Robles Diversion has been an essential component of the Matilija Dam Ecosystem Restoration Project (MDERP) since planning began in 2000.
Background:
The Robles Diversion and Fish Passage Facility is located on the Ventura River 2.3 miles downstream of the Matilija Dam. The Facility includes a timber diversion dam and control gates to divert flows from the upper Ventura River for storage in Lake Casitas. The lake serves as a primary and backup water supply for agricultural and municipal water users served by the Casitas Municipal Water District (CMWD).
The diversion was originally constructed by the U.S. Bureau of Reclamation in 1956 as part of the Ventura River Project which is built around Casitas Dam and Reservoir, located on Coyote Creek about two miles above its junction with the Ventura River. Casitas infrastructure also includes the 5.4-mile Robles-Casitas Canal, which conveys the diverted flow of the Ventura River into Lake Casitas; and the main conveyance system, which includes 34 miles of pipeline, five pumping stations, and six balancing reservoirs located throughout the project area. When originally constructed Lake Casitas had a capacity of 254,000 acre-feet, but recent estimates have reduced this to approximately 237,000 acre-feet.
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| Robles Diversion and canal Jan 2003, Rich Reid |
According to CMWD, Lake Casitas fills primarily from the natural flow of local creeks and the Robles Diversion. In any given year roughly 1/3 of the Lake’s water comes from the Robles Diversion.
Diversions are limited by the size of the canal, which can only convey 500 cubic feet per second (cfs) and regulations in a NOAA biological opinion that require adequate releases of water to ensure fish passage and other downstream water supplies.
The Coyote Creek watershed area that drains directly into the lake is only 3,500 acres. (In comparison, the similar sized Lake Cachuma (193,304 acre-feet) in the neighboring Santa Ynez watershed is supplied by more than 70 times the watershed area of 417 square miles (266,880 acres).) It was this small watershed area that necessitated the addition of diversion from the Ventura River to help maintain the water supply in Lake Casitas.
In 2005, a fish passage facility was constructed to allow migration upstream and downstream as well as to screen smolt and other small fish from being diverted into the lake. The components of this facility are shown below:
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| Robles Diversion components source: Casitas Municipal Water District |
Sedimentation Problems:
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| 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 |
As the Robles Diversion dam captures water for diversion in the large forebay, it also traps sediment. This sedimentation requires ongoing maintenance, including the costly mechanical removal and transport of sediment following a series of small storms or, in some cases, one big winter.
In January 2023 the upper Ventura River experienced a 50-year flood with flows over 20,000 cfs in the Robles reach. This occurred following the Thomas Fire of December 2017 which burned the majority of the upper watershed. A huge volume of sediment, including cobble and boulders, backed up behind the diversion dam filling the forebay. Flows became diverted to the east, eroding the earthen berm up until, fortunately, the timber wall failed. A similar event occurred in 1969, when 100-year flood flows were diverted both east and west (a phenomenon known as "avulsion") damaging both the diversion canal and the adjacent "riverbottom" neighborhood of Meiners Oaks. Diversion capability was not restored for the remainder of 1969.
As described in Matilija Dam: removal plans and sediment transport, this existing sedimentation problem will become worse as sand, gravel, and cobble begin to pass over the dam. Even if Matilija Dam remained in place, future equilibrium sediment loads will triple from what Robles has experienced over the past 70 years. And dam removal will create a short term sediment "wave" that could also overwhelm the facility.
Solutions to increased sedimentation:
The 2004 Army Corps of Engineers feasibility study for dam removal included a "High Flow Sediment Bypass" at Robles Diversion. This bypass consisted of additional gates that could be opened to allow sediment transported during peak flows to be flushed through this reach of the river, preventing the kind of backup that occurred in 2023.
This work has determined that
• Existing bypass is too narrow, less than 30% of natural river channel
• Matching natural river width at Robles would require additional bypass between 130 ft to 160 ft wide.
• It’s feasible to increase flow and sediment transport capacity at Robles
• Possible to adjust flow split between existing bypass and additional bypass as desired
by modifying invert elevation and forebay layout
• Various options to locate additional bypass and change approach flow conditions by
modifying forebay (narrowing, straightening, splitting)
Several alternatives were considered and narrowed down based on additional computer analysis. Currently the Bureau of Reclamation has built a physical scale model in their facility in Denver to test various configurations and determine the best approach for optimum sediment management.
The two primary alternatives #3 and #5 are illustrated below:
Fisheries considerations:
Although the initial intent of the Robles Diversion modification effort was to address the sedimentation concerns experienced during high flows, the resource agencies have a strong interest in improving fish passage around the diversion facility. Currently the fish ladder is the only opportunity for passage, even during times when the diversion is not operating. All river flows are currently routed through the diversion facility down the fish ladder with no bypass option, even outside the diversion season. And although the fish ladder has proven to be effective for trout, there is interest in ensuring other native species can pass. Additionally, the current operations are governed by NOAA's biological opinion, which defines how much water is allocated to the fish passage operations. All of this adds considerable complexity to an already difficult engineering task. Considerable resources are being directed to the exploration of additional fish ramps or other passage facilities to resolve these concerns.
Flooding concerns:
The original Corps of Engineers plan for this reach included construction of a new levee extending downstream from the Robles Diversion. This was based on coarse 1-dimensional flood modeling that predicted an incremental increase in streambed elevation due to dam removal.
Apart from the huge construction footprint, local residents did not welcome the prospect of a large wall complete with chain link fences, rodent control, and service road in their back yards. It turns out much of the flood concern was based on unrealistic backflow from the Ventura River.
Subsequent 2D modeling provided a more realistic understanding of the flood concerns at Meiners Oaks. As described above, sediment trapped by the Robles diversion dam creates a flood hazard to the adjacent community when the river is diverted by the blockage (avulsion.) The consulting team has conducted a preliminary study of how these high flow bypass designs will affect the downstream flood risk. Although the existing flood risk from the Ventura River may be characterized as relatively minor overtopping of the naturalized embankment, initial assessment shows that enhancing the flow path through this reach will help reduce this potential flooding. (Note that a considerable area of this neighborhood is at risk from overflow from Cozy Del creek, shown in yellow.) These preliminary results are promising, although the primary flood risk is potential stream bank erosion on the left (east) bank of the existing high flow channel downstream of the Robles Diversion. This downstream reach is directly affected by how flows are directed by the upstream modifications.
Summary:
The Ventura River Project constructed in 1959 provided Casitas Municipal Water District more than the 50 year lifespan of benefits from Matilija Dam. Over that period, the district was able to release water stored in Matilija reservoir for diversion at Robles into Lake Casitas. Sedimentation has rendered Matilija Dam completely useless for water supply, and structural degradation requires its timely removal. Most importantly, dam removal necessitates modernization of downstream infrastructure.
Modification of the Robles Diversion facility has been identified as the critical path component of the Matilija Dam project. Although complex and expensive, careful consideration of a solution that eliminates costly maintenance and periodic cleanout of sediment from the diversion facility as well as the need for costly maintenance of a permanent levee downstream will provide long term benefits to the community. Completing this component will clear the path to dam removal which is necessary to restore watershed resilience including access to headwater habitat that the endangered steelhead require.
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