Sensitivity analysis of the downstream flooding impacts of stormwater management ponds in the Sawmill River watershed using continuous simulation modeling

Numerous hydraulic modeling studies carried out since the 1970’s have shown that the impact of flow detention becomes 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 upstream ponds that can combine with other watershed drainage, negating the impacts of the ponds. Furthermore, previous research has generally concluded that ponds are the least effective at reducing downstream flows when they are located in the lower areas of the watershed, when they are installed on a lot level resolution and/or when the watershed has an elongated shape. However, most studies on the subject have only evaluated the downstream impacts of ponds based on synthetic design storm events and select historical storm events. 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 its watershed outlet at Sullivan’s Pond. The effectiveness of the upstream ponds to maintain pre-development peak flows discharged to Sullivan’s Pond was evaluated for multiple model scenarios to test the sensitivity of downstream peak flow reduction, which included varying the estimated maximum overland flow lengths of the watersheds, the estimated hydraulic conductivities of the soils, pond locations and development conditions.  Pond effectiveness was analysed for all rainfall events in the 61 year simulation period with a 1 in 2 month probability of occurrence and greater, providing results for a full range of storm durations and frequencies within approximately 900 historical rainfall events. Comparisons of pond effectiveness were then made between the various model scenarios for the different types of storm events. Overall, this study confirmed that the effectiveness of upstream ponds becomes reduced towards the watershed outlet, further validating the need for stormwater management systems to shift towards infiltration systems that maintain pre-development hydrographs, such that development does not increase runoff volumes and downstream peak flows.