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Matilija Dam Ecosystem Restoration Project, California

Matilija Dam

Matilija Dam

Matilija Dam is located on Matilija Creek, which joins with North Fork Matilija Creek 1/2 mile downstream of the dam to form the Ventura River. The Ventura River is predominantly a cobble bed river with a high sediment supply. The sediment production per area from the Ventura River watershed is one of the highest in the nation, at about 1 mm/yr. Sediment transport in the rivers and stream are dominated by large infrequent storms. According to sediment measurements, over 98% of the sediment transport in the Ventura River occurs in less than 1% of the time.

Matilija Reservoir Delta

Delta Behind Matilija Dam, from Paul Jenkin, Surf Riders

Matilija Dam was constructed in 1947 and has trapped approximately 5.9 million cubic yards of sediment. Matilija Reservoir currently has less than 500 acre-feet of capacity remaining and its ability to trap sediment and attenuate floods has been significantly decreased. Because of the large volume of trapped sediment, the major costs and impacts of the removal of Matilija Dam are primarily those associated with the management of this trapped sediment. There are four major alternatives being considered, and three of those have sub-alternatives

Alternative Sub-Alternative Description
No Action   No removal of dam or sediments
1   Full Dam Removal/Mechanical Sediment Transport: Dispose Fines, Sell Aggregate
2   Full Dam Removal/Natural Sediment Transport
  2a Slurry “Reservoir Area” Fines Offsite
  2b Natural Transport of "Reservoir Area" Fines
3   Incremental Dam Removal/Natural Sediment Transport
  3a Slurry “Reservoir Area” Fines Offsite
  3b Natural Transport of “Reservoir Area” Fines
4   Full Dam Removal/Sediment Stabilization on Site
  4a Permanent Stabilization
  4b Temporary Stabilization

Release of the trapped sediment carries important implications for downstream infrastructure and water quality including:

Casitas Levee

Casitas Levee Under High Flow
Courtesy of William Carey, Ventura County Watershed Protection District

A GSTAR 1-D model was developed to evaluate changes in bed profile under different alternatives. The model was calibrated to the Ventura River using the period from 1971 to 2000. Survey data was available at the beginning and end of this period and it was possible to compare the model results against measured values. Overflows were mapped for the 2, 5, 10, 50, 100, and 500-year return periods using the hydraulic model HEC-RAS.

Bed Profile No Action Thalweg Change

Initial Bed Profile and Change Under the No Action Alternative

An adaptive management plan and monitoring program is being developed to assist in planning managment of the sediment during and after dam removal.