OR/17/031 The debris flow susceptibility dataset

Bee, E J, Pennington, C V L, Dashwood, C and Lee, K. 2017. A User Guide for the GeoSure Extra: Debris Flow Susceptibility Model for Great Britain (version 6.0) . British Geological Survey Open Report, OR/17/031.

Background

The public’s understanding of the effect of ground conditions on the safety of their property and the implication for the value of their property is growing. Local councils are under increasing pressure from central government to provide environmental information. Information about geological hazards is needed, in particular, the identification of areas with a potential for ground movement.

In response to this, the BGS initiated a development programme to produce datasets that identified and assessed potential geohazards threatening the human environment in Great Britain: GeoSure. GeoSure ground stability data consist of six data layers in GIS format that identify areas of potential hazard in Great Britain. One of these six layers is concerned with landslides and best simulates shallow translational and rotational landslide types. In order to improve and widen this capability to include debris flows, the debris flow susceptibility dataset has been created.

After a series of debris flows in 2004 that affected main roads in Scotland, the Scottish Road Network Landslides Study was commissioned by the Scottish Executive (Winter et al., 2005). This review assessed the slopes adjacent to the trunk road network and identified areas that had the greatest potential for similar events in the future. The Debris Flow Susceptibility Model described here builds on the earlier work carried out, updates the data sources and extends the coverage to the whole of Great Britain.

What are debris flows?

The term debris flow refers to the rapid downslope flow of poorly-sorted debris mixed with water (Ballantyne, 2004). Debris flows are described by Hungr et al. (2014) as: 'very rapid to extremely rapid surging flow of saturated debris in a steep channel. Strong entrainment of material and water from the flow path'. They are a widespread phenomenon in mountainous terrain and are distinct from other types of landslides as they can occur periodically on established paths, usually gullies and first- or second-order drainage channels. Debris flows in Great Britain are most commonly found in upland Scotland but also in parts of Wales and the Lake District (e.g. Figure 1).

Figure 1 Examples of debris flows in Lairig Ghru, Cairngorms, Scotland. Photograph courtesy GoogleEarth: Getmapping plc 2016.

In Great Britain, there are two types of debris flow (Figure 2]; Cruden and Varnes, 1996; Ballantyne, 2004; Nettleton et al., 2005):

Hill Slope or Open-Slope Debris Flows form their own path down valley slopes as tracks or sheets and deposit material on the lower slopes where the gradient shallows.

Valley-confined or Channelised Debris Flows originate in bedrock gullies and are channelled for at least part of their length along the gully floor. The flows have the consistence equivalent to that of wet concrete and can be fronted by a boulder concentration or ‘head’.

The two categories are transitional; many valley-confined flows debouch on to open ground in their lower reaches, and hillslope flows often follow shallow gullies cut in valley-side drift, talus or regolith.

Figure 2 Hillslope (a) and channelised debris flow (b) Taken from Nettleton et al. (2005) © Crown Copyright 2005.

Figure 3 Debris flows in Lairig Grhu, Cairngorms, Scotland showing levée features. Photograph courtesy GoogleEarth: Getmapping plc 2016.

Debris flows consist of three main parts: a source area, track and depositional area.

Source areas may be initiated by a slide, debris avalanche or rock fall from a steep bank, or by spontaneous instability of the steep stream bed (Hungr et al., 2014). Irrespective of the mode of flow initiation, debris flows are generated when a build-up of pore water pressures in unconsolidated sediments causes a reduction in the shear resistance, leading to failure and sediment flow (Ballantyne, 2004). Most debris flows in Great Britain occur following a period of high magnitude precipitation events.

Debris flows tend to follow long, narrow tracks. The upper, erosional section of the flow consists of a gully that is continued downslope by parallel levées (e.g. Figure 3]) of dominantly coarse debris that enclose the track of the flow and often terminate downslope in one or more lobes of bouldery debris (Ballantyne, 2004).

Finally, the debris flow event is deposited on a debris flow fan. Characteristic features used to distinguish debris flow material from other sediment on a fan include high slope angle of the fan, very large individual particles, coarse levées and boulder trains, signs of impact loading on obstacles, U-shaped eroded channels and steep, debris-loaded channels upstream (Hungr et al., 2014).

Debris flow impacts

Debris flows are potentially very destructive as they can cause significant erosion of the substrates over which they flow, thereby increasing their sediment charge and further increasing their erosive capabilities (Nettleton et al., 2005). The Scottish road and rail networks in particular have been affected by debris flows. Recent disruptive events include:

• the A85 road at Glen Ogle where 57 people were stranded on the roadway between two debris flows (Figure 4; British Geological Survey, 2006);
• the western slopes of Stob Coire Sgriodain by Loch Treig, in the Scottish Highlands where a train was derailed (Figure 5; British Geological Survey, 2012); and
• the A83 Rest and Be Thankful Pass, the most widely reported locality for debris flows, much more than any other part of the trunk road network in Scotland (Figure 6 and Figure 7; Winter et al., 2013). The Rest and Be Thankful is the main route between Arrochar and Inverary through mountainous terrain. When this road is closed, a 55- mile detour and associated high economic consequences are regularly reported in the media. Wig-wag warning systems (Winter et al., 2013) were installed in 2011 and ten bespoke debris flow barriers in 2014 (Maccaferi, 2014).

Figure 4 A85 Debris Flow at Glen Ogle. 57 people were trapped in their cars. P5773044.

Figure 5 Stob Coire Sgriodain by Loch Treig, in the Scottish Highlands where a train was derailed. Right photograph used with permission of Route Asset Manager, Geotechnics Scotland.

Figure 6 Debris flow deposit blocking the A83 Rest and Be Thankful, August 2012. P785432.

Figure 7 Recent debris flows on the A83 Rest and Be Thankful Pass.

Who might require this dataset

Debris flows may lead to financial loss for anyone involved in the ownership or management of property, including developers, householders, loss adjusters, surveyors or local government. These costs could include increased insurance premiums, depressed house prices and, in some cases, engineering works to stabilise land or property. These hazards may also impact on anyone involved in the construction or maintenance of infrastructure networks (road or rail) or utility companies. Armed with knowledge about potential debris flows, preventative steps can be put in place to alleviate the impact of the hazard to people and assets (such as property). The cost of such prevention may be low, and is often many times lower than the repair bill following ground movement.

What the dataset shows

This addition to the GeoSure ground stability data consists of a single data layer in GIS format that identifies areas of potential debris flow hazard. It is essentially a national hazard susceptibility map. These data have been produced by engineering geologists and geospatial data analysts at the British Geological Survey and is presented as a digital spatial data layer. It is important to remember that the focus in this dataset was the identification of source areas and so not all of the track and area where material may be deposited will be covered for long run-out failures.

Debris flows occur when particular slope characteristics (such as regolith, gradient, drainage, sources of water, or the actions of people) combine to make the slope unstable. Debris flows are potentially very destructive and, due to the speed at which they take place, can for example rapidly block infrastructure routes and damage assets.