Te Puru Stream flood protection scheme - service level review

Report: TR 2019/26

Author: Duncan Grant

Abstract

A service level review has been undertaken of the flood protection assets on the Te Puru Stream. The process has included a reassessment of catchment hydrology and design flows, a complete rebuild of hydraulic models based on the available topographic datasets, and a comparison of revised flood profiles against surveyed asset crest levels (stopbanks and floodwalls). The review indicates the scheme can convey the design flow safely although there are some shortfalls in freeboard levels in the lower scheme. The shortfalls are possibly associated with aggradation of sediment in the lower stream channel near the mouth as captured in topographic datasets. Aggradation could result in less in-channel capacity which may increase flood levels. It is recommended that re-survey of the channel cross-sections is undertaken to compare with past records and rerun the hydraulic model to determine appropriate bed levels. Given the fixed nature of the assets (primarily floodwalls or stopbanks within confined areas) the recommended solution to maintain the service level and improve performance in the lower stream is to undertake channel excavation/maintenance to allow sufficient in-channel capacity and freeboard for the design condition.