The Tuggerah Lakes flood study was undertaken to determine flood behaviour for 1%, 5%, 20% and 50% annual exceedance probability (AEP) floods and an extreme flood event. The study area extends from Lake Munmorah in the north to Tuggerah Lake in the south. It is linked to the ocean through The Entrance at the southern end of Tuggerah Lake, see figure 1.
Data were compiled for historical flood events to enable the calibration of a mathematical hydrodynamic model of the lake system. Historical flood event water level and rainfall data was available in addition to recent continuous water level data and rainfall pluviographs.
Hydrographic survey information of the three lakes and two survey runs for The Entrance channel were available.
A depth averaged looped and branched fully dynamic numerical computer model was
applied to the lake system. Three historical flood events were used for calibration and
verification of the model. Analysis of wind data and wind setup was undertaken to
determine the importance of wind effects on lake levels. An entrance breach mechanism was developed for the calibration and design events.
The flood study results show that the 1% design flood level for Tuggerah Lakes is RL 2.23 mAHD. During floods there is a significant lag time between the flood peak upstream of Wyong and the peak water level in the lakes due to the retardation of flow through various road and railway crossings, and the substantial overbank flood storage areas in the lakes and flooplain downstream of Wyong.
The entrance condition is the single most important aspect controlling flood behaviour in the lakes. Little data on the development of The Entrance during floods is available, but predicted entrance behaviour has been based on available information at The Entrance, documented lagoon breakouts at other locations, and other flood studies where breakouts have been investigated. This is discussed in more detail in section 7.
The model as developed can be used as a basis for a Floodplain Management Study, and is used as the basis of a flood forecast system.
