The swale side slope BMP is used in combination with one of the three main channel BMPs (Swale Main Channel, Swale Main Channel with an underdrain, or Wet Swale). This BMP should not be used as a standalone BMP as the credits were determined as part of a swale system. The swale side slope must receive stormwater runoff from the direct watershed through sheet flow over the entire channel length. If the stormwater flow is channelized when it reaches the swale side slope, the swale main channel component should be used without the swale side slope component. The swale side slope BMP represents one side slope of a swale system. If a swale contains two side slopes that each receive runoff through sheet flow from impervious surfaces, then two swale side slopes should be used in the calculator and each routed to the corresponding swale main channel BMP. Credit toward the volume performance goal for a swale side slope is based on annual infiltration into the soils. Due to the fact that the performance goal is an instantaneous credit, a relationship between an annual volume reduction and the instantaneous credit was created. All pollutants in the infiltrated water are credited as being reduced.
For swale side slope BMPs, the user must input the following parameters to calculate the stormwater volume and pollutant load reductions.
If the following requirements for inputs into the MIDS calculator are not met, then an error message will inform the user to change the input to meet the requirement.
Required treatment volume, or the volume of stormwater runoff delivered to the BMP, equals the performance goal (1.1 inches or user-specified performance goal) times the impervious area draining to the BMP. Since no BMP can be routed to a swale side slope, no additional water can be routed to the swale side slope from an upstream BMP. Stormwater is delivered to the BMP instantaneously.
A swale side slope does not have storage capacity as do most other BMPs in the MIDS calculator. Volume reduction occurs only through infiltration as the stormwater travels over the side slope. To obtain an instantaneous Volume reduction capacity of BMP [V] for a swale side slope based on user inputs, annual volume reductions were determined and converted to a volume reduction capacity credit. The Volume of retention provided by BMP is the total instantaneous volume credit that can be claimed for that BMP, and is determined by comparing the Volume reduction capacity of BMP [V] to the Required treatment volume.
To determine the annual volume reduction credit given for a swale side slope, the P8 water quality model was used. 55 years of hourly rainfall data were modeled for swale side slopes with various configurations of side slope, flow path length, channel length, soil infiltration rate, and Manning’s n parameters. The model results provided annual average volume reduction rates. Multivariate regression was used to assess model results to determine the relationships between swale modeling parameters and annual volume reductions. The observed relationships are paired with the user-provided inputs to calculate an annual percent stormwater volume reduction for the swale side slope in the calculator.
To obtain the instantaneous Volume reduction capacity of BMP [V] side slope, the annual volume reductions are converted to an instantaneous volume reduction capacity. This is accomplished through the use of performance curves developed from a range of modeling scenarios. The performance curves use the annual volume reduction percentage, the infiltration rate of the underlying soils, the contributing watershed percent impervious area, and the size of the contributing watershed to calculate the Volume reduction capacity of BMP [V].
The MIDS calculator compares the Volume reduction capacity of BMP [V] with the Required treatment volume, and the lesser of the two values is used to populate the Volume of retention provided by BMP. This comparison between potential and actual treatment volumes ensures that the BMP does not claim more credit than is due based on the actual amount of water routed to it. The Volume of retention provided by BMP is thus the actual volume credit the BMP receives toward the instantaneous performance goal. For example, if the BMP is oversized the user will only receive volume credit for the Required treatment volume routed to the BMP.
Pollutant removal is accomplished through infiltration (volume reduction) in this BMP. Pollutant load reductions are thus dependent upon the volume of water retained by the BMP, and are calculated on an annual basis. The first step in calculating annual pollutant load reductions is to determine the Annual retention volume provided by BMP as discussed in the above Volume reduction section. A 100 percent pollutant removal rate is applied to this infiltrated stormwater for total suspended solids, dissolved phosphorus, and particulate phosphorus. Thus, while oversizing a BMP above the Required treatment volume will not provide additional credit towards the performance goal volume, it may provide additional annual volume and pollutant load reduction. Pollutants not removed through infiltration are routed to the downstream swale main channel.
NOTE: The user can modify event mean concentrations (EMCs) on the Site Information tab in the calculator. Default concentrations are 54.5 milligrams per liter for total suspended solids (TSS) and 0.3 milligrams per liter for total phosphorus (particulate plus dissolved). The calculator will notify the user if the default is changed. Changing the default EMC will result in changes to the total pounds of pollutant reduced.
The swale side slope BMP is designed to be part of a system with a corresponding swale main channel BMP. Therefore, a swale side slope BMP can only be routed to one of the three main channel BMPs: Swale Main Channel, Swale Main Channel (with underdrain), or Wet Swale. The swale side slope BMP should not be used as a standalone BMP. It should always be used in combination with one of the three main channel BMPs. The swale side slope BMP requires sheet flow over its surface area to receive credit. Therefore, no other BMP can be routed to the swale side slope BMP. It can only receive runoff from the direct watershed.
The following general assumptions apply in calculating the credits for a swale side slope. If these assumptions are not followed, the volume and pollutant reduction credits cannot be applied.
Swale side slope should be used in conjunction with a main channel swale. Examples are contained in the appropriate section for the main channel swales.