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Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/nhess-2019-11
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/nhess-2019-11
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 Feb 2019

Research article | 14 Feb 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Natural Hazards and Earth System Sciences (NHESS).

Significance of substrate soil moisture content for rockfall hazard assessment

Louise M. Vick1, Valerie Zimmer2, Christopher White3, Chris Massey4, and Tim Davies5 Louise M. Vick et al.
  • 1Institute of Geosciences, UiT The Arctic University of Norway, Dramsveien 201, Tromsø 9009, Norway
  • 2State Water Resources Control Board, 1001 I Street, Sacramento, California 95814, USA
  • 3Resource Development Consultants Limited, 8/308 Queen Street East, Hastings, Hawkes Bay, New Zealand
  • 4GNS Science, 1 Fairway Drive, Avalon 5010, New Zealand
  • 5Department of Geological Sciences, University of Canterbury, Christchurch 8041, New Zealand

Abstract. Rockfall modelling is an essential tool for hazard analysis in steep terrain. Calibrating terrain parameters ensures that the model results accurately represent the site-specific hazard. Parameterizing rockfall models is challenging because rockfall runout is highly sensitive to initial conditions, rock shape, size and material properties, terrain morphology, and terrain material properties. This contribution examines the mechanics of terrain scarring due to rockfall on the Port Hills of Christchurch, New Zealand. We use field-scale testing and laboratory direct-shear testing to quantify how the changing moisture content of the loessial soils can influence its strength from soft to hard, and vice versa.

We calibrate the three-dimensional rockfall model RAMMS by back analysing several well-documented rockfall events, adopting dry loessial soil conditions. We then test the calibrated dry model by adopting wet loessial soil conditions. The calibrated dry model over-predicts the runout distance when wet loessial soil conditions are assumed. We hypothesis that this is because both the shear strength and stiffness of wet loess are reduced relative to the dry loess, resulting in a higher damping effect on boulder dynamics. For realistic and conservative rockfall modelling, the maximum credible hazard must be assumed; for rockfall on loess slopes, the maximum credible hazard occurs during dry soil conditions.

Louise M. Vick et al.
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Louise M. Vick et al.
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Short summary
Rockfall boulders can travel long distances down slope and it is important to predict how far to prevent fatalities. A comparison of earthquake data from New Zealand during summer and full-scale rockfall experiments in the same soil during winter shows that during dry seasons boulders travel further downslope because the soil is harder. When using predictive tools, engineers and geologists should take soil conditions (and seasonal variations thereof) in to account.
Rockfall boulders can travel long distances down slope and it is important to predict how far to...
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