilitates the potential of each site by innovative measures that help minimize many design obstacles that may be experienced by late stormwater management remedies.

Disclaimer: The mention of trade names or commercial products does not constitute endorsement or recommendation for use.

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Shohreh Karimipour, PE, is an environmental engineer with the NYSDEC, Division of Water, Bureau of Water Permits. She is responsible for the support of the DEC’s technical standards for erosion and sediment control and stormwater treatment.

—Patricia Cerro-Reehil

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