Rangitaiki / Tarawera Catchments

• Geology and geography of the catchment
• Land use in the catchment
• Flood risk management in the catchment
• Significant floods in the catchment

Geology and Geography of the Catchment

The Rangitaiki Tarawera catchment covers a total area of approximately 4,000 km², with 3,005 km²in the Rangitaiki catchment and 990 km² in the Tarawera catchment.
The Rangitaiki Tarawera River floodplains comprises those low lying areas of river valley, adjacent to the river corridor, which is covered with water when the river overflows during floods. On this basis flood plains exist on the middle/lower Rangitaiki and Tarawera plains as well on the Waiohau and Galatea basins.

Tarawera

The total catchment of the Tarawera is approximately 990 km².
The headwaters of the Tarawera River include Lakes Okataina, Okareka, Tikitapu (Blue Lake), Rotokakahi (Green Lake) and Rotomahana. These lakes all drain into Lake Tarawera (elevation of just under 300 m), from here the Tarawera River begins.
Within 6.5 km of leaving Lake Tarawera the river has fallen to a level of 150 m a drop of 150m. From this point to Kawerau the river falls steadily and moderately steeply through deep pumice country.  Below Kawerau the grade is gentler and the bed is perched above the general level of the Rangitaiki Plains on a ridge formed from flood deposits.
The deep pumice of the Tarawera catchment together with the ponding effect of lake Tarawera regulates the runoff from heavy storms so the maximum-recorded floods are only two to three times the normal flow.
Significant tributaries are the Mangawhio, the Waiwhakapa and the Mangamate upstream of Kawerau, the Mangaone and the Ruruanga near Otakiri, and Awakaponga Stream near Matata. The Tarawera catchment also includes much of the drainage network on Rangitāiki Plains. Major canals in the current network, include the Awaiti, Omeheu, Awakaponga and canal 109. The Old Rangitaiki Channel, the path of the Rangitaiki before it was diverted into the cut at Thornton, also forms part of the Tarawera catchment.

Rangitaiki Catchment

The Rangitaiki River has its headwaters 130 km from the Bay of Plenty coast and 32 km east of Lake Taupo at an elevation of about 800 m above mean sea level.
From there it flows 64 km across the Kaingaroa Plains to Murupara. The Rangitāiki has been dammed towards the northern end of the plains, diverting some flow along the Rangitaiki Canal into the Wheao River, as part of the Wheao power scheme.
The flat pumice covered plains at Murupara are very absorbent and regulate runoff to such an extent that flood flows are only two or three times normal flow at this location.
Within 20 km downstream of Murupara, two major tributaries, the Whirinaki 527 km², and the Horomanga 218 km², enter the river on the eastern side. These rivers rise in the steep bush-covered Ikawhenua Ranges composed of greywacke rock, where the runoff is high and consequently they contribute relatively large flood flows (and quantities of shingle) to the main channel.
The river then continues over the Galatea Plains before it enters Lake Aniwhenua where it is used for electricity generation. Lake Rerewhakaaitu drains into the Rangitaiki at this point. Downstream of the dam, the main channel passes through a gorge and then out onto the Waiohau Plains. Several small tributaries arising in the Ikawhenua Ranges cross the Waiohau, contributing substantially to the flood flows downstream of Aniwhenua. The river travels for 13 km across the Waiohau Plains before it enters Lake Matahina where once again it is used for electricity generation.
Below Matahina the Rangitaiki passes through a well-defined valley before crossing the vast Rangitaiki Plains and entering the ocean just below Thornton. (Rangitaiki-Tarawera Floodplain Management Strategy Stage 1 May 2008)

Land Use in the Catchment

97.4% of the catchment is rural with the remaining 2.6% being urban. Details of the rural and urban land use are described below.
A land-use survey of the Rangitaiki-Tarawera catchment done in September 2007 is shown in table below.

Exotic (47.45%) and indigenous (native) forests (25.49%) are the most common land-uses accounting for 73% the overall catchment area. This is followed by exotic grassland which is farmed for dairy and/or drystock.
The majority of productive land (70% in exotic forest) is in the middle and upper reaches of the Rangitaiki River. The remaining productive land (30% in exotic grassland and orchard/cropping) is situated in the middle and lower catchment (Galatea basin and Rangitaiki Plains), closer to the coast.

Flood Risk Management in the Catchment

Prior to the construction of the Rangitaiki-Tarawera River Scheme, the Rangitaiki and Tarawera floodplains were subject to a risk of extensive flooding from bank overflow during high river flows or from the ad hoc stopbanks failing. Between 1944 and 1964, 15 floods occurred on the Rangitaiki River that caused extensive flooding to the surrounding areas. Ad hoc stopbanks along the Tarawera River breached in 1962.
The original Rangitaiki Tarawera River Scheme provided 100 year flood protection along the lower reaches of the Rangitaiki River between Te Teko and its mouth and the lower reach of the Tarawera River between State Highway 30 and its mouth. Construction of the main river scheme protection works occurred between 1965 and 1983.
In 1987, the Edgecumbe earthquake severely damaged stopbanks on the Rangitaiki River system, as a result an urgent review of the scheme was undertaken, with a particular interest in repairing the stopbanks around the fault scarp where a 1-2 metre discrepancy in top stopbank levels was left as a result of the quake.
The Rangitaiki plains also has a substantial drainage network that is designed to offer land some flood protection during minor floods (5-10yr events) and assist in keeping land drained to a level that helps the land stay profitable and good to farm.

View map of Rangitaiki / Tarawera stopbanks (pdf 17.5MB)

Link to more info on the Rangitaiki - Tarawera river scheme and scheme liason group notes
Link to more info on the Rangitaiki drainage scheme

Significant floods in the Catchment

Flood level data has been recorded since 1949. Prior to this period flood level information was obtained from local residents and it is apparent that exceptionally high floods occurred in 1906 and in 1925. Little is known about the 1906 flood but records of latter floods are available.
A very large flood occurred in the Rangitaiki River in 2004, this included a breach of stopbanks and many residences being evacuated from inundated homes.