Fluvial Modeling of Ventura River Responses to Matilija Dam Removal

A plan has been made to remove Matilija Dam on the Ventura River. With dam removal, the delta in the reservoir and the downstream channel are expected to undergo major changes in morphology. The FLUVIAL-12 model was employed to simulate reservoir and river channel responses after the dam removal. As the first step, the model was calibrated using the Ventura River data to establish its validity. In calibration, the model was used to simulate the fluvial processes starting from the time of dam completion. The simulated sediment deposition above Matilija Dam matches closely with the measured deposits by the U. S. Bureau of Reclamation. A large amount of sediment has been stored in the reservoir; some of the stored sediment will be transported downstream after the dam removal. An important consequence of dam removal is the major increase of sediment release to the river channel downstream. The sediment supply to the downstream reach is not only from the stored sediment in the reservoir but also from natural sediment inflow from the upstream watershed. Therefore, sediment supply to the downstream reach will exceed the natural sediment flow before the dam presence. This situation tends to overload the downstream reach with sediment to result in excessive deposition. The amount of sediment release from the area above the removed dam is closely related to the changes in reservoir morphology. It is necessary to model changes in channel bed profile and in channel width during erosion in order to determine the amount of sediment removal. The amount of sediment release may not be simulated using an erodible-bed model but it may be determined using an erodible boundary model.