Flood Protection

Lower Wairarapa Valley Development (LWVD) Scheme

The barage gates	The Ruamahanga River diversion

The area protected

The LWVD Scheme is a flood protection scheme for the Lower Wairarapa Valley.

It starts about 2 kilometres downstream of the Waiohine and Ruamahanga confluence to where Lake Onoke enters Palliser Bay and covers the valley between the western ranges and the eastern hills.  It includes the Tauherenikau River and all the tributary rivers that flow into Lake Wairarapa and Ruamahanga River.

The stopbanks and natural high land protect the area for a 20-year plus flood event. It is interesting to note that the system largely coped with the 50 year flood event in 2004.

Map of the LWVD Scheme area

Works programme

A works programme for the operation and maintenance of the scheme is managed by the Flood Protection Department of Greater Wellington Regional Council.

The Scheme has an Advisory Committee who provide advice on operations and liaise between landowners and Flood Protection staff. The programme involves work such as planting and vegetation control, stop bank maintenance and upgrades, fences, floodgates and channel work (alignment and bank protection), operation of the Barrage Gates and physical opening of the river mouth to the sea when it gets blocked.

Features of the LWVDS

There are several natural and man-made features that are used to provide flood protection to the area:

• The Hikunui and Awaroa Sills are low overflow banks that allow excess water in the Ruamahanga River to be diverted to Lake Wairarapa over an overland floodway system, locally known as Hikunui floodway, Awaroa floodway and Oporua floodway.
• The Tawaha floodway near Martinborough goes through Jenkins Dip and is designed to protect the Waihenga Bridge on State Highway 53. This floodway takes excess water from the Ruamahanga River at this point.
• The Barrage Gates regulate the flow of water between the Ruamahanga River and  Lake Wairarapa. Lake level data is recorded by GWRC. You can view the graphs of lake levels for Lake Wairarapa and Lake Onoke at specific monitoring sites (Burlings, Barrage North or Barrage South and Lake Ferry).
• The Ruamahanga Cut-off is where the river used to flow into Lake Wairarapa. The Ruamahanga River was diverted to by-pass the lake to avoid the flooding of land surrounding Lake Wairarapa.

Operation of the  Barrage Gates

There are four main scenarios under which the Barrage Gates operate:

1. No flood situation

Lake Onoke is open to the sea. The gates are operated to achieve the Target Levels specified in the Resource Consent.

2. Lake Onoke is blocked (spit closed)

The barrage gates are closed and the water level is allowed to rise until it is at a maximum allowable level. When this  level is reached, a cut is put into the bank of Onoke Spit. However, the Spit can’t be opened if the sea is rough in which case the gates are opened to backflow excess water into Lake Wairarapa. Once the Spit is open and the river level has dropped, the gates are opened to let the excess water out.  

3. Large flood

The gates are closed to stop Lake Wairarapa filling up. The Spit is open so water goes out to sea.

4. After a flood event

When the river peak has gone the Gates are opened to let the excess water out.

You might like to know…

There is a flood warning system in place for the landowners in flood-prone areas. They are alerted to manage stock and prepare for a flood.

In the past, large amounts of silt would have been deposited in the lake during floods; the silt would have been blown off during summer when the natural lake level would have been very low to form sandhills on the eastern side. Bulk of the silt is now carried to the sea during flood events.

The Flood Protection Scheme is funded 50:50 by benefiting landowners and general ratepayers.

Greater Wellington Regional Council is responsible for the management of the LWVD Scheme and more information can be found about the history and the Scheme on the GWRC website.