Farm

ISSUE:

Runoff from our farm often contains high levels of sediment, nutrient and e-coli. The water accumulates into an ephemeral river with significant volume and velocity which travels down the AB Lime hill blocks. Not long after this occurs the lower reaches of the property including the AB Lime office carpark, both cow underpasses and the main highway flash flood significantly.

SOLUTION:

From water testing we know that surface runoff contribution from the dairy farm is our main water quality concern and mitigations to combat this began with Clint Rissman and the Land Water Science team carrying out a hydrological survey and analysis of water flow paths on the farm and identifying high risk areas for sediment loss.

Land water science identified rough areas on the hill blocks that had the highest potential for sediment loss and would be better off being retired from dairy cattle grazing – this included 25ha of hill country which is now being progressively retired from grazing and will be planted in native vegetation to compliment 'Motu Ngahere'. These planted areas will help to slow down runoff in the future to decrease sediment and nutrient runoff and also see less of a flash flooding effect that has been seen in the lower reaches.

Planting 25ha in natives is a big job and so this will be done progressively over time and also native trees can take many years to establish so in the interim another mitigation measure was recommended which was the implementation of soil bunds to slow down and detain runoff for longer periods to allow the settlement of sediment and reduce contamination to surface water.

DESIGN:

Each bund will be designed to perform three functions to:

1. slow the ephemeral flow from the AB Lime hill country to lesson the effects of flooding on the lower reaches of the property
2. allow sediment to settle out behind the bunds to aid in reducing sediment loss to water ways and off site
3. assist with the further uptake of nutrients (nitrogen and phosphorous) found in runoff and the reduction of E-coli so these are not lost to waterways Some water retention bunds are located in retired production areas on the hill country and other bunds are located in areas that are still productive blocks grazed by livestock. Due to the two different areas these bunds will be located there are slightly different requirements for each

Retired Block Bunds – because these areas do not have stock near them these areas can be wetter for longer periods of time, resembling a constructed wetland and planted out in native vegetation. This can be done by using a less porous material in the construction of the bund or having smaller outlet pipes or no outlet pipes for water at all. Wetland vegetation in these bunds will assist in taking up nutrients, further slowing down the water flow and also helping to retain more sediment on the farm and not losing this off farm on to water ways.

Productive grazed area bunds – parts of these bunds will most likely be fenced off from cattle to maintain the bunds structural integrity but it is important that water does not have a residence time longer than about 3 days. In the end each bund is constructed slightly differently to best suit the area it is being built in.