The feedback from TRM inmates after the flood last weekend has been interesting with many contradicting theories depending on their success fishing. The results varied from disappointing to spectacular.

The highest catch number from one TRM inmate last Wednesday was 26 Rainbows, the biggest trout measured was 11.2 pound; we are waiting for the photos…

Every Tongariro fisho has a different theory, so to help explain with so many varying opinions, we researched some of the reports for the Tongariro River Flood Hazard Study to consider several other official engineering versions of predicting the likely Tongariro flood behaviour – although they fail to consider the most important factor – the detrimental effects on fishing!

A flood like that, over 700 cumecs, carries a massive sediment load now deposited across the wider slower, meandering lower river and filling in many older pools. This sediment includes sand, pumice, and ash from previous eruptions. Naturally trout avoid ash or pumice as they flash past any new deposits during spawning runs, so anglers now have to forget previous lies to read the water flow and try out every new spot. The inmates’ results confirm that some have already done that remarkably well…

Taupo District Flood Hazard Study – Tongariro River (extracts from OPUS Report dated  July 2011)

29 February 2004 flood

(Photos by Jason Goedhart of the Birches swing bridge after the 2004 flood)

The 29 February 2004 flood was the second largest flood recorded on the Tongariro River, only slightly smaller than that of February 1958. This event resulted in the flooding of houses, significant property damage, and some dramatic changes in river morphology and channel position.

February 2004 was a very wet month, particularly in the Tongariro catchment and the Taupo basin. Some rainfall stations received up to 480% of their usual monthly rainfall. The 36- hour rainfall totals resulting in this flood event were particularly high on the western side of the catchment. Rainfall totals in this area were up to 289mm (Mangatoetoe); approximately twice the total monthly rainfall in just 36 hours. Peak rainfall intensities were not particularly high (23.5mm/hr) but the rainfall was persistent with intensities of 7mm/hr or greater for 20 hours (Bowler, 2004).

The peak flow estimated at Turangi was approximately 1420m3/s. This is only slightly less than the peak flow in 1958 (i.e. 1470m3/s). From a planning and management perspective, the 2004 flood peak was 36% higher than the next largest flood in the instrumental record (1038m3/s in 1964).

Prior to the 2004 flood, the return period for an event of this magnitude was estimated at approximately 1 in a 100 years. Given that there were then two events of this magnitude in the previous 50 years, the return periods were revised after the 2004 event. The 2004 flood was subsequently thought to be a 1 in 55-year event, and the 1958 flood a 1 in 60-year event. These flood estimates have been revised again in 2010 using an additional six years of flow data. The relative lack of flood activity over this 6-year period has led to a reduction in the magnitude of specific design floods. For example, the 100-year event estimated in 2010 is 1451m3/s; the 1958 flood becoming a 107-year event and the 2004 flood becoming a 97-year event.

The 2004 flood took approximately 2 hours to travel the 35km from the Waipakihi confluence to Turangi. Therefore, the flood wave was travelling at almost 5m/s which is high enough to keep boulders up to 400 mm in motion (Bowler, 2004).

The main residential areas affected by flooding were houses down Herekiekie St, Hirangi Rd, Awamate Rd, the river end of Koura and Poto Sts, Bridge Lodge, Tongariro Lodge, and farm land on both sides of the delta (Figure 3.5). On Herekiekie St, up to 90 persons were evacuated and at least one house was subsequently condemned (Bowler, 2004).

A flood of this magnitude has the ability to erode and transport large amounts of material. As a result, significant changes to the form and position of the river are to be expected. The most dramatic change was in an area known as the Breakaway Pool. Here the river created a new channel through what was previously a heavily vegetated island. While this new channel had been developing slowly during previous flood events, the river permanently changed its course during the 2004 flood. This change did not just affect the position of the river. The new channel is approximately 30% shorter than the old, and therefore the gradient of the river has steepened. This has increased the velocity of flow which has also increased the river’s ability to erode and transport material.

Bed degradation was common during the 2004 flood. The high flows during the event moved much of the lose material down stream. This material is likely to have been deposited on the Tongariro delta where river gradients and velocities decrease. Environment Waikato estimate that as much as 150,000m3 of material may have been deposited downstream of the State Highway Bridge. This is consistent with the amount of bed degradation observed up stream where bed levels dropped by 300-500mm (Bowler, 2004).

The instability of the lower reaches of the channel are indicative of potential future changes. It appears that the overflow channels down towards the river mouth (across Grace Rd to Stump Bay and across Awamate Rd towards Deep Stream) have become more established following the 2004 flood. Recent observations at Awamate Rd show that even during quite low flows (around 40m3/s) water is flowing from the main Tongariro River channel towards Deep Stream. It is possible that during the next large flood the Tongariro River may move from its present course and cut a new channel to Lake Taupo (Bowler, 2004).

Photos after the 2013 floods when De Latour Pool was bypassed.

Channel stability

The Tongariro River carries thousands of tonnes of sediment each year. While some of this is boulders and gravel (on average about 11,700 tonnes per year) ten times this amount is sand-sized or finer. Some of this material comes from the Kaimanawa Range with large amounts of volcanic ash, pumice, and lava fragments coming from the volcanoes in the west of the catchment. Although the Tongariro is a gravel-bed river until just downstream of Turangi, the volcanic material is mainly sand-sized or finer and this tends to be washed onto the delta. The 1995-96 eruptions of Mt Ruapehu deposited nearly 7 million tonnes of material into the Tongariro catchment. Two-thirds of this was sand-sized or smaller. Much of this fine material has aready washed down to the Tongariro delta, but subsequent floods release new ‘waves’ of sediment into the river. Between eruptions, when sediment supply from upstream is reduced, floods can erode previous deposits along the bed and banks (Smart, 2005).

The Tongariro is therefore a highly dynamic river that undergoes significant changes in response to floods, and the input of sediment from erosion and eruptions in the headwaters. The river transports large amounts of sediment through the upper reaches and deposits this material on the river’s delta down stream of Turangi. Over the last 1850 years the Tongariro delta has grown at an average rate of 2.6 million tonnes per year. This is around twenty times the present rate. The relatively slow rate of growth under current conditions has important implications to the flood risk. The fact that the township of Turangi is located at the head of the delta, perhaps the most dynamic location in the entire catchment, adds significantly to the flood risk and the difficulties of managing this risk.

Prior to forming its present delta mouths, the Tongariro River flowed between Turangi and the oxidation ponds, significantly west of its current position. It then entered Lake Taupo along the line of the present Tokaanu Stream. Earlier river mouths can be seen in the bathymetry of Stump Bay. The river mouth and delta area are therefore highly dynamic. They have been subject to significant changes and shifts in the past, and are likely to change again at some stage in the future.

The lower delta currently shows that the river is close to breaking out of its present channel and forming a new path to Lake Taupo. This is a natural process that has occurred many times in the past. A significant volume of water is currently being lost from the present channel, even during relatively low flows as discussed above. Floodwaters spill from the river upstream of De Latours Pool. This water then flows east to Stump Bay, and west towards the Tokaanu tail race via Deep Stream. The most likely future breakout route for the river is from Downs Pool to Tokaanu Bay via Deep Stream (Smart, 2005).

Besides the impact of variable rates of sediment supply, the Tongariro delta is also affected by subsidence. This lowering of the delta is partly a function of the compaction of the sediments, but it is also caused by tectonic deformation. The net affect of these processes is that the delta is subsiding relative to the northern end of Lake Taupo by approximately 3mm a year. This has made, and is making, the southern shores of the lake and the Tongariro delta more flood prone.

The dominant controls on the growth and dynamics of the lower delta have been the frequency and magnitude of floods and eruptions, the level of Lake Taupo during flood events, and willow tree growth. The effects of these controls can be considered either beneficial or detrimental depending on whether one is considering wildlife habitat or human infrastructure on the delta (Smart, 2005).

An early aerial photo of original flow before Tokaanu Power Station and Turangi were developed.