Gawler 2012-13 Annual Report - page 157

Town of Gawler Annual Report 2012/13
Page 157
introduction • summary • strategic directions •
statutory information
• index
Gawler River Floodplain Management Authority
Annual Report 2012 – 2013
Flood Events
4
th
and 5
th
September 2010
Heavy rains fell across the catchment, up to 70 mm, estimated to be approximately a 1 in 7
year storm event resulted in strong flows in the North Para River. Water began to rise behind
the Bruce Eastick North Para Flood Mitigation Dam on Saturday and at 10.00am on Sunday,
reached a maximum height of 70 metres AHD, just below the high-level outlet pipes. Dam
waters estimated to be 1.2 Gl extended approximately four kilometres upstream, passed
through the low level outlet pipe at a controlled flow, returning to stream level at midday the
following day. The real time catchment rainfall, river flow data and height of water in the dam
were able to be monitored on the Bureau of Meteorology website.
Both the effect of the dam, and filling the South Para Reservoir, mitigated the flow in the
Gawler River that caused flooding of grazing land just east and west of the new Port
Wakefield Highway. No property damage was reported.
8
th
December 2010
Following intense storms in the early hours where several centres recorded 24 hour falls of
70 to 80 mm, that gave rise to strong flows in the North Para River that caused the Bruce
Eastick North Para Flood Mitigation Dam to operate for the second time in four months.
Water rose to 72.5 m, which caused the two High Level Outlet Pipes to operate for the first
time. Dam waters estimated to be 1.8 Gl extended over 4 kilometres upstream passed
through the low level outlet pipe at a controlled flow returning to stream level by late the
following afternoon.
Had the Bruce Eastick North Para Flood Mitigation Dam not been in place it is estimated that
a breakout just west of Bakers Road to the north would have occurred, flooding a number of
horticultural properties.
These events confirmed that the dam operated as designed and provided protection by
mitigating the peak flow to a level that could be passed by the lower Gawler River, without
overtopping the banks causing flooding as was the case in 2005 and 1992.
Minor Works
A replacement Turretfield Flow Gauge Weir has been constructed 800 metres downstream
from the dam on the North Para River. The former flow gauge weir just upstream of the dam
wall is drowned out in a flood event by the dam waters and has been decommissioned. The
replacement weir is smaller in design, gauging flows up to approximately 300 cubic metres
per second. This gauge weir downstream of the junction with Walker Creek provides flow
data from the western part of the catchment. Data from this flow gauge will be added to the
online real time data on the Bureau of Meteorology web site.
Two additional survey monuments have been constructed at the dam site, to provide a total
of four monuments to provide a survey base to monitor the deformation stability of the dam
wall and spillway. Part of the maintenance program is to periodically survey the structure to
ensure that there is no movement. In particular this is performed after major flood or seismic
events.
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