California harbor dredging: History and trends

摘要

California is a major shipping point for exports and imports across the Pacific Basin, has large commercial and recreational fisheries, and an abundance of marine recreational boaters. Each of these industries or activities requires either a port or harbor. California has 26 individual coastal ports and harbors, ranging from the huge sprawling container ports of Los Angeles and Long Beach to small fishing ports like Noyo Harbor and Bodega Bay. Almost all of California's ports and harbors were constructed without any knowledge or consideration of littoral drift directions and rates and potential future dredging issues. Rather, they were built where a need existed, where there was a history of boat anchorage, or where there was a natural feature (e.g. bay, estuary, or lagoon) that could be the basis of an improved port or harbor. California's littoral drift rates and directions are now well known and understood, however, and have led to the need to perform annual dredging at many of these harbors as a result of their locations (e.g. Santa Cruz, Oceanside, Santa Barbara, Ventura, and Channel Islands harbors) while other harbors require little or no annual dredging (e.g. Half Moon Bay, Moss Landing, Monterey, Redondo-King and Alamitos Bay). California's coastal harbors can be divided into three general groups based on their long-term annual dredging volumes, which range from three harbors that have never been dredged to the Channel Islands Harbor where nearly a million cubic yards is removed on average annually. There are coastal harbors where dredging rates have remained nearly constant over time, those where rates have gradually increased, and others where rates have decreased in recent years. While the causal factors for these changes are evident in a few cases, for most there are likely a combination of reasons including changes in sand supply by updrift rivers and streams related to dam construction as well as rainfall intensity and duration; lag times between when pulses of sand added to the shoreline from large discharge events actually reach downdrift harbors; variations in wave climate over time; shoreline topography and nearshore bathymetry that determine how much sand can be trapped upcoast of littoral barriers, such as jetties and breakwaters, before it enters a harbor; and timing of dredging. While there is virtually nothing that can be done to any of these harbors to significantly reduce annual dredging rates and costs, short of modifying either breakwater or jetty length and/or configuration to increase the volume of sand trapped upcoast, thereby altering dredging timing, they are clearly major economic engines, but come with associated costs.