10.7.5: Beach widening and creation

Increasing urbanization of coastal areas often conflicts with natural shoreline fluctuations. An example is the Mediterranean, where overdevelopment often leads to a need to widen the beaches by nourishments, which in principle is a once-only measure. Shoreface nourishments mainly contribute to the sediment balance of the active surf zone, but are not very efficient for immediate beach widening for recreational purposes. Beach nourishments are about twice as expensive as shoreface nourishments but directly benefit the beach. Lifetimes of beach fills can be extended by using coarser sediments.

More far-reaching is the situation that a coast has to be extended for a considerable distance (say in the order of magnitude of \(2\ km\) in the cross-shore direction and over \(20\ km\) in the alongshore direction). It is assumed that no serious structural erosion occurs in the existing situation. The area is intensively used as beach recreation area. One of the requirements is that after extension of the coast, recreation beaches are again available. This means that protection of the newly reclaimed area by a dike or revetment is not an acceptable option.

A more-or-less zero option for land reclamation would be an entire shift of the cross-shore profile of \(2\ km\) in the seaward direction. This holds not only for the waterline, but in principle also for all other depth contours to a water depth where natural adaptations of the profile are hardly to be expected (say \(15\ m\) below MSL). In order to achieve a \(2\ km\) shift of the coastline, per running metre along the shore \(2000\ m \times 20\ m = 40000\ m^3/m\) is required. The factor \(20\ m\) in the calculation is found by assuming a lower limit of MSL−\(15\ m\) and an upper limit above MSL of \(5\ m\). With an alongshore extension over \(20\ km\), the total required volume of sediment is 800 million \(m^3\), which means a very large project.

Figure 10.38: Cross-shore profiles in the case of large-scale land reclamation. The shaded volume is saved if the choice is made for a ‘perched’ beach supported by a submerged breakwater.

With the zero option, the new coast has a foundation that is identical to the old coast. A large part of the calculated volume is needed to make the new foundation. In order to restrict the volume of sediment needed for reclamation in the zero option, one could consider an alternative. For example, the upper part of the cross-shore profile after reclamation is ‘supported’ with the help of a submerged breakwater (Fig. 10.38). The lower end of the profile can then be left as it was; a large reduction of the volume of sand is achieved in this way. This solution is called a ‘perched beach’.

Figure 10.39: Application of structures to confine sand.

Figure 10.40: Submerged breakwater serving to perch the beach nourishment at the Pellestrina barrier island, in Venice, Italy. On the left side the Venetian Lagoon and on the right side the Adriatic Sea. Photo by Ni_Giri

Figure 10.41: Real-life example: tombolos behind emerged (i.e. having their crests above MSL) offshore breakwaters in Malaga, Spain. Photo from SCNE

To prevent large alongshore losses, groynes can be applied. The seaward limit of the reclaimed area can also be moulded with series of detached breakwaters to confine sediment (Figs. 10.39 to 10.41). The design of this type of reclamation should include a thorough analysis of the coastal processes and impacts.