IV. Retreating Shorelines
241
This is actually similar to what has been found so far, but more details have
yet to be worked out. In reality, grain sizes will not become that
differentiated because storms are composed of a spectrum of waves, not
regular waves, and storms vary in intensity. Each storm tries to produce its
own grain size distribution, as indicated in table 3.
The distribution in a less severe and in a severe storm may be as shown
below for three different depths (table 4a).
l Sev°r°
l °4ram
1 °3mm
t °2ram
I
Less severe
0.3 mm ......
0:22 mm
0.15 mm
Table 4a. : Distribution of grain sizes in function of storm severity.
In addition, small storms will carry the smaller grain sizes out in less deep
water for temporary deposition. This results in that bottom material in
values which become somewhat mixed and is what is actually found, e.g. in
Iceland. The ultimate result is still a slow migration of all grain sizes away
from land until the single grain sizes come to a 'test" Silt and clay sizes
will, however, continue their transfer to waters deep enough to make
settling possible, if ocean currents do not interfere with the process. Grain
sizes, therefore tend to become a little smaller than predicted by the ratioprocedure ° and no wonder : samples were all taken during the summer
season, when the coarser grains move shoreward, while the finer materials
tend to move oceanward to deposit in the calmer offshore waters. This
decreases the mean diameter and the sorting of the offshore material. In
practical coastal-engineering planning, we must use the winter grain sizes
corresponding to maximum exposure as design criteria, because they
determine the erosion pattern (Bruun, P., 1988, Profile nourishment : its
background and economic advantages : J. Coast. Res. 4.2, 218-228).
Dumping material on the beach is probably not the most economic
nourishment procedure. Profile-dumping with material of selected grain
sizes, depending on the availability of material, will undoubtedly prove to be
more practical (and of course more economical) as it provides a higher
degree of stability from the beginning 33.
3.1.7
Supplying the nourishment material
Mention has been made earlier of various approaches in securing the
nourishment material. Such material may come from land sites but offshore
dredging is perhaps currently the most commonly used method.
241
This is actually similar to what has been found so far, but more details have
yet to be worked out. In reality, grain sizes will not become that
differentiated because storms are composed of a spectrum of waves, not
regular waves, and storms vary in intensity. Each storm tries to produce its
own grain size distribution, as indicated in table 3.
The distribution in a less severe and in a severe storm may be as shown
below for three different depths (table 4a).
l Sev°r°
l °4ram
1 °3mm
t °2ram
I
Less severe
0.3 mm ......
0:22 mm
0.15 mm
Table 4a. : Distribution of grain sizes in function of storm severity.
In addition, small storms will carry the smaller grain sizes out in less deep
water for temporary deposition. This results in that bottom material in
values which become somewhat mixed and is what is actually found, e.g. in
Iceland. The ultimate result is still a slow migration of all grain sizes away
from land until the single grain sizes come to a 'test" Silt and clay sizes
will, however, continue their transfer to waters deep enough to make
settling possible, if ocean currents do not interfere with the process. Grain
sizes, therefore tend to become a little smaller than predicted by the ratioprocedure ° and no wonder : samples were all taken during the summer
season, when the coarser grains move shoreward, while the finer materials
tend to move oceanward to deposit in the calmer offshore waters. This
decreases the mean diameter and the sorting of the offshore material. In
practical coastal-engineering planning, we must use the winter grain sizes
corresponding to maximum exposure as design criteria, because they
determine the erosion pattern (Bruun, P., 1988, Profile nourishment : its
background and economic advantages : J. Coast. Res. 4.2, 218-228).
Dumping material on the beach is probably not the most economic
nourishment procedure. Profile-dumping with material of selected grain
sizes, depending on the availability of material, will undoubtedly prove to be
more practical (and of course more economical) as it provides a higher
degree of stability from the beginning 33.
3.1.7
Supplying the nourishment material
Mention has been made earlier of various approaches in securing the
nourishment material. Such material may come from land sites but offshore
dredging is perhaps currently the most commonly used method.