Matilija Dam Ecosystem Restoration Project, California
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.
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 Under High Flow
Courtesy of William Carey, Ventura County Watershed Protection District
- Inundation of property including residences and farm area
- Water delivery reliability
- Operation of diversion structures
- Pipeline access and security
- Inundation of water treatment facilities
- Levee adequacy
- Restoration of beach areas
- Recharge of the ground water table
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.
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.