TOPANGA CREEK AND LAGOON RESTORATION FEASIBILITY STUDY: A WATERSHED-SCALE APPROACH

摘要

Topanga Creek is a relatively small watershed within southern California that drains into the Pacific Ocean. It is one of the few natural coastal watersheds remaining near urban Los Angeles, with large land areas publicly owned and protected. A historically large population of the endangered steelhead trout is among its extensive resources. Several significant problems within the watershed require action, including the loss of habitat for endangered and sensitive species, severe flooding, erosion, and poor water quality in the downstream lagoon. Several specific locations throughout the watershed were targeted for intense scrutiny, as they were identified as having significant impacts on creek dynamics. Potential solutions for these sites were conceptually designed, then tested for their performance using numerical modeling and analytical methods to identify the preferred altemative(s) for consensus-based decision making. A feasibility study for the restoration of this watershed and lagoon took a broad view. It addressed the problems at a watershed scale with the goal of restoring a self-sustaining creek and lagoon system and their associated habitats. In California, this approach is long overdue, and the agencies involved are commended for taking the steps necessary to implement this unique approach. This study was funded by the California State Coastal Conservancy and implemented by the Resource Conservation District (RCD) of the Santa Monica Mountains. The engineering scope of the study included an opportunities and constraints analysis, quantifying existing conditions of hydrology, hydraulics, sediment transport, and water quality, and developing alternatives for restoration throughout the watershed. Extensive use was made of the Mike-11 numerical model to characterize the flood regime.