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7 HIGH HAZARD AREAS AND EARLY OPPORTUNITIES IN SCOTLAND

by M G Winter, F Macgregor and L Shackman

With the bringing together of such a range of geotechnical specialists, the majority of whom have a detailed knowledge of the Scottish trunk road network, the Project Workshop presented an excellent opportunity to identify what might be termed 'at-risk' sites on the network, where early investigation of potential debris flow occurrence would be likely to be productive.

The early identification of high hazard sites, on a subjective basis by acknowledged specialists, would serve the joint functions of assisting prioritisation of areas for action under Part 2 of the study, whilst providing, in parallel, a shortlist of sites appropriate for validating the debris flow hazard model in its development phase.

A listing of the areas considered to present a sufficiently high hazard to warrant concern was set out. Subsequent to the Project Workshop, Digital Ordnance Survey mapping at the 1:50,000 scale was used to inspect the areas identified in both plan form and also using a digital elevation model built into the software. The identified areas have been described in terms of their layout relative to adjacent steep slopes, watercourses, lochs and other features. Approximate distances between significant locations along the road have also been given.

The sites identified (in the order in which they were suggested at the Workshop, and not in any order of perceived hazard or hazard ranking) are set out in the following section.

7.1 AREAS OF HIGH PERCEIVED HAZARD

A83 Ardgarten to Loch Shira (29km)

This includes the stretch of road between the Rest and be Thankful and Cairndow and is characterised by the steep slopes above and below it, to the north-east and south-west respectively.

From Ardgarten (NN 27500 03150) the road ascends to the Rest and be Thankful (NN 22960 07450), a distance of around 7km, first in the base of the valley and then, as it gains elevation, on sidelong ground high above and to the north-east of the valley floor. The road then passes above Loch Restil and along the valley floor, before running on sidelong ground along the north side of Glen Kinglas. With the steep slopes of the Binnein an Fhidhleir rising above, this is where debris flows occurred in August 2004. The road then follows Glen Kinglas down to its mouth on to Loch Fyne (NN 18420 11300). This latter section of the route is some 9km long.

By this point the road is within 10m of sea level and close to the Loch Fyne shore, with steep slopes still rising above. At NN 19400 12500 the road turns sharply onto the low-lying ground around the head of Loch Fyne for a distance of less than 1km. The shore line is then rejoined (NN 18700 12600) and the route continues, generally no more than 20m above sea level, on sidelong ground with steep slopes above, until Loch Shira - an area of known landslide potential. This lochside stretch of the road covers approximately 13km.

A84 South of Strathyre (8km)

Heading south from Strathyre, the A84 follows the valley floor to the head of Loch Lubnaig. The road then follows a course just a few metres away from the eastern margin of the loch (NN 56350 15300) for 7km with the steep slopes of Beinn Each and associated hills above. At the end of the loch (NN 58600 10700) the slopes slacken near St Bride's Chapel before steepening again through the Pass of Leny (NN 58790 09160 to NN 60200 08650) for a further 1km. This is where the river runs through the Falls of Leny, immediately below the road.

A85 Glen Ogle (6km)

Running north-west, the A85 leaves Lochearnhead (NN 58870 23820) and follows Glen Ogle for a distance of just under 6km. (NN 55830 28400) Along this length the road runs on sidelong ground alongside the river. Running first to the west of the river and then to the north-east, the road runs up to 40 m above the river in some places. The August 2004 debris flows occurred in this section some 3km out of Lochearnhead.

Some debris flow activity has been observed to the north of Strathyre (18 August 2004) between the two preceding sections. It may be prudent to include this section in any early evaluations.

A87 Glenshiel (18km, plus a possible further 17km)

The A87 in Glenshiel is characterised by the steep mountain slopes on either side of the route. Running south from Kintail Lodge (NG 94450 20230) the road runs initially along the northerly shore of Loch Duich, turning into the mouth of Glenshiel (NG 93300 19100) after 1.5km. From this point the road crosses Shiel Bridge and then runs first along the south-west side of the small Loch Shiel and then the River Shiel, in Glen Shiel itself, as far as the Glenshiel battle site (NG 99150 13170). This section of around 7.5km runs mainly on the valley floor, but occasionally ascends to around 30m above the river. At the battle site the road once more crosses the river and then follows the north side of the glen as it turns to run eastwards. Through this approximately 8.5km stretch the road is mainly within a few metres of the valley floor as far as the pass that separates Glenshiel from Loch Cluanie (NH 03890 11700).

There may well be a case for adding the further 8.5km stretch of the A87 alongside Loch Cluanie (NH 09200 12060 to NH 16320 10700) to the list of high hazard areas. This length of road is backed by steep slopes to the north and runs close to and just above the loch throughout much of this stretch. A similar case could also be made for the 8.5km stretch of the A87 that runs alongside Loch Duich from just south of Eilean Donan (NG 88500 25550) to south east of Inverinate (NG 94400 21150) near Morvich. Although, relatively little drift material is present in parts of this area close to the road, debris flows have been known in this area with source material from high level slopes. It is however recognised that additional assessments may be required using ROSHI ( see Section 6.3.3).

A82 Fort Augustus to Lochend (29km, plus a possible further 9km)

As the A82 runs north out of Fort Augustus (NH 38250 10200) it follows a route close to the westerly side of Loch Ness, rarely rising more than 20 m above the loch until it turns inland towards the bridge over the river at Invermoriston (NH 41950 16500). This section is around 8.5km long. From Invermoriston, (NH 42050 16800) the road returns close to the lochside and runs parallel to it for some distance, again rarely rising more that 20m above the level of the loch itself. At Achnahannet (NH 51100 25750) the road then begins to follow a route further from and higher above the loch before turning inland at Urquhart Castle for Drumnadrochit (NH 52880 28450) some 17km from Invermoriston. After Drumnadrochit (NH 52650 30000), the road then rejoins the lochside, remaining close to it as far as Lochend (NH 59650 37950). This section is some 13.5km long.

The section from Drumnadrochit to Lochend has been the subject of recent inspections by the Operating Company. These are believed to have revealed only limited drift deposits close to the road and this section may be more suitable for assessment using ROSHI ( see Section 6.3.3). However, it is not clear how far up the slopes of the adjacent hillsides the inspections reached and therefore how relevamnt they are to debris flow hazard. Similar inspections are planned for the section south of the Drumnadrochit in 2005.

While there are noticeable variations in the steepness, extent and ground cover of the hills bounding this section, these generally rise sharply above the road right along the lengths detailed. For this reason, there is insufficient information on the basis of a simple map-based survey to rule out any of the sections from being of high hazard. Indeed there is a strong argument for including the 9km length of road alongside Loch Lochy which runs down from Laggan Locks (NN 28750 96150) to Letterfinlay (NN 24750 90800). Debris flows were experience in this area in early 2005 ( see Section 2.2).

A835 Ullapool to Braemore Junction (16km)

To the south of Ullapool (NH 15100 92050) the A835 follows a line that is frequently close to and just above the shore of Loch Broom and then latterly the River Broom all the way up the Corrieshalloch Gorge to Braemore Junction (NH 20920 77720), a distance of some 16km. Steep slopes are in evidence above this entire length of road.

A9 Dunkeld to Drumochter (22km)

From a point just north of Dunkeld, where the road crosses the River Tay (NO 00450 43900), the A9 runs for approximately 3km (NO 00200 47150) with the river close by below and the old A9 on the steep slopes above. To the north, the slopes above the road slacken and the hazard level diminishes. It is not until just to north of Pitlochry that the slopes steepen again as the road enters the Pass of Killicrankie (NO 91620 60750). After 2km the end of the pass is reached (NN 91920 62350) and the slopes slacken to lessen the hazard levels once more. In this area the slopes beneath the A9 are also steep and lead down to the old A9, the railway and the River Tay. The slopes above the road steepen once more at Shierglas (NN 88480 64320) and do not slacken for around 3.5km, beyond Blair Atholl, until Balnansteuartach is reached. At the Pass of Drumochter the slopes above the road steepen once more at The Wade Stone (NN 69142 71730) only slackening in steepness 13km later in the locality of North Drumochter Lodge (NN 63000 79700).

A95 Craigellachie (1km)

The hills in this area are significantly less steep and less high than in other areas identified in this listing. However, there may be a relatively high hazard level locally where the A95 between Maggieknockater and Craigellachie passes over a hill (NJ 30150 44750) and then sweeps downwards on sidelong ground through Birchbank downwards to the Spey at Craigellachie itself (NJ 29380 45150).

A86 Spean Bridge (5.5km)

Debris flows are known to have occurred in the area of the National Trust for Scotland site at Achaneich/Inverroy (NN 24600 81600) around 1999/2000. This area is characterised by relatively shallow slopes (for the area), but a high density of streams which could carry debris flow. Above the spring line the slopes steepen significantly. In addition the 3km length of road to the east of Spean Bridge exhibits particularly steep slopes (between a point just to the east of Roybridge, NN 27970 80900, and a second point to the east of Achluachrach, NN 31000 81200). It is suggested that the entire section between NN 23100 82000 to the east of Spean Bridge and NN 31000 81200 to the east of Achluachrach be considered, with the exception of the short stretch on the flood plain at Roybridge.

A87 (Skye) Gleann Torra-mhichaig to South of Raasay ferry (1.5km)

This section commences about 1.5km north of the Sligachan Hotel (NG 49650 30550), running generally southwards. After passing the junction with the A863, the A87 then runs north-east, skirting the base of the very steep slopes of Glamaig round past Sconser, and then heads southwards into Gleann Torra-mhichaig terminating where the road crosses the river Abhainn Torra-mhichaig (NG 53750 30700). For the initial part of the section in question the road runs just above the shore line, thereafter entering the glen, where it runs above the river.

7.2 EARLY OPPORTUNITIES

After availability of the GIS assessment data ( see Section 6.1) during Study 1, Part 2, a comparison will be made with the sections of road identified above. Such an exercise will enable a selection of different types of potential failure to be used in the evaluation and validation of the system for hazard ranking which is to be developed as a key objective of Part 2.

In addition to identifying the sites as listed above, the Project Workshop also gave an ideal opportunity to consider actions which could be carried out in the short term, either to minimise the build-up of potential factors which might give rise to unstable slope situations on the network, or to improve systems to collect and use meaningful data which might assist in the assessment or prediction of slope failure events in the future.

In the realm of minimising potential contributory factors, some retargeting of maintenance actions could be productive. Checking of gullies, ditches and catchpits, with a wider view to that of merely keeping the roadway itself clear of water, could be undertaken as part of regular inspections. Where ineffectiveness of the system, or underperformance under updated drainage criteria, is suspected, this should be considered in conjunction with the inspection regime for the roadside side slopes and remedial action addressed via an appropriate structured asset management plan. The principles of such a management approach are set out in HD 41/03 ( DMRB 4.1.3). Additionally, critical review of the alignment of culverts and other conduits close to the road ought to be carried out as part of inspection and reporting procedures.

Certain monitoring measures are already under consideration - for example, the installation of a rain gauge close to the A85 - but the use of any such data gained, in conjunction with longer-duration data available from the Meteorological Office, needs to be managed appropriately to serve a worthwhile and consistent function. At a later stage, informed selection of locations for discrete placement of additional rain-gauging facilities could be productive, and should be considered in the light of experience of managing the information from current sources.

An important action which could be introduced on an early basis is bringing NADICS, including both the current and proposed future network of variable message signs, into the management loop with regard to route advice when weather conditions conspire to create situations where sections of the network might be considered 'at-risk'.

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