Calculating credits for stormwater ponds

This site is currently undergoing revision. For more information, open this link.
This site is under construction. Anticipated completion date is March, 2015.

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual BMP or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

• providing incentives to site developers to encourage the preservation of natural areas and the reduction of the volume of stormwater runoff being conveyed to a best management practice (BMP);
• complying with permit requirements, including antidegradation (see [1]; [2]);
• meeting the MIDS performance goal; or
• meeting or complying with water quality objectives, including Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs).

This page provides a discussion of how constructed basins can achieve stormwater credits.

Overview

Schematic showing characteristics of a constructed pond or constructed wetland.

Stormwater ponds and Stormwater wetlands are the most common types of constructed basins with a permanent pool of water that are built for the purpose of capturing and storing stormwater runoff. These basins are constructed, either temporarily or in a permanent installation, to prevent or mitigate downstream water quantity and/or quality impacts. Several types of constructed basins and wetlands (stormwater basins, constructed stormwater ponds, wet ponds, forebays, wet sedimentation basins, wet detention ponds, constructed wetlands, stormwater wetlands, etc) are included in this general category. Generally Stormwater Ponds do not have a significant area of vegetation. Stormwater Wetlands do have significant vegetation that enhances the nutrient removal of the basin. Not included in this BMP category are dry basins without a permanent pool. Also not included are oil/water separators, swirl concentrators, and other manufactured devices with a permanent pool of water in the device.

Pollutant Removal Mechanisms

Constructed basins rely on physical, biological, and chemical processes to remove pollutants from incoming stormwater runoff. The primary treatment mechanism is gravitational settling of particulates and their associated pollutants as stormwater runoff resides in the permanent pool. Stormwater wetlands provide an additional mechanism for the removal of nutrient and other pollutants through the uptake by algae and aquatic vegetation. Volatilization and chemical activity can also occur in both ponds and wetlands, breaking down and assimilating a number of other stormwater contaminants such as hydrocarbons (WEF, ASCE/EWRI).

The longer stormwater runoff remains in the permanent pool, the more settling (and associated pollutant removal) and other treatment will occur. After the particulates settle to the bottom of a pond, a permanent pool provides protection from re-suspension when additional runoff enters the pond during and after a rain event (WEF, ASCE/EWRI).

Location in the Treatment Train

Stormwater Treatment Trains are comprised of multiple Best Management Practices that work together to minimize the volume of stormwater runoff, remove pollutants, and reduce the rate of stormwater runoff being discharged to Minnesota wetlands, lakes and streams. Under the Treatment Train approach, stormwater management begins with simple methods that prevent pollution from accumulating on the land surface, followed by methods that minimize the volume of runoff generated and is completed by Best Management Practices that reduce the pollutant concentration and/or volume of stormwater runoff. Constructed basins are typically located at the end of the stormwater treatment train, capturing all the runoff from the site.

Methodology for calculating credits

Constructed basins provide pollutant removal associated with settling of particulates normally present in stormwater runoff, and serve the purpose of reducing peak stormwater flows for channel protection and overbank flood control. Pollutant removal is accomplished by the maintenance of a permanent pool of water that serves to both settle and store the particulates. The necessity of the permanent pool negates the ability to infiltrate runoff; therefore no volume credit can be obtained for basins and wetlands. Consequently there are also no credits for TSS or TP removals associated with volume reduction.

Credits Based on Models

Users may opt to use a water quality model or calculator to compute volume, TSS and/or TP pollutant removal for the purpose of determining credits for stormwater ponds and wetlands. The available models described in this section are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency. Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

1. Model name and version
2. Date of analysis
3. Person or organization conducting analysis
4. Detailed summary of input data
5. Calibration and verification information
6. Detailed summary of output data

Model Selection

Table 3-1 contains a list of water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal by the BMP or a series of BMPs.

The information contained in Table 3-1 is current as of January, 2015. Because model developers are frequently issuing updates to their models, users are encouraged to follow the web links to obtain the most current information and capabilities for a specific model.