EXAMINATION OF PREDICTIVE MODELS FOR DREDGED MATERIAL DISPERSION IN OPEN WATER

The behaviour of dredged material when dumped in open water is currently of great interest from the point of view of turbidity generation and environmental effects. This paper presents a review of existing mathematical models for predicting the short-term fate of dredged material in open water and then compares the predictions to field data collected in conjunction with a U.S. Army Corps of Engineers hopper dredge. Several models were reviewed and then the Koh-Chang model was selected for a sensitivity analysis to examine the sensitivity of its output to changes in input. Predictions were made of the descent, collapse, and long-term diffusion phases of the material dump. Since the mathematical models have not been field verified, a review of data on open water dump sites was conducted and this data was compared to the predictions of the Koh-Chang model. A field program was also conducted to collect data on the turbidity plume behind a hopper dredge. The plume was sampled, in 3 dimensions, using water samplers and from a towed transmissometer. Predictions were then made, using the Koh-Chang model, and compared to the field results. Based upon the mathematical model review and sensitivity analysis, the review of past data from existing open water dump sites, and the field measurements of the turbidity plume behind a dredge, the state-of-the-art of open water prediction of short-term fate of dredged material is assessed and recommendations are made as to the needs for future research. The areas in which the predictions agreed with the field data are discussed as are the areas in which agreement was not achieved. Also discussed are the problems inherent in field verifying large mathematical models such as the Koh-Chang.