Complexities associated with constructing a high fishway: the Tallowa Dam experience.
The Sydney Catchment Authority and NSW Fisheries are conducting a study to investigate and ameliorate the impacts of Tallowa Dam in fish communities in the Shoalhaven River. Studies to date have shown that fish communities upstream and downstream of the dam, which were once largely continuous, now differ significantly. Large numbers of migratory fish now accumulate immediately downstream of the dam, and ten species are believed to be locally extinct upstream of the dam.
An investigation of the broad options available to help restore these communities concluded that the construction of a high fishway is the most appropriate option available. A detailed feasibility study is now underway to examine possible fishway designs and determine the viability of constructing and operating a fishway at Tallowa Dam, which is capable of restoring fish communities in the Shoalhaven River. The feasibility study will provide important information for future studies into high fishways at other dams across Australia.
A number of significant challenges need to be considered in the design of a suitable high fishway at Tallowa Dam, particularly as no fishway of a similar height has ever been built in Australia. Any fishway constructed at Tallowa Dam will need to ensure that the safety of the dam and its operational performance is not compromised. The fishway will also need to consider other works and activities at the dam, including the release of environmental flows and the potential for hydro-electric power generation.
To be effective in rehabilitating upstream fish communities, the fishway needs to accommodate all species attempting to migrate past the dam, ranging in size from 11 mm for juvenile gudgeons to over 1000 mm for large eels. For practical purposes, the fishway should operate in 95% of flows and be able to withstand flood events. The location and design of the fishway entrance are critical for fish to find and enter the fishway. Protection from predators may also be an issue considering the large size range of fish potentially using the fishway. The existing structure of Tallowa Dam presents a complex flow environment, with multiple attractant flows of differing temperatures from the spillway and outlet valve. This creates challenges in identifying the best location for the fishway entrance to ensure maximum attraction for all species.