Long breakwaters, series of groynes or detached offshore breakwaters can be used to solve structural erosion problems. The principle of these measures is that they locally affect (reduce) the existing longshore sediment transports and hence change the longshore transport gradients. Note that even for a well-designed protection scheme, lee-side erosion is unavoidable; downdrift of the protection scheme, longshore transport gradients will increase, leading to (additional) erosion. The coastal zone manager has to take these (adverse) consequences fully into account in the decision making process.
The use of seawalls or revetments parallel to the shore, built along the front slope of the dunes or the firm land, does not provide an adequate solution to a structural erosion problem. This is because the cause of the erosion problem, namely longshore transport gradients, is not taken away. Although initially the loss of dunes or land is indeed prevented, the erosion of the beach and underwater profile will continue. The end result may be a complete loss of beaches and damage and failure of the seawall or revetments, exemplified in Fig. 10.21 and further discussed in Sects. 10.6.2 and 10.6.3.
Figure 10.21 shows the cross-shore and longshore damage of an incorrectly applied revetment. While looking to the north here, south of this location a non-periodic, episodic high river discharge event 25 year ago served as a source of sediment for two decades, counteracting sediment losses from Cua Dai beach in northward direction. Around the beginning of the century the provincial authorities allowed resorts to be developed to the south of this location. However, the absence of a new episodic source event eroded the beaches in front of the resorts and the resorts started to act as revetments and/or groynes. Hence, the coast north of the resorts started to erode, so that the beaches – important for tourism – were disappearing. Although this was a problem of alongshore transport gradients causing erosion, it was chosen to halt erosion by geotextile bags, preventing the cross-shore supply of sediment to resolve the long-shore transport erosive gradients. As a result two damages occurred: 1) although the erosion was diminished by the geotextile bags, wave attack on the bags caused toe erosion and damage to the bags stability and the beaches in front disappeared; 2) the geotextile bags were initially successful in preventing further feeding of sediment to the alongshore transports but shifted the alongshore transport gradients further to the north causing erosion.