Pragmatic Practice for River Management - the Tongariro and Tauranga-Taupo Rivers

Best practice for River Management is an idea worth pursuing, but not always attainable. The goals should be kept firmly in view, but invariably the outcome will be a pragmatic combination of achievable components to achieve the Best Practicable Option (BPO).

The two most active contributories flowing into Lake Taupo are the Tongariro and Tauranga-Taupo Rivers, both rising in the greywacke rocks of the Kaimanawa ranges but with significant inputs from volcanic sources, particularly into the Tongariro. Both rivers flow through and impact on the established townships of Turangi and Oruatua/Te Rangiita respectively, and have caused significant flooding in recent years, including floods in excess of the 1% AEP on the Tongariro River.

The prime aims of river management have therefore been pragmatic approaches to protecting assets, in many cases ill advisedly located on the flood plains, by controlling flooding and erosion, and managing gravel deposits. The physical issues which were evaluated in the extended processes of providing asset protection included remedying uncontrolled river diversions and breakouts, gravel management, upgrading of previous bank works with rock revetments and extended stopbanks, with the design processes assisted by a succession of computer models.

The paper discusses the investigations and designs leading to resource consents and construction, and the requirements for ongoing river management. Reference is also made to the value and limitations on modeling with MIKE 11, MIKE 21 and MIKE FLOOD.