Dr. Francesco Comiti Faculty of Science and Technology
Free University of Bolzano-Bozen (Italy)
SEDIMENT AND WOOD TRANSPORT IN MOUNTAIN RIVERS:
DO WE KNOW ENOUGH TO MODEL IT ?
«Misura ciò che è misurabile e rendi misurabile ciò che non è misurabile» «Measure what it is measurable and make measurable what it is not»
Galileo Galilei (1564-1642)
Outline of the presentation
Mountain rivers: main characteristics Few words on debris flows
Bedload transport in mountain rivers: state of the art
Wood transport: hic sunt leones !
Conclusions
• Channels flowing within montane valleys (connection with hillslope processes) • Slope > 0.2-0.3 % (but also lower) • Poorly-sorted bed sediments (gravel, cobbles, boulders, and sand patches)
Mountain rivers
A subset of mountain rivers: steep channels
Montgomery and Buffington (1997)
Montgomery and Buffington (1997)
small W/D ratio (relative width) W
D
h
D small h/D ratio (relative submergence)
• Slope > 3-5 %
• Step-pool and cascade
Characteristics of steep channels
strong bed armouring and structuring
bedforms are rearranged only infrequently (up >30-50 yr)
possibility to feature debris flows
• T. Gadria (A=6.3 km2, S=0.16, Italian Alps) • Q ~ 80-90 m3s-1 (peak Cv ~ 50-60 %) • RI ?
A new monitoring station for debris flows
Video courtesy of Aut. Prov. Bolzano (Dr. P. Macconi)
Rainfall thresholds for debris flow occurrence
1 km
• Very small scale of relevant rainfall events
• Rainfall intensity-duration differ among raingauges
• Critical aspect for early
warning systems
Comiti et al (online)
Transitional processes in SC: debris floods
• Rio Cordon (A=5 km2, S=0.11, Italian Alps) • Q ~ 10 m3s-1 (peak solid Cv ~ 8-10 %) • RI ~ 50-70 yr
Video courtesy of ARPA Veneto (Dr. G. Scussel)
Bedload transport: are MR as lowland rivers ?
ncs Sqqcq ⋅−⋅= )(
• Shear stress (MPM-like) eqs.
• Unit discharge (Schoklitsch-like) eqs.
Equations based on unit discharge preferable in mountain channels
• Unit stream power (Bagnold-like) eqs.
What is the “right” water depth/hydraulic radius ?
Bedload assessment in MR: incipient conditions
• τc* (D50) not constant, increases
with slope up to 0.1-0.2 !
• In SC, hiding/protrusion effects are strong but not enough to lead to equimobility
bici DDa )/( 50
* =τ
Bunte et al. (2013)
Mao et al. (2007, Geomorphology)
b ~ -0.75
• Recent results on a glacier-melt river show near-equimobility conditions (b=-0.9)
• Bedload formulas overestimate ordinary bedload rates in MR by one or more orders of magnitude (Rickenmann 2001)
• Better prediction for large flood events and at lower slopes
(D’Agostino & Lenzi, 1999)
Bedload assessment in MR: rates and volumes
(Rickenmann & Koschni, 2010)
• Is overestimation due to form resistance, limited sediment supply, or both ?
Bedload assessment in MR: transport rates
• Stress partinioning between immobile/mobile grains (Yager et al 2007)
• Energy slope reduction from grain/total resistance ratio (Meyer-Peter & Müller 1948; Nitsche et al., 2011)
Correcting shear stress for form resistance
Bedload assessment in MR: transport rates Accounting for limited bed sediment supply
• Inclusion of bed armouring (Bathurst 2007)
• Surface area of mobile grains (Yager et al 2007)
Bedload assessment in MR
Are we set then ?
Bedload assessment in MR
Not really! Sediment supply from hillslopes and tributaries !
Bedload assessment in MR: at-a-site rating curve
Valid for a given reach (same slope and grain size)
bs QaQ ⋅=
Bunte et al (in preparation)
Highly non-linear (b=2÷20)
Large differences among sites
Lower exponent in steeper channels
Long term bedload transport «efficiency» in the Rio Cordon
0.1
1
10
100
Jan-87
Jan-89
Jan-91
Jan-93
Jan-95
Jan-97
Jan-99
Jan-01
Jan-03
Jan-05
Jan-07
Bed
load
Vol
ume/
Effe
ctiv
e R
unof
f
0
0.5
1
1.5
c fa
ctor
= (H
s/Ls)
/S
BV / ER
14 Sept 1994 flood
11 May 2001 flood
c factor
Ordinary flood events;Capacity and supply limited conditions
Ordinary flood events;Capacity limited condition;Overtime reduction of sediment supply
Extraordinary event;Capacity and supply unlimited conditions
Bedload assessment in MR: at-a-site variations
(Lenzi et al 2004)
b~3
b~8
About same flow discharge, so I guess bedload rates must
be comparable…
Late June Late August
Bedload assessment in MR: at-a-site variations
Saldur basin (18 km2)
Matsch/Mazia glacier
Bedload assessment in MR: Saldur River Direct method: Portable bedload traps (Bunte et al. 2005)
Indirect method: “pipe hydrophone” (Mizuyama et al, 1997)
Collected size > 4mm
Detected size > 2-4 mm
Dell’Agnese et al (in press)
Bedload assessment in MR: Saldur River
2011 acoustic pipe data 2011-2013 Bunte samples
June b~12
July b~10
August b~3
Qc
bs QaQ ⋅=
Small b: high sediment supply already at low flows Large b: low sediment supply, to be eroded from channel/banks
Bedload assessment in MR: Saldur River
June/ July
Aug/ Sept
Mao et al (2014)
A new sediment monitoring station: Solda River
Passo Stelvio
Ortles (3905 m)
Station (1114 m)
Geophone plates
Acoustic pipe
Turbidimeter
Automatic sampler
Conductimeter
“AQUASED” project • CISMA and Mountain-eering srl • Bolzano and Trento Universities • Support from the Aut. Prov. Bolzano
A=130 km2, 18 km2 glaciers
Calibration by a truck-operated modified “Bunte” trap
A new sediment monitoring station: Solda River
Foto: Provincia di La Spezia, AdB Magra.
Wood transport: hic sunt leones !
Wood transport: the Magra-Vara flood event Recruitment mostly from
floodplain erosion (60-70%), landslides also relevant
Wood from floodplain erosion
LW re
crui
ted
(m3 k
m-1
)
L
W d
epos
ited
(m3 k
m-1
)
Wood deposition
Lucia et al (in preparation)
Wood transport: when and how far ? Rienz River (630 km2) Tagged logs tracking along a 5 km reach:
- 55 already present - 51 introduced L=2-10 m D= 0.1-0.5 m
Peak water depth / log diameter
Local morphology and jamming
Lucia et al (in preparation)
Wood transport: volumes prediction
Rickenmann (2014)
Wood volumes recruited during large floods in Switzerland
Wood transport: when and how much ?
Q
Qw
• Hydraulic-based (Bocchiola et a., 2006; Crosato et al., 2013) mobility threshold exceeded frequently every year in most channels
• Channel morphology (width and roughness) controls mobility for a given log size at low-moderate flows
• During large floods, wood supply is the limiting factor
• Supply from landslides less related to Q than bank erosion
?
Qc,w Qbank erosion
Conclusions: so do we know enough of MR ?
• Simplified, unit discharge-based variables better in mountain rivers
• Key role of supply timing and magnitude (stochastic coupling with hillslope, glaciers and tributaries)
• Modelling sediment/wood transport is required to make rational predictions
• Errors in bedlod prediction can be 1-2 orders of magnitude for low-moderate floods. For wood even more !
• Very little field data available to formulate/validate complex models
• Long term monitoring needed, in different settings/hydrological regimes
Conclusions: good data wanted !
• Deployment of surrogates methods (acoustic, tracking)
• Monitoring channel dynamics WITH basin-scale processes
• Flume tests representing the complexity of real mountain rivers (GSD, rough banks, flashy events, bed history)
“Everything must be made as simple as possible. But not simpler than possible”
Albert Einstein (1879 - 1955)
Thank you