Impact of vegetation on channel migration rates under changing flood conditions in an experimental braided river.

BAS BODEWES; FERNANDEZ, ROCIO LUZ; STUART MACLELLAND; DANIEL PARSONS

Conferencia; European Geosciences Union; 2019

Underpresent day climate change predictions, with both higher magnitude and alteredfrequencies of flood events, uncertainties exist in how the behaviour of riversystems will change in the future. This uncertainty is greater when consideringthe impact of vegetation on the dynamics of fluvial systems, which itself mayalso vary because of climate change. Physical modelling can be a valuable toolto understand the processes on a smaller scale, in particular by simulating theeffects of vegetation using surrogates that can grow rapidly, thereby mimickingthe forcing induced by vegetation growth. Previous studies using surrogate vegetationin flume experiments on braided rivers (e.g. Tal and Paola, 2007; Bertoldi etal., 2015),have shown the potential for this modelling approach. Forthis research, we allowed braided river systems to evolve under constantconditions in two 2.5 m wide by 10 m long flume channels, set-up in the TotalEnvironments Simulator at the University of Hull. Once equilibrium conditionswere reached under bare sediment conditions, Alfalfa was periodically seededand allowed to germinate and then grow for different time periods. After eachperiod of growth, we used a sequence of high and low magnitude flood events inorder to capture change in morphology arising from the different sequences ofevents. Morphological changes were captured using a terrestrial laser scannerto collect over 50 DEMs. TheseDEMs demonstrate the impact surrogate vegetation has on both stabilizing theriver system and modifying the patterns of bank erosion and channel migrationin a braided system. Furthermore, frequent spatial analysis of erosion anddeposition based on the flow direction within the channel network allowed us todetermine erosion and deposition width for individual segments of the riversystem. Collating these segments yields a large dataset that indicates both bankerosion rates and volumetric channel migration rates for different stages of vegetationgrowth/maturity under different sequences of flood events.