Sand Capture In Southern California Submarine Canyons

摘要

Recently seven southern California submarine canyons have annually captured between 500 and nearly 1,000,000 cubic yards of sand. Coastal sediment budgets were proportionately impacted. An understanding the processes involved, and why the rates differ so greatly, provides useful information if ever an attempt is made to reduce or eliminate these losses. The availability of sand for capture is constrained by the net longshore sand transport rate, Q_n. In canyons very near or at the downcoast ends of their littoral cells, the capture rate is Q_n. Elsewhere it is less than Q_n . Transport into a canyon is directly proportional to the frequency, duration, and intensity of winter storms, inversely proportional to the width of the littoral zone landward of the canyon headwall, and substantially dependent on the shape of the nearby shoreline. All else being equal, canyons located off indentations in the shoreline capture much more sand than canyons located off bulges. The construction of a breakwater, jetty, seawall, the placement of a huge beachfill, and the artificial relocation of the mouth of a large river, altered the sand capture rate in four of the canyons. Where the impact was adverse, it has been substantially mitigated.