In this example, we’ll focus solely on Total Suspended Solids (TSS) as a key water quality parameter. TSS is a common pollutant in urban stormwater, often found at high concentrations. Reported TSS EMCs range from 180 to 548 mg/L (U.S. EPA, 1983) and 225 to 400 mg/L (UDFCD, 2001), depending on land use. TSS can impact receiving waters by altering habitats, increasing turbidity, and reducing recreational and aesthetic appeal. Additionally, TSS may contain toxic elements like heavy metals and adsorbed organics. We’ll discuss how to modify the model from Example 2 (file Example2-Post.inp) to consider TSS buildup, washoff, and transport in the post-development site.
Define the Pollutant
To start, we define TSS as a new pollutant in VOSWMM’s Quality category, measured in mg/L. We assume a small presence (10 mg/L) in rainwater. This example doesn’t consider TSS in groundwater, decay, or co-pollutants.
Define Land Uses
In this example, we have three land uses: Residential_1, Residential_2, and Commercial. Residential_1 applies to low and medium-density residential areas (lot types “L,” “M,” and “M2”), Residential_2 to high-density apartments and duplexes (lot types “DL” and “S”), and Commercial to lot types “T” and “RT.”
Land uses are defined in VOSWMM’s Quality category in the Data Browser. We won’t consider street sweeping in this example, so we won’t specify sweeping parameters. Each subcatchment area will have a mix of these land uses, assigned by indicating the percentage of surface area for each land use in the Land Use Assignment dialog, typically estimated visually from the study area map. You can find the detailed land use assignments in Table 5-4 later in this example.
Defining Pollutants and Land Uses in VOSWMM
Pollutants: To define a pollutant in VOSWMM, under Simulation tab click on Climatology >> Pollutant. The properties include the pollutant’s concentrations in external sources (like rainwater and groundwater), its decay rate (measured in day-1), and the name of a co-pollutant it depends on for buildup.
Land Uses
Different land uses generate pollutants at varying rates. In VOSWMM, land uses are configured within the Quality category of the Data Browser. The Land Use Editor has three categories: General, Buildup, and Washoff.
- The General tab contains the land use name and street sweeping details.
- The Buildup tab selects a buildup function, parameters, and normalizer variable (curb length or area) for each pollutant.
- The Washoff tab defines the washoff function, parameters, and removal efficiencies for pollutants, including those from street cleaning and BMPs.
- under Simulation tab click on Climatology >> Land Uses.
Specify a Buildup Function
In this example, we’ll select one of VOSWMM’s buildup equations to model the accumulation of TSS during dry weather. However, determining the best functional form is not always clear, even with available data. Although many buildup data sources suggest linear buildup over time, this assumption isn’t always valid. Often, the buildup rate decreases over time. So, in this case, we’ll use an exponential curve with parameters C1 (maximum buildup) and C2 (buildup rate constant) to represent the buildup rate B as a function of time t.
TSS buildup data suggests that commercial and residential areas tend to produce similar amounts of dust and dirt that make up TSS, although there can be variations. High-density residential areas usually generate more of this pollutant than low-density ones. Typical dust-and-dirt buildup rates nationwide study are shown in Table 5-2.
TSS buildup data suggests that commercial and residential areas tend to produce similar amounts of dust and dirt that make up TSS, although there can be variations. High-density residential areas usually generate more of this pollutant than low-density ones. Typical dust-and-dirt buildup rates nationwide study are shown in Table 5-2.
Table 5-3 displays parameters C1 and C2 used in equation 5-1 for each land use defined earlier. Figure 5-2 shows the exponential buildup model with these parameters. In VOSWMM, you set the buildup function and parameters for each land use in the Land Use Editor’s “Buildup” page. We use the “Exp” Buildup Function, with C1 as Max. Buildup and C2 as Rate Constant. The Power/Sat. Constant field is not used with the Exponential model.
Figure 5-2
These parameter values for TSS buildup are literature-based, but it’s recommended to define them based on site-specific data.
In this example, we normalize buildup in all subcatchments by curb length, chosen for each land use in the Land Use Editor. they should match those in Table 5-4. Ensure that curb length units (e.g., feet or meters) align with those used for the buildup rate (e.g., lbs/curb-ft or kg/curb-m) to avoid unit mismatches.
Figure 5-2
To initiate the simulation with initial buildup, we assume 5 dry days before the start. The program uses this timeframe with TSS buildup functions to calculate an initial TSS load for each subcatchment. You can specify the “Antecedent Dry Days” parameter in VOSWMM’s Simulation Options dialog.
EMCs
You can estimate Event Mean Concentrations (EMCs) using data from the EPA’s Nationwide Urban Runoff Program (U.S. EPA, 1983), which reports a median TSS EMC of 100 mg/L for urban sites. In this example, EMCs from Table 5-5 at the end of this section are used. To incorporate them into the model, you enter “EMC” in the Function field and input the concentration from Table 5-5 into the Coefficient field for each defined land use using the Land Use Editor’s Washoff page. The resulting SWMM input file is saved as Example5EMC.inp.
Exponential Washoff
SWMM employs an exponential washoff function defined as follows:
where:
W represents the rate of pollutant load washed off at time t in lbs/hr,
C1 stands for the washoff coefficient in (in/hr)-C2(hr)-1 units,
C2 signifies the washoff exponent,
q indicates the runoff rate per unit area at time t, measured in in/hr,
B denotes the remaining pollutant buildup on the surface at time t, measured in lbs.
In sediment transport theory, the exponent C2 typically falls between 1.1 and 2.6, with a common value of about 2. It’s reasonable to assume that areas with higher imperviousness, like commercial and high-density residential zones (Commercial and Residential_2 land uses), release pollutants faster than areas with individual lots (Residential_1). Thus, C2 is set to 2.2 for Residential_2 and Commercial, and 1.8 for Residential_1.
Determining washoff coefficient values (C1) is more challenging due to their significant variability. Monitoring data is essential for estimation. In this example, we assume C1 is 40 for Residential_2 and Commercial, and 20 for Residential_1.
Table 5-5 summarizes these C1 and C2 coefficients for each land use under the exponential washoff model. To input these values into the model, use the Land Use Editor’s Washoff part for each land use. Enter “EXP” in the Function field, input the C1 value from Table 5.5 into the Coefficient field, and the C2 value from the table into the Exponent field. Leave the remaining fields as 0. Save the resulting VOSWMM input file as Example5-EXP.inp.
