Resuspension and Redistribution of Sediments in Hamilton Harbour

Abstract

The size distribution of sediments in Hamilton Harbour is a result of both natural and human processes. The objective is to predict the behavior of sediment particles in the harbor, assuming wind driven resuspension to be the major natural factor controlling grain size distribution. In this approach hydraulic sorting of sediments was assumed to be entirely predictable from 10 years of wind direction and velocity data used to calculate fetch over the extent of the harbor. Bottom velocities were estimated assuming a universal velocity defect law and the direction of bottom currents were assumed to parallel the depth contours. Although simplistic in many regards, the model returns adjustment parameters that are consistent with accepted values and is sufficiently articulated to be used as a reference to evaluate the cause of sediment transport in the harbor. Known local disturbances associated with human activity in the harbor could be predicted by mapping the differences between sediment distribution and actual core data from surficial sediments. A large part of the sediment in the deeper part of the harbor however contained more silt and clays than predicted. We assume this excess to be caused by a uniform disturbance equivalent to a 4.8 cm/sec bottom current and results from the combination of small bottom currents and other transfers of energy from the mixed layer. Historical records show that this situation existed long before the industrial period. Man-made disturbances on the overall distribution of bottom sediments are therefore less important compared to changes due to agricultural practices at the turn of the century.