Most South-east Queensland streams and rivers suffer from poor riparian health and, as a result, their banks are the major source of rural diffuse sediment and nutrient pollution. Waterways are in an urgent need for restoration at a time when traditional Government funding is limited. Innovative approaches are needed to fund and deliver catchment restoration.

Nutrient offsetting provides an example of a potential new funding source: The potential exists to optimise social investment in the health of receiving waters by expanding the economic options available to sewerage treatment plant (STP) owners to satisfy environmental outcomes to include the use of riparian restoration. Done correctly, the potential exists to optimise social investment in riparian restoration while reducing sewerage costs.

The Department of Environment & Heritage Protection (DEHP); regional STP operator, Queensland Urban Utilities (QUU); and regional Natural Resource Management body, SEQ Catchments (SEQC) have come together to deliver a nutrient offsetting pilot study involving river restoration and use the learnings to help inform policy development. The pilot study investigates the potential to reduce rural diffuse pollution sources through river restoration in lieu of engineering solutions to reduce end-of-pipe, STP discharges.

This paper is presented in three parts: Part (a) deals with the estimation of sediment pollution avoided through river restoration: while Part (b) focusses on the estimation of nutrient pollution avoided: and Part (c) deals with lessons for policy. The Partners engaged geomorphologic specialists, Alluvium Consulting, to assist.

The direct estimation of sediment erosion avoided through river restoration is problematic and, as a result, modelling is needed. This paper assesses the potential of catchment (regional) and bank (Reach) models for modelling and assessing avoided bank erosion from Reach-level river restoration and concludes that bank models provide the needed granularity.

The partners then investigated various bank erosion models and determined that BSTEM best modelled the fluvial scour at the Pilot Study. BSTEM was then calibrated and used to develop relationships between expected bank erosion for the unremediated Reach, the newly restored bank and the restored bank one, two, five and ten years following revegetation for various design runoff events. These relationships were then integrated to determine an erosion avoided estimate.