BUILDING WITH NATURE including ASPECTS OF DREDGING & COASTAL DEFENSE
When coastal extensions and/or artificial islands are considered, it is essential to apply as much as possible the principle of building with nature. This means using more than before the materials and forces present in nature. The material being the loose mobile material sand, silt and gravel which can be present in the beaches, the dunes and/or the seabed.
The forces acting on this material, being the action of tides (ebb and flood), waves (specifically in the breaker zone), swell, sea currents, river outflow (referring to both sediment and water), gravity & wind and the interaction vegetation-sand (the root-system of grasses, herbs, shrubs, trees, keeping the sand together). Another factor which can be considered is the (complex) interaction between marine organisms and sand/silt/clay particles in beach and nearshore.
On the basis of careful studying the interaction of these materials and the forces acting on them, it is in many cases possible to create new flexible dynamic equilibrium coastlines, in which accretion and erosion are more or less balancing each other, be it that often a maintenance factor has to be taken into account, in the form of a beach nourishment in order to compensate for losses caused by longshore and/or offshore sand-transport. So, it is often possible to use sand from the sea-bed in order to integrate land in sea. Nature has to be helped in this process using certain dredging methods. Often a two-way process is preferred in the sense that land is integrated in the sea, but also water in the new land (creating salt & fresh water lagoons & lakes, and fresh water lenses under the newly acquired dunes). This approach has a number of advantages, it is attractive to live and work near waterfronts, it can be very attractive for creating new nature reserve areas, and fresh water lenses can increase the fresh water supply under the dunes and can be an instrument fighting salt water intrusion at the same time.
Very important is the interaction vegetation-sand. The vegetation has to be carefully chosen and applied and has to be in harmony with the geomorphology, the soil characteristics and the climate of the region.
The sand needed for the coastal extensions of offshore islands should in most cases be obtained from the sea-bed. This should be done in such a way that the disturbance of the marine environment is minimized (and as far as possible causing only temporarily disturbance). Analysis & monitoring of Plankton-Nekton-Benthos and of life in the adjacent terrestrial part of the coastal zone is necessary, before and after the dredging operation. Special attention for food chains and food webs within the ecosystem.
Various types of sea-beds should be taken into account. The presence of corals can be a reason for the imposition of strict conditions on the extraction of sand in certain areas. Living corals can be very sensitive to the deposition of overflowing solid particles. Effects of fishing and especially overfishing should be considered, since their effects can be very serious. Mitigating measures such as the construction of artificial reefs, - if well designed and carefully executed -, can create an interesting environment for marine organisms as has been proven in many cases over many years.
It is also possible to use dredged material in order to create a variety of wildlife habitats, especially aquatic habitats for benthic organisms and fish, bird habitats (upland habitat and nesting islands) and wetlands. Fishery resource improvement can take many forms. For example, bottom relief created by mounds of dredged material may provide refuge habitat for fish. In shallow or inter tidal waters, subject to erosion, mounds composed of fine grain sediments can be stabilized by planting sea grasses or capping with shell or other coarse material. These can also enhance the habitat.
