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Velocity attenuation of debris flows and a new momentum-based load model for rigid barriers

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An Erratum to this article was published on 11 July 2016

Abstract

Effective design of mitigation measures against debris flow hazards remains a challenging geotechnical problem. At present, a pseudo-static approach is commonly used for the calculation of impact load acting on a rigid debris-resisting barrier. The impact load is normally calculated based on the maximum velocity observed in the transportation zone under free-field conditions without considering debris-barrier interaction. In reality, the impact load acting on a barrier varies with the change of debris momentum flux but this is seldom considered in barrier design. To provide a scientific basis for assessing debris momentum flux during impact, this paper presents results from a study of debris-barrier interaction using physical flume modelling. This study showed that, following the first stage of impact, the accumulated debris behind a barrier formed a stationary zone and caused the remaining debris to slow down in a run-up process. In the experiments, the peak debris momentum was 30 % lower compared to that observed under free-field conditions. A new momentum-based model was developed to take into account attenuation of momentum flux for predicting debris impact load on rigid barriers. The new rationalised model was assessed using data from the notable Yu Tung Road debris flow in Hong Kong. The assessment showed that the design bending moment at the base of the barrier wall could be reduced more than 30 % using the proposed model, compared with the current design approach. The adoption of the proposed model could offer a new opportunity for practitioners to optimise the design of rigid barriers.

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Abbreviations

α :

Dynamic pressure coefficient

β :

Empirical factor to account for the dynamic effect of flow impact

ρ :

Debris density

θ :

Channel slope angle

θ d :

Deposition angle

ϕ :

Internal friction angle

δ :

Basal (interface) friction angle

g :

Gravitational acceleration

h :

Free-field debris thickness

h d :

Debris run-up height

h ret :

Design retaining height of debris

m :

Mass of debris

t :

Time

v :

Debris velocity

v p :

Peak debris velocity

v :

Free-field debris velocity (no barrier)

w :

Barrier width

BM:

Bending moment per unit width of barrier

E R :

Energy loss due to frictional resistance

F :

Total force acting on a barrier

F d :

Dynamic impact force

Fr:

Froude number

F ru :

Run-up impact force

F s :

Static force due to lateral earth pressure of the deposited materials

L :

Debris’ travel distance during the run-up process

M :

Momentum flux

N :

Weight of debris acting on slope surface

R :

Frictional resistance between moving and stationary debris

R m :

Momentum reduction factor due to debris-barrier interaction

R ru :

Velocity reduction factor due to frictional resistance between moving and stationary debris and conversion from kinetic to potential energies during run-up

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Acknowledgments

This paper is published with the permission of the Head of the Geotechnical Engineering Office and the Director of Civil Engineering and Development, Hong Kong SAR Government. The work described in this paper was supported by a grant from the Research Grants Council of the Hong Kong SAR (HKUST 06/CRF/12R and T22-603/15N). The authors would also like to acknowledge the support of the HKUST Jockey Club Institute of Advanced Study.

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Correspondence to R. C. H. Koo.

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An erratum to this article is available at http://dx.doi.org/10.1007/s10346-016-0730-6.

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Koo, R.C.H., Kwan, J.S.H., Ng, C.W.W. et al. Velocity attenuation of debris flows and a new momentum-based load model for rigid barriers. Landslides 14, 617–629 (2017). https://doi.org/10.1007/s10346-016-0715-5

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