Project Work completed for LandWISE Inc.

The “Integrated storm water management plan for the Arawhata sub-catchment” project was one of eight that formed an overall work programme for the Lake Horowhenua Freshwater Clean-up Fund programme.

The Arawhata is a small, intensively farmed catchment entering Lake Horowhenua. Sediments and accompanying nutrients can be carried by flood waters and overland drainage from farmed land into the Arawhata Stream and thereby the lake. This has adverse effects on the lake’s ecosystems. An integrated catchment drainage and sediment control plan was seen to have potential to increase farm production and profitability and decrease the loss of sediment and nutrients to the lake.

Twenty six farm cropping land drainage assessments were completed for properties in the Lake Horowhenua watershed with 355 ha of 446 ha being in the Arawhata sub-catchment. Block size ranged from 2 ha to 117 ha.

Tractor RTK-GPS and aerial LiDAR survey data were processed in OptiSurface® flow modelling and land shaping software. Computer modelling showed changing row direction, adjusting headlands and removing raised traffic lanes would address the majority of ponding and farm induced erosion risk factors in the Arawhata sub-catchment.

Twenty Drainage and Erosion Management Plans (DEMPs) were produced covering 82% or 368 ha of the total area assessed. In the Arawhata area, 15 plans covered 81% of the estimated 404 hectares cropped in the sub-catchment. (The blocks without specific DEMPs were very similar to nearby properties operated by the same grower.)

A wide variety of recommendations were included in the DEMPs. These ranged from ensuring the revised community drainage system allows for water discharge from the block to taking areas permanently out of production.

To avoid ponding, row orientation on several blocks was changed and growers have seen production benefits and reduced risk of drainage blow-outs, helping stop sediment generation and flow to waterways.

Those blocks with headland ponding issues received DEMP recommendations to lower the headland and establish vegetation before the drain edge. The concept of vegetated headlands has been accepted by growers and is being followed although there can be confounding issues. The performance of grassed headland systems is being independently assessed by Landcare Research and AgriLink.

A few blocks have steeper areas where heavy rain events can generate rill erosion or channel water causing gullying. In some cases, the area slopes to an on-farm drain that has a controlled exit point. The drain is monitored for sediment. Structures are being put in drains to slow and capture sediment which is removed and returned to the paddock.

One of the steeper Arawhata farms discharges surface water to a natural ponding/detention area formed by a drive causeway. This was used as a case study for a detention dam sediment trap that in its current state it would service about 7 ha of cropped land, the area discharging to it.

In a few cases, managing risk on cultivated soil was deemed impractical and land has been retired from cropping. Permanent grass has been established to hold soil together.

Additional farm walks with property owners to review their own plans for erosion management of other blocks covered a further 109 ha. Their plans were sound and highlighted increased grower understanding of erosion risks and mitigation methods. Satisfactory mitigation has been implemented on farms visited to date.

Clearly evident was the critical role of community drainage network failure in generation and transport of sediment to lake. Major failures have removed very large volumes of soil which ultimately travels to Lake Horowhenua. Such community drainage failures have continued to impact farms.

Failures happen at discrete points, channelling large volumes of water at erosive velocities on to crop land.  Farmers have made interim on-farm attempts such as stop banks to keep water in drains to control such flooding. The best solution requires completion of the revised drainage network infrastructure.

The Fresh Start for Fresh Water project was funded by the Ministry for the Environment, Horizons Regional Council and Tararua Vegetable Growers with support from Horticulture New Zealand.