Event-controlled changes in hydrologic connectivity in an urban watershed impacts streamflow response

Abstract:

Urban watersheds are characterized by impervious surfaces, compacted soils and decreased vegetation cover, resulting in reduced infiltration, evapotranspiration and greater surface runoff. Surface runoff from urban watersheds is transported rapidly to the stream through storm sewers, or via a combined sewer network to a Wastewater Treatment Plant (WWTP). A combined sewer conveys both sanitary water and surface water runoff to a WWTP if sufficient capacity exists. However, during large wet weather events, the system can become overwhelmed and result in combined sewer overflows to the stream. Recent research has found that urban land use and sewer-to-stream hydrologic connectivity is a dominant factor increasing flood severity and contaminant fluxes. However, relatively little research has investigated the role of the distribution of stormwater infrastructure on event hydrologic response of heavily urbanized, meso-scale watersheds. The objectives of this research are: (1) use geospatial sewer data to map changes in the impervious area connected to a stream during storm events; (2) examine the relationship between antecedent wetness, storm characteristics, and stream hydrological response. To address the research objectives, watershed and sewershed delineation was conducted for the Black Creek watershed, located in the Greater Toronto Area, Canada. Discharge and precipitation data from 2008-2018 were used for hydrometric analysis using the HydRun toolbox in MATLAB. Surface drainage from storm and combined sewers increased the total catchment area by 11.6 km² (18.5 %). Of this 11.6 km² increase, 9.82 km² of surface drainage contributes flow during wet weather events when the combined sewer becomes hydrologically connected. Mean event streamflow response represented by the runoff ratio was 0.27±0.2. Streamflow increased linearly as a function of event precipitation (P_event), however the influence of 3-, 5- and 7-day antecedent precipitation was negligible. These results suggest antecedent catchment wetness may be insignificant and that P_event dominates streamflow generation for this study site. However, groundwater infiltration into storm sewer pipes may also be an important process generating streamflow, which should be considered in urban water resource planning and management.