Weir design set to improve safety

17/05/2019 Posted by admin

The design of works to reduce the dangers of the South Dubbo weir is “proceeding satisfactorily”, a Dubbo City Council director reports.

Stewart McLeod provided the update this week nine months after the council committed to safety improvements of the site where two teenagers have died since 2008.

At the time of the resolution made in March the construction of a rockfill ramp was considered likely to trigger state legislation requiring a fishway, so a fishway was also included in the project, taking the costs from an estimated $2 million to $4 million.

This week Mr McLeod said in a report that a less costly fishway design had emerged as the preferred option – but not because of price.

The rock ramp fishway option scored the highest in an evaluation against a number of criteria that did not include financial outlay, Mr McLeod, council technical services director, said in a report to the works and services committee.

But it also “represented the least constructions costs” while a previously identified option “represented the greatest cost”, he said.

In the report on the agenda for the committee last night, Mr McLeod said a design workshop held on November 14 had evaluated five alternative options for fishways.

Previous reports had identified a vertical slot fishway as preferred, but this was before the council resolved to construct a rock ramp downstream of the existing weir crest for public safety reasons, Mr McLeod said.

The designs were evaluated in the areas of functionality, operation, environmental issues and risk, the director said.

A rock ramp fishway with lateral ridges was considered the best by the workshop, scoring 240 points, two more points than a rock ramp fishway with random rock placement, while a vertical slot fishway only achieved a score of 203.

“The cost of each option was also estimated, but only for the purpose of differentiating between options,” Mr McLeod said.

“It was considered the rock ramp fishway options represented the least construction costs while the vertical slot fishway represented the greatest cost.

“As the rock ramp fishway is the best and least expensive option, it has now been adopted as the preferred option.

“This option will see a series of pools down the sloping surface of the rock ramp.

“These will be designed so that fish can swim from pool to pool and pass upstream.”

Mr McLeod said a choice between the two types of rock ramp fishway could be made later during the detailed design phase.

A budget allocation of $4 million for construction was expected to be sufficient for the new preferred option, but a detailed estimate would be prepared after detailed design and before going to tender, he said.

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