Floodplain Mapping in the San Lorenzo Creek Watershed

Zone 2

The San Lorenzo Creek Watershed in Castro Valley is one of the largest watersheds in the District. It includes San Lorenzo Creekand tributaries Crow Creek, Cull Creek, Castro Valley Creek, Chabot Creek, Eden Canyon Creek, Palomares Creek, and Upper Sulphur Creek.

Flood control structures along San Lorenzo Creek and its tributaries were originally designed to handle a 25-year flood. Extensive development in Castro Valley over the years has caused greater runoff to flow into the creeks. The San Lorenzo Creek Watershed includes several large areas that have the potential to flood from a large storm.

A US Geological Survey (USGS) hydrology study completed in 2003 (KA & JH-cannot find this 2003 report) indicated an increase in stormwater flowing through flood control channels within the watershed. With public safety in mind, the District verified USGS results by completing its own, even more precise hydrologic study.

While the District’s study helped to remove some properties from FEMA’s flood maps, it is clear from both the USGS and District studies that the flood control infrastructure in both the San Lorenzo Creek and Castro Valley Creek watersheds is inadequate to carry large stormwater flows.

The District is now studying the best and most cost-effective solutions to provide greater flood protection to properties within the watershed.

One possible solution is to make modifications to Don Castro Reservoir so it can carry more stormwater flows through the channel. The District will likely need to construct floodwalls in several locations along San Lorenzo Creek, as well.

Another possible solution is to increase the water-carrying capacity of San Lorenzo Creek and its tributaries.

The District has proposed comprehensive upgrades to improve San Lorenzo Creek flood control. This multifaceted project has been incorporated into the Bay Area Integrated Regional Water Management Plan, but the District is still seeking funds.

There is no simple or inexpensive fix. Tens of millions of dollars will likely be required to make the improvements.

Cull Canyon Reservoir and Don Castro Reservoir and Dams

Map of San Lorenzo Creek watershed, including the 14 catchment areas within its boundaries. San Lorenzo and Castro Valley (Zone 2)

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