Friends of San Lorenzo Creek

Engineers Are Recalculating San Lorenzo Creek Overflow Risk

MAY 11, 2007

Engineers from the Federal Emergency Management Agency met with Alameda County Public Works counterparts Wednesday to share their newest mapping of flooding from San Lorenzo Creek. They found that the creek is likely to overflow its banks in the biggest storm in a century, and they mapped where the water would flow.

The agencies will be making flood insurance recommendations in the fall, said Hank Ackerman, principal civil engineer with Alameda County Public Works Agency Flood Control Program.

The program aims to make the insurance recommendations as accurate as possible. San Lorenzo Creek, which collects at the Cull Canyon Reservoir and Don Castro dams, runs through Hayward, unincorporated Hayward and San Lorenzo, where it forms the southern border of San Leandro until emptying into San Francisco Bay. After major flooding in 1954, the U.S. Army Corps of Engineers used data it had collected since the mid-1940s to complete a concrete bed and levee building project in 1959, Ackerman said.

Back then, the Corps designed the levees to hold more than the highest predictable flow, which it estimated at 10,400 cubic feet per second. But in 2003, the U.S. Geological Survey, armed with nearly 50 more years of data, calculated the flow from a hundred-year storm to be much greater, at 15,290 cubic feet per second. More data in 2005 caused the USGS to make a still higher prediction: 16,100 cubic feet per second, Ackerman said.

With this volume of water in San Lorenzo Creek, engineers know it will overflow its banks west of Interstate 880, with the worst area between the freeway and Washington Avenue, but with flooding also between Washington and the railroad bridge near the Bay shoreline. While San Lorenzo Creek's watershed was mostly farmland in the 1950s, now it is densely populated.

The flood control program aims to determine who would be affected when there is a deluge. "Let's find out truly which area will flood," Ackerman added.

Alameda County engineers used a detailed model to estimate where the water would flow, which takes into account the land elevation and where buildings and roads are. The FEMA model takes a more uniform approach, modeling the flood plain on the assumption that everywhere around the creek is the same elevation. The FEMA model contends a flood will cover two to three times the surface area as the county version, explained Rohin Saleh, a county hydrologist.

Around the lower San Lorenzo Creek, for example, FEMA has calculated that both the north and south banks will overflow, but the local public works model shows only the south side flooding. Moses Tsang, another county hydrologist, said that some 2,000 parcels would be flooded in the hundred-year storm. "We're talking with FEMA and will work to create something that's more realistic," Ackerman said.

Ackerman also explained that raising the height of the walls in the part of San Lorenzo Creek that could overflow would cause the water to back up in the channels that feed the creek and possibly into surrounding areas, where it would be difficult to drain without pumps.

FEMA cannot require people to buy insurance unless they have a federally backed loan, but private lenders can decide to adopt the federal recommendations.

Source: Daily Review