SEDIMENT AND WOOD TRANSPORT IN MOUNTAIN RIVERS: DO … · Mountain rivers . A subset of mountain...

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