Model Results

Runoff Comparison

The objective of this tutorial was to examine the effects of two commonly used LIDs: filter strips and infiltration trenches on site’s runoff. Figure 2.7 compares the resulting influent and effluent hydrographs for filter strip W13-FS for each of the design storms. Figure 2.8 does the same for infiltration trench W14-IT. Results for the other LIDs look similar to these.

Table 2.9 and 2.10 list the runoff coefficients for each filter strip and infiltration trench respectively. The coefficients are taken from the Subcatchment Runoff of the Status Report.

Table 2.9 : Runoff Coefficient for Filter Strips

Table 2.10 : Runoff Coefficient for Infiltration Trenches

The results show that filter strips offer minimal runoff control, with outflow rates nearly equal to inflow rates across all storm intensities. This indicates their primary function is pollutant removal, rather than reducing runoff flow or volume. In contrast, infiltration trenches demonstrate substantial reductions in runoff volume, especially during smaller storm events. It's important to note that, in this case, the trenches do not include a vegetative layer above the gravel backfill. Incorporating such a layer could potentially impact performance, depending on its design.

Figure 2.9 compares the discharges simulated at the outlet of the study area for each design storm (2-yr, 10-yr and 100-yr return period) both with and without LIDs.

Figure 2.9 compares the discharges simulated at the outlet of the study area for each design storm (2-yr, 10- and 100-yr return period) both with and without LIDs. For each design storm, LIDs reduce both runoff volumes and peak discharges. As the storm event becomes larger, LIDs become less effective and the attenuation of their volumes and peak-discharges is reduced. These percent reductions in outlet volumes and peaks are compared in Figure 2.10. It shows how the benefit of LID controls decreases with increasing size of storm.

Major Outcomes

The major outcomes of this tutorial are:

• A filter strip can be modeled as a rectangular 100% pervious subcatchment whose depression storage is the effective pore volume depth of the trench.
• Modeling these types of LIDs can require a finer level of subcatchment discretization to properly account for their localized placement.
• Infiltration trenches (without a top soil layer) are more effective than filter strips in reducing runoff volumes and peaks.
• The effectiveness of LIDs at reducing runoff volumes and peaks decreases with increasing size of storm event.