Streambank ‘Catchment modelled method’ computation
November 2023 release
To support the ‘Catchment modelled method’ of determining streambank erosion rate, parameters and outputs from the Great Barrier Reef (GBR) Dynamic SedNet models (Report Card 2019, RC10) were harvested and processed.
The primary data element to be calculated for Toolbox use is an estimate of an annual average mass of fine sediment (kg) eroded per unit of representative stream length (km) in each GBR Dynamic SedNet subcatchment. Using the RC10 prediction of annual average streambank erosion mass (fine sediment, kg per year) it is possible to directly calculate the required streambank erosion rate using the model’s internally stored stream length parameter. However, it is acknowledged that the density of the stream network represented by the GBR Dynamic SedNet models is much sparser than the existing stream network providing streambank sediments throughout much of the GBR catchments. A search for a more representative stream network data set led to the identification of the HydroRIVERS layer (part of the HydroSHEDS collection).
The HydroRIVERS data set was intersected with the modelling subcatchments from the GBR Dynamic SedNet models (RC10), with the length of intersected river segments calculated. On average, the length of intersected HydroRIVERS stream segments is 3.3 times greater than the relevant GBR Dynamic SedNet stream length.
Where the GBR Dynamic SedNet fine sediment erosion mass was greater than zero, an initial fine sediment erosion rate (kg/km) was calculated using the derived HydroRIVERS stream lengths (for convenience, erosion rates for particulate nitrogen and particulate phosphorus were calculated in the same manner). Recognising that the custodians of P2R Projector wanted no instances of zero streambank erosion represented (preferring a low rate represented) in the Toolbox, each of the 6 GBR NRM Regions (Cape York, Wet Tropics, Burdekin, Mackay Whitsundays, Fitzroy, Burnett Mary) had the initial fine sediment rates analysed for a suitable minimum value to be applied.
Using Microsoft Excel, the fine sediment erosion representing the 15th Percentile was selected as the minimum value to apply in low and zero erosion modelling subcatchments for each NRM Region. Similarly, to provide an upper cap of fine sediment erosion rates in P2R Projector Toolbox, the value representing the 97th Percentile was selected. It is these fine sediment erosion rates (kg/km, constrained by the 15th and 97th Percentile of regional rates) that support the P2R Projector implementation of the Toolbox Catchment Modelling Method. The same process and percentile constraints have been applied to the erosion rates of particulate nitrogen and particulate phosphorus.
To compliment the infilled streambank erosion rates, any GBR Dynamic SedNet subcatchments with a zero or missing stream length have had a default of 1 km provided for Toolbox use.