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Summary of key note speech for 6th International Conference on Debris Flow Hazards Mitigation
Debris-flow hazard assessment and methods applied
in engineering practice
Dieter RICKENMANN
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1 Swiss Federal Research Institute WSL, Birmensdorf, Switzerland
Debris flows constitute a major natural hazard in mountainous regions. The
main elements required for a practical hazard assessment include the following
steps: (i) estimation of potential initiation zones and sediment sources; (ii)
establishment of a relation between the magnitude and frequency of expected future
debris-flow events; and (iii) assessment of the flow behavior and delineation of
areas potentially endangered by flowing debris.
In the first part I will discuss some approaches that are being used to estimate
the magnitude of a so-called “design” debris-flow event. Some studies proposed to
determine a (water) runoff hydrograph based in rainfall-runoff simulations, and
then to use a bulking factor to estimate the entrained sediment volume, assuming
that the resulting debris-flow “sedigraph” has a similar shape as the runoff
hydrograph. Although water input may be a limiting factor in very small
catchments regarding total sediment entrainment by debris flows, the mechanisms
leading to debris-flow formation are typically more complex than implied by the
above simplified approach. As a result, the maximum discharge of a debris-flow
surge may not be linked to the peak runoff discharge produced by the triggering
rainfall event. Quantitative methods to estimate the magnitude of contributing point
sediment sources (slope instabilities) or to determine sediment erosion and
entrainment along a channel are still very limited. Therefore, it appears to be a
common engineering practice to a perform a field-based geomporphologic
assessment of these two main processes to arrive at a “design” event magnitude
with an assumed occurrence probability. It is often an equally challenging task to
establish a link between debris-flow magnitude and event frequency. For this,
historical information on past debris-flow activity is important, even if event sizes
are often only known semi-quantitatively at best. Other techniques exist for
debris-flow dating of past events but they are typically costly and may only provide
part of the required information.
In the second part I will focus on the flow “behavior”, first looking at simple
models of runout modelling and then discussing the application of numerical
simulation models. Simple empirical approaches to estimate the runout distance are
often based on the potential volume of a debris-flows event. The use of simple
approaches demonstrates the importance of the total debris-flow volume and of the
topography on the runout distance and on the depositional pattern. More detailed
information on potentially affected areas may be obtained by using more physically
based numerical simulation models. It is recognized that the flow behavior is very
complex. A difficulty related to selecting an appropriate simulation model is the
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large variability of material compositions and water contents. Rigorous criteria are
largely lacking to distinguish between appropriate flow regimes for the spectrum of
debris flows which can be expected in a given catchment. Most simulation models
require some calibration of the parameters. Before performing simulations to
predict the flow intensity and the affected areas on a fan, it is highly recommended
to make a prior calibration of the model parameters based on observations of past
field events on the same fan if possible. An example application for a Swiss debris
fan illustrates the variability of the results when using three different debris-flow
simulation models, even though all three models were first calibrated based on the
observed deposition areas of a past event.
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