STRATEGIES ADDRESSING HYDROMODIFICATION IN CHANNEL SEGMENTS WITHIN HIGHLY-EROSIVE OR UNSTABLE TERRAINS

Urbanization alters rates and timing of runoff, often leading to long-term downstream changes in channel morphology and functions. No matter how stable the substrate may be within the envelope of land-use change, altered patterns of runoff can act on unstable channel segments further downstream, including those underlain by very weak rocks or destabilized by legacy uses. While our culture's approach to mitigating these effects is still evolving, careful geologic and geomorphic analysis can lead to measures minimizing or offsetting further disturbance. Such analysis includes:

• Establishing a sediment-transport baseline

• Characterizing geologic units or complexes of varying strength and stability

• Identifying the key geomorphic and vegetational dynamics affecting slope and channel stability within each unit, including responses to major historic events

• Systematic delineation of existing mass movements, incising reaches and other sediment sources

• Complete mapping of bed and bank stability, segment by segment

• Rapidly-revisable runoff modeling with segment boundaries matching geologic or geomorphic contacts

• Site-specific approaches to runoff management, planned for the sandiness, cohesiveness, cementation, and gravel content of the bed and banks

In this paper we present two case studies east of San Francisco Bay in watersheds with similar geology, rainfall, vegetation and grazing histories, but with different sediment-delivery processes and existing expressions of instability. At one site, channels are deeply incising through sandy-clay alluvium; at the other, channels have cut to resistant bedrock, but have several metastable banks formed of landslide-toe debris. Our objective was to address downstream hydromodification resulting from urbanization. In both cases we arrived at similar strategies for mitigating hydromodification— reduce storm-flow peaks and volume, coupled with treating a reasonable number of the existing point sources of sediment predominantly through biostabilization. Thresholds for runoff control and criteria for bank protection and repair differed markedly at the two sites.