Environmental Engineering Reference
In-Depth Information
The idea of depositing a large quantity of sediment at a selected point on the
beach and allowing wave action to spread it along the shore to renourish beaches
downdrift was tested at San Onofre, California, in the 1980s. A 200,000 m
3
sand
lobe was deposited, and surveys showed that at first there was some beach erosion
downdrift, because the lobe was acting much like a breakwater, but this came to an
end as the sand was distributed alongshore. The apex of the lobe migrated at about
2 m/day, and as it moved the sand lobe diminished rapidly in size, shrinking at the
rate of about 50 % every 300 days, and becoming asymmetrical, attenuated and
narrow as it prograded the beach downdrift (Grove et al.
1987
).
More recently this concept has been applied on a large scale to the Dutch
coast, where 21 million m
3
of sand were deposited in a hook-shaped lobe (Royal
HaskoningDHV
2009
; Stive et al.
2013
). The aim was to produce growth of dunes
and beach in the coastal section between Hoek van Holland and Scheveningen,
increasing coastal protection from flooding as well as potential for habitat creation
and recreation development. This method of design has been termed a 'sand engine',
but this term should be used with caution as it could imply that there is a mechanical
component to the concept. There is in fact no mechanical placement or modifica-
tion of the of beach sediment following the initial placement. The main expectation
of a 'sand engine' is that the placed beach sediment will stabilise the coastline in
its present position and feed sand about 10 km downdrift over an extended period
(20 years) as the emplaced sediment is shaped by waves, wind and currents.
Results of numerical modelling and observed data from the first year were pre-
sented by Stive et al. (
2013
). This included the stormy winter of 2011-2012, when the
shape of the deposited sand peninsula changed considerably (Fig.
4.19
). The maximum
width decreased from 0.96 to 0.84 km, while its length alongshore increased from 2.4
to 3.6 km. The sediment volume at the location of the initial peninsula decreased dur-
ing this first year by about 1.4 million m
3
, while adjacent coastal sectors showed an
increase in sediment volume of 0.9 million m
3
, confirming the longshore distribution.
Renourishment of the beach on the German North Sea island of Sylt began in
1972, using 770,000 m
3
of sand obtained by shallow dredging and deposited as a
large lobe protruding from the shore. This sand was gradually distributed downdrift
by wave action. Progress was monitored on profiles surveyed at 500 m intervals,
which showed that after 5 years more than 60 % of the sand deposited in the lobe
had moved on to the beach downdrift. On the basis of this experience it was decided
that the optimum site for lobe deposition should be a kilometre updrift of the site cho-
sen initially. The project showed that renourishment by means of redistribution from a
deposited lobe was feasible, and that the location of such a lobe should be well updrift
of the sector to receive the beach renourishment. A similar principle guided the dump-
ing of urban rubble on the shore near Odessa to renourish beaches downdrift.
Several beach renourishment projects have used the principle that longshore
drift interrupted by a tidal inlet with strong transverse ebb and flow currents (which
act like a breakwater) can be restored by sealing off the inlet or cutting a new one.
This has been illustrated on Seabrook Island, South Carolina, where beach erosion
became severe when southward drifting of sand was impeded by Captain Sams Inlet,
with interception on the northern (updrift) side. In due course the sand accumulating
