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Coastal Geomorphology of the Forth River Mouth at Turners Beach: Analysis of Recent Sedimentation Changes

Ellison, JC, Arkley, K, Bester, C, Dell, G, Taylor, E, Hannaford, R, Holland, S, Clark, Michelle, Atkins, C, Foster, I and Morrison, BVR 2002 , Coastal Geomorphology of the Forth River Mouth at Turners Beach: Analysis of Recent Sedimentation Changes.

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A coastal geomorphological analysis was conducted at the mouth of the Forth River and Turners Beach to assist the Turners Beach Coastcare Group in understanding recent morphological changes occurring in this area. These changes include erosion of the backshore dunes at the eastern end of Turners Beach, extending to the BBQ hut in the mouth of the Forth estuary, and sediment deposition inside the mouth, both as a shore attached spit and as mid-channel banks. The aims of this study are to establish the sources of the sand deposition at the mouth of the Forth River, and to determine if it derives from Turners Beach, the eroding dunes, or catchment sources.
Two sites were selected for survey, one at the shore attached spit inside the mouth of the Forth, known as the “neck” of the river, and the second at the eroding dune front on the eastern end of Turners Beach. Two transects were surveyed at each location. The purpose of beach profile measurement was to examine beach cross section morphology and beach slope, for comparison with sediment grain size, shape, and mineraology characteristics. Surveys were conducted in a manner to allow future resurvey and evaluation of change. Historical air photographs of the area were analysed to show changes over the last few decades, and compared with an 1882 map.
The paired sand spits in the mouth of the Leven Estuary were not present on the 1882 map, but air photographs from 1946 and 2000 show that they existed throughout this later period. These sand spits moved southward between 1976 and 1984, were smoothed north by a river flood between 1984 and 1989, and developed southwards again more recently. There were limited mid channel sand banks in 1976, but by 1984 deposits extended as far south as the Highway bridge. A river flood cleared the channel between 1984 and 1989, and by 2000 mid-channel sand banks were again well developed. Air photos showed erosion of the foreshore reserve from 1976 to 2000.
There is good evidence that sediment being eroded from the dune face scarps on the eastern end of Turners Beach is being deposited at the spit inside the Forth mouth at the neck. Sediment analysis showed very similar grain size characteristics and distributions, sediment mineralogy and particle shape. This study repeated a previous profile measurement from 1990, showing an 8.3 meter retreat of the seaward scarp of the dune. The dune deposits consist of shingle rich Pleistocene glaciofluvial outwash deposits, hence dune erosion increases shingle deposits on the beach.
Erosion of the backshore dunes at Turners Beach is at too rapid a rate to be explained by sea-level rise in the last few decades. Erosion has been exacerbated by a combination of onshore storm action and human disturbance. At the mouth of the Forth, the southerly development of the neck sand spit and the mid-channel sediment are caused by strong flood (incoming) tidal currents, and reduced effective floods from the Forth catchment. With continued flow characteristics of the Forth river, relocation of the boat ramp upstream towards the highway bridge is recommended. It should be noted that increased beach erosion and estuary mouth widening are expected to occur with future sea-level rise, and any major foreshore construction may in future require expensive maintenance.

Item Type: Report (Technical Report)
Authors/Creators:Ellison, JC and Arkley, K and Bester, C and Dell, G and Taylor, E and Hannaford, R and Holland, S and Clark, Michelle and Atkins, C and Foster, I and Morrison, BVR
Publisher: University of Tasmania
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