Report: TR04/28

Author: Greg Ryan

Abstract

The Waitomo District Council asked Waikato Regional Council to investigate flood levels in the Mangaokewa Stream affecting land parcels in Te Kuiti that are likely to be the subject of future urban development.

Environment Waikato have undertaken this investigation by constructing a one-dimensional hydraulic model of the Mangaokewa Stream, extending from a point upstream of the Te Kuiti township to a point approximately 4 km downstream of the Te Kumi Station bridge.

The 1 in 5, 10, 20, 50 and 500-year flood flows used in the hydraulic model were taken from a flood frequency analysis completed using data recorded by the Waikato Regional Council water level/flow recorder at Te Kuiti. The 1 in 100 year flood flow used in the hydraulic model was conservatively taken from the 1983 review of the Lower Waipa Waikato Control Scheme. The results from the hydraulic model were calibrated using the stream flow rating table produced for the channel cross section at the recorder site.

The results from the investigation include a set of flood level profiles for different flood events in the Mangaokewa Stream within the Te Kuiti urban area, with particular focus on the parcels of land identified by Waitomo District Council.

It is recommended that these levels be adopted for identifying the flood hazard areas when considering future development. It should be noted that further hydraulic assessments will be required if flood protection works are proposed by way of stopbanks or filling (raising ground levels) the flood plains, as these will change the flood profiles upstream and downstream of the works.

It is also recommended that topographic surveys of each land parcel are completed to allow the full translation of the flood level profiles into flood extent plans.

Hydraulic Investigation: Mangaokewa Stream, Te Kuiti Urban Area [PDF, 1.4 MB]