In preparation for the next Matilija Dam Design Oversight Group (DOG) meeting on December 4th, we have been out looking at the slurry disposal sites. The issue is summarized in notes from the last DOG meeting on October 2nd.
In our comment letter, we pointed out that one of the proposed disposal areas (MODA) is a very popular public access point, of which there are few within the watershed. We are also concerned that disposal in this area would be permanent, rather than located in an area where the river would be allowed to wash the fine sediments downstream during future floods.
An analysis of erosion potential is available at matilijadam.org: Bureau of Reclamation Hydraulic Analyses of Sediment Disposal Sites Presentation
MODA (Meiners Oaks Disposal Area) is shown in this image. The blue shading is the area which the river could potentially erode in the future during a 100-yr flood. Note that the planned levee upstream of the site would effectively prevent the river from accessing this portion of the floodplain.
This photo is taken from the bluff near the trailhead, and shows the 70 acre floodplain area. The entire 2.1 million cubic yards of sediment could be deposited here at an average height of 35 feet.
The upstream Baldwin Rd site (BRDA1) is within the floodplain. Because these are active river channels there is less old growth vegetation (oaks and sycamores) and more riparian floodplain vegetation. The analysis shows that this site would erode in the future (blue shading).
The second Baldwin Rd site (BRDA2) is County property that used to be the jail honor farm. The site includes active channel and floodplain terrace, much of which was used for grazing. The river would also erode much of this site in the future.
Discussion of slurry disposal will be continued at the December 4th DOG meeting.