Conclusion

This tutorial introduced modeling of water quality treatment in an urban catchment area using GeoSWMM. It demonstrated TSS removal considering in both local LID source controls as well as in a regional detention basin. The key points demonstrated in this tutorial are:

i. LID controls can be modeled as distinct subcatchments with a single landuse that has a constant removal efficiency assigned to it. GeoSWMM applies this removal efficiency to the runoff that the control receives from upstream subcatchments.

ii. Treatment within a detention pond is modeled with a user-supplied Treatment Function that expresses either the fractional removal or outlet concentration of a pollutant as a function of inlet concentration and such operational variables as flow rate, depth, and surface area.

iii. An exponential treatment function was used to predict TSS removal within this tutorial’s detention pond as a function of a removal constant and the pond’s water depth, where the removal constant reflects the settling velocity of the particles to be removed.

iv. GeoSWMM’s use of constant removal efficiencies for LID controls makes its LID treatment performance insensitive to size of storm.

v. Treatment performance for detention ponds decreases with increasing size of storm due to an increase in pond depth. It also decreases with decreasing size distribution of the sediments that constitute the TSS in the runoff.