Tuesday, July 16, 2024

Riverbed changes after 2023 flood

The aerial photos shown in January 2023 Flood Overview illustrate how the flood of January 9, 2023 ripped out large areas of vegetation and "re-set" the floodplain.  In response to this 50 year flood, in many places the primary flow of the river shifted into historic channels within the wide floodplain.

This was the first major flood since the Thomas Fire of December, 2017.  As is typical for this watershed, erosion of thousands of acres of steep mountains following the fire introduced large amounts of sediment into the headwaters of the Ventura River.  This is documented in Post fire storm event, Jan 9 2018, Retrospective on the Thomas FireVentura River post-fire sedimentation 2019, and Matilija canyon sedimentation.

Planning for the removal of Matilija dam has been underway since 1999, with the primary concern being predicting the fate of both the 9+ million cubic yards of sediment trapped upstream of the dam as well as the renewed sediment supply from Matilija Canyon once the dam is gone.  Planners and engineers have been relying on a variety of computer models to estimate changes in the riverbed with increased sediment supplies. Until now there has not been any real world data to groundtruth these predictions.    

A July 2024 report Post-Flooding Numerical Model Update and Review - Technical Memorandum generated for the Upper Ventura River Groundwater Sustainability Agency (UVRGSA) provides a hindsight analysis of channel changes following the 2023 flood based on LIDAR and satellite imagery.  The concern of the GSA was whether the changes in the riverbed would affect their groundwater computer model. Although the focus of this work was on calibration of the UVRGSA groundwater-surface water model, the document includes an interesting review of the changes in channel alignment and bed elevation summarized below.

Of interest is the conclusion that, 

"The changes in the channel characteristics generally depended on the location within the basin. The upstream areas were found to more likely increase in streambed elevations in response to sediment deposition and the downstream areas were more likely to observe decreases in streambed elevation in response to scour."


Of note: 

Upstream of Robles;

In the Kennedy hydrogeologic area, there was minimal change in the lateral position of the updated channel network’s position in comparison to the original channel network. However, the streambed elevation increased by approximately four feet on average (Figure 2.2a), indicating significant sediment deposition in this area. The accretion within the channel network increases moving downstream, with a maximum elevation difference of over eight feet occurring at the most-downstream reach. 




Ventura River preserve:

In the Robles North hydrogeologic area, the lateral position of the channel was minimally changed in comparison to the original channel in the northern half of this area. A new braid was formed in the southern half of this area (Figure 2.2b). Similar to the Kennedy zone, the overall channel within the Robles N zone increased in elevation by up to approximately three to five feet in some areas.

 

Within the Robles South hydrogeologic area, the lateral position of the updated channel network is generally in the same location as the original channel network (Figure 2.2c). However, the updated channel network is less tortuous than the original channel network. There is a small vertical change in the streambed elevation. On average, the updated channel network is approximately one foot lower than the original channel network. This zone represents a transition between sediment accretion in the upper portion of the basin and erosion in the lower portion of the basin. 

 


Live Oak Levee;

In the Santa Ana North hydrogeologic area, the updated channel shifted both westward and eastward in different reaches of the stream in addition to the formation of new braids (Figure 2.2d). The stream location begins westward of the original channel in the upstream areas and then crosses over eastward of the original channel until roughly rejoining the original channel. The updated channel network is consistently around two feet lower than the original channel network in this area, indicating the overall erosion of the streambed. 



Also of note, the “initial flush” from the Thomas Fire consisted of very fine sediment (ash and silt) that deposited in the riverbed following erosional flows up to 5,000 cfs (see Ventura River post-fire sedimentation 2019.)  The following winter the fine sediment was flushed and replaced by sand and gravel moving in flows up to 1500 cfs. (March rains.)   Then the big 2023 flood produced flows over 20,000 cfs which flushed the sand up onto the floodplain terraces and out to the beach, with the new channels almost entirely lined with cobble and boulders (including what some call “VW sized boulders”.)    

The 2024 storms had peak flows over 6,000 cfs which was enough to move this slug of sediment further downstream, and the active channel shifted dramatically again in response.  The image below is the 2023 main channel in Robles South that was abandoned in 2024 when sediment plugged its junction upstream.  

Former primary channel, Ventura River above Baldwin Rd, March 2024



Ventura River Flood flows, 2024 



Computer modeling for the Matilija Dam Ecosystem Restoration Project has predicted a sediment wave moving downstream following dam removal.  The results of this analysis demonstrate this effect, with a rise of 5-8 ft elevation in the riverbed seen moving into the upper reaches of the Ventura River following the 2023 flood.  It is likely that a followup analysis using LIDAR data after the 2024 floods would show this sediment moving further downstream, once again shifting the active channels and raising the riverbed in areas that were scoured in 2023.  

The Ventura River is dominated by the regional drought and flood cycles. This steep watershed has the natural capacity to transport huge volumes of sediment generated by the highly erodible sedimentary geology of the the transverse ranges of California.  Much of the Ventura River features a broad floodplain with many channels that shift in response to the ever changing flow of water and sediment.  For over 70 years Matilija Dam has trapped a large portion of the sediment yield from the upper watershed, and this deficit creates a net erosional environment downstream as seen in the downstream reaches of this study.  Although it's hard to quantify the volume, the pulse of sediment following the Thomas Fire is behaving much as predicted for the removal of Matilija Dam.  More on this to come...  
 


Reference:

Post-Flooding Numerical Model Update and Review - Technical Memorandum, Prepared for: Upper Ventura River Groundwater Agency, July 2024


On this blog:


Tuesday, May 7, 2024

Matilija canyon sedimentation

This winter’s rains deposited more sediment into the former reservoir created by Matilija dam. A large 36” drain pipe has been rehabilitated to keep the water level seven feet below the crest in compliance with dam safety requirements. 


Matilija reservoir  4-01-2024

Matilija reservoir 4-25-2024

The photos above show the effects of the limited drawdown.  By April 25, Matilija Creek had reestablished a channel through the reservoir sediment.  Although the water entering the reservoir was visibly clear, some erosion near the outlet was still affecting water quality downstream.  




drone image showing the outlet of Matilija Dam, 4-25-2024
(Smitty West on Facebook)

drone image showing reservoir sediment behind Matilija Dam, 5-28-2024
(Smitty West on Facebook)

The photo below shows the confluence of clear water flowing in from North Fork Matilija Creek on the left and Matilija Creek downstream of the dam entering from the right.  These impacts are less than when Matilija Reservoir Drained in 2020 because of the earlier season and higher base flows through the reservoir.   


Matilija-NF Confluence 4-25-2024



The upper reservoir has experienced significant aggradation in the past two wet years.  Large quantities of sediment initially mobilized by the Thomas Fire has arrived at the relatively flat plain upstream of the dam. The extensive riparian forest growing on reservoir sediments has trapped the coarse sediment, cobbles and boulders as well as logs and debris, raising elevations above the level of Matilija Canyon road.  County road crews built an earthen berm to try to keep Matilija Creek out of the road, but the low spot floods with rainfall.  Local residents have created a temporary detour for lower clearance vehicles while trucks just plow through the puddle.  Matilija Canyon road remains closed to non-residents limiting public access to the National Forest at the head of the canyon.


Matilija Road upstream of the dam 4-1-2024


Matilija Road upstream of the dam 4-1-2024

Matilija Road upstream of the dam 4-1-2024


Further upstream, Matilija Creek has meandered and undercut the road.   

Damage along Matilija Canyon Rd, 4-1-2024

Damage along Matilija Canyon Rd, 4-1-2024


These locations are shown on the aerial image below.  It appears that Matilija Dam is having an impact upstream along Matilija Creek as the riverbed slope adjusts to accumulating sediment. 

Overview of Matilija Creek above Matilija Dam
(Google Maps May 7, 2024)


Also of interest is the large alluvial fan deposit at the mouth of Rattlesnake Canyon.  This also buried the road, but tributary inputs like this will ultimately shape the alignment of Matilija Creek once the dam is removed.  Of course, leaving the dam in place will further exacerbate all of these upstream problems.

Overview of Matilija Creek/Rattlesnake Canyon confluence area
 of Matilija Reservoir (now forested) above Matilija Dam
(Google Maps May 7, 2024)

Monday, April 29, 2024

Lake Casitas filled

 For the first time since 1998 Lake Casitas has filled to 100% of capacity.  Capacity was recently re-evaluated and current estimates are 238,000 acre feet.  The artificial reservoir created by the dam on Coyote Creek originally held 256,000 acre feet, which is estimated to provide a "safe yield" of 20 years for communities in the Ojai Valley, parts of the city of Ventura, and coastal areas along the Rincon parkway.  In many areas the reservoir serves as backup to primarily groundwater supplies.  The back to back wet winters have also saturated the groundwater basins.  


Ojai Valley News, April 23, 2024


Diversions from the Ventura River were stopped on April 23 at the Robles Diversion, which supplies approximately one third of the inputs to the lake.  


Stream gage hydrographs for Matilija Creek and Foster Park,
April 16-23, 2024,  USGS

Flows at Foster Park, downstream of the diversion, jumped by 75 cfs increasing flows in the Ventura River by 50%.


Ventura River above Hwy 150, April 23, 2024


In the news: 

Diversions stop, lake 100% full, Ojai Valley News, Apr 23, 2024

'Our insurance policy:' Water spills from Lake Casitas for first time since 1998, Ventura County Star, April 27, 2024


 

Thursday, April 25, 2024

Surfrider's Climate Action Program and Surfers' Point

On Earth Day the Surfrider Foundation announced  the Climate Action Program that aims to track and highlight on the great work of local Surfrider chapters around the country who have been working to restore their local beaches  



The Ventura County Chapter has been a leader in coastal ecosystem management, and the Surfers' Point Managed Shoreline Retreat is a featured project in the new program.  Although it took 15 years for the first phase of the project to be constructed, in 2011 the Chapter "adopted" the dune restoration portion of the project.  Hundreds of volunteer hours were dedicated to planting and maintaining the native dune habitat that is crucial to stabilizing wind-blown sand to naturally maintain the dunes.  We are hopeful that with the increased attention of the Climate Action Program that other chapters can use Surfers' Point as a model for restoring resilient shorelines around the country.     


Surfers' Point Managed Shoreline retreat project Phase 2:


The diagrams below are from the City's upcoming public outreach and illustrate the plans to relocate the damaged bike path and parking lot back to Shoreline Drive.   



Erosion at Surfers' Point continues
 requiring relocation of the bike path into the parking lot
Feb 8, 2024

Get Involved:

With construction still on track for Fall 2024, the City of Ventura is seeking public input on the public art component of the project:  

A community workshop for the Surfers Point Managed Retreat Project will be held on Thursday, May 2, at 5:30 p.m. at Santa Rosa Hall, located at the Ventura County Fairgrounds. Spanish interpretation will be available. 

"We are thrilled to involve our community in this pivotal stage of the Surfers Point Managed Retreat Project," said Mayor Joe Schroeder. "After receiving over $16 million from the State of California’s Coastal Conservancy to fund this project, community input and collaboration are crucial in creating a space that reflects the needs and aspirations of our residents and visitors for this iconic location."

This workshop presents a valuable opportunity for community members to engage in interactive discussions, activities, and collaborative sessions that will shape the amenities and potential public art themes at the Surfers Point site, among other aspects.


More on this blog:  Surfers' Point





Monday, March 11, 2024

El Niño winter swells 2023-2024

 

The 2023-2024 winter season has had similar coastal impacts as the 2016 El Niño event.  Higher sea levels and a signature strong Pacific storm track has focused wave energy and impacted beaches and infrastructure up and down the California coast.  In Ventura County, the beaches were stripped of sand exposing those areas most vulnerable to future sea level rise.

On December 28, 2023, the leading edge of a building pacific swell sent a storm surge over protective walls in the Pierpont neighborhood.  


A video of people and cars being flushed up the street went viral and made national news.  Ocean water and debris flooded the lanes in the Pierpont neighborhood, and there was some damage to docks at a marina in the Ventura Harbor. 

    


In response to this event, the City of Ventura and County firefighting units built three miles of sand berms the length of the developed beaches in Ventura and Oxnard. These berms were removed during the week of March 4 with authorization from the California Coastal Commission and Army Corps of Engineers.

A pierpont resident was quoted in the news saying, "We did have a breach, as the surge came over into my backyard in December. Then the next day, they put the berm up, and I had no worries after that. It totally disrupted my view, but that's a small price for the protection having all of your stuff ruined."  This echos the sentiment of beachfront residents who have resisted efforts to build permanent protective dunes and prevailed in a lawsuit requiring the City of Ventura to remove windblown sand accumulating in front of their properties.

Ventura County Fire Dept removing sand berm
Oxnard Shores, 3-5-2024


Commentary:

This event was predictable using the advanced weather and wave models currently available.  Surfers regularly use these models to know when the best conditions will occur.  In this case the swell had been monitored and tracked as it developed off the coast of Japan and built all the way across the Pacific Ocean.  The leading edge of these swells typically have a very long period indicative of the huge amount of energy from the extended "fetch" of high winds transferred to the sea surface.  The irony was the "too little too late" emergency response from local government.  Better awareness of ocean conditions and long term planning is clearly needed as climate change fuels ever bigger storm systems and rising sea levels.  Local tide gages were registering around one foot above the predicted astronomical tide, primarily a result of thermal expansion from the warm water throughout the Pacific Ocean fueled by El Niño.  These events are already happening before significant sea level rise.  According to the California Coastal Commission we can expect "as much as a 66-inch increase in sea level along segments of California's coast by the year 2100."  Under that scenario the Pierpont community will be under water much of the year.

  

Stormsurf wave prediction for 12-28-2023

Reference:  

watch the Stormsurf videos to learn more about El Niño and ocean wave generation.  

Sea Level Rise - California Coastal Commission


In the News:

Rogue wave injures eight, damages coastal motel in Ventura, KCLU | By Lance Orozco, Published December 28, 2023

Temporary sand berms intended to prevent coastal flooding are being removed in Ventura County, KCLU | By Lance Orozco, Published March 5, 2024

See damage from heavy surf in Ventura's Pierpont neighborhood, VC Star, Dec 29, 2023


On this blog:

Surfers Point - first real test


Thursday, January 25, 2024

Watching the dams come out: Klamath River

Last year the removal of four dams on the Klamath River began.  This week the fourth dam, Copco No. 1, was breached setting the stage for the physical removal of all four dams this spring and summer.

Video of the drawdown demonstrates what is becoming one of the the standard dam removal methods, blasting a hole in the lower part of the dam to drain the reservoir and release the sediments trapped upstream.




In the news:



Monday, January 22, 2024

More Watershed Education

 Since 2021 the Merito Foundation has organized the Ventura River Action Network for 6th through 12th graders.  

https://www.meritofoundation.org/venturariveractionnetwork

 

The V-RAN program includes Professional Development (PD) outdoors in the field, PD webinars, Science Curricula, and stipends to science teachers of VUSD enrolled in the program. The teachers' students (~600-700 per school year) are participating in in-class science activities, virtual and in the field youth community science experiences at Ventura River Watershed, and project-based learning through the EECCOA Challenge (a green STEM competition) with cash and in-kind prizes for students, and funds to implement the most cost-effective proposal to reduce the carbon footprint of the school campus authored by the students. 

The program includes field trips to monitor the river and visit Matilija Dam.  Visit the Story Map to learn more:



https://storymaps.arcgis.com/stories/4d93e82e5977448996aa64ba1e3d18a2