The role of vegetation and large wood in braided rivers dynamics Walter Bertoldi Department of Civil, Environmental and Mechanical Engineering University of Trento Séminaire Gestrans, Grenoble, 21 Novembre 2012

Vegetation and wood in braided rivers This work has been possible thanks to the help of many collaborators: Leverhulme Project: Angela Gurnell, Nick Drake, Alex Henshaw Trento Lab work + Tagliamento field work: Matilde Welber, Sandra Zanella, Martino Salvaro, Marco Redolfi, Lorenzo Forti Hydralab project Hull: Luca Mao, Michal Tal, Francesco Comiti, Diego Ravazzolo, Matilde Welber, Sandra Zanella, Rob Thomas + discussions with Peter Ashmore, Hervé Piégay, Nicola Surian

Vegetation and wood in braided rivers 3/30 BRAIDING & VEGETATION

Vegetation and wood in braided rivers 4/30 Gran and Paola, 2001 Tal and Paola, 2007, 2010 Eaton et al Murray and Paola, 2003 Perona et al., 2009 Crosato and Saleh, 2011 BRAIDING & VEGETATION

Vegetation and wood in braided rivers 5/30 TAGLIAMENTO RIVER, ITALY ~ 20 km LONG ~ 800 m WIDE PINZANO GORGE ~ 120 m WIDE

Vegetation and wood in braided rivers 6/30 LiDAR ANALYSIS m Canopy Surface ModelFiltered and de-trended DEMDEM + vegetation height

Vegetation and wood in braided rivers 7/30 Sub-reach 3  Vegetation cover = 1.3% (~0% taller than 5m) Sub-reach 13  Vegetation cover = 16% (~8% taller than 5m) SUB-REACH COMPARISON

Vegetation and wood in braided rivers 8/30 FREQUENCY DISTRIBUTIONS The vegetation induces fine sediment deposition and island aggrading The flow is constrained in narrower and deeper channels (Bertoldi et al., WRR)

Vegetation and wood in braided rivers 9/30 BED STATISTICS Area proportional to tall vegetation % SKEWNESS Positive Skew Negative Skew (Bertoldi et al., WRR)

Vegetation and wood in braided rivers 10/30 Sub-reach 3Sub-reach 13 Euler Gamma pdf (Paola, 1996; Nicholas, 2000) EFFECT ON SEDIMENT FLUX?  ~ 5 ~ 3

Vegetation and wood in braided rivers flow direction CORNINO BANK EROSION AND TREES INPUT 11/30 Where is vegetation coming from?

Vegetation and wood in braided rivers A)26/10/ B)03/11/ C)28/12/ A)B) C) position of island edge BANK EROSION AND TREES INPUT flow direction 12/30

Vegetation and wood in braided rivers A) 18/12/ B) 26/12/ C) 15/01/ position of island edge on 18/12/2009 flow direction A)B) C) BANK EROSION AND TREES INPUT 13/34

Vegetation and wood in braided rivers (A)(B)(C) (E)(F) (D) m N trunk diameter (cm) # of trees flow direction TREES DISTRIBUTION (Bertoldi et al., Geomorphology) 14/30

Vegetation and wood in braided rivers m N m 0.8 m BED ELEVATION DEPOSITED TREES AB flow direction TREES DISTRIBUTION 15/30 (Bertoldi et al., Geomorphology)

Vegetation and wood in braided rivers ELEVATION DISTRIBUTION 16/30 (Bertoldi et al., Geomorphology)

Vegetation and wood in braided rivers ELEVATION DISTRIBUTION 17/30 (Bertoldi et al., Geomorphology)

Vegetation and wood in braided rivers 3 m 25 m 18/30 MODELLING WOOD DISPERSAL Is it possible to model these processes in the lab?  PhD work of Matilde Welber

Vegetation and wood in braided rivers Wood scaling issues: -Diameter (2-6 mm) -Length (4-12 cm) -Density -Presence of roots, branches,... 19/30 MODELLING WOOD DISPERSAL

Vegetation and wood in braided rivers BAR APEXALONG BANKS WOODDEPOSITIONPATTERNS WOOD DEPOSITION PATTERNS 20/30

Vegetation and wood in braided rivers SPATIALVARIABILITY SPATIAL VARIABILITY 5 RUNS WITH THE SAME PARAMETERS COMPLEX SYSTEM + LONG TIME SCALE INPUT 21/30

Vegetation and wood in braided rivers FLOW bar 1bar 4bar 2bar 3 BAR - WOODINTERACTION BAR - WOOD INTERACTION 22/30

Vegetation and wood in braided rivers Q=0.9 l/s Q=1.2 l/s Q=1.5 l/s Q=1.8 l/s Braiding formed with Q = 1.8 l/s ELEVATIONDISTRIBUTION ELEVATION DISTRIBUTION 23/30

Vegetation and wood in braided rivers 2D  high variability 3D  narrow range 24/30 ELEVATIONDISTRIBUTION ELEVATION DISTRIBUTION

Vegetation and wood in braided rivers THE MORPHODYNAMIC IMPACTS OF VEGETATION AND LARGE WOOD IN FLUVIAL SYSTEMS PI:Luca Mao Participants: W. Bertoldi, N. Osei, D. Ravazzolo, M. Tal, M. Welber, S. Zanella UoH staff:S. McLelland, B. Murphy, R. Thomas

Vegetation and wood in braided rivers flow WOOD/VEGETATIONINTERACTION WOOD/VEGETATION INTERACTION Tagliamento 26/30

Vegetation and wood in braided rivers ’Aerial’ pictures to map  Planform configuration  Wood deposit  Vegetation growth EXPERIMENTSSETUP EXPERIMENTS SETUP 27/30

Vegetation and wood in braided rivers Flume 3 Flume 1 WEEK 1WEEK 2WEEK3WEEK 4 flow 28/30 PRELIMINARYRESULTS PRELIMINARY RESULTS

Vegetation and wood in braided rivers Wood dynamics changed significantly even in the first hours with low vegetation  Decrease in wood deposited as single logs from 40% to 25%  <10% in long term  Decrease in wood moved after one hour from 40% to <20%  Increase in wood deposited in already existing groups from 80% in long term 29/30 PRELIMINARYRESULTS PRELIMINARY RESULTS

Vegetation and wood in braided rivers average Low flood effect Low vegetation growth single runs 30/30 average High flood effect Vegetation grows fast single runs  Similar distributions  Different distributions Modelling vegetation after a sequence of floods (Gurnell et al., Earth-Science Reviews) VEGETATION AND FLOODS

QUESTIONS ?

9 positions (4 EU + 5 Non EU) Application deadline: January 15, 2013

Vegetation and wood in braided rivers 33/34 FLOW-VEGETATION INTERACTION (Gurnell et al., Earth-Science Reviews)

Vegetation and wood in braided rivers 34/22 ~ 1 m Courtesy of Jonathan Laronne BRAIDING AT DWARF SCALE