Offshore Dredging Systems For Beach Nourishment Projects

Beach erosion is a serious international problem. In the United States alone, it has been reported by the Corps of Engineers that measures to control erosion appear justified along 2700 miles of coastline. The cost of this work is estimated at $1.8 billion. The natural forces causing beach erosion are complex, involving effects of wind, and upland erosion. A discussion of these factors is beyond the scope of this paper. Suffice it to say, beach erosion is with us to stay. It is possible in many cases to overcome the problem by modifying current patterns through use of artificial structures, and by replacing the lost sand with sand dredged from the sea floor. This paper deals with an analysis of several dredging systems that can be effectively employed for rebuilding eroded beaches, a process referred to as beach nourishment. Beach nourishment can be accomplished with both permanent, and portable, dredging systems. 1.1 An example of the former is a system installed in Italy along the Liguria Sea between Massa and Carrere. The need for a permanent installation is the exception however, and in most instances replenishment can take place on a periodic basis, using portable dredging plant. 1.2 If sand is available onshore immediately near the beach to be replenished, it can be pumped directly to the beach site through a delivery pipeline, using established dredging techniques. In many cases, however, the sand is only available immediately seaward of the beach. This paper will review the various dredging systems which can be employed in this latter case. To provide a means for selecting the dredging system most appropriate for a given replenishment project, this paper will present calculations of (1) the cost per cubic meter of sand deposited on the beach, and (2) the return on investment in dredging plant. 2. INFLUENCING FACTORS Generally speaking t the following factors must be considered in selecting the appropriate dredging and transportation system.2.1 The location of the offshore sand deposit and the characteristics of the deposit2.2 The number of cubic meter of sand required for replenishment2.3 The sand delivery site2.4 Type of dredge 2.1 Sand deposit As mentioned in the introduction t this study pertains only to cases in which the source of sand for beach replenishment lies offshore in the open sea. In all cases the supply site should be a sand deposit of sufficient volume and suitable grain size characteristics. A "suitable sand composition" is defined as follows:(Equation Available In Full Paper) Selection of dredging system t and the cost per cubic meter of sand deposited on the beach t are determined to a large extent by the distance between the offshore sand deposit and the sand delivery site. If the distance is not too great t and the dredge is equipped with machinery of sufficient power t it may be possible to pump sand directly through a pipeline from the offshore deposit to the beach.