On-Line Detention Structures and Fish-Friendly Design Criteria

Abstract

For several years Engeny has been involved in a masterplan flood assessment intended to manage flood runoff within a waterway in a developing urban setting. Flood attenuation can be achieved via two key strategies involving off-line or on-line structures. Off-line structures operate by attenuating peak flow runoff prior to reaching the Lawful Point of Discharge, whilst on-line attenuation is undertaken within the receiving waterway. Typically, smaller developments adopt off-line structures, whilst larger developments are more frequently managed using on-line structures. On-line attenuation strategies introduce new constraints in the form of additional design criteria regarding passage of marine and aquatic life. Waterway barrier works applications are assessed by the Department of Agriculture and Fisheries (DAF) and are typically undertaken at the Operational Works stage of a development application. Engeny has received an existing design which has been approved by Local Council and now needs to be refined to achieve the additional relevant criteria and address the constraints of fish-friendly design for a number of waterway crossings. The design approach for enhancing fish passage will typically contrast with the design approach for impounding water for the purposes of design flood peak attenuation. This presentation/paper will discuss the impact of fish passage requirements for on-line detention structures and the various obstacles when refining approved/existing designs to achieve both flood peak attenuation and fish-friendly design metrics. This paper will also investigate a case study where this has been successfully implemented and approved.