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Sedimentation in Submarine Canyons in San Diego County, California, 1984 - 1987

  • Author(s): Moffatt and Nichol, Engineers
  • et al.
Abstract

Three submarine canyons have an effect on the coast north of San Diego, California. Scripps and La Jolla Canyons extend almost to shore and permanently trap sand at the south end of the Oceanside Littoral Cell. They are also responsible for enhancing the local, long-term shoreline retreat rate as evidenced by the embayed shoreline adjacent to each. Carlsbad Submarine Canyon, in the central portion of the Oceanside Cell, extends shoreward across the Continental Shelf to a water depth of about 100 ft. Littoral sand is not carried to the canyon head at that depth. The effects of wave refraction over Carlsbad Canyon have resulted in a reduction in the local rate of shoreline retreat and produced a slight bulge in the nearby shoreline.

The first objective of the field investigation described in this report was to quantify the rate at which littoral sand was carried to and deposited in the shallow heads of Scripps and La Jolla Canyons between December 1984 and June 1987. The second objective was to establish, for the same period, the rate at which the deposited material was flushed down the axes of the canyons. Littoral sand, once it is flushed to deep water, is unrecoverable. The frequency, magnitude and duration of storms, the characteristics of the local longshore sediment transport regime, and the location and shape of the heads of the canyons control the entrapment rate, how much sand the canyon head can hold before it is flushed out, and the frequency of flushing.

Sand Entrapment Rate. Scripps Canyon has at least 6 shallow-water tributaries that trap sand. The most active four were investigated in this study. La Jolla Canyon has a single, but much larger, O.7-mi long, shallow-water head. Canyon heads filled when sand moved seaward from the littoral zone into nearcoast sediment depressions. Depressions are shallow, relatively steep, saucer-shaped region located above canyon gorges. Gorges are comprised of rock headwalls, rock sidewalls that in some places are vertical or even overhanging, and rocky, seaward-sloping floors.

Sand that had the same size distribution as sediments in the nearby littoral zone was deposited in the depressions. Very fine-grained sands, micas, and organic debris passed over the depressions and were deposited in the gorges. As the prograding deposits in the depressions of Scripps Canyon were funnelled into the narrow (10 to 200-ft wide) gorges, they moved on top of the finer, lower-specific-gravity material. Relatively little passed over the sidewalls. In La Jolla Canyon about equal amounts of sand passed into the gorge over the rim of its wide headwall, and through several chute-like depressions or reentrants that pierce the headwall.

Sumner and South Branches are presently the most active tributaries of Scripps Canyon. The upper boundaries of their depressions are closest to shore and they intercepted more sand than tributaries that began in deeper water. Sumner and South Branches are located near the north end of the tributary system of Scripps Canyon and preferentially filled when longshore sediment transport was to the south. The north re-entrant of La Jolla Canyon was the most active part of that canyon, even though the gorge there is farther from shore than it is elsewhere.

In both canyons a much larger volume of littoral sand was initially deposited in the depressions than in the gorges. The ratio was about 20:1 in Sumner and South Branches.

Over 80 percent of the sand was transported seaward into the canyon heads between November and May during wave storms, probably more the result of storm-induced downwelling than transport in rip currents. Shore-normal transport was dominant. A relatively small quantity of sand entered the canyon heads parallel to shore. Only small amounts of littoral sand were carried into the depressions during the summer and autumn, but large quantities of mica and especially kelp and ,sea grass debris was deposited in the gorges at all times of year. Organic debris was transported by rip currents over the headwalls, and by longshore currents over the sidewalls.

An average of about 29,000 yd3/yr of littoral sand was deposited in the shallow heads of the canyons between December 1984 and June 1987. Only about 1,000 yd3/yr of that was trapped in La Jolla Canyon. In Scripps Canyon an average of about 22,000 yd3/yr was deposited in Sumner Branch; about 2,000 yd3/yr was deposited in South Branch. The long-term rate of littoral sand entrapment in or adjacent to Scripps and La Jolla Canyons appears to be approximately equal to the net longshore sediment transport rate at the south end of the Oceanside Littoral Cell.

Sand Flushing Rate. Once littoral sand passed over the upper edge of the depression it was carried downslope in small surface slumps and by wave-induced bottom oscillations coupled with gravity. In this way a critical slope of about 18 degrees was maintained at the seaward face of the prograding deposit. The normal load created by this deposit increased greatly as its toe prograded onto the steeply-dipping deposit in the gorges of Sumner and South Branches. When the normal load exceeded the internal shear strength of the deposit a massive downslope movement of sediment occurred. The slumps and slides were also controlled, in part, by the decomposition of organic debris that reduced the strength of the deposit near the floor of the gorges. The heads of Scripps Canyon apparently reach a critical volume of sand at which time the deposit is susceptible to failure. In Sumner Branch the critical volume is about 50,000 yd3 while in South Branch it is about 5,000 yd3•

Flushing occurred when storms moved large quantities of sand onto the upper part of the depressions, thereby increasing the normal load. Sumner, South and Shepard Branches flushed on 13 December 1984. Sumner and South Branches filled and again flushed in early spring 1987, so their flushing frequency during the field investigation was O.4/yr. The flushing frequency of other Scripps Canyon tributary valleys is estimated to be O.025+/yr.

Flushing occurs most often in gorges with steeply-sloping floors that fill rapidly because they head close to shore. Wave loading may be a factor in reducing the strength of the deposit during storms.

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