One of the main factors influencing debris flow occurrence is water, with heavy and/or sustained rainfall in particular triggering the majority of landslide events. At present in Scotland the amount of rain falling during storm events, or in the period preceding the occurrence of a landslide, is relatively unquantified. It is generally accepted, however, that debris flows can be initiated either by long periods of rainfall or shorter intense storms.

Following the events of August 2004 it was concluded that a system of rain gauges should be installed at key locations in Scotland, with the intention of facilitating a greater understanding of the amount of rainfall which would cause instabilities at debris flow risk sites. Detailed scrutiny of the rainfall leading up to debris flow events would enable the development of a rainfall ‘trigger level’ above which it is likely that debris flow will occur. This would, in turn, allow the forecasting of periods during which such events might take place.

In the long term, a management strategy is required. This would include protocols for action, increased surveillance when predetermined levels of rainfall are exceeded and the potential for road closures to protect road users.

Forecasting Methods

Rainfall analysis is the most frequently adopted approach for forecasting landslides and worldwide observations have helped to identify the minimum and maximum volume of rain required over various periods of time to trigger these events.

It is widely accepted that Scottish debris flow events are usually preceded by extended periods of heavy, antecedent, rainfall prior to the storm that may trigger the event itself. Evidence of the influence of rainfall on landslide events has been gathered following events in many places in the world.

Scotland’s rainfall patterns can be broadly divided into two zones covering the east and west of the country. The Met Office indicates that in the east rainfall generally peaks in August while in the west the maximum rainfall levels are reached during the wider period September to January. While rainfall levels in the west are relatively low in August they do increase from a low point in May.

Soil may therefore undergo a transition from a dry to a wetter state around August, leading to increased potential for debris flow and other forms of landslide activity at that time.

The analysis and interpretation of climate change models for Scotland suggests that there may be the potential for an increase in future debris flow activity in Scotland, as rainfall becomes more concentrated as well as becoming more intense (i.e. it is concentrated into fewer, higher magnitude events).

A Trigger Threshold for Scotland

Forecasting of conditions which could lead to debris flow is limited at present, as the rainfall gauge network in Scotland is sparse in most of the areas of interest. Although the existing system does cover some of the areas of interest, the outputs are not sufficiently detailed and more accurate data would be required.

Three hypothetical threshold levels have been outlined:

  • A threshold level above which debris flow might be expected to occur.
  • A lower threshold level at which a warning could be issued and action taken, giving adequate lead-in time for these to be effective.
  • A still lower threshold level is set at which instruments are checked and key personnel alerted that conditions likely to lead to debris flow are developing, essentially the last step before issuing a warning.

Work has been undertaken to back analyse rainfall data from past debris flow events in order to develop a threshold level above which debris flow might be expected. Ongoing work is aimed at analysing more recent events in order to allow the further development and validation of this threshold level. The first such analysis, that of the storm which led to the debris flow event at the A83 Rest and be Thankful on 28 October 2007, indicates that the tentative debris flow threshold developed from the back analysis shows some promise for practical use.

High quality data from a variety of geographical locations will be needed in order to validate and/or change the threshold prior to its introduction as a management tool for the road network. Due to the frequency of such major events in Scotland, this process may take approximately five years.