Use of remote imagery to verify modelled streambank retreat rates

The Source Catchments model attributes fractions of the sediment load carried by the rivers discharging into the Great Barrier Reef (GBR) lagoon, to specific sources – namely, hillslope, gully and streambank erosion. This study was conceived to determine whether it was possible to derive estimates of streambank retreat rates in a river discharging into the GBR lagoon, by using remote imagery. This remote imagery was to be in the form of readily available historical aerial photographs and more recent satellite imagery. Any usable estimates of streambank retreat rates that were obtained, were to be evaluated in regard to their suitability for the verification of the algorithms in the streambank erosion module in the Source Catchments model. The streambank mapping exercise was undertaken using sets of historical aerial photographs and more recent satellite imagery covering some 150-kilometres in the middle reaches of the Mary River. This river carries a disproportionately large sediment load relative to its volumetric discharge. The Source Catchments model attributes this differentiating characteristic to above-average rates of streambank erosion in the Mary River. The likely inundation extents associated with bankfull discharge were delineated in the various sets of historical imagery. This process yielded a time series of bankfull discharge polygons covering the subject reaches. Estimates of bankfull width were then obtained by laying transects at 500-metre intervals along and at right angles to the river centerline, in each set of images. This provided measurements of bankfull width at each transect location in each polygon in the time series. Digital elevation data obtained from airborne LiDAR surveys was used to aid verification of the bankfull width estimates derived from contemporaneous remote imagery. Streambank retreat rates were then determined on the basis of the change in bankfull width in the intervals between the sets of images in the time series. The statistical significance of the long-term rate of streambank retreat at each transect location was assessed by fitting the non-parametric Theil-Kendall Robust Line to the data. Changes in bankfull width varied substantially along the studied reaches, but did show a somewhat remarkable lack of temporal variability at the various transect locations. Spatial variability appeared more closely related to stream sinuosity than it did to presence of upper storey riparian vegetation. Notwithstanding the observed uniformity of the long-term trends, temporarily accelerated streambank retreat rates were associated with major floods. This study confirmed that the streambank retreat rates in the Mary River are relatively high when compared to other rivers discharging into the GBR lagoon. Furthermore, the broadscale retreat rate values presently adopted in Source Catchments model were found to be reasonably accurate. However, at a finer scale the modelled and measured estimates of retreat rates were somewhat disparate. The nature of this study precluded definitive identification of causative factors, but indicated that some refinement of the streambank erosion algorithms, or the applied data values, might be required if this disparity was to be addressed.