Development and application of SedNetNZ

Smarter targeting of erosion mitigation

SedNetNZ is an erosion model that predicts the generation and transport of sediment through river networks, based on a relatively simple physical representation of hillslope and channel processes at small sub-catchment scale (average c. 40 ha). The model provides estimates of long-term average annual sediment load (t yr^–1) and yield (t km^–2yr^–1). It improves on available erosion models in New Zealand by providing estimates of sediment load generated by different erosion processes (landslides, gullies, earthflows, surface, and bank erosion) and sediment deposition on floodplains.

This allows improved targeting of erosion mitigation to the key contributing processes, and analysis of the linkages between upstream sediment generation and downstream sediment loading. It is also highly suited for scenario analysis of changes in land management and implementation of erosion mitigation practices. Several regional councils have recently commissioned SedNetNZ analyses of large catchments to support land and water policy development.

SedNetNZ was used in Hawke’s Bay to estimate current sediment load for each major sub-catchment (see Fig. 1 for an example). The outputs from SedNetNZ were intersected with farm boundaries from AgriBase to estimate farm sediment loads and the potential for reducing sediment loads from hillslope erosion processes through the adoption of soil conservation farm plans, assuming a 70% reduction in sediment where farm plans were fully implemented. In the Tukituki catchment, it was estimated the current sediment load is 761,000 t yr^–1 but this could be reduced by 27% to 556,000 t yr^–1 by implementing farm plans on the 100 farms with the greatest area of highly erodible land. The model was also used to calculate the potential reduction in bank erosion that could be achieved by fencing to exclude stock from riparian areas, assuming a reduction in stream bank erosion of 80% as a result of stock exclusion. In the Tukituki catchment the current sediment load from bank erosion was estimated to be 162,000 t yr–1 but this could be reduced to 34,000 t yr^–1 if all streams were fenced.

Hawke’s Bay Regional Council is using the SedNetNZ results to help identify priority areas of erosion/sediment production and relate this information back to stakeholder/farmer groups involved in land and water plan change processes, and to help implementation of the National Policy Statement for Freshwater Management. It has allowed Council to supply stakeholders with estimates of the scale of the sediment issue, estimates of costs associated with reducing sediment, and possible ways this may be achieved. The modelling is also helping direct policy discussions on setting sediment reduction targets and whether they can be achieved through non-regulatory methods.

In Northland, SedNetNZ has been used in a project to develop a model for assessing costs and benefits of sediment mitigation scenarios for the Kaipara Harbour Catchment. SedNetNZ provides the annual sediment loads from each erosion process so that reductions in sediment loads can be calculated for different mitigations that have variable costs. Approximately 52% of sediment in the catchment is estimated to come from hillslope sources (landslide, gully, earthflow, and surficial erosion), while the remaining 48% is from bank erosion. SedNetNZ shows that implementing catchment-wide soil conservation works for hillslope and bank erosion could reduce sediment loads to Kaipara Harbour by c. 40%. Sediment loads were translated to freshwater sediment attributes (visual clarity, euphotic depth, and suspended sediment concentration) through a sediment concentration-discharge rating curve and used to predict how changing sediment load after implementation of erosion mitigation would change these sediment attributes (Fig. 2). Sediment loads resulting from the various mitigations were also translated into annual average sedimentation rates at nine depositional zones within the Kaipara Harbour. SedNetNZ was also used in the development of a farm management tool to be used by land management advisors to formulate farm-scale sediment mitigation plans that target critical source areas of sediment under different land uses.

The results from the project will assist Northland Regional Council and Auckland Council with making decisions about current and new regulatory and non-regulatory initiatives to manage sources of sediment in the Kaipara Harbour Catchment. It will also help the community better understand erosion and its effects on fresh and coastal water quality.

Horizons Regional Council has used SedNetNZ to assess the impact of soil conservation work to date, and possible future work under the Sustainable Land Use Initiative (SLUI), on sediment load, as well as assess the impact of climate change on sediment loads. In addition the impact of SLUI works on water clarity, nitrogen, phosphorus and E. coli loads was evaluated.

This showed that SLUI works to date have reduced sediment load by 835,000 tonnes (−6%). With no further SLUI works the sediment load for the region is predicted to decrease by 16% by 2043, but with ongoing implementation of SLUI the sediment load could be reduced by 30%. SLUI works are predicted to increase mean visual clarity by between 11% (with no further SLUI works) and 29% (with ongoing implementation of SLUI) by 2043. Climate change is predicted to increase sediment yields, with the extent of increase depending on the climate change scenario, between 41% and 179%. To offset these predicted large effects of climate change, continued, and arguably increased, investment in erosion mitigation will be vital.

To date SLUI has reduced phosphous load by about 6%. By 2043 reductions in P load are predicted to range from 30% (with no further SLUI works) to >50% (with ongoing implementation of SLUI). SLUI has made very little difference to nitrogen loading to streams (<0.1%) and even with complete stock exclusion on SLUI farms there would only be a reduction of <1%. The average reduction in median E. coli values as a result of SLUI fencing to date 2.6%, however if all streams were fenced then median E. coli values could be reduced by 34%.

The results from this work will assist Horizons Regional Council to advocate for ongoing investment in the SLUI programme.

The Hawke’s Bay work was funded by Hawke’s Bay Regional Council. The Kaipara work was funded by the Ministry for the Environment, Northland Regional Council, and Auckland Council and was carried out in association with Streamlined Environmental, NIWA, University of Otago, and University of Maine. The work in the Manawatu-Wanganui region was funded by the Horizons Regional Council.

MBIE funding from 2018 of a new programme “Smarter targeting of erosion control” will allow further development of SedNetNZ, including improving the temporal resolution of the model from long-term average annual estimates of sediment load to annual or event-scale estimates.