Drone Based Quantification of Channel Response to an Extreme Flood for a Piedmont Stream

George Heritage,Neil Entwistle

doi:10.3390/rs11172031

Abstract

The influence of extreme floods on the form and functioning of upland systems has concentrated on the erosive impact of these flows. They are seen to be highly competent with coarse sediment transport rates limited by upstream supply and moderated by the ‘blanketing’ effect of an armour layer. This study investigates the effect of extreme events on the upland sediment cascade subjected to a recent extreme rainfall-induced flood event. The drone-based survey generated orthophotography and a DEM surface, which was compared with historic LiDAR data. This allowed erosion and deposition to be quantified and the surface micro-variation used to characterise stable and mobile sediment. The idealised model of sediment residence time increasing downstream is questioned by the findings of this study as relatively little coarse bedload sediment appears to have been transferred downstream in favour of initial local channel erosion (moderated by legacy large sediment), mid-reach palaeo-channel reactivation, sub-channel infilling and downstream deposition of the majority of mobilised sediment across berm and bar surfaces within the active inset channel margins. Channel margin erosion was largely limited to fine sediment stripping moderated by the re-exposure of post-glacial sediment. Only a weak relationship was found between local channel slope and deposition, with storage linked more to the presence of inset berm and bar areas within the inset active channel. Downstream fining of sediment is apparent as is a strong contrast between coarser active sub-channels and finer bar and berm areas.

Highlights

It is generally argued that change processes are at their greatest during high magnitude, low frequency, extreme flood events when sediment yields can increase by orders of magnitude, even when averaged over centennial to millennial timescales [1,2]
This study investigates the upper Liza Beck, Cumbria, UK piedmont stream response to a recent extreme rainfall-induced flood event (Storm Desmond, December 2015) through the use of images derived from an unmanned aerial vehicle and structure from motion photogrammetry
Aerial imagery spans a shorter time-period with photographs available from 2004 and 2014, and the orthophoto developed from the 2018 drone survey (Figure 4)

Summary

Introduction

It is generally argued that change processes are at their greatest during high magnitude, low frequency, extreme flood events when sediment yields can increase by orders of magnitude, even when averaged over centennial to millennial timescales [1,2]. Selective transport of glacial and fluvio-glacial material over long time periods has meant that many upland fluvial systems in the UK have been said to exhibit coarse sediment dominated features that are relict from past climates, when these systems were far more energetic than today This legacy influence was well noted by [4] where he discussed the influence of glacial and fluvio-glacial processes in the formation of immobile boulder dominated channel beds along upland watercourses in the UK and further exemplified by [14] for Harris Creek, an upland cobble-gravel stream in British Columbia, Canada with low sediment supply and a strongly seasonal, nival flow regime. All changes are discussed with respect to flood energy and relict sediment deposits

System Form

Error Handling

Surface roughness estimation

Results

Discussion