HOW DOES STREAM RESTORATION AFFECT FLOODWAVE PROPAGATION IN URBAN CHANNELS?

This research investigates the geomorphic and hydraulic controls on floodwave propagation in two urban headwater streams in the Baltimore metropolitan region using field measured hydrologic and topographic data and 2-d hydrodynamic models. A spectrum of channel and riparian zone morphologies and conditions were explored to assess the impacts of restoration strategies on floodwave propagation. These include exploring the impacts on floodwave behavior of 1) end-member floodplain designs for a highly urban watershed with a degraded non-alluvial headwater channel and of 2) a long, broad forested alluvial floodplain in a suburban watershed. The effects of these floodplain settings on floodwave properties were analyzed for 1) a range of storms with variable intensities and durations producing floods from the 1- to 5-year recurrence intervals and 2) low to high floodplain frictional resistance values.

The non-alluvial stream experienced almost pure floodwave translation under existing conditions and also when routed through a two-stage restoration design. A third scenario where the upland corridor was removed and the floodplain was widened to an extreme extent resulted in modest changes to the hydrograph peak (~10%) and duration of the hydrograph (<10 minutes). The only significant attenuation observed in the non-alluvial stream was in connection with a scenario involving large ponds in the floodplain and a reduced culvert that caused backwater. Floods that occur naturally in the alluvial stream overwhelmed the storage capacity of the floodplain and experienced translation. Significant attenuation was observed, however, when the flashy hydrographs from the non-alluvial stream were transposed and routed through this alluvial valley, resulting in up to 60% peak reduction and an increase in hydrograph length up to 60 minutes. Even with a broad, connected alluvial riparian zone with a minimally incised channel, an approximate doubling in watershed area was needed to effectively attenuate a floodwave generated from a small urban headwater stream. The results of these analyses show that restoration in headwater basins is unlikely to accomplish significant mitigation of the floodwave, whereas wide, low-gradient floodplains have greater capacity to store and attenuate flashy urban hydrographs.’