Discussion
• Sediment sorting and morphology of the experimental debris flows is similar to natural debris flows (Fig. 7).
• Width-to-depth ratio of the experimental debris-flow channels is in the range of natural debris flows.
Runout length (or travel distance) is relatively restricted and at the lower range of natural debris flows (Fig. 8).
• Lobe height generally determined by the frontal accumulation of coarse particles, not by yield strength.
Key experimental results
• The small-scale experimental debris flows comprised multiple surges, coarse particles accumulated at the flow front, and were subsequently shouldered aside to deposit in lateral levees by a more liquefied flow body. This resulted in strong sorting, with the coarsest particles concentrated in lateral levees and at the frontal margins (Fig. 3).
Conclusions
• We experimentally created unconfined small-scale debris flows with self-formed levees and a marked depositional lobe.
• Flow dynamics, deposit morphology and sediment sorting were similar to natural debris flows.
• Debris-flow composition has a profound effect on runout distance and depositional mechanism.
Therefore, compositional effects should be incorporated in runout distance predictors.
• There is an optimum runout distance for gravel and clay fraction, whereas runout increases with water fraction (latter result not shown on this poster).
• Debris-flow deposition is primary governed by friction at the flow front in most debris flows, but in debris flows with a very high clay content high viscosity and yield strength govern deposition.
Acknowledgements: Support by the Physical Geography Laboratory at UU (Chris Roosendaal, Henk Markies and Marcel van Maarseveen) was essential for this reseach.
Funding: TdH is supported by NWO grant ALW-GO-PL17-2012 to MGK.
Future experimental work:
• Debris-flow fans: studying the autogenic dynamics of debris-flow fans (Fig. 9).
• Debris-flow erosion: studying the erosive potential of debris-flows of various composition (Fig. 10).
0 0.5 1 1.5
Runout distance (m )
0 1 2 3
Flow velocity (m/s)
0 0.01 0.02 0.03 0.04
Levee height (m )
0 0.02 0.04 0.06 0.08
Lobe height (m )
0 0.1 0.2 0.3 0.4
0 0.1 0.2 0.3 0.4
Lobe width (m )
Clay fraction (−)
0 0.1 0.2 0.3 0.4
0 100 200 300
Yield strength (Pa)
Clay fraction (−)
0 0.1 0.2 0.3 0.4
0 0.002 0.004 0.006 0.008 0.01
Diffusivity (m
2/s)
Clay fraction (−)
a b
c d e
f g
10 −2 10 −1 10 0 10 1 10 2
10 −3 10 −2 10 −1 10 0 10 1
Channel depth (m)
10 0.2 10 0.4 10 0.6
10 −1 10 0
Elevation difference (m )
Travel distance (m)
L/E = 20 L/E = 5 L/E = 1
L/E = 2
W/D = 1
W/D = 5
W/D = 10 W/D = 25 W/D = 50
Channel width (m)
Experimental debris flows Rickenmann, 1999
De Ruig & Hoozemans, 1986 Bulmer et al., 2002
L/E = 10 Typical range of natural debris flows
a b
c d
Fig. 9) Debris-flow fan after 54 stacked debris flows. Fig. 10) Debris-flow erosion experiment. We use an initial bed layered with colored sand, in order to determine the erosive depth in the runout zone.
Fig. 7) Comparison between sediment sorting of experimental (a,c) and natural (b,d)
debris flows. Fig. 8) Comparison of the dimensions of experimental and unconfined and confined natural debris flows.
Fig. 5) Flow, morphological and geotechnical properties as a function of gravel fraction in otherwise
the same conditions. The solid line connects the values averaged by gravel fraction class. Fig. 6) Flow, morphological and geotechnical properties as a function of clay fraction in otherwise the same conditions. The solid line connects the values averaged by clay fraction class.
Fig. 3) Morphology and sediment sorting of selected debris flows. F
gdenotes gravel fraction, F
cdenotes clay fraction.
• Clear optimum between runout distance and gravel fraction (Fig. 5). Low gravel fraction: levees insignificant , causing lateral spreading and small runout length. More gravel: increased collisional forces, enhanced levee formation, longer runout. Very high gravel fractions: reduced runout by large resistive coarse-grained flow front. Deposition induced by frontal resistance.
• Clear optimum between runout distance and clay fraction (Fig. 6). Clay fraction up to 0.2: clay suspension in pore-fluid, liquefying the flow and increasing runout. Larger clay fractions: viscous flows, very high yield strength, strongly decreased runout distance. Deposition induced by viscosity and yield strength in clay-rich flows.
Martian
Effects of debris-flow composition on runout distance and depositional mechanisms in laboratory experiments
Tjalling de Haas 1,P , Lisanne Braat 2 , Jasper R. F. W. Leuven 2 , Ivar R. Lokhorst 2 & Maarten G. Kleinhans 1
1) Utrecht Univeristy, Faculty of Geosciences, Department of Physical Geography. 2) Students Earth Sciences, Utrecht University. P) Presenting author, t.dehaas@uu.nl www.geog.uu.nl/fg/mkleinhans
NH51B-3846
Introduction
Debris-flow composition
• Debris-flow composition (i.e., rheology) is generally neglected in runout distance predictions.
• The effects of debris-flow composition on debris-flow depositional mechanisms is poorly understood.
Need for small-scale experiments
• The effects of debris-flow composition on runout distance and depositional mechanism have been largely neglected for practical reasons.
• Experiments enable detailed control of boundary conditions, such as debris-flow composition. However, unconfined experimental debris flows with self-formed levees and a marked lobe have only been formed in the large-scale USGS laboratory flume, and have not been formed in smaller-scale flumes to date.
Objectives
• We aim to:
- Experimentally create unconfined small-scale debris flows that show similar flow behavior, grain segregation and deposit morphology as natural debris flows.
- Evaluate the effects of debris-flow composition on runout distance and depositional mechanisms.
Tjalling de Haas
a b c
d Decreasing gravel content e f
Increasing clay content
F g = 64, F c = 0.88 F g = 47, F c = 1.3 F g = 18, F c = 4.4
F g = 47, F c = 1.3
F g = 16, F c = 14 F g = 14, F c = 23
0 0.5
1 1.5
Runout distance (m )
0 1 2 3
Flow velocity (m/s)
0 0.01 0.02 0.03 0.04
Levee height (m )
0 0.02 0.04 0.06 0.08
Lobe height (m )
0 0.2 0.4 0.6 0.8
0 0.1 0.2 0.3 0.4
Lobe width (m )
Gravel fraction (−)
0 0.2 0.4 0.6 0.8
0 10 20 30 40 50
Yield strength (Pa)
Gravel fraction (−)
0 0.2 0.4 0.6 0.8
0 0.02 0.04 0.06 0.08 0.1
Diffusivity (m
2/s)
Gravel fraction (−)
a b
c d e
f g
Methods
Experimental setup
C Z
a b
a
θ
~ 3 m
0.12 m 1.45 m
1.20 m
2.0 m
0.5 m 0.3 m
outflow plain
channel
PLAN VIEW
CROSS-SECTIONAL VIEW
mixing tank hatch
hatch
mixing tank
0.3 m
