Beach Renourishment Principles - Beach Renourishment - page 78

those formed of gravel (pebbles or cobbles) last about five times as long as those
of fine sand, which disappears quickly offshore (Roellig 1989 ). Practical beach
experiments were conducted at Tankerton in southeast England to monitor beach
evolution. Five compartments between groynes were filled with significantly dif-
ferent material: locally recycled, very fine, very coarse, an experimental cap and a
standard replenishment mix (Clarke and Brookes 2008 ). The results demonstrated
a continued loss of fine sediment, with over double the amount removed when
compared to the coarser material. However, there is marked reluctance of people
used to sandy beaches to have them replaced by more stable, coarser shingle.
In addition to median grain size, grading can affect the morphology (especially
profile) of the beach, which is related to porosity and permeability (She et al. 2007 ).
Rapid losses have occurred on beaches renourished with unsuitable sediment.
In 1982 the 12 km beach fronting the seaside resort at Ocean City, New Jersey,
was renourished using poorly sorted shelly sand dredged from a nearby tidal delta,
but this was quickly washed away by storm waves (Pilkey and Clayton 1987 ). An
example of the successful use of coarse sediment after finer sediment had been
washed away occurred on the Presque Isle Peninsula on the Pennsylvania shore
of Lake Erie. A beach that had been losing sand was renourished in 1960-1961
by dumping 525,000 m 3 of sand, but this was soon depleted because the fill had a
higher proportion of fine sand than the preceding natural beach. When the beach
was renourished in 1965 with 12,700 m 3 of coarser sand it became more stable
(Berg and Duane 1968 ).
Demands for the restoration of Hampton beach in Victoria, Australia, resulted
in 1975 in the pumping of sand from the floor of nearby Sandringham Harbour
through a pipeline on to the shore. The renourished beach was washed away by
storm waves within a few days, because the sand taken from the harbour floor sand
was much finer than that on the natural beach. This was because the sand that had
drifted into the harbour had previously been sorted by wave action, which had left
coarse sand on the beach and withdrawn the finer fraction to the sea floor, whence
it was transported by nearshore currents into Sandringham Harbour (Bird 1996 ).
Several other beach renourishment projects have shown that if the sand used in
beach renourishment is too fine it will quickly wash away. At Wrightsville, North
Carolina, sand dredged from a nearby estuary was used to renourish the beach five
times between 1939 and 1970. In each case the sand was removed during storms
and deposited offshore (Pearson and Riggs 1981 ), and it was realised that the sand
taken from the estuary was too fine for long-term retention. Renourishment with
coarser sediment in 1972 was more successful.
On the island of Norderney the authorities renourished the beach in 1951
using sand dredged from nearshore shoals. The dredged sediment included a
higher proportion of fine sand than was present on the natural beach, and this was
soon extracted by wave action and washed back out to the nearshore zone (Eitner
and Ragutzki 1994 ). Later renourishment projects have taken account of the fact
that this sorting would occur, and that only the coarser fraction would persist on
the beach, the finer material moving out to maintain nearshore bar formations
(Kunz 1990 ).