Challenges of Restoring an Altered Reservoir/Debris/Detention Basin to its Maximum Potential Flood Mitigation Capacity - A Case Study in Ventura County, California

摘要

Ventura County Watershed Protection District (VCWPD) owns and maintains a number of debris/detention basins and dams/reservoirs that mainly serve for flood control and debris collection purposes. Most of these facilities have been developed and built jointly by the County and Federal partners like Natural Resources Conservation Service (NRCS) since the 1960s, and later delivered to the County for future Operation & Maintenance (O&M). The extent of these facilities has required development of a manual for these facilities called Debris and Detention Basin Manual. Over the years, the manual has been updated to include changes in hydrology and sediment yield among other factors. However, the manual generally does not include the history of the basins development and an O&M protocol for every basin. A dam/reservoir project that may have originally been conceived as a water supply facility (e.g., Runkle Dam in Simi Valley) and may not have met the life cycle and level of service expectations as planned due to various reasons (urban development and land use changes, large water transfer projects, etc.), has become a debris/sediment collection facility with some detention benefits by default. Over three decades after construction, the basin had collected significant sediment and consequently lost about half its storage volume due to lack of protocol for basin maintenance. The challenge is that a major portion of the basin has been filled up and vegetation has established over time that cannot be simply removed due to environmental regulation enacted in recent decades (California Environmental Quality Act, CEQA). Since the project was promoted by Ventura County Flood Control District, nowadays called VCWPD, one could not simply discount the flood reduction benefits of the facility either. The restoration of the basin to its full capacity as demanded by a public stake holder would be difficult because of stringent environmental requirements and mitigation costs to remove the vegetations, and the multitude of other stakeholders that need to be involved (environmental permits from multiple agencies, development interests, etc.). Although the basin had not been cleaned up in the early years of post dam construction, in recent years the lower portion upstream of the dam is cleaned up such that the storage for sediment and water is about half the original design. The upper parts of the basin are filled up with sediment and vegetation growth that require extensive and costly off-site mitigation if the basin is to be restored to its full capacity. Therefore, finding a scheme that would allow a multi-purpose functionality (debris collection and temporary flow detention to mitigate flooding downstream of the dam, potential water supply, off site mitigation, providing fill materials for a planned development upstream of the basin, etc.) would be a preferred approach for this facility in the future that meet all stakeholder's interests. Additionally, development of an Operation & Maintenance protocol that would be economically feasible and accepted by the stakeholders was a major goal of the project. This paper discusses the history of the basin and alternatives considered for the restoration of the basin to provide maximum flood mitigation for downstream areas.