DESIGN AND BEHAVIOUR OF SANDTRAPS IN REGIONS OP HIGH LITTORAL DRIFT

Siltation in harbours and their approach channels is one of the major problems connected with the development of harbours. The economics of harbours are directly related to their annual maintenance dredging, and as such a proper assessment of the quantity of siltation and provision of adequate measures for the maintenance of depths would form an important part of planning the development programmes. Siltation could occur due to various reasons viz. deposition of littoral drift which is interrupted by the approach channel, deposition of sediments brought into suspension by wave action (including during storm/cyclone) Whenever the alongshore drift is large, wave action obviously is quite substantial which renders the maintenance of depths during this period by dredging difficult. In such cases it would be necessary to make adequate provision to ensure that the depths are not deteriorated to any substantial extent by the movement of the drift. One of the common means for achieving this is the provision of sandtraps on the updrlft side of the approach channel which would 'store' the drift material temporarily and from where the material could be dredged at convenience. The design of the sandtrap would be governed by a number of factors such as the extent over which a major part of the drift takes place, quantity of material transported, size distribution of sediments, velocity of currents, mode of dredging etc. Waves of moderate to high intensity occurs along the eastern coast of India from south and south west direction during south west monsoon period from May to September and from North Easterly direction during North East monsoon beginning from November. In view of this climate the direction of drift along the shore changes with Mason. The quantities of drift during these periods are also different owing to the magnitudeand periods of wave action which differs between the two seasons. The net drift along this coast varies from 0»60 million m3 at Madras on the south which increases progressively to 1,00 million m3 at Paradip further North (Fig, 1),