Lagoon Restoration near Ephemeral River Mouth

Abstract

Coastal lagoons have been restored for environmental enhancement. If the site for lagoon restoration is also near a river mouth, then the riverine system is changed into an estuarine system. Such a change may result in unmitigatible consequences unless certain special measures are employed. The proposed lagoon restoration plans for San Dieguito River were evaluated based on mathematical modeling to assess the river channel behavior in its existing state and in response to human modification. The coastal river is a supplier of beach sand that directly affects the shore protection and beachfront properties. In the initial plan proposed for lagoon restoration, the river system would be changed into a lagoon system. Under this plan, the sand flow in the river would be disrupted. The resulting impacts on scour and sediment delivery are found to be significant. While certain impacts can be mitigated with large expenditure, other impacts simply are not mitigatible. However, the difficulties in mitigation can be avoided if the riverine system is preserved in the estuarine system. In the revised plan proposed for lagoon restoration, tidal basins will be created and berms will be used to maintain the effective flow area of the river channel to bypass the tidal basins. This scheme maintains the sand flow through the river reach to avoid potential scour impacts by the project. The revised proposed plan was evaluated; the results show that the project will not change the potential scour for the infrastructure and that it will maintain the present sediment delivery to the beach.