Flood Mitigation
Corps, State Agencies Prep for Flood Risks From California Snowmelt Runoff
The US Army Corps of Engineers are preparing to use the Isabella dam’s service spillway to release water for the first time since 1983 in order to handle expected water level rise from snowpack melt.
Photo courtesy US Army Corps of Engineers Sacramento District
In the wake of record-setting snowfalls in the Sierra Nevada Mountains over the past six months, California and federal officials are preparing to handle the flooding threat posed by imminent meltwater runoff. The efforts include releasing water from the state's key dams to allow for the expected capacity, preparing emergency responses for imperiled area's in the state's Central Valley and launching groundwater recharge projects.
On May 1, California's Department of Water Resources (DWR) conducted its fifth seasonal snow survey at Phillips Station in El Dorado County. The survey recorded a snow depth of 59 in. and a snow water equivalent of 30 in., 241% of the average for that location. The last year of measurable snow on May 1 at the site was in 2020 when only 1.5 in. of snow and 0.5 in. of snow water equivalent were recorded.
"The snowpack will not disappear in one week or one month but will lead to sustained high flows across the San Joaquin and Tulare Basins over the next several months," said DWR Director Karla Nemeth. "While providing a significant boost to California's water supplies, this year's massive snowpack is posing continued flood risks in the San Joaquin Valley."
According to the agency, the forecasted amount of snowmelt runoff and water flow into California river basins are expected to exceed previous April-July records. As reports of increased flood risks come in, DWR is coordinating responses with other agencies across the state.
In late April, the department provided technical assistance through its State-Federal Flood Operations Center (FOC) deploying an 11-person incident command team to support county emergency response officials in the Tulare Basin. This team included flood fight specialists and hydrologists providing technical assistance on flood fighting efforts as well as planning for the coming snowmelt. The FOC also worked to supply requested flood fighting materials, including 217,000 sandbags and 17,700 supersacks.
"Beyond the current response efforts, DWR is looking ahead to help prepare the area for the coming snowmelt flood risk," said Jeremy Arrich, Manager of DWR's Division of Flood Management. "As the 2023 snowmelt season begins, DWR will continue to work closely with local county officials and USACE to determine what actions are necessary to protect public safety."
California Governor Newsom announced on May 11 that the state is providing funding to raise the Corcoran levee to 192 feet to further help protect the Tulare Basin region. The funding is part of $290 million designated for statewide flood control measures in the governor's proposed budget. That is in addition to $202 million proposed in January.
Isabella Dam Preparations
On May 9, the U.S. Army Corps of Engineers announced that Lake Isabella, situated on the Kern River approximately 42 miles northeast of Bakersfield, is expected to exceed its gross pool of 568,000 acre-ft this summer, requiring its dam's service spillway to release water for the first time since 1983. The lake is currently at 60% of capacity but once it fills the resulting release could last for several weeks. The lake's fill plan will be adjusted to accommodate this year's historic snowpack.
"The fill plan sets reservoir filling rate targets, but since we can't control snowmelt volume, we will have to make adjustments in real-time," said Mike Ruthford, a lead engineer with the USACE South Pacific Division Dam Safety Production Center. "We will coordinate these adjustments with downstream water users, as we've always done."
Water volume at Lake Isabella had been limited to 361,000 acre-feet since 2006 when issues regarding seepage, overtopping, and seismic safety at the dam were identified. USACE received approval to implement a fill plan allowing the lake to increase capacity following the March completion of Phase 2 of the Isabella Dam Safety Modification Project.
While it may be necessary to have prolonged high-volume releases at the Isabella Dam, Corps officials say there is no expectation that water levels will reach the top of the dam's new labyrinth weir or spill into the emergency spillway.
In addition, the Corps is collaborating with DWR and other partners to evaluate and convey any possible effects of the expected snowmelt runoff on Bakersfield and the Tulare Basin.
Groundwater Recharging
Another DWR measure to relieve pressure on California's rivers and reservoirs are plans with groundwater sustainability agencies to implement and manage groundwater recharge projects. The projects capture water in rivers and streams during higher flows and transport it to local aquifer recharge basins allowing runoff to percolate into the aquifer.
DWR is also working to fund, secure and deploy temporary diversion equipment. On April 25, the Fresno Irrigation District deployed the first set of temporary pumps and siphons to reduce downstream flood impacts in the Tulare Lake Region and expand groundwater recharge efforts by diverting water from Kings River to existing recharge facilities or to existing agricultural lands.
To expedite the process for groundwater recharging projects, Governor Newsom recently signed Executive Order N-4-23, temporarily lifting regulations and setting conditions for diverting flood stage water without permits to boost groundwater recharge storage. In a statement Newsom said, "California is experiencing extreme rain and snow, so we're making it simple to redirect water to recharge groundwater basins.”
