Geomorphological effectiveness of floods to rework gravel bars: Insight from hyperscale topography and hydraulic modelling

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.

The Geomorphic Impact of Outburst Floods: Integrating Observations and Numerical Simulations of the 2000 Yigong Flood, Eastern Himalaya

“…The hydraulics of infrequent, high‐magnitude floods may therefore exert a strong control on channel incision and development of channel width and the river longitudinal profile. Taken together, our observations highlight the potential for background discharges and outburst floods of different sizes to promote not only different amounts, but also different patterns of bedrock erosion (Reid et al, ).…”

Ground Control Point Distribution for Accurate Kilometre-Scale Topographic Mapping Using an RTK-GNSS Unmanned Aerial Vehicle and SfM Photogrammetry

“…Digital photography and Unmanned Aerial Vehicle (UAV) 2 of 21 hardware technologies [10][11][12] coupled with Global Navigation Satellite System (GNSS) placed ground control points (GCPs) and Structure-from-Motion with multiview stereo photogrammetry (SfM MVS; hereafter together referred to as SfM photogrammetry) [13] has become a widely established survey technique at this spatial scale [13][14][15][16][17][18]. The challenges for applying these technologies in dynamic environments compared to alternative techniques, such as static [19] and dynamic [20] terrestrial laser scanning, are: survey logistics including flight permissions, flight duration, weather and access for GCP placement [11,[21][22][23][24]; imagery acquisition to mitigate structural errors [25,26]; and, postprocessing of point clouds to reconstruct accurate submerged topography [27,28] and classify vegetation [29]. The recent development of Direct Georeferencing (DG) of imagery captured by UAVs [30][31][32] potentially offers the opportunity to reduce the logistical challenge and time-consuming nature of GCP field operations [33].…”

Bed Particle Displacements and Morphological Development in a Wandering Gravel‐Bed River

“…It appears that riffle‐pool channels have limited transport distances compared with other channels, presumably a result of bars and riffles constraining particle movement. Over the long‐term, path lengths are therefore likely to be limited by the rate of bar development and re‐working, which in turn plays an important role in overall channel evolution and stability (Reid et al., 2019; Rice et al., 2009).…”