The Medowie area, located five kilometres to the north-east of Raymond Terrace in NSW, has in recent years undergone considerable residential development and potentially more may occur in the future. Rural and rural residential lots are being sub-divided for suburban lots and in some locations new development is proposed. A challenge that faces planners is that within both the Campvale and Moffats Swamp catchments which comprise Medowie, some areas are subject to inundation by flooding in relatively small flood events (1Y to 5Y ARI). A further complication is that the Campvale catchment is a drinking water supply catchment and so drainage solutions must take account of water quality impacts.
Campvale Swamp has historically exhibited two main flooding related issues. Firstly, areas upstream of Ferodale Road have been subjected to relatively short duration flooding of property and roads, with over floor level flooding in some cases (particularly along Kirrang Drive). Secondly, downstream of Ferodale Road flooding of the Campvale Drain Inundation Area (CDIA) can be sustained over weeks and tends to disrupt/damage agricultural activities rather than flood residences (although some residences are flood affected). Although a problem of real concern, as it is related to the duration of inundation rather than the peak flood level achieved, it is noteworthy that this issue could not be a focus of a flood study carried out under the NSW State Government Floodplain Risk Management Program. Inundation within the CDIA is aggravated by the restricted outlet capacity and by unmitigated upstream development, which by converting pervious areas to impervious areas removes catchment storage.
In order to assess the flood liability of Medowie, WMAwater were given the task of carrying out a drainage and flood study for the Moffats and Campvale Swamp catchments. As part of this work various data sets were collected, previous reports were reviewed, a community survey was carried out, calibration data was collected and a hydraulic model was built, calibrated, validated and then applied to design flood modelling. Calibration/validation work demonstrated a good match between the model and a number of reliable flood levels. It also demonstrated a close match to a wide range of observed flood depths collected during the extensive community consultation phase of the project.
Results indicate that numerous properties will suffer some degree of inundation. Most flooding will occur, not on the fringes of the swamp areas in Campvale and Moffats, but rather in upstream areas where flows moving downstream are not able to be contained in overland flow paths and drains. A feature of such flood liability is that due to limited upstream catchment area flood depths are unlikely to change dramatically for events of different probability. That is, whilst property may be liable to flooding in relatively small events, flood depths will not dramatically increase in rarer events such as the 1% AEP event for example.