Foreshore Nourishment-More Sand but Lower Costs

摘要

The Dutch people have a long history of creating new land and defending their properties against the sea. A short historical view of the Dutch policy on coastal defense is given. In some more detail the policy of the last decade is highlighted. In 1990 the Dutch government decided to maintain its coastline by means of beach nourishment projects. Since 1990 nearly 300 USS million is spent on replenishing the sandy coast of the Netherlands. In most of these projects conventional beach nourishment methods were used. The conventional method of using midsize trailing suction hopper dredges, which pump the sand ashore through a temporary pipeline is shortly described. Recently a new method of nourishment has been used for coastline maintenance, so called foreshore nourishments. The sand is not placed on the beach but dumped on the foreshore, five meters below sea level. This method will save costs significantly. No pipeline is required and operations can be carried out during the recreational season. Since the sand is dumped using bottom doors instead of pumped ashore, the cycletime of the dredge and consequently the costs decrease. In the paper the research, design, dredging operations, monitoring and current results of some foreshore nourishment projects are described. A significant part of the costs of a contract is related to risk management. Due to the specific conditions in a highly dynamic coastal zone with changing weather conditions, working on a nourishment project is not always possible and sandlosses may occur, causing a risk premium up to 40%. In the period 1997-1999 the Dutch Ministry of Transport and Public Works carried out five foreshore nourishment test projects in which they took all the operational risk. This resulted in a further decrease of costs. The Ministry of Transport and Public works managed the risk by directing the vessels to inland channel maintenance projects during time of bad weather conditions. The paper describes the contractual aspects of projects. Currently the Ministry of Transport and Public Works is preparing a partnering or alliance contract for a nourishment project. The basic idea behind this is an optimal use of the strengths of the parties involved and their ability to manage risks. This may result in flexible long term contracts for an adequate and cost effective coastline maintenance.