Case Studies
Artificial/Enhancement of wetlands - Lake Okaro
Okaro wetland just after construction in 2005 (Photo: C. Tanner)
A 2.3 ha wetland has been constructed by NIWA and Opus International Consultants to protect Lake Okaro from diffuse nutrient inputs entering the northern end of the lake. The wetland was designed to filter nutrient inflows from the surrounding farmland, focusing on the removal of nitrogen. The photos below show the change in vegetation in the wetland over time.
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For more information:
c.tanner@niwa.co.nz
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Riparian plantings - Waiwhakareke or Horseshoe Lake
This lake is the focus of a large urban restoration project that aims to improve the trophic state of the lake and enhance the natural values of the surrounding area. The riparian plantings should not only reduce nutrient levels entering the lake but also add to the network of wildlife corridors around Hamilton city.
Riparian vegetation has been chosen carefully, by considering which native plants are the most suitable for different aspects. For example naturally swampy areas are planted with plants adapted to living in wetlands. Planting began in September 2004 and monitoring plots have been established to monitor the success of the plantings, and how effective they are at removing different types of nutrients. These results should help provide guidelines for building a riparian margin to ensure it will remain effective over the long term (Grove et al., 2006). Read more
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Silt traps - Lake Ngaroto
An extensive restoration project is being conducted on Lake Ngaroto, one of the Waipa peat lakes. Fencing and riparian plantings have been completed around the majority of the lake and silt traps have been placed on three of the four main inflows. The silt traps widen and deepen the inflow drain, in an attempt to slow down the water enough that the sediment particles will drop out of the water column and settle to the bottom of the trap. These sediment traps must be deepened on a regular basis to remain effective and the sediment must be disposed of in a suitable manner, to prevent it from entering the lake.
The design of sediment trap described above and used for Lake Ngaroto, is fairly basic and was designed to target the heavier bed load of sediment travelling along the base of the drain rather than the suspended sediment in the water column (Greenwood, 2001). During a flood event the water would be moving too quickly for the sediment trap to be effective. However, flood events are assumed to flush the lake system, therefore they sediment transport into the lake at this time will be balanced by sediment transport out of the lake.
There are other sediment trap designs that target both the suspended sediment and bed load sediment. One of these is located on the major inflow of Lake B (Lake Kaituna) near Lake Kainui. This sediment trap was constructed by digging the drain much deeper about 6 m from where it entered the lake and using this earth to completely fill in the drain entrance to Lake Kaituna, apart from a small tunnel for fish transport. During flood events this drain would flood part of the riparian strip of Lake Kaituna before entering the lake. (Photo supplied by W. Powrie)
