The final meeting of the Matilija Dam Fine Sediment Workgroup was held on Wednesday April 30. This was the fourth all-day meeting intended to "agree upon possible solutions to manage the approximately two million cubic yards of fine sediments" currently stored in the defunct reservoir.
The final consensus from the workgroup, which included representatives from each of the stakeholder groups, was to pursue two paths:
I) Interim Notching Design
II) Hybrid Alternative Analysis
The design and analysis identified through the stakeholder process will define the immediate data gaps necessary to achieve stakeholder and regulatory clearance to move forward with both short-term notching and the long-term dam removal plan.
The details of this plan are as follows:
I) Interim Notching Design:
Ventura County will pursue investigations into the feasibility of notching the dam down to the silt line, independent of the federal Corps of Engineers process. The intent is to lower the height of the dam to limit the capacity for further accumulation of sediment in the upstream reservoir area.
The Feasibility Study revealed that Matilija Dam has the potential to continue to trap sediment transported down Matilija Canyon from large storms. This accumulation (up to 3 million cubic yards, or an additional 50% over the estimated 6 million cubic yards) is changing the 2004 baseline conditions. And each year that the dam traps more sediment, the ultimate cost of the dam removal project increases.
Because funding is scarce, and it may be many years before federal appropriations are available for the full dam removal process, it was agreed that this is a prudent 'interim' step that is acceptable to the stakeholders.
II) Hybrid Alternative Analysis:
During the third meeting, the Working Group identified the potential for a hybrid alternative that could reduce the cost of the project while minimizing the potential for requiring congressional reauthorization. This 'hybrid' would consider the potential for optimizing sediment management with three components:
A. Reservoir Area - storing sediment upstream of dam within upland terraces & possibly under the roadbed
B. Slurry - transport fines downstream to BRDA disposal areas identified in Feasibility Plan
C. Notching - opportunistic 'natural transport'
A. Reservoir Area Design:
The group agreed to first prioritize the Reservoir Area Design. The Feasibility Plan called for excavation of a 100-ft wide meandering stream channel through the sediment upstream of the dam. This channel would re-create a natural gradient for the riverbed, which would allow for controlled natural release of sediment from the temporary storage areas over time. During Feasibility it was assumed that all 2 million cubic yards of fine sediment (silt and clay) would be slurried downstream - but the cost and location of the slurry disposal areas hung up the project.
The Reservoir Area Design will reconsider the potential for incorporating some or all of this fine sediment into the temporary storage areas identified in the Feasibility Plan. This design will;
- Determine upstream storage capacity in keeping with the Feasibility Plan
- Develop a re-vegetation and natural stream-bank stabilization plan
- Determine potential incremental impacts as compared to the Feasibility assumptions
This design process will incorporate 'natural' sediment stabilization to the maximum extent, and minimize soil cement stream-bank hardening. The workgroup agreed that the intent is to develop a design to re-vegetate the sediment storage areas to minimize erosion of the fine sediments that may create a chronic water quality problem downstream. It was made clear that this was not intended to re-visit the "Upstream Storage Areas" that raised objections last year.
B. BRDA Design
Once the Reservoir Area Design is complete, the remaining quantity of fine sediments that may need to be slurried downstream will be better defined. This will determine the area needed and drive the design for the downstream slurry disposal areas.
BRDA is the 'Baldwin Road Disposal Areas' for fine sediment slurried from reservoir as identified in the Feasibility Plan. Reducing the quantity of sediment that needs to be slurried may reduce the costs that increased for this project component during final design. Design for these areas will include consideration of temporary storage and revegetation of more permanent areas.
C. Natural Transport
Although there was much discussion of the potential for utilizing Natural Transport within the project time frame, this was not identified as a priority study in the final meeting. This was largely due to the unpredictable nature of our climate and the need to prioritize limited funding for studies. The general idea was to be prepared to take advantage of a large storm event if it occurred during the 2-3 year construction operations. Studies to consider this option would include analysis of how to notch the dam and control the release of sediments, and a determination of the subsequent downstream impacts to water quality and biological resources.
One of the potential means to mitigate any impacts may come from the "Double Barrel Bypass," as proposed by Matt Stoecker. This concept would take advantage of the slurry pipelines to deliver clean water diverted from upstream of the project area directly to water users downstream.
Optimization of Hybrid Alternative
Once the technical feasibility and data gaps are complete for the three components above (Reservoir Area Design, BRDA Design, and Natural Transport), an optimized solution may be developed based upon the associated cost estimates. The final disposition of the 2 million cubic yards of fine sediment will be determined by the capacity, costs, and environmental impacts of each of these management scenarios.
The next steps in the process are;
- Outcomes Document - the Study Group will be reviewing a draft report that documents the outcome of the facilitated process sometime later this month.
- Studies - the studies outlined above will be commenced using the limited funding available and utilizing expertise discussed in the Workgroup meetings
From the meetings over the past months it was clear that the group is supportive of the project moving forward as soon as possible. There was consensus on the outstanding issues and the priorities for next steps, but the overshadowing dampener is the clear lack of funding in the near term.
With that in mind, this process cleared the way for the local agencies to move forward with a plan to notch the dam to prevent further sediment accumulation.
At the same time, the plan for dam removal can be refined in order to realize the ecosystem restoration objectives and reduce overall project costs in preparation for future funding opportunities.
In the News:
New Proposal on How to Tear Down Matilija Dam Unveiled
After negotiations over how to tear down Matilija Dam reached a stalemate last year, a new proposal is being examined on how to remove the massive amount of sediment built up behind the defunct dam near Ojai.
Each of the stakeholders provided written comments in response to a questionnaire posed to the group. These documents were used to consolidate the issues and develop the path forward, and may be downloaded from http://matilijadam.org/.
The Feasibility Plan is described in a simple poster here: http://www.venturariver.org/2010/04/matilija-dam-poster.html
Other presentations and information is also available at http://matilijadam.org/
Matilija Coalition comments are online at http://matilija-coalition.org/comments2.htm
And the recent history of the project is documented on this blog: http://www.venturariver.org/search/label/Matilija%20Dam