Identification of structures restricting tidal flow in New South Wales, Australia

Abstract

New South Wales is the most populous state in Australia with 80% of its people living 100 km or less from the coast. One legacy of settlement has been the construction of structures that interrupt tidal flow to coastal wetlands. We initiated a four-phase operation to locate these structures. In the first phase, 148 1:25,000 scale topographic maps were examined for six types of structure with the potential to reduce tidal flow: bridges, causeways, culverts, floodgates, fords and weirs. In the second phase, anecdotal information on the presence, impact and suitability for modification of these structures was sought from commercial fishers, oyster farmers and district fisheries officers. In the third phase, field investigations were carried out to confirm the presence of the structures, gain a first-hand impression of rehabilitation potential, and gather site-specific information not available from the maps, such as the diameter of culverts. The fourth phase was a review of the anecdotal and field data by the commercial fishers and conservation managers working for NSW Fisheries. A database was constructed with catchment and sub-catchment data, type of habitat problem, rehabilitation potential and other relevant comments. Over 5,300 structures between mean sea level and the +10-m contour were located on the topographic maps; approximately 2,000 structures were not inspected (1,000 were above tidal limit and another 1,000 were inaccessible within the constraints of the project). Field inspections identified 500 structures that were incorrectly labeled. Another 1,000 structures were identified from sources other than the topographic maps, giving rise to a total of over 4,200 structures with potential for structural modification to rehabilitate estuarine habitat. Of these, 1,388, including 1,035 floodgates, 185 culverts, 91 weirs, 46 causeways, 26 bridges and 5 fords, were considered modifiable. Investigations are continuing to set up a ranking system by which to initiate rehabilitation activities. The results have relevance to habitat managers along the eastern seaboard and elsewhere in Australia, and the method by which the results were obtained may have application elsewhere.