Physical Modelling of William Slim Drive Retarding Basin Outlet, ACT

Client: Cardno Young
Year: 2006
Project Reference: 04145
WRL Technical Report: Physical Modelling of William Slim Drive Retarding Basin Outlet, Canberra, ACT (2006/17)

It is planned to construct a PMF retardation basin on Ginninderra Creek floodplain adjacent to William Slim Drive and upstream of the suburb of McKellar. This basin in combination with minor upgrading works of the downstream Lake Ginninderra dam embankment that are being undertaken by Roads ACT; will reduce estimated PMF flows at the dam embankment to “safe levels”. The location of the retarding basin is approximately 8 km north-north-west of Canberra City. Cardno Young engaged the Water Research Laboratory to undertake a physical modelling study of the outlet structure of the retarding basin.

WRL constructed and tested a 1:50 scale hydraulic model of the William Slim Drive retarding basin outlet structure. The initial retarding basin design was formed by an earthfill embankment approximately 650 m long extending across the Ginninderra Creek floodplain at an elevation of 596 m AHD. The outlet for the retarding basin consisted of two Humes 217 arched culverts, each measuring 21 m wide at the base and 7 m high at the centre. Flows exiting the arched culverts were trained by downstream earth fill embankment flume walls, extending for approximately 190 m downstream of the outlet. Subsequent to the hydraulic rating of the initial design, several design modifications were implemented to alter the retarding basin outlet. The most significant design changes included altering the Humes 217 arched culverts to Humes 186 arched culverts (18 m wide at the base and 6 m high in the centre), and reducing the length of the downstream flow training walls from approximately 190 m long to 60 m long.

The physical modelling program undertaken by WRL for this project included detailed analysis of:

The structure rating (upstream head versus discharge)
Air entrainment, vortex formation, and vortex classification
Downstream velocities and flow directions
Downstream energy dissipation measures

The results of the modelling investigation allowed considerable cost reductions for the retarding basin outlet design, and identified required design modifications, in order to provide “safe” downstream flow conditions.