Experiments by Invited Researchers


The morphodynamic impacts of vegetation and large wood in fluvial systems

Project acronym: HyIV-Hull-01
Name of Group Leader: Luca Mao
User-Project Title: The morphodynamic impacts of vegetation and large wood in fluvial systems
Facility: TES
Data Management Report: Report

User-Project Objectives

Both riparian vegetation and dead logs play a crucial role in shaping and determining the morphodynamics of rivers systems. However, vegetation and logs seems to exert opposite effects, the former reducing channel width and braiding index, and the latter increasing channel width and dynamics. The planned experiments are aimed at overcoming the substantial lack of evidences on the contrasting effects of vegetation and logs in braiding systems. A series of experiments will be run using vegetation (alfalfa) and wood pieces separately and simultaneously in order to address the following specific objectives: •To determine the role of vegetation, large wood and of both together on modifying the planform and the bed configuration of gravel-bed braided rivers, particularly channel width, depth and braiding index; •To quantify the morphological impacts (channel width and braiding index) as well as the degree of wood dynamic motions (uncongested/congested) and wood jams patterns under different conditions of log feeding rates.

Short description of the work carried out

Three parallel channels were set-up in the TES to address the proposed objectives of the project. The first stage of the project involved the development of steady-state braiding in each of the channels. Then simulated large woody debris were introduced upstream at different feed rates for each channel (0.6, 1.3 and 2 logs per minute respectively) to establish the behaviour of large woody debris within a braided system. The large woody debris were then cleared from the channels each channel was seeded with alfalfa to simulate the growth of vegetation. The seeding was repeated twice at 4 day intervals to achieve a dense cover of vegetation. Then simulated large woody debris was fed into two channels using the lowest and highest rates from the previous tests. During the experiments detailed photogrammetry was used to map the interactions among morphology, woody debris and vegetation. At the beginning and end of each stage a laser scanner was used to enable a DEM to be produced. Three key difficulties were encountered during the experiments due to the time available: (i) it was difficult to fully test whether equilibrium conditions had been achieved, (ii) the number of laser scans that could be completed was limited which had an impact on the final scan resolution, and (iii) it was difficult to predict the correct seeding density and to maintain the healthy growth of the vegetation. Despite these difficulties the primary objectives were achieved.

Highlights of important research results

The initial analysis of data from these experiments has produced the following results: •The input of large wood alone causes minimal change to the braiding index; •In a channel without vegetation, the near-equilibrium storage of wood depends directly on the wood input rate; •Vegetation encroaching onto the floodplain reduces braiding index and channel width; •The input of large wood to a channel with a vegetated floodplain leads to a further decrease in the braiding index; •The wood storage changes substantially when riparian vegetation encroaches the banks, and depends on the degree of organization of the jams; •Wood jams are less frequent but larger in size in a channel with riparian vegetation. Further analysis of the laser scan surveys will be used to assess in detail the concurrent effects of riparian vegetation and wood on channel morphology. The vertical time-lapse imagery from the photgrammetry will be used to investigate the temporal evolution of vegetation encroachment and removal as well as the size, pattern, and spatial distribution of wood jams. Initial results were presented at the EGU conference in Vienna during April 2012.

Bertoldi, W., Mao L., Comiti F., Gurnell A., McLelland S., Ravazzolo D., Tal M., Welber M., Zanella S.The effect of input load on large wood dynamics in braided streams: preliminary results from physical modeling , 2013 8th International Conference on Geomorphology, held 27-31 August, 2013 in Paris, France, p.810.
Mao L., Bertoldi, W., Comiti F., Gurnell A., Gurnell, A.M., McLelland S., Osei, N., Ravazzolo D., Tal M., Welber M., Zanella S. The morphodynamic impact of vegetation and large wood on fluvial systems , 2012 EGU General Assembly 2012, held 22-27 April, 2012 in Vienna, Austria, p.4369.
Walter Bertoldi, Matilde Welber, Luca Mao, Sandra Zanella and Francesco Comiti A flume experiment on wood storage and remobilization in braided river systems , 2014 Earth Surface Processes and Landforms, Volume 39, Issue 6, pages 804?813, May 2014