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New South Wales Government
Both natural and trained coastal entrances can create a variety of hazards. Natural entrances tend to migrate along the beach in response to freshwater flooding and coastal storm effects, so threatening any adjacent developments and the amenity of affected beaches. Training works will stabilise the location of an entrance, but may engender significant changes to the estuary and nearby beaches. Some training works in New South Wales have had detrimental effects of an unforeseen nature.
The location of untrained coastal entrances can vary markedly in response to freshwater floods and coastal storm conditions. This is illustrated in Figure C4.1, which shows breakthrough of an entrance spit caused by river erosion. Similar behaviour on a smaller scale is readily evident at many beaches, where stormwater discharges break through the normally closed entrances of creeks and lagoons.
Narrow low sand spits separating an estuary from coastal waters are vulnerable to breakthrough. This is especially so when the hinterland catchment has a rapid rainfall response time, which ensures high freshwater discharges of maximum erosion potential.
Whilst entrances to major estuaries can remain stable for many years, major floods can cause breakthrough at unexpected locations. This is illustrated in Figure C4.2, which shows the breakthrough at the Shoalhaven River entrance that occurred in 1988.
The breakthrough of low sand spits can also occur from the ocean. This was dramatically illustrated in the storm of 1974, when breakthrough and wave runup on a spit of the North Arm of the Brunswick River virtually destroyed the village of Sheltering Palms (PWD, 1978). In extreme cases, the entire spit can be destroyed by breakthrough in a single storm event, as occurred at Myall Point, Port Stephens, in 1927 (PWD, 1987).
Even where an inlet is stabilised by training works, entrance scour due to major floods can have considerable short and long term impacts on the estuary and adjacent shorelines. Flood scour can damage structures built in the active scour zone, as was illustrated dramatically in 1971 by the loss of Hancock Bridge which spanned the Bega River Entrance (see Figure C4.3). Other effects of entrance scour include improved estuary flushing, a drop in water levels along the estuary, the exposure of previously submerged estuarine fringes, and improved navigability (Williams, 1981).
Figure C4.3b Bega River Entrance After the 1971 Flood.
Infilling and Closure
Entrance infilling refers to the buildup of sediment in coastal entrances. Under certain conditions, the entrance may be completely closed (this generally occurs only for small creeks and lagoons). Entrance infilling can restrict tidal flows and navigable access, reduce flushing and foster nutrient buildup and associated algal growth in the estuary. In addition, infilling provides a mechanism for the loss of sand from the beach compartment.
Infilling at the Bega River entrance, after scouring of the entrance during the 1971 flood, caused general erosion at nearby beaches (see Figure C4.3). In this case, beaches eroded as sand was lost to the scour "sink" (PWD, 1980).
Training works at coastal entrances can markedly change the hydraulic behaviour of the estuary: the tidal range, water levels, current speeds, flushing behaviour and salinity levels all may be altered. In addition, training works may reduce or prolong freshwater flooding and can significantly alter waterborne sediment transport processes, both within the estuary and along nearby beaches.
The hazards of training works are demonstrated by the impact of the northern breakwater used to train the entrance to Wallis Lake at Forster/Tuncurry. Figures C4.4 shows photographs of the entrance and estuary prior and subsequent to the construction of the breakwater, which was constructed in 1966. The presence of the breakwater led to a number of major changes in the estuarine/beach system. First, significant and rapid scour of the entrance bar and shoals occurred. This threatened the foundations of the road bridge resulting in expensive remedial works to the bridge abutments. Second, there was massive sand relocation from the estuary to the margins of adjacent beaches. Finally, the tidal range of the estuary increased threefold (Nielsen & Gordon, 1980).
Figure C4.4b Wallis Lake Entrance, 1974.
These changes are still occurring and it is estimated that it will take 50 years for the estuary to adjust to the altered entrance conditions.
Entrance breakwaters or training walls that project seawards may interfere with littoral transport along the coastline. Accretion may occur on updrift beaches or within estuary deltas with a resulting reduction in sand supply to down drift beaches (see Figure C4.5).
Estuaries with multiple entrances may also be sensitive to entrance works. Two examples serve to illustrate this effect.
First, the training of one entrance may cause another to close, such as occurred on the Macleay River at Grassy Head.
Second, the interconnection of entrances may result in a substantial redistribution of flows, as has occurred between the Crookhaven and Shoalhaven Rivers. To use the more navigable entrance of the Crookhaven River, Alexander Berry organised the cutting of a small canal between the Crookhaven and Shoalhaven estuaries in 1822. The original canal was 6m wide and 200m long. Today, that canal is some 200m wide and 16m deep, and has resulted in a 50% division of flood flows between both river systems, i.e. 50% of the floodwaters in the Shoalhaven River pass out through the Crookhaven entrance. The enlargement of the original canal was caused by natural scour and by dredging works last century (PWD, 1977).
Detailed studies are required to assess the effects of training works on coastal processes and the nearby coastal zone. A sediment budget analysis is required to assess the significance of all forms of waterborne sediment transport. Detailed measurements of estuary and entrance currents may be required. Computer models can be used to investigate likely response to changed entrance conditions. Physical models may also be required to investigate changed erosion and accretion patterns.
Nielsen, A.F. and Gordon, A.D., (1980). "Tidal Inlet Behavioural Analysis". 17th International Conference on Coastal Engineering, Sydney, March 1980, ASCE.
PWD, (1977). "Engineering Studies of Shoalhaven/Crookhaven Estuary". Review undertaken by Coastal Branch, Public Works Department of New South Wales, Report No. PWD 77026, September 1977.
PWD, (1978). "Byron Bay/Hastings Point Erosion Study". Report prepared by Coastal Engineering Branch, Public Works Department of New South Wales, Report No. PWD 78026. November 1978. ISBN 7240-2691-6.
PWD, (1980). "Tathra Erosion Study". Report prepared by Coastal Engineering Branch, Public Works Department of New South Wales, Report No. PWD 79015, February 1980.
PWD, (1987). "Jimmys Beach Erosion Study". Report prepared by Coastal Engineering Branch, Public Works Department of New South Wales, Report No. PWD 85042. February 1987. ISBN 7240-2964-8,
Williams, G., (1981). "Investigation and Management of Estuarine Lake Inlets". First Australian Conference on Hydraulics in Civil Engineering, Sydney, October 1981. I.E.Aust.