Bottom Composition and Sand Movement

at the Proposed Sinking Site

1.0 Overview

A detailed review of the surrounding area was conducted by the Naval Ocean Systems Center (NOSC) in the late 1950’s for the construction of the NEL Tower. The studies included the bottom from approximately 60 feet in depth to the edge of the continental shelf. Data was obtained using bottom coring samples and acoustic soundings to determine the depth, composition and thickness of bottom layers.

2.0 Bottom Composition

Mapping around the tower showed recent marine sands overlying an older sediment layer from the San Diego River. This sediment layer tapers and ends midway to the proposed sinking site. The sinking site is composed primarily of fine sand that is approximately 30 feet thick, this layer was deposited during the Holocene period as the glaciers melted. The uppermost parts have been modified by present day currents and wave action. Under this layer is several feet of weathered conglomerate which sits on top of the subsurface bedrock. The bedrock is thought to be composed of cretaceous sandstone and shale, like those cropping out on nearby Point Loma. Exact composition of this bedrock was not determined by coring, but by using explosive sounding data that determined substrate densities. True granitic bedrock does not occur until past 6,000 feet in depth.

3.0 Sand Movement

Long term observations in the area using scuba and television recordings identified cyclic variations in ripple mark heights corresponding to seasonal variations in waves, currents, and animal populations. In winter ripple heights are highest at about 6 to 8 inches. They gradually reduce to 2 to 3 inches in summer due to decreasing water motion and increased activity of marine organisms. Actual sand depth does not vary to any degree during these cycles. Sand movement around a structure on the bottom would be influenced by the position of the structure relative to the current or surge direction. Sand would tend to pile up on the lee of the structure much as a snow drift. This drift would then be scrubbed when currents and wave action change direction.

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