Erosion of dredged material islands due to waves and currents

Abstract

The purpose of the research reported here was to develop empirical capability of predicting the erosion rates of cohesionless dredged material islands for wave and wind environment similar to that of the Upper Laguna Madre, Texas. The field and the historical data for a selected dredged material island (Crane Island) were employed in conducting the preliminary movable-bed model study. The degree of reliability of the preliminary model studies was established by comparing their results with the prototype results. Next, a general model study was conducted to investigate the separate effect of various factors such as different water depths, island spacing, island orientation and presence of small magnitude current on the island erosion rates. The increase in the water depth caused an increase in the island erosion rates. Once the eroded material settled down on the bottom, it experienced very little disturbance. The island migration rate and hence the channel silting rate under the higher water depths (0.72 ft and 0.80 ft) were found to be insignificant. The island erosion rates decreased with decreasing spacing. The islands having spacing of 50 ft and built parallel to the incident wave crests (0° orientation) offered the most economical solution in terms of the long-term maintenance dredging. Next, the islands were tested for two more orientations, 45° and 90° with respect to the incident waves. The overall erosion of the islands was affected only slightly by different orientations. The islands built at 0° orientation offered slightly more resistance to wave erosion than the islands at 45° or 90° orientation..