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.
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
Coastalwiki
