Model Setup

In this tutorial, Total Suspended Solids (TSS) will be the sole water quality parameter analyzed. TSS is a prevalent pollutant in urban stormwater, often found at high concentrations. According to the U.S. EPA (1983), Event Mean Concentrations (EMCs) for TSS range from 180 to 548 mg/L, while the City of Bellingham, WA (2010) reported values between 225 and 400 mg/L depending on land use. TSS can significantly impact receiving waters by increasing turbidity, altering aquatic habitats, and reducing recreational and aesthetic value. Furthermore, the suspended solids may carry harmful substances, including heavy metals and adsorbed organic pollutants. This section outlines how to update the models from Tutorial 05 to simulate the buildup, washoff, and transport of TSS across the post-development urban catchment.

Define the Pollutant

The first step is to define TSS as a new pollutant under the Quality category in GeoSWMM’s Data Browser. Its concentration units will be mg/L, and a small amount (10 mg/L) is assumed to be present in rainwater (as shown in 2.3). Concentrations in groundwater as well as a first order decay are not considered in this tutorial, nor will any co-pollutant be defined for TSS.

Define Land Uses

Four different land uses will be considered in this tutorial: Developed_Open_Space, Developed_Low_Intensitty, Developed_Medium_Intensity and Forest. The land cover data has been collected from the National Land Cover Database 2011 (NLCD 2011) of USGS. The data set was originally developed by the U.S. Geological Survey (USGS) in part by Multi-Resolution Land Characteristics Consortium (MRLC). The land cover data is a raster based GIS data that also includes land use pattern across the United States at a spatial resolution of 30 meters. This data was processed in ArcGIS to determine the overall land cover in each Catchment. The different land use types are shown in Figure 2.4.

The Land uses are defined in GeoSWMM in the Quality Block under the object panel. Street sweeping is not considered in this tutorial so sweeping parameters are not defined. A mixture of land uses will be assigned to each subcatchment area. This is done by opening the Property Editor for a given subcatchment, selecting the Land Use property and clicking the ellipsis button as shown in Figure 2.5. A Land Use Assignment dialog will appear where one enters the percentage of surface area that is assigned to each land use. Percentages are estimated visually from the study area map. Table 2.5, shown later in this tutorial summarizes the assignment of land uses in each of the subcatchments.

Specify a Buildup Function

One of SWMM’s buildup equations will be selected to characterize the accumulation of TSS during dry weather periods. Unfortunately, the choice of the best functional form is never obvious, even if data are available. Even though most buildup data in the literature imply a linear buildup with time, it has been observed that this linear assumption is not always true (Sartor and Boyd, 1972), and that the buildup rate tends to decrease with time. Thus, this tutorial will use an exponential curve with parameters C₁ (maximum buildup possible) and C₂ (buildup rate constant) to represent the buildup rate B as a function of time t:

Buildup data for TSS reveal that commercial and residential areas tend to generate similar amounts of the dust and dirt that comprise the TSS (again, there is a large variation for different cases). Similarly, high-density residential areas tend to produce more of this pollutant than low-density residential areas. Typical values of dust-and-dirt buildup rates based on a nationwide study by Manning et al. (1977) are shown in Table 2.3.

Table 2.3 : Typical Dust and Dirt Buildup Rates

Table 2.4 shows the parameters C₁ and C₂ used in equation 2.1 for each land use defined earlier. A graphical representation of the exponential buildup model with these parameters is shown in Figure 2.4. In GeoSWMM the buildup function and its parameters are defined for each land use on the “Buildup” page of the Land Use Editor. The Buildup Function used here is Exp, the constant C₁ is entered in the field Max. Buildup and constant C₂ is entered in the field Rate Constant. The field Power/Sat. Constant is not defined when the Exponential model is used.

The values of the parameters used in this TSS buildup function were obtained from the literature. No other justification supports their use and it is strongly recommended that modelers define them based on site data specific to their project.

Table 2.4 : Parameters for TSS buildup

Buildup in all the subcatchments will be normalized in this tutorial by the subcatchment area. This choice is specified for each land use in the Land Use Editor (See Figure A.1). As the GeoSWMM is integrated with ArcGIS Pro projection system, the corresponding areas of the subcatchments are automatically populated in GeoSWMM. The areas and the land uses for each subcatchments are listed in Table 2.2.5: Area and land uses for each Subcatchment. These values are assigned to each subcatchment by using the Property Editor (See Figure 2.5)

Table 2.5 : Area and land uses for each Subcatchment

Finally, in order to start the simulation with some initial buildup already present, it is assumed that there were 5 days of dry antecedent conditions before the start of the simulation. The program will apply this time interval to the TSS buildup functions to compute an initial loading of TSS over each subcatchment. The Antecedent Dry Days parameter is specified on the Dates section of the Simulation Options dialog in GeoSWMM.

Specify a Washoff Function

Two methods are used in this tutorial to simulate washoff: Event Mean Concentrations (EMCs) and an Exponential washoff equation (EXP). The following sections explain how these are added to the model.

EMCs

An estimation of the EMCs can be obtained from the Nationwide Urban Runoff Program (NURP) conducted by EPA (U.S. EPA, 1983). According to this study, the median TSS EMC observed in urban sites is 100 mg/L. Based on the general observation that residential and commercial areas produce similar pollutant loads, and taking into account the differences among land uses, this tutorial uses the EMCs shown in Table 2.6. These EMCs are entered into the model using the Land Use Editor’s Washoff section for each defined land use. The entry for the Function field is EMC, the concentration from Table 2.6 is entered in the Coefficient field and the remaining fields can be set to 0. The resulting GeoSWMM input file is saved as Tutorial_05_EMC.gdb.

Exponential Washoff

The exponential washoff function used in SWMM is:

where:

W = rate of pollutant load washed off at time t in lbs/hr .

C₁ = washoff coefficient in units of (in/hr)-C₂ (hr)-1.

C₂ = washoff exponent.

q = runoff rate per unit area at time t, in/hr .

B = pollutant buildup remaining on the surface at time t, lbs.

According to sediment transport theory, values of the exponent C2 should range between 1.1 and 2.6, with most values near 2 (Vanoni, 1975). One can assume that commercial and high-density residential areas (Developed_Open_Space, Developed_Low_Intensitty, Developed_Medium_Intensity and Forest), because of their higher imperviousness, tend to release pollutants faster than areas with individual lots (Developed_Medium_Intensity). Thus, a value of 2.2 is used for C2 in the Developed_Medium_Intensity and Forrest land uses, 1.8 is used for the Developed_Open_Space and 2 is used for the Developed_Low_Intensitty.

Values of the washoff coefficient (C1) are much more difficult to infer because they can vary in nature by 3 or 4 orders of magnitude. This variation may be less extreme in urban areas, but is still significant. Monitoring data should be used to help estimate a value for this constant. The current example assumes a C1 equal to 40 for Developed_Medium_Intensity and Forest, C1 equal to 20 for the land use Developed_Open_Space and C1 equal to 20 for the land use Developed_Low_Intensitty.

Table 2.6 summarizes the C1 and C2 coefficients used for each land use under exponential washoff. These are entered into the model using the Land Use Editor Washoff page for each defined land use. The entry for the Function field is EXP, the C1 value Table 2.6 is entered in the Coefficient field, and the C2 value from the table is entered into the Exponent field. The remaining fields can be set to 0. The resulting SWMM input file is saved as Tutorial_05_EXP.gdb

Table 2.6 : Washoff characteristics for each land use

Simulation Option Setting

Both the EMC washoff model (Tutorial_05_EMC.gdb) and the exponential washoff model (Tutorial_05_EXP.gdb) were run for the 0.1 in., 0.19 in. and 2-yr rainfall events under the following set of analysis options:

Table 2.7 : Simulation Options for Tutorial 05