Controls on Reach Scale Hydrologic Response in Urban Streams and Implications for Restoration Design

This dissertation explored the nature of hydraulic controls on flow paths and residence times in the channel and riparian zone in two highly urban and two suburban 2nd-3rd order streams in the Baltimore metropolitan region. The influence of urban development and in-stream infrastructure on flow patterns across the flow frequency spectrum were compared to natural features in the channel and riparian zone that have similar effects on flow patterns. This work was motivated by the recognition that the urban stream syndrome paradigm of simplified channel geometry associated with extremes of low and high flow does not account for the full range of interactions between urban watershed hydrology, riparian zone morphology, and urban infrastructure. First, to overcome technical obstacles associated with measuring discharge in flashy urban streams, an innovative methodology is presented in Chapter 2 for extrapolating the high end of discharge rating curves in four urban streams, the results of which were used to more accurately quantify the hydrologic response of the watersheds subsequently studied in this dissertation. In Chapter 3, the goal was to assess how a spectrum of channel and riparian zone morphologies affect the behavior of floodwaves generated from an urban landscape. A degraded urban non-alluvial headwater stream experienced almost pure floodwave translation under existing conditions and also when routed through a planned restoration design intended to improve connectivity with the adjacent riparian zone. Even in a third scenario where the upland corridor was removed and the floodplain was widened to an extreme extent there was little change in the hydrograph routed through the restoration reach. Only in an extremely unrealistic scenario in which the water was stored without release was there a significant change to the hydrographs. For these designs, simply increasing floodplain connectivity did not significantly alter the hydrographs. Next, the effects of a long, broad alluvial riparian zone in a suburban watershed on the properties of floodwaves were assessed. Floods that occur naturally in this watershed overwhelmed the storage capacity of the floodplain and experienced translation, while significant attenuation was observed when hydrographs with large peaks and narrow time bases from a highly impervious headwater urban watershed were routed through this alluvial valley. In Chapter 4, the relationships between longitudinal channel geometries, urban infrastructure, channel-spanning blockages, and mean hydraulic residence times under base flow conditions were also explored. Results show that road crossings create abrupt, non-erodible knickpoints in the channel which exert a strong effect on the distribution of elevation drops in water surface, similar to the controlling influence of channel blockages on channel forming processes. Modeling results across the spectrum of base flows indicate that these urban structures can play a role in controlling flow patterns and residence time, and that channel-spanning blockages and other restoration structures have a significant influence on estimates of in-channel residence time. This work is an important contribution to our understanding of the mechanisms controlling the propagation and storage of surface waters in urban stream networks and adjacent riparian zones, and helps to better define the efficacy of stream restoration practices