Instream dissolved oxygen sensitivity to baseflow and the use of baseflow timeseries in SWMM and WASP modeling of urban streams

The dissolved oxygen concentration (DO) in urban streams during extended dry weather periods typically is depleted by microbial and algal respiration. Simulating in-stream reaeration processes is critical to the development of a comprehensive model of the DO budget. For this study, water quality models for DO were developed using the USEPA SWMM and WASP models for the non-tidal extents of tributaries to tidal portions of the Delaware River in Southeastern Pennsylvania. The motivation for this work is to better understand the sources, transport and fate of oxygen demanding substances as well as their influences on the DO budget in these urbanized watersheds. The watersheds are highly impervious and include areas drained both by combined and separate sewers. The streams exhibit rapid runoff response to rainfall and snowmelt at all times and depleted baseflow conditions during the summer.

The SWMM5 model was used to simulate watershed hydrology and hydraulics, streamflow hydrodynamics, and pollutant loading and routing. The SWMM5 results establish boundary conditions for the WASP-based stream DO models. Water quality routing and processes within the stream channels were simulated in WASP, which uses velocity and depth inputs to estimate reaeration rates for the DO budget. Commonly represented in watershed models as an annual or monthly average value, baseflow conditions, and the influence on velocity and depth, are important aspects of the model formulation. A sensitivity analysis of baseflow on dissolved oxygen in the stream channel was performed to determine the necessary level of detail to reliably represent in-stream DO conditions during the critical summer months. Results of that analysis suggest a need to more properly represent baseflow in the stream channel models with a greater spatial resolution than that provided by a monthly average. Therefore, baseflow separation was performed on flow data available for the streams at an hourly timestep. The resulting estimate of hourly baseflow is used in an area-weighted timeseries, along with velocity and depth, to better simulate reaeration rates. This method allows for a more accurate representation of dissolved oxygen during periods of low flow between storms and as baseflow recedes during summer months. This paper will describe the processes used in the analysis as well as lessons learned.