Restoring the hydrologic response to pre-developed conditions in an urbanized headwater catchment: Reality or utopia

The conversion of forested areas to impervious surfaces, lawns and pastures alters the natural hydrology of an area by increasing the flashiness of stormwater generated runoff, resulting in increased streamflow peaks and volumes. Current stormwater infrastructure fails to offset the impact of increased impervious surfaces and loss of natural vegetation. The lack of adequate stormwater facilities along with increasing urbanization and population growth illustrates the importance of understanding urban watershed behavior and low impact development (LID) that focus on restoring the hydrology of an area. In this study, we developed a lumped urban ecohydrology model (UEM) that represents vegetation dynamics, connects pervious and impervious surfaces and implements various bioretention treatment scenarios. The model is applied to an urban headwater subcatchment located in the Newaukum Creek Basin. We evaluate the hydrologic impact of controlling runoff at the source and disconnecting impervious surfaces from the storm drain through model sensitivity analysis with varying sizes of bioretention cells. The model response to changes in storm depth, impervious area fraction, and treated area fraction of the urbanized catchment is assessed to identify the sensitivity of catchment variables in meeting hydrologic targets and goals. The effectiveness of the bioretention cells are quantified as the reduction in hydrologic indicators, high pulse count (HPC) and high pulse range (HPR), of the simulated basin outflow as well as changes in the flow duration curves. Indicator results of model simulations show increasing bioretention cell size reached a 'threshold of effectiveness' for stream health improvement. In other words, after an initial improvement in indicator values associated with improved stream health, implementing larger bioretention cells to capture runoff from the same fraction of the basin did not significantly reduce indicators to further improve stream health. The bioretention cells were more effective at improving watershed conditions in drier climates or when total impervious fraction of the basin was reduced.