Model Results

EMC Washoff Results

Figure 2.7 and Figure 2.8 show the runoff TSS concentrations simulated at different subcatchments both for the 0.1 in and the 0.19 in. (Figure 2.9) show the results for the 2yr storm. The concentrations are constant and correspond to the summation of the constant concentration in the rain (10 mg/L) and the EMCs assigned to the land uses within each subcatchment. Once the surface runoff ceases, the TSS concentration goes to zero. That is why, eventually, concentration for all the subcatchments reached zero after a certain time periods for individual subcatchments as they generate no runoff. Note that with EMC washoff, the size of the storm has no effect on a subcatchment’s runoff concentration.

Figure 2.10 shows the TSS concentration over time (pollutograph) simulated at the study area outlet for each of the three storm events (0.1, 0.19, and 1.0 in.). The outlet concentration reflects the combined effect of the TSS washoff produced from each subcatchment and routing through the conveyance network. The peak-concentrations and shapes for the pollutographs are very similar. Compared with the washoff concentrations generated by the individual subcatchments (Figure 2.7, Figure 2.8 and Figure 2.9), the outlet concentrations are not constant but attenuate over time. This attenuation is caused primarily by the longer time taken by the runoff from the lower EMC subcatchments (Such as W8) to reach the outlet. Some of it is also a result of the numerical dispersion in the model resulting from the assumption of complete mixing within each conveyance conduit during the pollutant routing process.

Figure 2.10 also shows that TSS concentrations continue to appear at the outlet for an extended period of time after the end of the storm event. This is an artifact of the flow routing procedure wherein the conduits continue to carry a very small volume of water whose concentration still reflects the high EMC levels. Thus, the concentrations appear high, yet the mass loads carried by these small discharges are negligible. This is evident when the outlet hydrograph is plotted alongside the outlet loadograph for a given storm. A loadograph is a plot of concentration times flow rate versus time. An example for the 0.1 in. event is shown in Figure 2.11. This plot was generated by exporting the time series table for Total Inflow and TSS concentration at the outfall node O1 into a spreadsheet, using the spreadsheet to multiply flow and concentration together (and converting the result to lbs/hr), and then plotting both flow and load versus time. Note how the TSS load discharged from the catchment declines in the same manner as the total runoff discharge.

Exponential Washoff Results

Figure 2.12 shows the simulated TSS concentration in the runoff from different subcatchments using the 0.1 in. storm and the Exponential washoff equation. These concentrations vary up to a certain period and then become constant. Figure 2.13 shows the same plots but for the 0.19 in storm. Note two significant differences with respect to the results obtained for the 0.1 in storm. The maximum TSS concentrations are much larger (around 10 times) and the generation of TSS is much faster, as seen by the sharper-peaked pollutographs in Figure 2.123. Finally, Figure 2.14 shows the same graphs for the larger 1-in., 2-yr storm. The TSS concentrations are slightly larger than those for the 0.19 in. storm but the difference is much smaller than the difference between the 0.1 in and 0.19 in storms. Similar results hold for the pollutographs generated for the watershed’s outlet as seen in Figure 2.15.

Even though the EMC and Exponential (EXP) washoff models utilize different coefficients that are not directly comparable, it is interesting to compute what the average event concentration in the runoff from each subcatchment was under the two models. The resulting averages are shown in Table 2.8 for the case of the 0.19 in. storm. The point being made here is that even though the pollutographs produced by the two models can look very different, with the proper choice of coefficients it is possible to get event average concentrations that look similar. Although the results of the Exponential model are more pleasing to one’s sense of how pollutants are washed off the watershed, in the absence of field measurements one cannot claim that they are necessarily more accurate. Most GeoSWMM modelers tend to use the EMC method unless data are available to estimate and calibrate the coefficients required of a more sophisticated buildup and washoff model.

Table 2.8: Average TSS concentration for the 0.19 in. event