Location: Ventura County, CA
Clients: Ventura County Watershed Protection District and AECOM
Project Manager: Derek Booth
Stillwater Sciences has been working collaboratively with URS (now AECOM), the California Coastal Conservancy, and Ventura County to provide a fresh look at the 20-year old Matilija Dam removal project in southern California. Upon reviewing more than a dozen previously discussed dam removal alternatives and concepts, the team realized that all the previous dam removal alternatives have been based on the assumption that more than 2 million cubic yards of fine sediment deposit needed be mechanically excavated prior to dam removal in order to minimize its downstream impact, which results in very high projected project cost and difficulty for the disposal of the mechanically removed sediment. As a result, Stillwater Sciences redirected that focus towards alternatives for sediment management and dam removal that would minimize potential fine sediment impacts without requiring mechanical sediment removal.
With the help of sediment transport modeling and watershed-based evaluation of hydrology and sediment yields, the team eliminated various prior dam removal concepts, particularly phased dam removal alternatives, due to their relatively high cost and multiple impacts, leaving only three potential alternatives for further consideration. Two of these alternatives, including the preferred alternative selected by the stakeholder group, recommend a one-time release of the impounded sediment by natural erosion to minimize the duration of the downstream impact. This would be accomplished by timing the dam breaching and resulting sediment release to coincide with a high-flow event. Stillwater Sciences was able to quantify the likely duration and intensity of the downstream impacts on in-stream biota, compare those impacts to what a natural high-flow event would generate even without the additional reservoir-impounded sediment, and demonstrate the feasibility and benefits of this approach to the client and stakeholders.
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