Wednesday, February 8, 2023

January 2023 Flood Overview

Aerial images provide perspective on how the Ventura River responds to flood.  The wide floodplain is typical of high sediment yield watersheds, featuring braided channels that shift with each major flood event.  The flood of January 9, 2023 ripped out large areas of vegetation and "re-set" the floodplain.   These images were taken shortly after the storm and are organized downstream to upstream. 

Ventura River near Foster Park - flows overtopped the bank near the City of Ventura Nye Wells

Flooding near Foster Park - several homes were damaged 

Ventura River near Oak View - stream bank erosion damaged the Ventura River Trail and broke the main sewer line - combined with a similar broken line on San Antonio Creek 14 million gallons of sewage were estimated to have spilled into the river closing beaches for more than a week

Ventura River downstream of Santa Ana Bridge

Ventura River at Santa Ana Bridge - the bridge was recently replaced widening the river to accommodate high flows and future increased sediment transport with Matilija Dam removal

Live Oak levee upstream of Santa Ana Bridge - similar damage occurred in the 2005 flood - this levee is currently in final design for reconstruction as part of the Matilija Dam Ecosystem Restoration Project

Ventura River at Highway 150 bridge

Ventura River above Highway 150
the Ventura River Preserve protects open space for recreation and  natural floodplain management 

Robles Diversion - the flood filled the forebay with sediment and breached the timber cutoff wall
Casitas Municipal Water District quickly implemented emergency repairs to restore water
 diversions to Lake Casitas

Ventura River upstream of Robles Diversion
stream bank erosion exposed and broke the Matilija Conduit which supplies Casitas water to property owners upstream

Ventura River near Ojala - this view looking upstream includes Matilija Canyon and Matilija Dam - flood flows severely damaged the Camino Cielo crossing which provides access from Highway 33 to several properties - a new bridge is being designed as part of the Matilija Dam Ecosystem Restoration Project

Photo published in the Ojai Valley News shows the extent of debris flows on Highway 33 - the Highway was closed indefinitely for repairs 

Damage to Highway 33 along North Fork Matilija Creek - photo published by CalTrans

(all photos courtesy Rich Reid except where noted)

Hindsight Analysis of the 2023 Flood

Forecasters predicted 8 inches of rain for the January 6 storm.  However, after the first 8 inches accumulated on already saturated ground another 8 inches fell in less than 8 hours on the ridges and in Matilija canyon.  The the third in a succession of atmospheric rivers, this storm ultimately delivered over 18 inches to the steep mountains of the Ventura River watershed.

The USGS flow gage at Foster Park is the primary indicator for flood flows on the Ventura River.  This hydrograph includes the upstream USGS gage below Matilija Dam.  Unfortunately neither of these gages measured the peak of the flood on January 6.

Ventura County Watershed Protection District provides a chart with flood impacts expected for specific flows at certain gages.  The chart below shows these impacts for the Foster Park gage.

Extrapolating these impacts it is reasonable to assume that with the closure of Highway 101 Foster Park probably experienced peak flows over 40,000 cfs.  According to the USBR chart below this corresponds to perhaps a 20 year event in the lower part of the watershed.

However, based upon Casitas Municipal Water District reports of over 23,000 cfs at the Robles Diversion it is likely that the upper watershed experienced a 50-year event.  This would be consistent with the damage experienced in Matilija Canyon and along Highway 33, which is likely to remain closed for a year.  (The last time the cutoff wall at Robles Diversion failed was 1969.) 

Note that Watershed Protection estimates that damage to the homes near Foster Park would not occur until the 60,000 cfs flow level.  As illustrated in the photos above this year's flooding did impact those residents suggesting that possibly vegetation and debris backed up flows under the Casitas Vista bridge below Foster park.  This neighborhood was recently removed from the FEMA 100-yr floodplain.

Also note that, as predicted, flows over 25,000 cfs resulted in damage to the Ojai Sanitation District sewer line along the bike path.  (Another sewer line also failed on San Antonio Creek along with significant property damage in that tributary.)  

The flood of January 6, 2023 was a historic event with significant impacts to infrastructure and property in the Ventura River watershed.  (The chart below was compiled before the 2005 flood)

On June 1, WPD hydrologist Scott Holder provided an overview of the flood for the Ventura River Watershed Council.  His flow estimates and other data are presented in the slides below:

And the Ventura River Water District published their June 1 water supply update showing lake and aquifer levels: