Vegetation age (size) reduces the morphological impact of river flooding

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

Auckland

Simposio; River, Coastal and Estuarine Morphodynamics; 2019

NZ Rivers gropu, IHAR

Flow-stage variation is known to impact braided channelpattern parameters more than single thread channels,because braided rivers are characterized by higherwidth/depth ratios and are strongly dependent onexposure of bar-scale topography (Bridge, 1993). Asfuture variability in hydro-climate is set to increase overlarge parts of the globe due to the impacts of climatechange, variability in flow stage will likelydisproportionately alter the morphodynamics of braidedchannel systems. It is also now widely accepted thatfluvial process cannot be well understood withoutconsidering biological influences. Changes in vegetationas a result of climate and land use change will also likelyimpact river systems and their longer-term response.Indeed, scaled flume studies have shown how changes invegetation density can have a significant control onhydrogeomorphic processes (Gran and Paola, 2001).The interactions of changing flood frequency, droughtand alterations in vegetation growth patterns on fluvialsystem response remain very poorly understood. Thisstudy addresses this gap in understanding with the mainresearch question: how do the channel planform andchannel dynamics in a braided river reach respond to aflood sequencing during vegetation growth stages? Wemodelled the hydro-morphodynamics of a river reachsubjected to a periodic change of the discharge (floodsequence) whist simultaneously growing vegetation onbar surfaces, to address this research question. Theexperimental procedure tested the impact of timescaleson recovery and explored how multiple perturbationsimpacted morphodynamic responses to flood sequences.Experimental Set-upThe experimental data was collected in the TotalEnvironment Simulator (TES), University of Hull. TheTES was divided into two mobile bed flumes (2.5 mwide, 10 m long) with constant longitudinal slope(0.015) and uniform grain size (ds = d50 = 0.45 mm). Theflow discharge was controlled at the source pointupstream and was adjusted for changes in time tosimulate a flood sequence of low and high floods. Twosediment supply conditions were considered:equilibrium (balance between transport capacity andsupply) and deficit. For the vegetation, alfalfa seedswere uniformly spread on the bed (1 seed cm-2) andallowed to grow before the release of the sequentialfloods. Four different age stages of the vegetation wereinvestigated along with a vegetation dying phase. Abovethe flume, a 3D laser scanner enabled us to produce 1mm precision digital elevation maps (DEMs) of themorphology after each flood event.