PROJECT DESCRIPTION
Living Water is a partnership between the Department of Conservation and Fonterra. The focus of the partnership is to find game-changing and scalable solutions that will enable farming, freshwater and healthy ecosystems to thrive side-by-side. Five catchments across five regions throughout New Zealand were selected for this initial investment. WSP is proud to be involved with Living Water in the Wairua River Catchment in Northland. WSP Engineers were engaged to design, build and monitor Water Treatment Bunds in flow paths to collect and slow the release of stormwater on two farms in the catchment.
The ability of these structures to reduce peak flow and capture sediment in the Northland farming landscape is currently being assessed. This trial will gauge their effectiveness and inform whether these freshwater improvement tools can be used at a catchment wide scale to improve water quality and freshwater ecological outcomes.
CLIENT BRIEF/PROJECT CHALLENGES
Focused on the interface between dairy farming practice and freshwater ecosystems, the Living Water collaboration seeks to prioritise on-farm actions, improving water quality, and reconnecting lowland habitats via freshwater corridors. The aim is to highlight and advance sustainable farming systems in the New Zealand agricultural sector.Sediment is identified as a major freshwater contaminant throughout New Zealand and a significant issue for the Northland Region.
A Wairua catchment ecosystem services report by Landcare Research (Landcare, 2017) identified widespread use of detention bunds or water treatment bunds as a potential solution for improving the state of the catchment. This initiated the Wairua Catchment Water Treatment Bund Project to trial the effectiveness of water treatment/detention bunds for slowing water and trapping sediment to improve freshwater ecological outcomes. Monitoring of the bunds effectiveness in regards to cost and water quality was a key consideration and outcome of the project.
Treating a large volume of water running off a catchment at peak flows presents a significant challenge. This is a common scenario in Northland, which is subject to frequent heavy and occasionally high intensity rainfall events (NIWA, 2013). The Northland region gets 35 to 40 percent of its annual rainfall in the period June to August (NIWA, 2013). Often during these winter months, soils are already at field capacity meaning there is limited limited ability to accommodate these deluges without runoff. These high intensity rainfall events result in rapid overland flows, localised and widespread flooding, and erosion where sedimentis mobilised into streams and on to rivers, harbours and estuaries.It is expected that sediment-related limits will shortly be included within National Policy Statement for Freshwater (MfE, MPI, 2019).
It is anticipated it will become compulsory for regional plans to include policies, objectives, limits and methods for sediment related attributes. Water Treatment bunds are a potential mitigation solution available to farmers and industry to help treat the effects of farming activities in Northland and nationwide.
OUR SOLUTION
While water treatment bunds or detention bunds have been used in other parts of the country such as the Rotorua lakes catchment they had not been constructed and trailed in Northland Conditions. Northland has challenging clay rich soils, complex geologies and topography and high rainfalls. WSP was able to design, build and monitor the devices and report back to Living Water. Quantifying the performance of these water treatment bunds to date has yielded results that showed significant reductions in total suspended sediment (TSS) concentrations throughout the ponding events and moderating hydrological pathways during peak flow events.
OUTCOME/CLIENT BENEFITS
Monitoring continues
LIST OF PRIMARY SERVICES
Design, build, survey, construction supervision. Monitoring of bunds effectiveness.
