Using continuous simulation to analyse the downstream impacts of stormwater management ponds in the sawmill river watershed

Abstract

Numerous hydraulic modeling studies carried out since the 1970s have demonstrated that the impact of flow detention is reduced as the flow is routed through the downstream watershed. Since stormwater management ponds delay and prolong peak flows, the downstream watershed receives increased runoff volumes and sustained peak flow rates from the ponds that can combine with other watershed drainage to increase downstream peak flows. This study uses continuous simulation modeling in PCSWMM to analyse the impacts of stormwater management ponds located throughout the Sawmill River watershed in Dartmouth, Nova Scotia on downstream peak flows near the watershed outlet at Sullivan's Pond. The effectiveness of the upstream ponds to maintain peak flows at Sullivan's Pond was analysed for all 903 historical rainfall events within a 61 y simulation period that had a 1 in 2 month probability of occurrence or greater, providing results for a full range of rainfall frequencies, rainfall amounts, rainfall durations and initial flow conditions. Pond effectiveness was evaluated for multiple model scenarios, which included varying the maximum overland flow lengths, the hydraulic conductivities of the soils, pond locations and development conditions. Overall, the study confirmed that the effectiveness of upstream ponds is reduced towards the watershed outlet, demonstrating the need for stormwater management practices that preserve runoff volumes and timing characteristics in addition to peak flows.