Planning Considerations - Beach Renourishment - page 108

damaging, but ecological recovery is much quicker on shallow excavations. This
was illustrated during beach renourishment on the island of Sylt (Sect. 4.3.2 ,
p. 62), when sand was excavated from the nearshore sea floor by a hopper dredge,
which cut to a depth of about 2 m and disturbed large areas. Deeper dredging from
an anchored hopper could restrict disturbance to a much smaller area by cutting to
a depth of 40 m, but it was decided that deep excavations could have more severe
adverse impacts on wave processes and marine ecosystems (Dette 1990 ).
When sea floor sources of sand were sought for the renourishment of Mentone
Beach, Port Phillip Bay, Australia, in 1976 (Sect. 4.2.7 , p. 49) there were fears that
the dredging of sand would have adverse effects on sea floor ecosystems, notably
seagrass communities, a habitat for fish and shellfish. However there was reassur-
ance from Watson ( 1973 ), who had found that excavation of a trench 5 m wide
and up to 3 m deep to carry a gas pipeline across the floor of Port Phillip Bay in
1972 had caused only temporary depletion of benthic organisms, and may even
have enriched the local fishery. In the event, the replenishment of Mentone beach
does not appear to have adversely affected the sea floor ecosystems in Port Phillip
Bay, although corrosion of the pipeline may yet pose a problem.
Dredging of sediment from the sea floor may release toxic chemicals. At Bogue
Banks, North Carolina, sediment dredged from a harbour was found to be laden
with hydrogen sulphide, which caused much intertidal and nearshore turbidity,
modifying the habitat and killing many invertebrates. The sea floor ecosystem
began to recover only slowly after beach dumping ended (Reilly and Bellis 1983 ).
Monitoring of sea floor plant and animal communities has shown that many
gradually recover after dredging has ceased. In Florida surveys showed good
recovery of sea floor biota 5 years after the dredging areas off Hillsboro Beach
(Marsh and Turbeville 1981 ) and in the Tyrrhenian Sea, Italy, analysis of the
effects of dredging on macrobenthic fauna demonstrated that although the rich-
ness and diversity of species was reduced during dredging re-colonisation could
be observed a few months after dredging ceased (La Porta et al. 2009 ). This study
also found that re-colonisation was more rapid for areas after one dredging, com-
pared with areas after two, agreeing with other studies (Cooper et al. 2007 ) that
the re-colonisation of benthic assemblages is related to the intensity of dredging.
Sea floor ecosystems may also be damaged by burial or increased turbidity
when sediment dredged from harbours or harbour approaches is dumped offshore.
It may be better to use dredged sediment for beach renourishment or land rec-
lamation instead of dumping it offshore, where it can have ecologically adverse
impacts on the vegetation that sustains the sea floor fauna, including fisheries.
6.1.2 Impacts During Transportation
Sediment dredged from the sea floor has to be transported to the shore, either in
pipes or on boats. Leakages from pipes during pumping or losses as boats are
loaded, navigated and unloaded can cause turbidity in the sea, the coarser gravel