Thursday, June 5, 2025

Matilija Dam: removal plans and sediment transport

 

Matilija Dam, aerial view of impounded sediment, May 2024

Planning for the removal of Matilija Dam has been ongoing since 1998 (see chronology report).  As with all dam removal projects, sediment management is the primary concern.  Questions include how much sediment has accumulated, how best to remove the sediments to restore the natural stream gradient, and what are the downstream impacts when sediment is released from the former reservoir?  

This last question is particularly relevant to the Ventura River where significant floodplain and water supply infrastructure has been developed since Matilija Dam was constructed in 1947.  Concerns about potential impacts to aging and deficient levees downstream as well as the Robles diversion have led to a multicomponent project called the Matilija Dam Ecosystem Restoration Project (MDERP.)  Although complicated by the multiple agencies, interests, and components, this complex endeavor will ultimately enhance watershed resilience through upgraded infrastructure and restored natural processes.

Map of MDERP project components,
Ventura County 2024 

Field studies have confirmed the general profile of sediments trapped upstream of the dam.  Over the decades, the remnant reservoir accumulated fine silt and clay while the larger "coarse" sediment accumulated in the delta and upstream channel.  


Note that over the past 3 decades since studies began, it is estimated that approximately 50% more sediment has accumulated behind the dam, primarily due to the 2018 Thomas fire and the floods of 2022-2023.  Currently, sand and gravel has overlain the fine reservoir deposit and filled to the level of the outlet on the back side of the dam so that sand and gravel are now being transported through and over the dam.  

Current planning for dam removal includes engineering two 12ft diameter orifices that will be opened during a flood to allow high flows to head-cut a channel upstream through the sediment (AECOM, March 2022).  This initial flush will mobilize a large percentage of the fine sediments stored in the former reservoir area resulting in temporarily high turbidity (i.e. "muddy" water) downstream.  


Dam removal concept for Matilija Dam

The plans call for complete removal of trees and other vegetation upstream of the dam and excavation of a "pilot channel" to initiate head cutting through the middle of the reservoir area to maximize evacuation of the fine sediments during this initial flush.  It is expected that this channel will become relatively stable and erosion will diminish once the flow cuts down to the approximate depth and alignment of the historic channel.  A storm prediction analysis (AECOM, November 2022) determined the preceding conditions necessary for a 3-yr or greater flood event, equivalent to about 3,300 cfs deemed necessary to adequately establish the new upstream channel and flush the fine sediment down the Ventura River and out to the ocean. 




channel through sediments upstream of Matilija Dam resulting from head-cut erosion during flood
alignment based on historic streambed (Stillwater 2016)




After this initial (phase I) flush, future floods will incrementally move more of the coarse sediment (gravel, cobble, boulders) down the Ventura river.  Based on other similar dam removal projects, it is assumed that a maximum of 80% of the trapped sediment will eventually be transported downstream.  In other words high terraces will exist adjacent to the restored stream channel through Matilija canyon that will gradually erode over time feeding sand gravel and cobble into the Ventura River in addition to the restored natural "background" sediment flowing down Matilija Creek.


Estimates of sediment available for transport after dam removal


A computer model was developed to simulate the downstream response of the Ventura River to the release of sediment resulting from dam removal (Stillwater Sciences, February 2020, March 2025).  The table below shows the sediment volumes applied to this computer simulation.  Note that the impounded sediment represents approximately 7 times the "background" supply from Matilija Creek, which makes sense since the dam has been trapping that supply for decades.  Also note that the supply from North Fork Matilija Creek (which runs up Hwy 33) is just one third of the background volume from Matilija Creek.

Sediment supply applied to computer models of the Ventura River


The change in sediment load in the Ventura River may be conceptualized by the graph below.  The current trend with dam in place is for increased sediment to pass through/over Matilija Dam until the background load from Matilija Creek is added to the current sediment transport in the river.  The removal of Matilija Dam will generate a large pulse of sediment that will be transported down the river with each flood until equilibrium is reached.  Note that during this time there may be 6 or 7 times the current sediment load, and in the long term equilibrium condition three times as much sediment will be transported down the Ventura River.  While this will provide tremendous benefits to the riverine ecosystem, migrating endangered steelhead, and ultimately the rivermouth delta and beaches of Ventura County, some of the downstream infrastructure will need to be modified to accommodate these changes.  

In order to model sediment transport, several "synthetic hydrographs" were developed based on historic flows in the Ventura River.  Because the "flashy" watershed is characterized by long periods of dry conditions with occasional large winter floods, hydrographs were developed to compress time and represent those periods when sediment is moving down the river.



 The "Run 2b" hydrograph represents the most rapid transport of sediment by repeating the extreme floods of 1969 back-to-back four times.  The "Run 2e" hydrograph was used to help predict what may happen under a more realistic scenario.   The graph below illustrates the rapid flushing of sediment from Matilija Dam and the relative magnitude of this volume compared with the background inputs from Matilija Creek and other tributaries.


The map below gives an overview of the extent of the computer model.  Topography was based upon Lidar imaging to provide a basis for the channel configuration.  Note that modeling is limited to a "fixed bed" so the natural scour and avulsion (channel changes) are not taken into account. 




The graph below shows the maximum expected downstream changes to the riverbed and 100-year flood elevation with the removal of Matilija Dam.  As one might expect, the biggest changes are in the first two miles directly downstream of the dam where the river channel is narrow and constrained.  Below the Robles diversion the floodplain widens and becomes braided with a natural capacity to absorb increased sediment loads.  Note that the model assumed that modifications are made to both the Robles Diversion and Santa Ana Bridge to accommodate the increases in sediment.  (Santa Ana Bridge widening project was completed in 2022 and planning is underway for a high flow bypass at Robles.)  




Over the course of the project, several computer modeling approaches were used beginning with a one dimensional model, the DREAM2 dam removal model, and several iterations of the 2-D model which was developed to help provide greater resolution for areas of concern.  As illustrated below, all of these models show similar trends.    



These results may be translated into more practical terms by projecting the predicted 100-yr water surface elevation onto the floodplain topography to visualize potential changes in flooding.  Below is one example of the worst case scenario in the reach directly downstream of the dam.  




The next step will be translating all of this data into a risk and liability approach that considers all downstream interests.  Developing a realistic assessment of these risks will be important step toward dam removal.

 
Commentary: 

The Matilija Dam Removal Task 2.9 2D Modeling Report summarizes more than two decades of analysis directed at determining the transport and fate of the more than 8 million cubic yards of sediment currently sequestered in the former reservoir of Matilija Dam. This unique dam removal project in a steep flashy coastal drainage is complicated by the developed floodplain downstream of the dam. The need to assess the risk of the project to infrastructure and communities downstream has required an ongoing evolution in the state of the art hydrologic and geomorphic computer simulations.

This body of work concludes that that the primary impact of dam removal will occur in the first few miles downstream of the dam within a timeframe bounded by the first two 100 year flood events (perhaps many decades or more than a century into the future). In the upper reach of the study area below the dam the floodplain is geologically constrained and the pulse of sediment released from behind the dam will temporarily increase streambed elevations and water surface elevations. This effect diminishes downstream of the Robles/Meiners Oaks area as the broad undeveloped floodplain will naturally absorb and meter out this sediment pulse through avulsion and channel migration processes beyond the capacity of this modeling effort.

The Phase II SRH-2D model improves on the prior 1D analyses. However, the Phase III SRH-2D model that was developed to add sand transport to the simulations probably overpredicts streambed aggradation. The models developed here for Matilija Dam, particularly the Phase III analysis, represent an advancement in the state of the art, therefore there is not yet a similar case where this has been applied for comparison. However, to the extent that other large dams have been removed, there is some hindsight into how such predictions have played out. One important conclusion from the referenced review of dam removal (Major et al 2017) is:

Rivers are resilient. After dam removals, rivers have largely achieved apparent stability – at least within their range of natural variability – within a few years rather than decades. Despite concerns about rapidly releasing possibly large volumes of sediment to river systems, most removals to date have shown adverse geomorphic impacts (such as complete bed-texture transformation, enhanced flood risk, or damage to fish habitat) are short-lived and have been neither as damaging nor as long-lasting as feared.


References:

Matilija Dam Ecosystem Restoration Project, Chronology Report, Ventura County Public Works Agency and Rincon Consultants, April 2023

Matilija Dam Removal, Sediment Transport, and Robles Diversion Mitigation Project, Dam Removal Concepts Evaluation Report, AECOM and Stillwater Sciences, January 2016

DRAFT 65% Design Report for Matilija Dam Removal, AECOM, March 2022

Predictability Assessment of Flushing Storm Event, Matilija Dam Removal 65 Percent Design Planning Study, AECOM, November 2022



Matilija Dam Removal 65% Design Subtask 2.9: Hydraulic and Sediment Transport Modeling in SRH-2D, Stillwater Sciences, March 2025

Major, J. J., A. E. East, J. E. O’Connor, G. E. Grant, A. C. Wilcox, C. S. Magirl, M. J. Collins, and D. D. Tullos, 2017. Gravel-Bed Rivers: Processes and Disasters, First Edition, Chapter 13 Geomorphic Responses to Dam Removal in the United States – a Two-Decade Perspective.

More information and references are available at https://matilijadam.venturacounty.gov/


Wednesday, April 30, 2025

Surfrider Love Your Beach tour

On May 22, Earth Day 2025, the Surfrider Foundation stopped in Ventura with the "Love Your Beach" California tour  to raise awareness and defend the California Coastal Act.  

Paul Jenkin led a walking tour of the Surfers Point Managed Shoreline Retreat project currently under construction, followed by an informal gathering at Topa Topa Brewery in downtown Ventura.  Postcards were circulated to collect signatures and will be delivered to elected officials at this year’s Coastal Day in Sacramento.


Paul Jenkin explains how "managed retreat" makes room for natural beach processes


Paul Jenkin explains how windblown sand migrates across the dune-less area created for kiteboarding
 in Phase 1 of the Surfers Point Managed Shoreline Retreat project


In the news:

"Love Your Beach" tour stops at Surfers Point in Ventura, Surfrider calls on public to support Coastal Act, Coastal Commission, VC Star, April 24, 2025

The Surfrider Foundation spent Earth Day at a Ventura beach, one of the advocacy group's first stops on a tour to rally support for California's Coastal Act.  The decades-old law helps protect coastal resources and public access, said Jennifer Savage, Surfrider's California policy associate director. Over the years, those rights have become woven into the state's identity, she said. Now, some may not think they need to be defended.

During the local stop, Paul Jenkin, Surfrider's Ventura campaign coordinator, lead nearly three dozen on a tour of the shoreline restoration at Surfers Point.

Called managed retreat, the project is expected to make room for the beach by relocating a damaged path and eroding parking lot at the Ventura County Fairgrounds. Jenkin called it rewilding the coast.

He and others started advocating for the work back in the 1990s, pushing for an alternative to building a sea wall – a temporary fix that experts say can exacerbate issues. A first phase of the project wrapped up more than a decade ago, and the second phase started late last year.

"We’re allowing the beach to naturally come and go," Jenkin said. Because of that, the beach will be there long after others disappear, he said. 




Tuesday, March 4, 2025

Don't doubt the trout


On Saturday, March 1, the Ojai Valley Land Conservancy hosted an evening of music, film, and conversation at the historic Ventura River Steelhead Preserve.  The night began with a live performance by Par Avion, led by CalTrout Ambassadors Rebecca Ramirez and Bernard Yin, and a screening of Don't Doubt the Trout (directed by Ojai’s Isabela Zawistowska), which follows Rebecca and Bernard’s journey to understand Southern California’s endangered steelhead and how the experience deepens their connection to nature. The evening concluded with an outstanding panel of local conservation experts discussing fish, rivers, watersheds, and dam removal. 






PANELISTS 

Mark Capelli is with the National Marine Fisheries Service, where he is responsible for coordinating the development of recovery plans for the threatened and endangered steelhead trout of central and southern California (from Monterey to the U.S. Mexican border). He is the lead author of the "Southern California Steelhead Recovery Plan" and the most recent "2023 5-Year Review of Southern California Steelhead."

Rosi Dagit is Principal Conservation Biologist for the Resource Conservation District of the Santa Monica Mountains. She has initiated and coordinated numerous research, restoration and monitoring projects throughout the Santa Monica Mountains, starting with studies of Malibu Lagoon, Leo Carrillo State Beach, and moving inland to watershed level analysis and sensitive species monitoring. She recently led efforts to rescue tidewater gobies and steelhead trout in the Palisades Fire burn scar. Currently, Rosi leads the Topanga Creek Stream Team.

Paul Jenkin is the Ventura Campaign Coordinator for Surfrider Foundation, and founder of the Matilija Coalition. He serves on the Surfers Point Working Group, a multi-agency task force working to solve beach erosion problems near the mouth of the Ventura River. Paul has also served on the various technical work groups within the Matilija Dam Ecosystem Restoration Feasibility Study, and acts as the coordinator of the non-government efforts to remove Matilija Dam and restore the Ventura River watershed.

Russell Marlow is a CalTrout Senior Project Manager for the South Coast Region, leading the Ventura office. Russell previously worked for USFWS and the NPS as a biologist focused on native and endangered aquatic species before completing his graduate studies at South Dakota School of Mines studying aquatic macroinvertebrate assemblages. Russell’s project focus in the Ventura Office is on reconnecting critical habitat for the endangered southern steelhead through passage barrier removal or remediation.

Bernard Yin and Rebecca Ramirez serve as ambassadors for CalTrout, Patagonia Fly Fishing, conservation co-chairs for the Southern Sierra Fly Fishers Club, and perform music with their band, Par Avion.


More:

https://wildandscenicfilmfestival.org/film/dont-doubt-the-trout/







Thursday, December 19, 2024

Surfers' Point Phase 2 construction begins


Surfers' Point Phase 2 construction 12-16-24


At the June 1, 2023 meeting of the California Coastal Conservancy the board approved $16,200,000 to the City of San Buenaventura (Ventura) to construct Phase 2 of the Surfers Point Managed Retreat Project, a sea level rise adaptation project that relocates existing infrastructure landward and restores beach dune habitat, in Ventura. 

Final design was completed in 2024, with construction originally scheduled between Labor Day 2024 and Memorial Day 2025 as required by the Coastal Commission to avoid the summer months. 

On October 11, 2024 the City of Ventura hosted a groundbreaking ceremony for phase two of the Surfers' Point managed shoreline retreat project.  



After many delays, work began at Surfers Point on December 2, 2024.  Site clearing was well underway leading up to the Christmas holidays.

According to an article published in the Ventura County Star;

Work was scheduled to start in September, but there were delays due to the California Coastal Conservancy’s grant administration process and work approvals as well as the area being used for Mountain Fire operations, said Jennifer Buckley, spokesperson for the city of Ventura, in an email on Monday.

In June 2023, the conservancy approved $16.2 million for the second phase of work on Surfers Point, a project that began more than a decade ago. Its first phase was completed in 2013 with an approximate cost of $4.5 million and included $1.6 million from the coastal conservancy, Buckley said.

After receiving bids and factoring in contingencies such as construction changes, the estimated cost for the second phase is approximately $18 million, Buckley said.

...

Despite the challenges, the work of managed retreat is important, Jenkin said.

"The initial reaction to coastal erosion is to dump rocks and build a seawall," he said. "At Surfrider, that's one of our biggest concerns, especially with rising seas that we'll start losing our recreational beaches."

Jenkin said he does not want to see Surfers Point, an attraction close to downtown that draws visitors from all over, turned into a "concrete jungle" like other Ventura County coasts.

...

The construction is affecting the Ventura County Fairgrounds and its ongoing events. Dan Long, board president of the fairgrounds, said while he is in support of the project, it will negatively affect the fairgrounds as work is done.

“We’re going to lose quite a bit of parking and some RV parking,” he said. “While we’re very happy to see this done because it helps all of us ― the public, the city infrastructure and our infrastructure ― it is costly to us and we’re having to make a lot of different arrangements to try and make things fit.”



More:

On this blog Surfers Point

Project website: Surfers Point - Phase II City of Ventura

Surfers' Point Phase 2 Groundbreaking ceremony, Ventura.surfrider.org

What does it take to save Surfers' Point?, Surfrider.org

10 Waves Saved: Surfers Point, California, Surfer Magazine, Dec 24, 2024


In the news:

Surfers Point coastal pathway in Ventura to be relocated for $13.9 million, Ojai Valley News, Sept 6 2024

Revamp of Ventura's Surfers Point gets underwayVentura County Star 2024/12/10

Friday, October 25, 2024

Keeper of the Coast

 Thirty years ago I embarked on my journey with the Surfrider Foundation. I am so honored to be recognized as a "Keeper of the Coast," joining the ranks of those who have truly made a difference in protecting and restoring our ocean, waves, and beaches. Sincere thanks to Chad Nelson and Surfrider for supporting my life's work.





Wednesday, October 2, 2024

Historic Ecology: Oak Trees

One of the biggest changes to the Ventura River ecosystem came with the influx of settlers in the 1800s.  Some say the oak woodland that filled the valley was so dense that a squirrel could travel in the canopy from Ojai to the beach in Ventura without touching the ground.  But as people moved in, land was cleared to make room for agriculture with the wood exported to the growing city of Los Angeles, much of it to be burned as firewood. 

As the oak woodland was replaced with irrigated agriculture the water balance shifted from abundance to deficit.  Rather than capturing and infiltrating rainfall, the land now required that water be pumped from the aquifers to sustain crops and orchards.  By 1890, over 4,000 acres had already been deforested.  Today there is approximately 6,000 acres of irrigated land in the Ojai Valley.  

Recognizing the importance of maintaining and restoring the remaining oak woodlands, the County of Ventura and other jurisdictions throughout California have ordinances protecting oak trees.  Many organizations work to educate residents and coordinate volunteer efforts.  In the Ojai Valley this includes;

Once upon a Watershed celebrates OAKTOBER, Oak Awareness Month. 

Join OUW and other organizations and individuals across the state and country as we recognize the importance of oaks and oak ecosystems.  Every individual, organization, and community can play an important role in celebrating oaks and oak ecosystems throughout the month of October—OAKtober!

OAKtober_376U.jpg

 

Ojai Trees is an Ojai Valley community forestry group that welcomes people of all ages and backgrounds who want to do something tangible to help the environment. 


The following are excerpts from publications documenting the history of oaks in California:

Meiners Oaks

http://ojaihistory.com/he-got-meiners-o-for-unpaid-debt/

John Meiners, native of Germany, had come to the United States about 1848 and had established a successful brewery business in Milwaukee. He acquired this Ojai ranch in the seventies, sight unseen, as a result of an unpaid debt. When he heard that his friend, Edward D. Holton, a Milwaukee banker, was going to California for a brief trip, Meiners asked him to see the property he had acquired. Mr. Holton’s evaluation was perhaps it was the largest oak grove on level land in Southern California, much of it so dense that the ground was in continuous shade. Furthermore, to his surprise, Meiners discovered that the climate of the valley was good for his asthma.

The barn and livestock area on the Meiners Ranch. A fence surrounds the main oak grove seen in the distance.
The barn and livestock area on the Meiners Ranch.
A fence surrounds the main oak grove seen in the distance.
Ojai Valley Museum

Oaks of Southern California

https://www.kcet.org/shows/lost-la/the-oak-trees-of-southern-california-a-brief-history

When Europeans arrived, they noticed the beauty of the oaks and used them as a way to make sense of their novel surroundings. Upon summiting the Sepulveda Pass and looking out over the San Fernando Valley in 1770, a Spanish expedition called the expansive plain Valle de Santa Catalina de Bononia de los Encinos. ("Encino" is Spanish for live oak.) In central California, a later expedition named a oak-shrouded pass El Paso de Robles. ("Robles" referred to the area's valley oaks.) Later, as highly visible landmarks, some trees served as boundary markers between ranchos, appearing on diseños that recorded Spanish- and Mexican-era land grants.

But almost as soon as the Spanish enshrined the oaks in the region's place names, the more intensive uses of they land they introduced began to threaten the trees' survival. Farming, annual husbandry, and the arrival of non-native annual grasses stymied oak reproduction. Mature oaks were cut for lumber or fuel.

American land use practices only intensified the destructive processes. Like their Spanish predecessors, Americans would name their communities and streets after the trees (Thousand Oaks, Fair Oaks Boulevard, etc.) and then proceed to hasten their downfall. Because of the irregular shape of their trunks, oak trees were rarely felled for lumber, but oakwood came to be prized as fuel. The dense wood and lack of resin meant that the wood and resulting coals burned long and slowly.

Just as they had sustained Southern California's indigenous peoples, oak trees nourished residents of the booming city of Los Angeles, albeit in an indirect and unsustainable way. Demand in Los Angeles for hardwood drew loggers into the San Fernando, Santa Clarita, and San Gabriel valleys. As the loggers clear-cut thousands of acres of oak woodlands and savannas and delivered the firewood to Los Angeles, bakers tossed the wood into their ovens, feeding a city while denuding the countryside.

Oaks also fell to the axe as Southern Californians envisioned more profitable uses for oak-dominated landscapes. In the nineteenth century, citrus growers cleared oaks savannas to make way for orchards. Other oak habitats declined as groundwater pumping lowered the water table. Later, real-estate developers uprooted trees to build new houses and commercial properties for the expanding metropolis.



A Brief History and Guide to California's Native Oaks

http://www.ourcityforest.org/blog/2020/7/a-brief-history-and-guide-to-californias-native-oaks

The native oaks of California once dominated the landscape. Accounts of Spanish explorers mention their awe at the sight of the abundance of oaks around them. However, those with the strongest connection to the oaks of California were and are indigenous people.

Unfortunately, the arrival of the Spanish did not bode well for the native people nor the oaks of California. The Spanish introduced grazing animals and felled oak forests to make room for their agricultural enterprises. They also saw value in the lumber of oak trees, leading to even more deforestation. Before the native people could do anything to prevent them, the Spanish had dramatically damaged the relationship between the people and the oaks. 

The people native to Santa Clara Valley are known as the Ohlone, which is a name that encompasses 50 separate tribes ranging from the South Bay all the way down to Monterey. Native oaks of California are ingrained in their society as a resource both physically and spiritually. Acorns were the primary food source for the Ohlone prior to the arrival of the Spanish and were held in high regard amongst the native people. Anthropologists estimate 75% of native Californians relied on acorns in their daily diet. Their new year, a joyous occasion, was marked by the acorn harvest. The Ohlone people would dance amongst the oak groves each year to promote a good harvest. During the acorn harvest, entire families would go out and collect the acorns of a large tree, which took about a day. The women would then prepare the acorns by shelling them and using a mortar and pestle, grinding the acorns into a fine powder. After being ground, the acorn flour would undergo the lengthy process of leaching the bitter tannins from the acorns which made them unpalatable. After the tannins were leached, the acorn flour was much sweeter and easier to eat and could be used to make soups, mush, and even bread (I myself love acorn bread). Excess unground, shelled acorns could be stored up to 10 years. The preparation of acorns was not just fulfilling a necessity. It was also a time for social connection during which the women could talk amongst themselves and share stories of their lives and even gossip. 


The Powerful Survival Story of California’s Oaks

https://marinmagazine.com/feature-story/oak-stories/

The story that oaks tell about the impact of humans in California is mostly a sad one. Natural landscapes dominated by oak trees once covered more than a third of the state. Starting around 1850, clearing trees for agriculture and grazing decimated vast oak lands, and a century later the subdivision boom inflamed a trend that has never really ceased. Biologists now estimate that more than a third of California’s original 10 to 12 million acres of oak woodlands have been lost since settlement, and only about 4 percent of the remaining woodlands are protected. When oaks are lost, so are many of the wild creatures and other plant life that are part of the oak’s rich natural web — among the most biodiverse of the state’s ecosystems.


ACORNS: TRADITIONAL FOOD STAPLE

http://www.danielnpaul.com/CaliforniaNativeAmericans-Acorns.html

As late as 1844, when explorer John C. Fremont led an expedition to the Sacramento Valley, he described the north state foothills as "smooth and grassy; [the woodlands] had no undergrowth; and in the open valleys of riverlets, or around spring heads, the low groves of oak give the appearance of orchards in an old cultivated country." Similarly, a nineteenth-century visitor to the middle fork of the Tuolumne River near Yosemite Valley found it "like an English park-a lovely valley, wide and grassy, broken with clumps of oak and cedar."

Fires were used to insure good growth and healthy orchards:

Natives may have been setting fires for 5,000 years speculates Kat Anderson (an ethnobotanist with the Amerindian Studies Centre at UCLA), judging by how long fire-loving giant sequoias have been expanding their range.


The History of Oak Woodlands in California, Part II: The Native American and Historic Period

https://scholarworks.calstate.edu/downloads/sn00b2449

The open oak woodlands described in the accounts of Spanish explorers were in large part created by land use practices of the California Indians, particularly burning. Extensive ethnographic evidence documents widespread use of fire by indigenous people to manipulate plants utilized for food, basketry, tools, clothing, and other uses. Fire helped maintain oak woodlands and reduce expansion of conifers where these forest types overlapped.


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


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