HR: 10:20h
AN: H52D-01    [Abstracts]
TI: Urban Effects on Stream Hydrology and Geomorphology: Variations, Magnitudes and Implications for Stream Protection and Restoration
AU: Brown, S
EM: sbrown@balancehydro.com
AF: Balance Hydrologics, 841 Folger Ave, Berkeley, CA 94710, United States
AU: Hecht, B
EM: bhecht@balancehydro.com
AF: Balance Hydrologics, 841 Folger Ave, Berkeley, CA 94710, United States
AU: * Gartner, J D
EM: jgartner@balancehydro.com
AF: Balance Hydrologics, 841 Folger Ave, Berkeley, CA 94710, United States
AU: Owens, J
EM: jowens@balancehydro.com
AF: Balance Hydrologics, 841 Folger Ave, Berkeley, CA 94710, United States
AB: The hydrologic and geomorphic response to urbanization has become an increasing concern for regulatory agencies and practitioners of stream protection and restoration. However, there has been comparatively less attention to this topic in the research community. In his classic diagram of the effect of urbanization, Leopold (1968) illustrated how increases in impervious area and degree of ‘improved' storm drainage tend to shorten lag times and increase peak flows. This response, sometimes termed hydrograph modification or hydromodification, can have dramatic effects on stream morphology, inducing both accelerated erosion and/or deposition. We present field data for case studies in California that contrast hydrologic and geomorphic responses in sandy versus clay-rich soils with different underlying geology. The first case study compares the urban effect in paired watersheds with relatively sandy terrain in Southern California. Annual peak flows, measured by indirect methods, increased by up to an order of magnitude in catchments with urbanization compared to adjacent catchments without urbanization. Morphologic responses have included significant incision plus natural-levee deposition from overbank flows. The second case study, in areas with clay-rich soils in the San Francisco Bay Area, examines hydrographs in watersheds with and without urbanization. Watersheds with urbanized areas showed significant increases in stream flow in early season storms and moderate mid-winter storms, but less of an urban effect in large mid-winter storms. Near bankfull flow events capable of sediment transport also became more frequent following urbanization, especially early in the rainfall season and following mid-winter dry spells. Channel response to hydromodification included increases in bank recession and collapse associated with the early-season storms. A third case study, in the East San Francisco Bay Hills shows how even minor (approximately 1 percent) increase in impervious area, an index of urbanization, can cause up to an order of magnitude increases in early season storm flow peaks. In summary, these case studies illustrate variation in both the magnitude and type of responses due to site conditions, which can be used to guide methods of stream restoration and protection to the local setting.
DE: 1803 Anthropogenic effects (4802, 4902)
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1860 Streamflow
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting