Port breakwaters and coastal erosion

1-Introduction

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 analyze the effects of the port breakwaters depend on :

• General position of the port (ex : 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).

==2-Analysis of the effects of the port breakwaters on coastal erosion. 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 applie 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. ===2.1-Ports in "isolated environment" (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.

position of the port versus the sea

sea with tide

sea with big tide

I write after sea with tide and current sand coast rock coast