Those of us who have friends or relatives back east got the phone calls and text messages: “Are you guys OK?” Their local and national media represented our recent rainstorms as catastrophic events that swept all of us into Santa Monica Bay.
While folks who live in the foothills, canyons or downstream from recent burn areas always have reason for concern during a strong rain event, most of us just get annoyed by a flooded intersection, pothole or a drip from our ceiling.
Above: At Loyola Marymount University’s northern border, sandy soils that comprise the Westchester Bluffs are structurally weak. Add water, a steep slope and the weight of ill-advised iceplant ground cover, and whoosh! A textbook “slump” landslide.
The two large storm sequences that visited Southern California this February added sorely needed snowpack and runoff to our mountains and reservoirs. How large were they in historical and hypothetical context and would the Ballona Wetlands Restoration Project change things?
February’s storms were of the warm, atmospheric river variety, typical during our current El Niño period. Such storms bring much rain, but at medium intensity over several days. They don’t usually fill up our storm channels, but rather keep them flowing moderately for days.
Find out what's happening in Marina Del Reywith free, real-time updates from Patch.
In contrast to El Niño events, our “normal” cold storms that brew in the Gulf of Alaska slam quickly into the state, dropping all of their rain in heavy downpours in just a few hours. These are the big events our flood control channels are designed to hold, and our Marina-Playa area is at the “end of the pipe”, so to speak.
Above: The Ballona Creek storm channel can safely hold a once every 100 years, 46,000 cubic feet per second runoff event.
The first large storm this year peaked on February 4-5. It filled my yard bucket in Playa Vista with around 5 1/2 inches over two days. A neighbor on the Westchester bluffs reported 7 inches. That’s big rain for Los Angeles in a two day period.
Above: Hydrograph of the February 4-5 runoff event in Ballona Creek.
On February 19-20 we received another storm. How did these two storms rank in history and what about flooding in the Ballona Wetlands? Answer: Both storms were moderate in historical and hydrological contexts.
Above: The Ballona Creek watershed. All rainfall runoff in the shaded area flows into and through Ballona Creek to the ocean.
Ballona Creek’s largest recorded runoff event was on November 21, 1967. The peak flood flow in Ballona Creek was 32,500 cubic feet per second (cfs) that day. The present Ballona storm channel is designed to hold 46,000 cfs, plus a factor of safety.
Above: L.A. County flood data for February 4 show a flow peak of 18,578 cfs.
The peak flows in the Creek this February were just under 19,000 cfs on February 4. The storm of February 19-21 registered even less at its peak, only 6,656 cfs.
Our government flood agencies maintain a gauging station - a post of measuring instruments - installed in Ballona Creek near Sawtelle Boulevard. That station and its predecessor have recorded flood water surface elevation since around 1928.
Above: On the bike path near Sawtelle Blvd., stream gauging station F38C-R records Ballona Creek water levels and meteorological data.
Water flow, in cubic feet per second (cfs) can be calculated when the water surface elevation (above the channel bottom), flow velocity (speed) and channel dimensions are known. Computers hold all this data and crank out flow values every 15 minutes.
The Ballona Creek channel is designed to hold a 46,000 cfs flood flow, the 100-year event. The largest of our most recent flows, 18,578 cfs, didn’t come close to the historical peak or channel capacity.
The people who sued to stop the Ballona Wetlands Restoration Project made much ado about flood risk in their court petition. That, too, will turn out to be a big red herring.
A sympathetic judge threw them a bone and ordered the state to reanalyze flood impacts of the Restoration Project using a ginormous and highly unlikely flood peak of 68,000 cfs. The results, I expect, will be no change in impacts.
A flood that large, a roughly once in 600-year event, would swamp neighborhoods far upstream of the Ballona Wetlands. In our local Marina/Playa area, impacts would be limited to minor overtopping of the Ballona Creek levee near the UCLA crew launch ramp. That overtopping would occur even without the Wetlands Restoration Project, according to the initial Project analysis.
Above: A once in 600-year catastrophic flood would overtop the creek levee at the UCLA boat facility. Boats would get wet!
The proposed Ballona Wetlands Restoration will greatly expand the creek’s flood plain area, allowing peak flows to spread out between Fiji way and the Westchester Bluffs. Increased by hundreds of acres, the restored wetlands will act as a flood shock absorber, in effect. Today, flood flows are confined to the obsolete concrete channel built a century ago. The Restoration Project will change that for the better.
Above: Peak flood flows will have much more area to spread out in the restored Ballona Wetlands.
When the opponents of the Ballona Wetlands Restoration Project wave their arms and yell “flood”, know that they are all wet.
Enjoy your Ballona Wetlands!
References
(1) NOAA Advanced Hydrologic Prediction Service. https://water.weather.gov/ahps2/hydrograph.php?wfo=lox&gage=blnc1
(2) L.A. County Department of Public Works. Channel flow data. https://dpw.lacounty.gov/wrd/rainfall/Home/Flow
(3) L.A. County DPW. Annual Hydrologic Reports. https://ladpw.org/wrd/report/index.cfm
(4) How freeing rivers can help California ease flood risks and revive ecosystems. L.A.Times, 02/20/2024. https://www.latimes.com/environment/story/2024-02-20/california-floodplain-restoration
Author’s disclosure of affiliations:
Dr. David W. Kay served on the Board of Directors of the non-profit Friends of Ballona Wetlands from 2007 until 2015, and served as Board President in 2012-13. He presently serves on the Board of Ballona Discovery Park in Playa Vista. Dr. Kay is a staunch advocate for the state of California's plans to restore the Ballona Wetlands Ecological Reserve.
From 1984 until 2022, Dr. Kay was employed by Southern California Edison Company, exclusively in the company's environmental services organizations. His many responsibilities included restoration of the 440-acre San Dieguito Wetlands near Del Mar. He retired in 2022 as Senior Manager for Major Project Environmental Management at the company, after 38 years of service.
Dr. Kay earned bachelor and masters degrees in biology and a doctorate in environmental science.
See Dr. Kay’s Patch Community Contributor profile here.