North Creek has a catchment area of approximately 5.3 square kilometres and lies within the boundaries of the City of Lake Macquarie Local Government Area. It drains into Lake Macquarie at Warners Bay. Flooding of roads and residential areas within the catchment has occurred on several occasions in living memory. The most notable being June 1949 and February 1990. This report was prepared by Webb, McKeown & Associates on behalf of the City of Lake Macquarie Floodplain Risk Management Committee and details the hydrologic and hydraulic investigations carried out to determine the design flood behaviour (levels, flows and velocities). It represents the technical foundation in the process to provide a formal Floodplain Risk Management Plan for the catchment. All available rainfall, flood and topographic data were collected and analysed as part of the study. Whilst there is a reasonably good flood record around the Lake itself, there is generally a poor record for the North Creek catchment. The quantity and quality of the available data has influenced the hydrologic and hydraulic modelling approach adopted. A WBNM hydrologic model was set up to cover the entire catchment draining to the Lake at Warners Bay. A MIKE11 hydraulic model was structured to model the main creek channels and overbank areas within the designated study area. The downstream limit of the hydraulic model was Warners Bay. Due to the limited amount of available historical data the hydrologic and hydraulic models could not be rigorously calibrated to ensure that they accurately simulated recorded flood events. For both models parameter values from established texts and those found to be applicable in previous studies were therefore used in determining appropriate values for the present study. The available historical flood level information was then compared to the design flood levels. Design rainfall data were determined from Australian Rainfall & Runoff 1987 and input to the hydrologic model to derive flow inputs for the hydraulic model. Design flood levels were obtained by inputting the design flows and boundary conditions to the hydraulic model. Flood levels in the lower parts of the floodplain are influenced by a combination of: • flows discharging from the North Creek catchment, • elevated water levels in Lake Macquarie (investigated in Reference 1), • wind wave activity within Warners Bay (investigated in Reference 2). An “envelope” approach was used to determine design flood levels in these areas. The accuracy of the design flood levels at any one location is largely dependent on the availability of suitable historical flood data, the survey data, and the reliability of the design rainfall intensities. The relative accuracy of the design flood levels obtained from the Lake Macquarie Flood Studies are considered to be of the order of ±0.3. For the upper reaches of this North Creek study area the accuracy is more likely to be in the order of ±0.5 m due to the paucity of data available for model calibration and verification. It is recommended that Council install maximum height recorders in the catchment in order to accurately record all future flood events. This would greatly assist in increasing the accuracy of any future flood studies. The potential influence of wind wave effects on design flood levels near the mouth can be significant depending on the prevailing conditions. The results from previous studies suggest a wave runup value of 0.22 m should be added to the design 1% AEP “still” water lake flood levels to minimise the risk of inundation for new development affected by wave runup. Council’s current Flood Planning Level for new floor levels is the 1% AEP flood level plus 0.5 m freeboard. As the wave runup value is within the 0.5 m freeboard there is no need to increase the Flood Planning Level to take account of wave runup. However wave runup may influence the structural design or location of any proposed structure adjacent to the foreshore. Floor level data were collected as part of the present study and the number of buildings (residential and non-residential) inundated for the range of design events considered is summarised below along with the estimated tangible flood damages.