Geomorphological effectiveness of floods to rework gravel bars: Insight from hyperscale topography and hydraulic modelling |
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| Authors: | HE Reid RD Williams GJ Brierley SE Coleman R Lamb CD Rennie MJ Tancock |
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| Institution: | 1. Scottish Environment Protection Agency, Stirling, UK;2. Geographical and Earth Sciences, University of Glasgow, UK;3. School of Environment, University of Auckland, New Zealand;4. Department of Civil and Environmental Engineering, University of Auckland, New Zealand;5. JBA Trust, Skipton, UK;6. Department of Civil Engineering, University of Ottawa, Canada;7. JBA Consulting, Skipton, UK |
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| Abstract: | Bars are key morphological units in river systems, fashioning the sediment regime and bedload transport processes within a reach. Reworking of these features underpins channel adjustment at larger scales, thereby acting as a key determinant of channel stability. Despite their importance to channel evolution, few investigations have acquired spatially continuous data on bar morphology and sediment-size to investigate bar reworking. To this end, four bars along a 10 km reach of a wandering gravel-bed river were surveyed with terrestrial laser scanning (TLS), comparing downstream changes in slope, bed material size and channel planform. Detrended standard deviations (σz) were extracted from TLS point clouds and correlated to underlying physically measured median grain-size (D50), across a greater range of σz values than have hitherto been reported. The resulting linear regression model was used to create a 1 m resolution median grain-size map. A fusion of airborne LiDAR and optical-empirical bathymetric mapping was used to develop reach-scale digital elevation models (DEMs) for rapid two-dimensional hydraulic modelling using JFlow® software. The ratio of dimensionless shear stress over critical shear stress was calculated for each raster cell to calculate the effectiveness of a range of flood events (2.33–100 year recurrence intervals) to entrain sediment and rework bar units. Results show that multiple bar forming discharges exist, whereby frequent flood flows rework tail and back channel areas, while much larger, less frequent floods are required to mobilise the coarser sediment fraction on bar heads. Valley confinement is shown to exert a primary influence on patterns of bar reworking. Historical aerial photography, hyperscale DEMs and hydraulic modelling are used to explain channel adjustment at the reach scale. The proportion of the bar comprised of more frequently entrained units (tail, back channel, supra-platform) relative to more static units (bar head) exerts a direct influence upon geomorphic sensitivity. © 2018 John Wiley & Sons, Ltd. |
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| Keywords: | bar sedimentology wandering gravel-bed river fluvial morphodynamics sediment entrainment terrestrial laser scanning |
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