Modeling the effects of dam removal on coastal lagoon dynamics over multiple time scales

The Ventura River Estuary in southern California is a small bar-built coastal lagoon, fed by episodic discharge from the Ventura River and periodically open to the ocean through a coastal inlet. The lagoon, approximately 25 km downstream of the Matilija Dam, provides critical habitat for a range of species. Proposed removal of the dam would reestablish connectivity for steelhead and restore sediment loading to the relatively sediment-poor Ventura coastline. However, sediment loads associated with dam removal may impact inlet dynamics and lagoon habitat over a wide range of timescales. The initial sediment release following dam removal will occur over a few days, while restored sediment loading will modify dynamics over subsequent years to decades.

Both short- and long-term effects of dam removal on lagoon and inlet dynamics were evaluated using a multi-model approach. A high-resolution numerical hydrodynamic and sediment transport model was developed in Delft3D to resolve small-scale transport process in the lagoon. The high resolution lagoon model was used to simulate dam removal events (with upstream discharge and sediment loading from prior dam removal studies) to predict short-term impacts on water quality and sedimentation. Subsequent flood events were also simulated to evaluate stability of deposited sediments and the effect of restored sediment loading. The lagoon model was coupled with a long-term empirical inlet model, which utilizes a hydrologic mass balance approach to predict inlet breach conditions over long periods (decades). Substantive changes in inlet breach conditions (breach timing, frequency, and/or duration) due to dam removal can have significant impacts on species that rely on seasonal lagoon dynamics. The inlet model, validated with observed breach conditions, incorporated hydrologic fluxes (riverine discharge, wave overtopping, berm seepage and evaporation), berm elevation changes (fluvial inlet erosion and wave swash accretion), and lagoon specific geometry and hypsometry (relationship between lagoon storage and water elevation) and was used to evaluate changes to inlet conditions due to dam removal and sea level rise. The multi-model approach allowed for robust evaluation of worst-case dam removal effects on lagoon dynamics and habitat over multiple time scales.