Port breakwaters and coastal erosion

This article describes the impacts of breakwaters for three different types of ports on coastal erosion. The discussed ports are examples of ports in an "isolated environment", with a rivermouth in the sea and with the mouth of a large estuary.

Assessment framework

The analysis of the effects of the port breakwaters on coastal erosion cannot be approached without replacing the structure in its whole environment (Morphogic, Hydraulic, Lithologic).

In addition it must be approached thinking of the relationship between the "vulnerability" and the "risk" (if one is invulnerable one does not risk anything). The risk is related mainly hydraulics which is the erosive power (wave, tide currents) and a little to morphology. The vulnerability is associated to morphodynamics and lithology. Consequently the parameters to be taken into account to analyse the effects of the port breakwaters depend on :

• General position of the port (e.g. downstream estuary or not).
• Nature of the breakwaters, their positioning and their orientation (compared to the coast and compared to the directions of incident wave).
• Nature of the sea (with or without tide and tide current).
• Nature of the incident waves (main directions and intensity).
• Morphology and lithology of the zone influenced by the work (beaches or cliffs, sandy or rocky coasts).

The variability of the parameters is large and a port breakwater is practically always a prototype, from which one have great difficulty to determine general conclusions which apply in all cases. However, if we restrain to some specific standard cases, it is possible to apprehend the principal effects caused by the port breakwaters.

Analysis of the effects of the port breakwaters on coastal erosion

Three different types of ports are analyzed:

• in an "isolated environment"
• with a rivermouth in the sea
• with the mouth of a large estuary

Ports in an isolated environment

Ports in isolated environment include sea without tide and any, or very little, river contribution in the basins. In this case the breakwater are placed perpendicularly to the direction of propagation of the incidental waves which diffract (pass round the obstacle) on the pierhead. This phenomenon deviates the wave directions of attack of the waves at the coast, breaking an established balance and generating a more or less important erosion according to the lithological structure of the coast.

• If the coast is rocky, erosion is negligible.
• If the coast is sandy, erosion is important while remaining limited in space if there is no coastal current.

On the other hand, if the phenomena of erosion can remain limited, it is not the same of those related to pollution due to the harbour traffic, because the basins constitute structures relatively closed with any or little exchanges with the open sea.

Port with a rivermouth in the sea

Ports located at the mouth of a river in a sea are subject to tides and coastal currents. In this case, the breakwater of protection against the incidental waves should not impede the river flow. So they are built in order to prolong the bed of this one. The entry of the port therefore consists of two parallel breakwaters generally built perpendicularly to the coast. This structure induces the following phenomena:

Figure 1: Processes for a port with a rivermouth in the sea

• Even for no frontal waves, but because of the phenomenon of diffraction, these parallel breakwaters constitute a wave guide and by heavy weather a difficulty for the entry of the boats in the port. On the other hand, this same phenomenon of refraction attenuates the wave within the basins located at the bottom of this channel,
• These parallel breakwaters built perpendicularly to the coast:
• Generate a phenomenon of refraction which modifies the angle of attack of the waves on the coast downstream the work (see the former sub-section)
• Represent an obstacle to coastal sediment transfer.
• This last point is very important with regard to coastal erosion. Indeed, in the seas with tide and coastal current, the stability of the coast is due to a balance between erosion by transport of the sediments towards the downstream and fattening by contribution of sediments from the upstream. The construction of breakwaters perpendicularly at the coast blocks this coastal sedimentary transport and breaks this balance.
• Consequently, by refraction around the work, the coast is attacked hard by the waves downstream it, and there is a greater erosion due to the fact that the sediments transported downstream by the coastal current are not compensated by a contribution upstream, this one being blocked by the breakwater. For example, if the coast is made up of cliffs, the fact of destroying the estran of shingles at the feet of those (transport without recharging) induces a direct attack of the waves against the feet of the cliffs which are all the more fragile since the impact of the waves in the cracks of those deteriorates their in-depth structure.

Port with the mouth of a large estuary

Here, the important problem is not so much the stability of the feature of coast, but the safeguarding of the mudflat area of the estuary. Indeed, in the absence of human intervention, the pail part of an estuary generally constitutes a wetland of great ecological importance (zone of reproduction of species of all kinds and zone of migratory stages). See also Figure 2 and 3.

• By contribution of the sediments of the river these wetlands or mudflat areas migrate permanently towards the downstream.
• However to protect themselves against the silting up of their basins, the ports build breakwater which, in fact, canalise water of the river preventing any sedimentation and therefore any migration of the mudflat areas towards the downstream, whereas they continue to be filled upstream. It follows from there a disappearance of these wetlands however essential from the ecological point of view.
• Recently, the various partners of the management of the coastal zones became aware of this problem. That is why, for example, that at the time of the enlarging of the port of Le Havre (France) at the mouth of the Seine, compensatory solutions were elaborate in dialogue in order to preserve these wetlands without penalizing the economic development (Program Port 2000).

Figure 2: Temporal evolution without human intervention

Figure 3: Influence of port breakwaters

 

References

Pilarczyk K. and Zeidler R. Offshore breakwaters and shore evolution control.Balkema Edition

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