MEASURED AND COMPUTED COASTAL OCEAN BEDLOAD TRANSPORT

Abstract

A comparison is made between the measured infilling of two test pits off the coastline of San Francisco and predictions using a coastal bedload transport model. The model, based on the work of Madsen and Grant (1967), relates the bedload transport to the bottom shear stress through an empirical relationship based on laboratory experiments. The bottom shear stress is estimated from the bottom currents created by waves and low frequency currents. The model applies beyond the breaker zone in contrast to littoral transport. The test pits, dredged as part of the Southwest Ocean Outfall Project for San Francisco, were located 1.6 km (1 mi) and 3.2 km (2 mi) offshore in 13' m (42 ft) and 16 m (53 ft) of water. The depth of the pits relative to the natural seabed was about 8.4 m (25 ft). The comparison was conducted for a period up to 2 months in the fall of 1978. The paper discussed the quality and scope of available data required as input to the model and shows how regional wave data were trans formed to augment local measurements. Uncertainties in model results stemming from limitations in the input data are presented. With suitable adjustment of the scale of the gravitational term in the expression for the Shields parameter, overall agreement between computed and measured bedload was accomplished within the limits of accuracy of the bathymetric surveys. A sensitivity analysis of selected input conditions and coefficients was also conducted.