FLOOD RISK ASSESSMENT FOR A CATCHMENT SPANNING DIFFERENT DISTRICTS – A PILOT STUDY

The Waimapu Stream, which flows into the Tauranga Harbour estuary, has a flood-prone lower catchment area that lies within the Tauranga City area. Most of its catchment, however, lies outside of Tauranga City in the Western Bay of Plenty District. The entire catchment lies within the Bay of Plenty Region.

There has been concern expressed that flooding effects within Tauranga City can be exacerbated by changes in land use that occur within the Western Bay of Plenty District, over which Tauranga City Council has little or no control. Collaboration between the different authorities has been identified as fundamental principle to ensure a sustainable management of the catchment. Bay of Plenty Regional Council has initiated a Flood Risk Project to develop a Regional Flood Risk Management Framework and a Regional Flood Risk Management Strategy, using the Waimapu Stream catchment as one of the pilot studies.

Analyses have been undertaken in this catchment in an attempt to quantify the management constraints as well as opportunities that exist within the overall flood risk framework implementation. Of significance in the lower catchment are predicted flood depths and velocity-depth product (used for safety evaluation), while land use practices in the upper catchment require management to reduce the sediment load to the Tauranga harbour.

In this paper the approach to this pilot study is explained in detail. The study uses a risk-based assessment for identifying ongoing management of the catchment over the next 100 years to ensure that the current flood risk is maintained or mitigated to an acceptable level.