Quantifying river?s response over plant life stages during a sequence of floods

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

Conferencia; European Geosciences Union; 2019

With river flooding being more frequent due to climate change, the interaction betweenfluvial morphodynamics and riparian vegetation may depend in part on the sequence of theflood events. We report a laboratory study on the mechanism of bed morphology adjustmentin response to vegetation growth and flood sequencing. A set of 60 runs was performed in amobile bed flume (10 m x 2.5 m), with constant longitudinal slope (0.015) and uniform grainsize (0.46 mm) in the Total Environment Simulator at the University of Hull, UK, to identifythe critical role played by vegetation (alfalfa) during the release of a sequence of low andhigh magnitude flood events. Our setting did not allow recovery of neither morphology norvegetation in between floods, as the goal was to model the situation where the subsequentevent occurred before the system had fully recovered from the previous perturbation.Sediment conditions included runs with equilibrium loads and floods with sediment deficit(in which transport capacity exceeded supply) to quantify the sensitivity of the river?sresponse to sediment variations within the supply. In the runs with vegetation, four differentgrowth periods for the alfalfa were investigated (4,8,14 and 20 days) along with a dyingphase, to get insights into the relative role of the plant?s age (size) in the geomorphic impactof the sequential floods. The outcomes of each run were characterized by a detailed digitalelevation model, digital imagery and continuous monitoring of the sediment transportedthrough the flume outlet.Observations indicate that both the removal of plants by bank undercutting and sedimenttransport efficiency at the river reach scale adjust with the life stage of the plants. Thebraiding intensity (computed directly from topographic data) increases as the flood-survivalplants growth, and it becomes less sensitive to flow variability. Under this configuration, theimpact of vegetation patchiness becomes more important since it leads to localised erosion byobstructing the flow and encouraging bar development. To some extent, the antecedent flowconditions are important on the system response (filling/eroding), and the same lowmagnitudeflood impacts differently if it is before or after the high-magnitude flood. Further,an elevation-based distribution function approach (Redolfi et al., 2016) is used to quantify thecomplexity, and explaining the dynamic response of the channels to plant?s age at the reachscale. In particular, the cross-section shape of the rivers computed by the b(D) curve (whereb: width, D: depth) illustrates how higher vegetation might mimic the effect of channelconfinement.Overall, results indicate that vegetation growth can have a significant influence on channelpattern and planform dynamics, and a threshold in plant?s size can enhance the impact of thevariations in flood sequencing and reduce the resilience of the system. This knowledge isparticularly important for restoration and risk management of fluvial environments whencontemplating changes in flood magnitude and frequency due to climate change or otherlong-term drivers.