The Lower Wairarapa valley Development scheme is one of the largest and most complex ﬂood management systems in New Zealand. nearly 30 years after its completion there are still challenges in the management of the scheme.
Future increased sea level and ﬂoods, owing to the impacts of climate change, need to be fully understood. Enhancing the environmental impact of the scheme, while maximising the reduced ﬂooding eﬀects, will require considerable investment in research and works. The scheme has had a major impact on the environment. The area of wetlands has been considerably reduced and the hydrological regime of Lake Wairarapa has been altered significantly. The impacts of these changes are only now becoming apparent. The challenge for the future is to maintain this development but at the same time establish and maintain complementary environmental systems.
The Scheme is a complex of man-made constructions. Keeping the Ruamahunga River mouth opened is the most important element of the scheme.
The Ruamahanga River ﬂows into the sea at Palliser Bay through the Lake Onoke Spit. The mouth of the Ruamahanga River is called the lake opening. The natural opening generally migrates towards the eastern end of the sand bar and when this happens the ﬂow out to sea is reduced considerably. Under the scheme the opening is sited in line with Lake Ferry road to enable quicker drainage of the river.
Typically, between January and May the mouth gets blocked when there is low river ﬂow, combined with large southerly swells in Palliser Bay. However, it can get blocked at other times, too. In an extreme year, the mouth could be blocked as many as 18 times but on average there are between six and eight blockages a year.
It is the very straight section of the Ruamahanga river that cuts across the Rongotea and Pouawha lagoons, connecting Te Hopai with the mouth of the Tauanui River. The Diversion Channel has become a very popular area for jet boating and water skiing.
The objective is to stabilise the water level in Lake Wairarapa to a defined operating range and to allow sufficient freeboard to accept ﬂoodwaters from the Lower Wairarapa Valley Development Scheme ﬂoodways and tributaries. The Barrage Gates may be used to either lower or raise the water level in Lake Önoke, or in Lake Wairarapa, depending on the conditions. The seasonal levels at Lake Wairarapa has been determined after years of consultation with the interested parties such as the lake shore landowners, DoC, Fish and Game, the Iwi, LWVDS Advisory Committee, Ducks Unlimited, SWDC and the Regional Council. These levels are stated as “Target Levels” in the Resource Consent issued for the operation of the Barrage Gates, as they are not absolute levels that can be controlled. By and large the weather conditions determine the lake levels.
The Barrage gates are the largest mechanical asset of the Lower Wairarapa Valley Development Scheme. Owing to salty water, the gates need to be regularly repainted and the lifting ropes changed. They are operated mechanically and can be controlled electronically by the Council’s telemetry system.
Some 200 kilometres of stopbanks have been constructed to contain high river levels.
On the Ruamahanga River stopbanks have been built from just upstream of Martinborough down to Lake Onoke. Tauherenikau River is stopbanked from SH53 downstream to Lake Wairarapa. The lower reaches of eastern and western tributaries to the Ruamahanga River and Lake Wairarapa are also stopbanked.
The stopbanks have been built to diﬀerent standards primarily because of funding shortages.
Stopbanks between Martinborough and Tuhitarata were designed for 20-year ﬂood standard. This means, there is a 5 percent chance of ﬂoods occurring to this level in any one year. The occurrence of ﬂoods since the scheme’s construction has reduced this to a 10-year ﬂood standard due to the increased frequency of ﬂood events and deposition of gravel and silt. In 2004 a one in 40-year ﬂood occurred and some of the stopbanks were overtopped. Just one pre-scheme stopbank breached during that event.
Short-term overtopping of the stopbanks is not a major problem where they are well maintained, have a good batter slope, a good grass cover and have no stock tracks.
Vegetation, such as willows and some ﬂaxes and native grasses, are planted on the outside of the stopbanks, on the river berm. The roots of these plantings act as reinforcing to hold up the river edge and their leaves tend to slow down the current. Following extended ﬂoods, the berm area can be vulnerable to slumping, which can threaten the edge of the adjacent stopbank. Slumping is where the bank drops away, generally when the ﬂoods recede, due to its weight under very wet conditions. Removal of the build-up of silt, and good management of the trees reduce the chance of slumping.
Text by Ian Gunn, Principle Advisor Catchment Management, GWRC
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Content on this page was last updated: 09/03/2017 12:44pm