5 HAZARD ASSESSMENT AND HAZARD RANKING

This element of the study comprises two activities:

The desk-based interpretation of the GIS-based assessment which determines hazards on the trunk road network and associated site-specific assessments to give a hazard score.
The determination of the exposure of road users to the hazards identified, as in item (1) above, and the consequent ranking of the hazards to provide an analogue for the relative risk at each site.

Hazard Assessment

The debris flow hazards identified in Section 4 essentially relate to Scotland as a whole. The hazards extant on the Scottish trunk road network were determined using the GIS imagery, Ordnance Survey 1:50,000 mapping and low resolution aerial photography to understand plausible flow paths between hazard areas and the network. This highlighted the debris flow hazard areas in relation to the road network and shows which have the potential to affect the trunk road network – the main objective of the study.

Ordnance Survey mapping for Glen Ogle, presented conventionally in two-dimensions (left) and in draped over a digital elevation model (right) to illustrate relative heights. (© Crown Copyright. All rights reserved Scottish Government 100020540, 2008.)

Additional imagery, maps and survey results were viewed alongside the GIS data created in Section 4 to enhance the understanding of the nature of the land in those areas and assess their proximity to the road network. Particular attention was paid to the land between the areas of potential debris flow triggering and the road in order to ascertain whether any debris flow might reach the road.

The Scottish trunk road network is made up of some 3,200km of road, ranging from motorways to single track roads. Of this just over 600km was thought to be potentially at risk from debris flows. These parts of the route were further categorised for severity of potential hazard and ranked as Priorities 1 to 4, where 1 is most severe and 4 is of lesser severity.

Low resolution aerial photography for Glen Ogle, presented conventionally in two-dimensions (left) and in draped over a digital elevation model (right) to illustrate relative heights. The white space represents limitations in the available coverage.

Two sections of the road network were highlighted for separate assessment – the A82 through Glen Coe and the A87 on Skye. These stretches of the network, totalling 46km in length, display particular characteristics which require a different type of assessment and were felt to require more specialist study. However, these sections continue to be included in the routes with higher hazard, in order to give a true reflection of their status, as summarised in the table below.

Outcomes from the interpretation of the GIS-based imagery.

	Route Lengths Assessed (km)	Percentage of Main Study Route Lengths (%)	Percentage of Trunk Road Network (%)
Priority 1	135	22	4
Priority 2	154	25	5
Priority 3	160	26	5
Priority 4	112	19	4
Separate Assessment	46	8	1
Total	607	100	19

The table above shows the breakdown of the road network by Priority. These priorities were used to set the sequence of the subsequent site inspections. These inspections were carried out in 2007 on the Priority 1 and 2 sites for which good quality aerial photography was available. The outputs form these inspections supplemented and validated the findings of the desk-based study.

The inspection process for each site began with the survey team studying high quality aerial photography and maps to allow them to familiarise themselves with the site and to highlight features which would be looked at in more detail during the site inspections itself.

Site inspections were conducted initially by driving the route which had been highlighted as a priority, observing and noting features and taking photographs to record the findings. Detailed site inspections in the form of walkover surveys up and down the slopes were then carried out at all locations to record more detail about the make-up of the hillsides adjacent to the roads. Any new features or recent unrecorded changes to the landscape, such as deforestation, were observed and incorporated into the assessment.

Each site was rated to take account of four main features:

Water (i.e. stream patterns, drainage, accumulation of water).
Instability (i.e. evidence of recent instability).
Slope / topography.
Vegetation and land-use.

These scores were combined with the existing data to produce a finalised the hazard score for the particular stretch of road.

The assessments of the Priority 1 and 2 sites that took place in 2007 are intended to be part of an ongoing programme of site inspections that will be carried out over the coming years.

Hazard Ranking

As a final stage of the process, the data which had been collated and verified during the study was fed into a formula which would calculate the overall risk of debris flows to the road network and road users.

This formula factors in the hazards and the elements at risk, considering, for example, the level of traffic known to use certain stretches of road which effectively represents the risk to road users. Socio-economic factors are also incorporated, such as the length and viability of diversions in the event of the road being closed.

The map and table below show the sections of road which have the highest hazard rankings. A total of 67 sites are detailed, representing just over 380km of the trunk road network.

It is suggested that the GIS-based assessment and the associated interpretation should be refreshed approximately every 10 years to take advantage of any improvements in technology or data which will enhance the findings. It is also suggested that sites of higher hazard ranking should be subject to reassessment to take account of any changes on the ground.

Clearly debris flows are not the only hazards that may affect roads in Scotland and amongst the others are those presented by rock falls. Between 1994 and 1999 Transport Scotland (in a previous guise) initiated and operated a structured programme of rock slope risk assessment and management on the trunk road network.

A 2004 review recommended further action in this respect and Transport Scotland is currently assessing the future actions required to address those Hazard Rating surveys and re-inspections that remain to be carried out.

Sites with a hazard ranking score of 100 or greater

Route Code	OC Unit	Start-NGR	End-NGR	Length (m)	Priority	Initial Hazard Score (from GIS-Based Assessment and Interpretation)	Final Additive Hazard Score from Site Inspections	Finalised Hazard Score	Exposure Score	*Hazard Ranking (Risk) Score = Hazard Exposure**	Locality
A82-17	NW	NN 28766 96227	NN 21391 85632	13,400	1	80	20	100	2.5	250	Loch Lochy
A85-09	NW	NN 50672 28326	NN 38766 25266	12,900	2	60	40	100	2.5	250	Glen Dochart
A82-08	NW	NH 45761 19182	NH 43486 16747	3,410	1	80	10	90	2.5	225	N of Invermoriston
A82-37	NW	NN 34026 00456	NS 34556 97686	3,300	1	80	-	90	2.5	225	Inverbeg and N
A9-12	NW	ND 02175 14804	NC 93895 09663	10,200	1	80	-	90	2.5	225	S of Helmsdale
A9-35b	NW	NN 66562 72101	NN 69762 71546	3,310	1	80	10	90	2.5	225	N Glen Garry
A82-09	NW	NH 42981 16557	NH 42451 16667	581	1	80	00	80	2.5	200	Invermoriston
A82-26	NW	NN 05220 59568	NN 07550 58357	2,720	2	60	20	80	2.5	200	E of Ballachulish
A82-34	NW	NN 33296 20776	NN 31776 09196	13,500	1	80	20	100	2.0	200	N Loch Lomond
A85-08	NW	NN 58437 24970	NN 55677 29396	5,480	1	80	20	100	2.0	200	Glen Ogle
A9-11	NW	ND 08775 20794	ND 02860 15349	11,200	1	80	20	100	2.0	200	N of Helmsdale
A83-02	NW	NN 26901 03861	NN 23021 07837	6,310	1	80	-	90	2.0	180	Ardgarten to Rest & Be Thankful
A83-04	NW	NN 23421 09592	NN 19096 09927	4,360	1	80	-	90	2.0	180	Glen Kinglas
A9-44	NW	NO 00212 47141	NO 00472 43871	3,320	1	80	-	90	2.0	180	N of Dunkeld
A87-19	NW	NG 64039 23632	NG 48718 29902	26,100	Separate Assessment	80	-	90	2.0	180	Southern Skye - N of Broadford
A82-36	NW	NN 31916 04456	NN 34026 00456	4,610	2	60	-	70	2.5	175	S of Tarbet
A9-35a	NW	NN 63982 83957	NN 64987 73046	11,900	2	60	10	70	2.5	175	S of Dalwhinnie
A83-06	NW	NN 19221 12717	NN 11260 08848	9,170	2	60	25	85	2.0	170	Clachan to Strone Point
A82-05	NW	NH 52566 28987	NH 49631 23632	6,770	2	60	5	65	2.5	163	S of Drumnadrochit
A77-11	SW	NX 05214 72439	NX 08694 63338	9,990	2	60	20	80	2.0	160	S of Glen App
A82-02	NW	NH 60696 39243	NH 57346 34993	5,520	1	80	20	100	1.5	150	N end of Loch Ness
A83-05	NW	NN 18406 11247	NN 19406 12512	1,620	1	80	20	100	1.5	150	Cairndow
A87-12	NW	NH 03370 12016	NG 96289 14946	8,620	1	80	20	100	1.5	150	E Glen Shiel
A87-15	NW	NG 94469 21121	NG 88269 26106	8,650	1	80	20	100	1.5	150	Loch Duich
A87-09	NW	NH 11495 10731	NH 09725 11731	2,080	1	80	15	95	1.5	143	W Loch Cluanie
A830-05	NW	NM 90195 80853	NM 76679 82314	15,500	2	60	10	70	2.0	140	Glenfinnan to Lochailort
A9-45	NW	NO 03452 41486	NO 04062 40886	877	2	60	-	70	2.0	140	S of Dunkeld
A82-27	NW	NN 10700 58212	NN 27671 52992	19,900	Separate Assessment	80	-	90	1.5	135	Glen Coe
A828-01	NW	NN 05175 59653	NM 99145 54983	8,540	2	60	30	90	1.5	135	W of Ballachulish
A835-07	NW	NH 38284 70387	NH 28554 73906	11,400	1	80	10	90	1.5	135	Lubfearn to W Loch Glascarnoch
A85-15	NW	NN 13191 28352	NN 03135 29863	12,400	1	80	10	90	1.5	135	Dalmally to W Pass of Brander
A86-12	NW	NN 25591 81307	NN 22966 81947	2,770	1	80	10	90	1.5	135	Inverroy to Spean Bridge
A87-13	NW	NG 96259 14951	NG 94614 17946	3,790	2	60	30	90	1.5	135	W Glen Shiel
A82-07	NW	NH 47461 21012	NH 46411 19822	1,620	3	40	-	50	2.5	125	N of Alltsigh
A82-16	NW	NN 29996 98177	NN 28981 96572	1,960	3	40	-	50	2.5	125	Loch Oich to Loch Lochy
A82-23	NW	NN 04505 66337	NN 03765 65377	1,260	3	40	-	50	2.5	125	N of Corran Ferry
A82-24	NW	NN 02295 63258	NN 02645 62728	688	3	40	-	50	2.5	125	S of Corran Ferry
A82-38	NW	NS 34556 97686	NS 35196 87156	11,100	3	40	-	50	2.5	125	N & S of Luss
A83-18	NW	NR 84819 80506	NR 86284 74006	7,040	3	40	-	50	2.5	125	S of Inverneill
A83-20	NW	NR 86794 69696	NR 86529 69066	687	3	40	-	50	2.5	125	N Tarbet
A9-24	NW	NH 72341 35783	NH 75841 34579	4,040	3	40	-	50	2.5	125	N of Loch Moy
A9-27	NW	NH 82171 26569	NH 87652 24074	6,660	3	40	-	50	2.5	125	Slochd
M90-09	NE	NO 14377 13430	NO 13887 15335	3,200	3	40	-	50	2.5	125	N of Glen Farg
A82-04	NW	NH 52391 30037	NH 50831 30172	1,590	1	80	0	80	1.5	120	Drumnadochit
A86-03	NW	NN 67317 95722	NN 67162 95417	357	1	80	0	80	1.5	120	Glentruim House
A86-09	NW	NN 48856 87552	NN 47661 86407	1,730	1	80	0	80	1.5	120	Aberarder (Loch Laggan)
A86-10	NW	NN 47516 86247	NN 37536 81267	11,600	2	60	15	75	1.5	113	Loch Laggan and Reservoir
A86-11	NW	NN 33266 80957	NN 27646 81067	6,180	2	60	15	75	1.5	113	Tulloch to Roy Bridge
A7-06	SE	NT 40762 02692	NY 38842 96252	7,160	2	60	-	70	1.5	105	S of Teviothead
A835-09	NW	NH 19553 80586	NH 18168 85540	5,320	2	60	-	70	1.5	105	S of Loch Broom
A1-06	SE	NT 79571 67434	NT 85681 62704	8,630	3	40	-	50	2.0	100	Penmanshiel to Howburn
A7-01	SE	NT 48882 32523	NT 48142 31013	1,840	3	40	-	50	2.0	100	N of Selkirk
A76-04	SW	NS 85832 04117	NS 81022 07857	6,570	3	40	-	50	2.0	100	S of Sanquhar
A77-10	SW	NX 09284 77378	NX 05214 72439	6,640	3	40	-	50	2.0	100	Glen App
A83-01	NW	NN 29616 05036	NN 28391 03881	1,760	3	40	-	50	2.0	100	W of Succoth
A83-07	NW	NN 11260 08848	NN 11395 10083	1,260	3	40	-	50	2.0	100	E Loch Shira
A83-10	NW	NN 04495 04203	NN 02915 03179	1,910	3	40	-	50	2.0	100	E of Auchindrain Folk Museum
A83-12	NW	NS 01725 99834	NR 98995 97649	3,550	3	40	-	50	2.0	100	W of Furnace
A83-21	NW	NR 86034 68451	NR 85284 68076	839	3	40	-	50	2.0	100	W of Tarbet
A830-04	NW	NM 90855 80478	NM 90205 80848	867	3	40	-	50	2.0	100	Glenfinnan
A830-06	NW	NM 76679 82314	NM 71574 84404	6,080	3	40	-	50	2.0	100	Lochailort to Prince's Cairn
A835-04	NW	NH 43565 58802	NH 40650 59367	3,110	3	40	-	50	2.0	100	S of Garve
A84-03	NW	NN 57047 14530	NN 58487 13465	1,900	3	40	-	50	2.0	100	N Loch Lubnaig
A9-09	NW	ND 15325 29325	ND 13145 25995	4,350	3	40	-	50	2.0	100	S of Dunbeath
A9-10	NW	ND 12010 23055	ND 11670 22435	1,110	3	40	-	50	2.0	100	Berriedale
M74-09	M74 DBFO	NS 95997 16852	NS 96337 16502	492	3	40	-	50	2.0	100	Elvanfoot