The catchment of the Georges River is extensive and covers\r almost 1000 square kilometres. Although major floods\r generally result from prolonged heavy rainfall in the\r upper part of the catchment there is generally little\r advance warning of major floods.\r \r In fact, warning of the transition of a flood from\r nuisance status to a major 1 in 100 year event, could be\r typically of the order of only 3 to 4 hours. As a\r consequence there would be only limited opportunity for\r effective evacuation of the Moorebank-Milperra floodway, a\r situation which is compounded by the fact that road access\r into the area is cut at an early stage of a flood.\r \r Major flooding has posed a potential threat to development\r adjacent to the Georges River since the earliest days of\r settlement. A major flood was recorded in 1873 and\r several other large floods occurred prior to the turn of\r the century. However, the river has been relatively flood\r free throughout the twentieth century with the only\r significant flood occurring in 1956. Since then only\r minor floods in 1961, 1964, and 1978 have been\r experienced.\r \r This report addresses the impacts of the 20, 50 and 100\r year floods on the Milperra floodway. These impacts are\r presented primarily in terms of depths of inundation and\r flood velocities through the residential area. Table 1\r compares depths for these events with those which would\r have been experienced during the floods of 1873, 1875,\r 1889, 1956, 1961, 1964 and 1978.

This report details the flood gauging operations undertaken\r from 1700 hrs on March 21st, 1983 to 0200 hrs on March 22nd,\r 1983 and presents previous flood data collected on the\r Georges River.\r \r The Georges River has a total catchment area of\r approximately 650 square kilometres and flows into the\r south-western corner of Botany Bay about 15km south of\r Sydney. The tidal influence under low freshwater discharge\r conditions extends approximately 40km upstream and\r terminates at Liverpool Weir.\r \r Flood monitoring at selected sites as defined in Figure 1 is\r undertaken for floods in excess of a one in ten year\r frequency. Flood monitoring is commenced on receipt of\r Confidential/Preliminary Flood Warning advice when it is\r predicted that a stage level of 2.0 meters at the Liverpool\r Weir gauge (approximately 4.7m AHD) will be exceeded.\r \r The collection of this flood data has in the past and will\r in the future be used to calibrate physical modeIs which are\r constructed at the PWD Manly Hydraulics Laboratory and\r U.N.S.W. Water Research Laboratory.\r \r The current metering- lines, flood gauge location and\r automatic recorder locations are as follows:\r \r i) Current metering/flood level observations:\r \r 1. East Hills Footbridge\r 2. Milperra Road Bridge\r 3. Rabaul Road (gauge readings only)\r 4. Lansdowne Bridge - Prospect Creek\r 5. Irelands Bridge - Cabramatta Creek\r 6. Mountbatten Bridge - Warwick Farm\r \r ii) Automatic recorders are located:\r \r 1. Milperra Bridge (see note below)\r 2. Lansdowne Bridge\r 3. Cutler Road, Lansvale\r 4. Scrivener Street, Warwick Farm\r 5. Liverpool weir (upstream)\r \r Note on Milperra - The Milperra automatic recorder is located\r immediately upstream of the road bridge. It\r consisted of two measurement points, however only\r the downstream one was operating at the time of\r the monitoring. The recorder has since been modified and now has only one measurement point.\r \r

This report details the flood gauging operation undertaken on 5th and 6th August, 1986 and presents the data collected for the Georges River catchment in Sydney.

The Georges River catchment area is approximately 960 square kilometres and is shown in Figure 2.1. The main arm of the river begins its course near the Cataract Dam, and exits into Botany Bay. The main channel is joined throughout its course by minor streams and creeks, and also by the Woronora River; which exits into the Georges River.

The catchment area comprises a large part of the south western urban area of the city of Sydney, and the floods that have occurred in the catchment have at times caused significant damage to property.

To rationalise future development of the area in the face of potential flood threats, various numerical and physical models have been developed by both the University of N.S.W. Water Research Laboratory, and the Public Works Department's Manly Hydraulics Laboratory. The success of the models in predicting the flooding characteristics of various flood events depend upon collecting specific flood data over a series of floods· for model calibration and verification. To this end, all known information regarding previous floods has been documented (Ref. 1), but there is a need for more comprehensive flood data, especially for major floods.

Manly Hydraulics Laboratory's Data Collection Section was requested by the Department's Rivers and Ports Branch to co-ordinate and undertake flood monitoring at various sites over the area which is being modelled. These monitoring sites appear in Figure 1.1. A previous report (Ref. 1) details the results of gauging of minor floods undertaken in 1978 and 1983.

Flood gauging on the 5th and 6th August, 1986 was carried out by personnel from Manly Hydraulics Laboratory, Rivers and ports Branch and the Water Research Laboratory.

The general extent of the flooding can be seen in Plates 1.1 - 1.3.

The current metering lines, flood gauge locations and automatic recorder locations from which data was collected are as follows:

i) Current metering/gauge observations at:

1. East Hills Footbridge
2. Milperra Road Bridge
3. Mountbatten Bridge
4. Lansdowne Bridge

ii) Automatic recorders located at:

1. Milperra Bridge
2. Cutler Road, Lansvale
3. Liverpool. Weir.

Data collection from the Irelands Bridge current metering station and the Scrivener Street auto recorder is also specified under the Georges river flood data collection program, but inaccessibility to the bridge and equipment malfunction respectively, precluded data collection from these sites on this occasion.

The lower Georges River Valley is subject to widespread flooding from time to time. The Moorebank-Milperra area, which is centred around Milperra Bridge, has been identified as a major floodway hazard zone due to substantial residential development on floodprone land. Any restriction to the passage of floodwaters placed downstream of Milperra Bridge is likely to create an afflux (i.e. an increase in flood levels) in the Moorebank-Milperra area. The Department of Main Roads was therefore concerned as to the effects of its proposed South-Western Freeway crossing of the Georges River 3 km downstream of Milperra Bridge. The Public Works Department's Manly Hydraulics Laboratory was engaged to construct and test a physical hydraulic flood model of the area in order to determine the afflux caused by the bridge structure during a 1 in 100 year flood. This report provides details of the investigation and gives model test results and conclusions.