Kowai River, Leithfield Beach and Amberley Beach flood investigation Technical Report
- Author
- Wild, M.
- Year
- 2014
- Publisher / Organisation
- Environment Canterbury
- Number
- R14/99
- Pages
- 80
- Summary
- Flooding occurs on the Kowai River floodplain, and in the Leithfield Beach and Amberley Beach settlements, during prolonged periods of high intensity rainfall in the foothills and coastal catchments. In particular, local surface water runoff can cause considerable volumes of water to pass into the areas surrounding the Amberley Beach and Leithfield Beach settlements. In the case of Leithfield Beach, this flooding can also be exacerbated by overbank flows from the Kowai River. This study of the Kowai River floodplain and Leithfield Beach and Amberley Beach settlements used a combined one and two dimensional hydraulic computer model to estimate flood extent, depths, and flood levels for the 50 year and 200 year Average Recurrence Interval (ARI) flood events. Floodplain topography for the model was derived from a detailed LiDAR survey flown in 2012. The model was calibrated using the July 2008 storm event which recorded a 95 m3/s (5-10 year ARI) peak flow on the Kowai River North Branch at Grays Road (Site 66101). Kowai River flood hydrographs for the July 2008 event were based on the Kowai North Branch at Grays Road flood hydrograph, while local flood water flows draining into the Leithfield Beach and Amberley Beach settlements was determined from local rainfall records. The 50 year and 200 year ARI rainfall events were derived by scaling the 2008 flood hydrographs and local 24-hour and 48-hour rainfall totals. Several assumptions have been necessary when undertaking this modelling study. For example, the derivation of flood flows and tide levels, impact of human intervention to open lagoon outlets, and timing of high tides on the Kowai River mouth (and other outlets to the sea), have all been assessed and require assumptions to be made. There is also very limited calibration information in the way of measured water levels and no measured river and lagoon outlet flows (which can vary enormously due to tide levels, lagoon levels, and the resulting extent of scouring). Regular disturbances to some areas of the floodplain (e.g. due to quarrying), and antecedent conditions within the catchment, can also have a significant impact on river flows, lagoon and quarry ‘pond’ levels, and surface runoff. These results should therefore be interpreted and used with care. It is hoped the floodplain mapping and predicted flood levels will assist land use planning within the area, and provide information on appropriate minimum floor levels for proposed buildings. The modelled 200 year (and 50 year) ARI flood water levels for Amberley Beach and Leithfield Beach settlements have been determined to be 3.4 m (3.2 m) and 3.8 m (3.5 m) above mean sea level (Lyttelton 1937 Datum), respectively. These model results can also be used to determine 50 year and 200 year ARI flood levels for other locations within the model extent (e.g. Leithfield township). This modelling study assumes that the large design flood events will be widespread causing both the Kowai River and local catchments to generate the design flood flows simultaneously; the sea level is also raised to incorporate an additional 0.4 m storm surge. Wave run-up and overtopping of the sand dunes at Amberley Beach and Leithfield Beach has not been included in this study.