ASSESSING THE STABILITY OF INNER SHELF DREDGE SPOIL MOUNDS USING SPREADSHEET APPLICATIONS ON PERSONAL COMPUTERS

Three methods, viz. the HANDS and ALLISON (1991) method, predictions o f sediment threshold, and predictions of sediment transport rate, are used to assess the long term stability of dredge spoil mounds on the i nner shelf and compared with available observations of their behaviour . These methods me chosen for simplicity and ease of implementation wi th a spreadsheet application on a personal computer. Three main approa ches were followed: an evaluation of the annual, and daily limits, of onshore-offshore sediment movement near the spoil mound; a comparison between the theoretical thresholds of sediment motion and the annual w ave height and period joint distribution: and an evaluation of sedimen t transport rates and directions using semi-empirical relationships. T he HANDS and ALLISON (1991) method had the smallest data requirements: mean and standard deviation of the annual significant wave height dis tribution; and the median grain-size at the mid-point of the shoal zon e. Analysis of sediment threshold and comparison with wave climate req uired data concerning the annual joint wave height and period probabil ity distribution, and sediment textural characteristics. Sediment tran sport rate calculations required the most detailed information about t he site: annual joint wave height and period probability distribution; sediment textural characteristics; and mean unidirectional current ve locity. Using a spreadsheet, it is relatively easy to simulate a range of values (that span the likely conditions) with all three methods, i f the necessary data are not available. All the methods produced predi ctions consistent with available observations, and all were straightfo rward to implement within a spreadsheet application. The choice of met hod depends on the information required, with each method needing diff erent ditto and providing contrasting but complementary outputs.