Methods for Position Control and Beach Face Profiling

Abstract

Accurate, repetitive surveying of the subaerial beach and shallow nearshore out to depths of about −1 m relative to Mean Sea Level (MSL) was performed as part of Task 4E of the Nearshore Sediment Transport Study (NSTS). This surveying was used to measure changes in beach shape and volume caused by cross-shore and longshore sediment transport during the inter-survey periods. The surveys also provided a daily measurement of the height of surf zone instrumentation above the sand bottom. Because accuracies of better than 5 cm were required for this task, the survey technique selected was similar to that described by Nordstrom and Inman (1975). A self-leveling engineers level and fiberglass, extendable survey rod were used for measuring vertical changes in the beach profile. The self-leveling level had a standard deviation of approximately ±2 mm over 1.6 km of double-run leveling. It was water resistant, with a magnification of about 32x and minimum focus distance of about 2 m. The leveling rods were graduated in 0.01 foot increments, with a linear accuracy of better than 1 in 4000. The fiberglass construction insured that the rod did not swell when wet and affect instrument accuracy. The survey line was a thin, plastic-sheathed steel line graduated at 3.0 meter increments. The construction of the line minimized stretching/contraction which would detract from survey accuracy. The maximum separation between level and rod was maintained at less than 60 m where possible; the primary exception to this occurred in the seaward portions of the profile when the level could not be relocated in the swash zone.