Model Results

TSS Treatment at LIDs

First, we look at how effectively the LIDs treat TSS. Figure 2.9 shows a comparison between the TSS concentration in the treated runoff from filter strip W1-FS and the upstream runoff from subcatchment W1 during a 0.1-in storm event. For context, the runoff flow rates from both areas are also included.

The TSS concentration drops from 182.3 mg/L in the upstream runoff to 54.7 mg/L after passing through the filter strip; an exact match with the 70% TSS removal efficiency set for this LID. Similar levels of performance were observed across the other filter strips for all design storm events.

TSS concentration at Detention Pond with and without Treatment

The next comparison focuses on how effectively the detention pond treats TSS across different design storms. Instead of comparing the time series of influent and effluent TSS concentrations (which can be misleading due to differences in flow rates) we compare the effluent TSS concentrations with and without treatment for each storm event. The results are shown in Figure 2.10.

To generate these plots, the model was run for each design storm (0.1 in., 0.19 in., and the 2-year storm) twice—once with the treatment function enabled for the storage unit node SU1, and once without it. After each run, time series data for TSS concentration at SU1 were exported to a spreadsheet and used to create the graphs shown in Figure 2.8.

Key observations from these results include:

• For the WQCV storm (0.19 in.), the pond performs as designed. With a treatment rate constant (k) of 0.01 ft/hr (as used in Equation 2.3), the pond removes nearly all settleable solids within the 40-hour design period (Figure 2.8b).
• Treatment effectiveness decreases for larger, short-duration storms, such as the 2-year event, due to higher water depths, which reduce the settling efficiency.
• Across all three storm sizes, there is a noticeable delay before significant TSS removal begins. A 50% reduction in settleable solids takes approximately 35 minutes for the 0.1 in. storm, 27 minutes for the 0.19 in. storm, and 20 minutes for the 2-year storm (Figure 2.8a).

TSS Mass Load at Detention Pond with and without Treatment

Another way to assess the pond’s treatment performance is by comparing the total mass of TSS (Total Suspended Solids) it releases with and without treatment. Figure 2.9 illustrates this for the 0.19-inch WQCV storm, showing the TSS mass load and effluent released by pond SU1 TSS and Total Inflow at O1, see Results.xlsx for detailed calculations). Unlike the concentration-based results, the impact of treatment on reducing the total mass of TSS released is less pronounced. In the Tutorial_06_Final.gdb simulation, 1.671 pounds of TSS were washed off during the storm (as reported under Wet Weather Inflow in the Quality Routing Continuity section in Status Report). Of that, only 0.594 pounds were removed in the pond (shown as Mass Reacted in Status Report), resulting in a total mass removal of just 35.6%.

For the other storm events, mass removal rates were even lower—26.6% for the 0.1-inch storm and only 1.2% for the 2-year storm. These relatively modest results are due to the time it takes for solids to settle in the pond. During this period, the pond continues to release outflow, allowing some of the solids to be carried downstream before they can settle out.

TSS Treatment Performance for a higher value Removal Constant (k = 0.3)

The relatively modest performance of the detention pond shown in earlier results is based on a conservative assumption; a low removal constant (k) representing particles with very slow settling speeds. Specifically, this k-value reflects particles in the bottom 20% of settling velocities from a nationwide study. But what if the particles in the TSS runoff were larger and settled more quickly? To explore this, we can use a higher k-value of 0.3 ft/hr, which corresponds to the 40th percentile of settling velocities reported in the NURP study (US EPA, 1986). Figure 2.10 shows how this higher k-value affects TSS concentrations in the pond’s discharge, while Figure 2.11 illustrates the change in total TSS mass loading for the 0.19-inch storm. Table 2.10 provides a side-by-side summary of the pond’s treatment performance for both k-values.

These results highlight a key point: predictions of TSS removal are highly sensitive to the k-value used in the model. Unfortunately, as noted by the US EPA (1986), settling velocities for solids can vary widely not only from site to site but even from one storm to the next at the same location. This variability makes it challenging to accurately estimate how effective a detention pond will be in removing TSS.

Table 2.10 : Detention pond TSS treatment performance summary

Total TSS Load Discharge

Figure 2.12 shows a comparison of the total pounds of TSS discharged from the study area for each design storm under three different treatment scenarios These values come from the Status Reports (External Outflow under Quality Routing Continuity) generated by simulating the model for three scenarios:

(a) with both LIDs and the detention pond active,

(b) with just the LIDs (pond treatment turned off), and

(c) with no treatment at all, using the input file from Tutorial_06_EMC.gdb.

For this analysis, a conservative removal rate (k) of 0.01 ft/hr was used for the detention pond.

As shown in the Figure 2.12, TSS loads consistently decrease as more treatment is applied. Interestingly, the reduction provided by the detention pond is only slightly greater than what’s achieved by the LIDs alone. This might seem surprising, since the pond is a regional BMP treating runoff from the entire catchment, while the LIDs are local measures targeting smaller areas. The reason lies in the conservative k-value used for the pond. If a higher k-value were applied, representing faster-settling, larger particles—then the pond’s performance would improve, and the total TSS discharged would be even lower.

Major Outcomes

In this tutorial, GeoSWMM has been used to show the resulting TSS concentrations in the pond discharge with both conservative and higher k-value. TSS mass load discharge comparisons have been made among three different storm events as well as for three treatment scenarios. The major outcomes of the analyses made are listed below:

• For the treatment function used in this tutorial, the pond provided less incremental TSS load reduction than did the LIDs. This result, however, is completely dependent on the value of the removal constant k used within the pond’s treatment function Modeling these types of LIDs can require a finer level of subcatchment discretization to properly account for their localized placement.
• The large variability reported for particle settling velocities in urban runoff makes it extremely difficult to estimate a removal constant for a detention pond’s treatment function that can consistently provide reliable estimates of the pond’s treatment performance.