Mardi Creek is one of the smaller creeks draining into the Wyong River. It has a catchment area of approximately 6 square kilometres to the Pacific Highway (Figure 1). Upstream of the Pacific Highway there are several buildings within the township of Tuggerah which have experienced flooding in the past. Gavenlock Road has also been cut by floodwaters on many occasions. This report details the hydrologic and hydraulic investigations which have been undertaken to provide design flood levels along Mardi Creek. The study area was defined as the Mardi Creek floodplain from downstream of the P3 Freeway, to a lagoon situated on the Wyong River floodplain approximately 1 kilometre below the Pacific Highway (Figure 2). The Upper Wyong River Flood Study (Reference 1) was completed in February 1988. This showed that the area downstream of Tonkiss Street was liable to flooding from the Wyong River, and the 1% design flood level was approximately 5m AHD. Flooding from the Wyong River last occurred here in June 1964.

For the Upper Wyong River Flood Study, a calibrated Watershed Bounded Network Model (WBNM) and Cell Model were established to model the hydrology and hydraulics respectively. These models were subsequently expanded and then used to determine design flood level data downstream of the Pacific Highway to Tuggerah Lake in the Lower Wyong River Flood Study Review 1991 (Reference 3). The two Wyong River Flood Studies provide design flood levels downstream of Tonkiss Street on Mardi Creek for flooding from the Wyong River. In order to determine design flood levels from local runoff, in the absence of flooding from the Wyong River, separate hydrologic and hydraulic models had to be established. A WBNM model was set up to model the hydrology of the Mardi Creek catchment to the lagoon. Flow hydrographs from this model were input to a RUBICON hydraulic model to obtain design flood levels within the study area. The models were calibrated to the limited historical data available.