Environmental Engineering Reference
In-Depth Information
beach renourishment projects should be regarded as experimental, with improve-
ments based on experience gained from continued monitoring.
Mapping of changes on a renourished beach can determine patterns of move-
ment alongshore. In 1963 sand was dumped on the shore near Absecon Inlet, north-
east of Atlantic City, New Jersey. Repeated mapping showed that the beach fill was
shaped into a lobe that migrated south-westward along the shore at 2-3 m/day.
This was a smaller version of the 'sand engine' technique described on Sect.
4.3.2
, p. 62, 63. After 2 years this lobe arrived to augment the beach in the vicinity
of the main pier, but it continued to move along the shore in front of the board-
walk, and then beyond, so that the widening of the Atlantic City beach was only
temporary. The response in 1970 was to add a further 596,000 m
3
of sand near
Absecon Inlet, and this also moved alongshore to the Atlantic City pier, where the
beach was widened in 1972, and moved on south-westward (Everts et al.
1974
). It
was then clear that on this longshore drift-dominated coast a beach could only be
maintained at Atlantic City by frequent small injections of sand at the north-east-
ern end (Pilkey and Clayton
1987
). Groynes were placed on the shore to reduce
the rate of drifting to the south-west in an attempt to keep the beach at Atlantic
City (Weggel and Sorensen
1991
).
Monitoring of renourished beaches has generally been restricted to the
emplaced sector, to decide when and where further renourishment is necessary, but
there should also be mapping and monitoring of changes on adjacent shore and
nearshore areas to which eroded sediment may move. There is a risk that sediment
from renourished beaches will drift alongshore and accumulate in boat harbours,
or as shoals impeding navigation at the mouths of rivers and creeks. On the other
hand, it may prove beneficial in renourishing other beaches along the coast.
Monitoring can determine quantities of sediment lost from a beach. At Kirra, at
the southern end of the Gold Coast in Queensland, Australia, over 5 million m
3
of
sand was dumped on the beach in several phases between 1985 and 1990. A sur-
vey in May 1992 showed that 87 % of this renourished sediment was still on this
beach or in the nearshore region, the remainder having drifted alongshore to aug-
ment beaches to the north (Delft Hydraulics Laboratory
1992
).
Decisions on when and where a renourished beach should be further replen-
ished, and how much beach fill is required, can be made in terms of information
from such mapping and monitoring. There were rapid changes after renourishment
on the beach at Wrightsville, North Carolina, monitoring of 50 transverse profiles
showing rapid initial losses, some 66 % of the renourished sediment being lost
within the first year. The erosion rate slackened as the beach profile, originally a
terrace with a convex seaward slope, became concave, and there was then more
gradual recession, the beach maintaining a more or less constant seaward slope.
The initial rate of erosion on the renourished beach was ten times that of the pre-
ceding natural beach erosion (Pilkey and Clayton
1987
), but the loss rate on the
renourished beach declined to the long-term natural erosion rate after 8 years. This
was the effective residence time of the beach fill, and indicated that if the renour-
ished beach were to be maintained here it would need to be renourished at inter-
vals of about 5 years.
