CHALLENGES TO UNDERSTANDING LITTORAL SAND BUDGETS ALONG ACTIVE MARGIN, HIGH ENERGY COASTLINES

Active margins are generally characterized by coastal cliffs and mountains, and steep streams that overall have coarser sediment loads and higher sediment yields than streams draining passive margins. The great majority (75-99%) of beach sand in California is derived from fluvial sources. This commodity has become increasingly important to California’s economy, both as a recreational resource as well as the most cost-effective buffer to the state’s highly developed coastline from wave attack. However, there are a number of shortcomings in our understanding of littoral sediment budgets that must be resolved if we are to make decisions that will allow us to sustain these systems over the long-term. Methods for quantifying source inputs, littoral drift rates, losses to individual sinks, and long-term changes in beach volumes are primitive by the standards of other disciplines. We can measure concentrations of contaminants in sediments to parts per billion, yet our annual estimates of fluvial sediment loads are typically considered to be accurate within ± 30%. Considerable data on suspended sediment exist for major rivers, but relatively few data have been collected on bedload transport and many smaller streams have no sediment discharge data at all. Sediment transport measurements are also limited during high flows when most sediment is transported, adding significant uncertainty to the use of sediment rating curves. Research is underway to determine sand budgets for all of California’s littoral cells using harbor dredging as a proxy for littoral drift at specific points within littoral cells. One initial, surprising finding highlights our poor understanding of littoral budgets. At the Santa Barbara Harbor at the downcoast end the Santa Barbara littoral cell, the long-term dredging volumes do not reflect significant reduction in littoral sand transport, despite substantial impoundment of the major updrift sand-supplying rivers. Improving our ability to quantify littoral sand delivery or production from different sources, littoral transport rates and long-term changes in beach volumes is necessary if we are to understand how human activity has altered the beaches of individual littoral cells.