The details for the events on this recent timeline are summarized on this blog. The short version is that the project got bogged down in 2007-2008 over the proposed management of the fine sediment that is trapped behind Matilija Dam. The federal feasibility study included a dredge-and-slurry system to physically move this fine sediment downstream of the Robles Diversion Dam which diverts water into Lake Casitas. Construct-ability and cost concerns led to the formation of the Fine Sediment Study Group which recommended further analysis of alternatives to the slurry system. These studies are now underway under the guidance of the TAC. The work is being performed by a combined consultant team from two firms, URS and Stillwater Sciences, both of whom have considerable experience with other major dam removal and river restoration projects in the western United States.
The meeting included an overview of the study process from URS, as well as more technical presentations from Stillwater Sciences.
Seth Genzler, project manager for URS, explained the current scope of work for the studies that are expected to be completed in early 2015.
- Task 1: Dam Removal Plans and Cost Estimates
- a) Task 1.1: Background Review & Concept Matrix
- b) Task 1.2: Initial Options Screening
- c) Task 1.3: Full Dam Removal Concepts Evaluation
- Task 2: Sediment Analysis
- a) Task 2.1: Hydrologic Assessment
- b) Task 2.2: Sediment Characterization
- c) Task 2.3: Initial Options Sediment Analysis
- d) Task 2.4: Full Dam Removal Sediment Analysis
- Task 3: Robles Diversion Mitigation
- a) Task 3.1: Background Material Review
- b) Task 3.2: Hydrologic Assessment for Water Supply
- c) Task 3.3: Robles Mitigation Concept Evaluation
Derek Booth (Stillwater) presented a summary of the recently completed Hydrology and Sediment Characterization Reports that provide an overview of the river hydrology and sediment transport with the goal of providing some insight to reservoir sediment management under various dam removal scenarios. He proposed a set of hydrographs to be used in the sediment transport modeling to represent varied water years. The analysis demonstrated that flows at Matilija Dam are approximately 1/3 of those downstream at Foster Park. Note the very steep hydrographs typical of our "flashy" river system. This illustrates both the very high peak flows (with high potential for sediment transport) as well as the very short window of opportunity for natural sediment transport.
One of the primary considerations in plan formulation for the removal of Matilija Dam is the operations of the Robles Diversion Dam, located about 2 miles downstream. An analysis of actual diversion rates over the past two decades revealed that Robles accounts for approximately 28% of the water stored in the reservoir. (Note that previous planning had been based upon the assumption of 45% contribution from Robles.)
Stillwater also conducted an analysis of the sediment currently impounded upstream of Matilija Dam.
Based on recent LIDAR surveys and pre dam topography Stillwater estimates there are now 6.8 million cubic yards of sediment contained in the "wedge" upstream of the dam. (Analysis conducted a decade ago estimated 6 million cubic yards, indicating that additional sediment may have accumulated since then.) As illustrated below, approximately 65% of this is a mix of fine silt, clay, and sand, while the 35% further upstream is coarser grained cobble, gravel, and sand.
The question above was posed based on an analysis that predicts 99% of natural sediment transport in the river to be fine sediment. The answer is that the dam has actually trapped very little of the fine sediment from Matilija Canyon, the majority of which flushes over the dam during flood events as illustrated in this photo.
The afternoon Technical Advisory Committee meeting reviewed the Initial Screening Process and a Concept Matrix developed by URS to summarize all the previous dam removal concepts that have been considered as well as develop some new ideas based on discussions with the technical team.
Previous analyses covered many different approaches to dam removal, and have been described and analyzed in the following technical documents:
- Appraisal Report (USBR 2000)
- Feasibility Study (USACE 2004)
- Upstream Sediment Disposal Areas (USA; USACE 2010)
- Fine Sediment Study Group Final Report (FSSG 2011)
- Double Barrel Bypass (Stoecker 2011)
All of the previous concepts were combined along with several new concepts in a large table and then consolidated into a flowchart based on sediment management approach. From these, several concepts (or groups of similar concepts) were carried forward into a potential initial options list.
Note that these are not the final options, as the Technical Advisory Committee had some input and suggestions during the afternoon meeting, and will be reviewing and commenting on these to further refine this list. The options selected will be carried forward into the next level of analysis that will be directed toward determining the approximate cost and feasibility for a comparison of the concepts.
These graphics are excerpted from the presentations and technical memos available at MatilijaDam.org
Previous history of the project may be found on this blog: http://www.venturariver.org/search/label/Matilija%20Dam