RESTORATION OF AGING CONCRETE CHANNELS THROUGH A WATERSHED-SCALE APPROACH TO GREEN INFRASTRUCTURE: “A FIFTY-YEAR PLAN”

As flood control districts in suburbanized watersheds seek to address aging concrete flood control channels, existing development patterns limit opportunities to restore channels and the processes that support self-sustaining creeks. Current approaches to urban creek restoration often target opportunistic, site-based projects resulting in isolated reaches that fail to connect to former floodplains or address impacts of urbanization in upstream contributing areas. Planning for restoration of urbanized lowland drainage networks requires a strategic approach to consider how underlying biophysical conditions throughout a watershed can support cost-effective “green” infrastructure (GI) that re-integrates infiltration, groundwater recharge, seasonal floodplains and sediment transport processes into the urban footprint.

Using accessible geospatial data and analysis tools, our GI suitability analyses delineate strategic target areas in Walnut Creek watershed to support key functions of urban watersheds: flow conveyance, seasonal flood storage, and infiltration for groundwater recharge. Informed by previous research and regulations, we developed suitability criteria for creek naturalization, infiltration, and floodplain expansion. We identified areas with overlapping ecological and social benefits of creek naturalization, and mapped their intersection with parcel-based opportunities to develop recommendations for creek and floodplain expansion restoration strategies. Our infiltration analysis indicates limited opportunities for shallow surface infiltration due to low permeability soils, liquefaction, and steep slopes; however, permeable quaternary deposits present opportunities for deep stormwater infiltration where shallow infiltration may be infeasible. Previous research shows that shallow infiltration and deep infiltration areas can support infiltrating GI to treat stormwater runoff, increase recharge, and reduce peak flows; our next step is to test these hypotheses. If implemented in contributing drainage areas of high priority channel reaches we foresee improved habitat conditions for salmonid populations through connected migration corridors and floodplains, improved water quality, increased base flow and cooler summer water temperatures.