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1 INTRODUCTION

Following a period of heavy rainfall in August 2004 a number of landslides closed sections of the trunk road in Scotland. The largest of these happened on the A85 near Lochearnhead and trapped 57 people, who had to be evacuated by helicopter. Nicol Stephen MSP, Minister for Transport, commissioned two studies to consider issues arising from these landslides.

Figure 1.1 - Landslide on A85 near Lochearnhead, August 2004.

Photograph © Perthshire Picture Agency.

The first of these studies considers the causes of landslides and what measures could be taken to manage the risks of these occurring and is reported elsewhere. The second of these studies considers the effect that climate change might have on the design and operation of roads, to identify whether any changes in current practices are required. The Scottish Executive commissioned Jacobs Babtie via the BEAR Scotland Ltd Operating Company terms commission to undertake this study, with a brief to:

'Undertake a desktop analysis of the potential trends in climate change in Scotland and determine the detailed implications for the operation and management of road networks. This will consider all elements of weather to include temperature, rain, snow and ice, wind, fog and coastal flooding'

The study team also included members of the climate change team at the Met Office and liaised closely with key stakeholders within the Scottish Executive, other governmental bodies and parties involved in the operation of the road network.

This report presents a summary of the findings of the second study. It is intended to provide the reader with an overview of the climate change predictions and the subsequent recommendations for design and operation of the road network. A full technical report on this study is also available, which gives more detail than is presented here. The summary report outlines the study methodology, reviews the historical records for weather events, outlines the predicted trends in climate change and identifies the consequent recommendations for the future design and operation of the road network

The study was undertaken in three stages, as outlined in the following sub-sections.

2.1 ANALYSIS OF WEATHER EVENT IMPLICATIONS

An initial analysis was undertaken to identify the weather events of particular importance to this study, by considering how weather events influence aspects of design and operation of the road network. This analysis was reviewed at a workshop involving key stakeholders, to confirm that the areas identified and the further assessments proposed were appropriate.

2.2 IDENTIFICATION OF PREDICTED TRENDS IN CLIMATE CHANGE IN WEATHER EVENT CATEGORIES

Once the weather events of interest had been identified it was necessary to obtain information on the predicted trends in these weather events associated with climate change. In obtaining this information the study team made use of the climate change models developed at the Met Office's Hadley Centre. These models were developed as part of the UK Climate Impacts Programme ( UKCIP), established by the Department of Environment, Transport and the Regions ( DETR). These models have been developed taking account of the Intergovernmental Panel on Climate Change ( IPCC) scenarios of change. These scenarios are based on a range of possible emission outputs, which have been consolidated into four categories, termed 'Low', 'Medium-Low', 'Medium-High' and 'High'. It is important to note that all of these scenarios may be considered equally likely to occur. The carbon dioxide emissions for each of the four scenarios are shown in Figure 2.1.

Figure 2.1 - Emissions of CO ₂ (giga-tonnes carbon per year) in four emissions scenarios. Source: Hadley Centre Technical Note 44.

The most recent set of Climate Change Scenarios published, in 2002, under the UKCIP are known as UKCIP02. The results presented were based on the latest regional climate model ( RCM) developed by the Met Office, HadRM3. RCM's are used to enable more detailed modelling of specific areas, as they have a horizontal grid of 50km, compared to a global climate model ( GCM) grid of 300km. The RCM is 'embedded' in the GCM, from which it takes all of its information for the surrounding area.

The improved accuracy of the RCM compared to the GCM is demonstrated in Figure 2.2, which illustrates how each of these models compares with current observations for rainfall of 10, 20 and 30mm/hr intensity.

Figure 2.2 - Comparison of Observations, Regional Climate Model ( RCM) predictions and Global Climate Model ( GCM) predictions for UK summer rainfall.

Source: Met Office.

A more detailed RCM was developed for the British Irish Council ( BIC) report (Jenkins et al, 2003), which had a 25km grid. This enables the varied topography of Scotland to be represented more accurately and also permitted some of the Scottish islands to be included in climate modelling studies for the first time.

The UKCIP02 report presented the predicted trends in climate change for three 'time-slices' or thirty year periods centred about the 2020's, 2050's and 2080's. The changes are assessed in relation to a modelled present day climate, which is the 30 year period between 1961 and 1990. In developing these predictions it is important to note that there are three areas of uncertainty within the modelling. These are:

• Emissions scenarios, which have been discussed previously. While all scenarios must be considered possible, within this report reference will generally be made to the Medium-High emissions scenario, which has the greatest range of information available. As a rough guide, this scenario may be considered to approximate to 'business as usual'. As a consequence of the range of emissions scenarios, different levels of climate change may be expected in the modelling outputs. However, it may be noted that the choice of scenario makes little difference over the next 50 years, with significant divergence occurring after that time. This provides increased confidence in the reliability of the predictions for the near future.
• Scientific uncertainty, which underlies the ability of the current modelling capability and the knowledge of climate behaviour to provide reasonable estimates of future trends. A number of climate models are in existence which function in different ways and as a result provide different outputs. It must also be recognised that the knowledge of climate behaviour may not be sufficiently accurate to enable any of the models to be considered completely accurate. The HadCM3 GCM used in this study is globally respected among the climate modelling community and is an established world leader. Results from the model generally fall within the mid-range of predictions rather than the extremes.
• Natural variability, which describes the changeable nature of climate systems irrespective of climate change effects. This variability may be seen on year to year and decade to decade periods. It is highly likely that in the future there will be periods when natural variability combines with climate change to produce periods of extreme warming or summer drying. It is equally likely that the two factors will combine at another time to produce a relatively cold or dry winter. Figure 2.3 shows winter mean precipitation over Scotland from one integration of HadCM3, the GCM, using the Medium-High emissions scenario. Although there is a general trend towards wetter winters, there are still individual years or short periods when winter precipitation is below the modelled present day climate, even towards the end of the period when average changes typically exceed 20%. Note that this is one evolution of the model and is purely to demonstrate the impact of natural variability. It is not a year on year prediction of rainfall and should not be taken to be a forecast.

Figure 2.3 - Winter mean precipitation change (%), relative to 1961 to 1990 average, over Scotland, Medium-High emissions scenario, one HadCM3 ensemble member. Each bar is a winter mean, the red curve is a longer term running mean, and the stars denote a model year which is 'record breaking'. Source: Met Office.

The full technical report prepared for this study presents a more detailed discussion of the methodology used in climate change prediction.

2.3 EVALUATION OF PREDICTED TRENDS IN TERMS OF IMPACT ON THE ROAD NETWORK

Following confirmation of the predicted trends in climate change, the implications of these were assessed by reference to current guidance on the use of weather event data in the design and operation of the road network. In addition, consultation was undertaken with parties responsible for managing the road network. To provide a broad range of geographical experience, together with views on roads ranging from major urban motorways to rural single carriageways, this consultation was undertaken with the two parties responsible for maintaining the North-West, North-East, South-West and South-East trunk road units on behalf of the Scottish Executive. In addition, in response to emerging findings from the study process, consultations were undertaken with NADICS.

3.1 INTRODUCTION

The full technical report prepared for this study presents a detailed discussion of the historical context of the weather variables under consideration. The following sections outline the main issues identified.

3.2 TEMPERATURE

Temperature is one of the underlying parameters that affect a number of areas of road design and operation. The conditions in which road surfacing materials may be laid and concrete placed are dependent on temperature and the expected long-term performance of these materials is based on the occurrence of a certain temperature environment. In addition, the growing season is temperature dependent and assumptions relating to the need for maintenance of landscaping areas are also based on the occurrence of a certain temperature environment.

There is evidence from analysis carried out on temperature records in Scotland (Jones and Lister, 2004) that long-term statistically significant seasonal warming, except for the winter (Dec-Feb) season, has been observed. The mainland Scotland records show a mean increase in annual temperature over the period, expressed linearly, of 0.69 ^oC. The north and north-western isles records show an increase on the same basis of 0.64 ^oC.

There is some evidence from the historic records that the growing season is lengthening (Hulme et al, 2002). Based on data available for central England for the period between 1930 and 2000, the thermal growing season in Scotland may be expected to have increased by on average one-third of a day a year over that period.

3.3 RAIN

Rain is one of the most important factors affecting the design and operation of the road network. It affects the design of drainage systems that collect and discharge surface water. It also affects the sizing of river bridges/culverts. Rain also creates a hazard to road users when it is not shed sufficiently quickly from the carriageway and is a frequent contributing factor in road accidents. Rain can cause landslide events, both through large volumes of surface water eroding the land surface, and through changes in groundwater levels reducing the stability of cuttings. In addition, rain, together with temperature, can significantly alter the soil moisture condition within a catchment, creating a situation where the volume of water that the catchment sheds may be much higher than the 15% to 50% currently used in the design of drainage systems. The rainfall events currently used in road design are based on historical records and therefore there is a concern that if rain is expected to increase, these records may no longer correctly describe the design storm events.

Analysis of records shows that recent changes in Scottish rainfall may be identified. Figure 3.1 shows the Scottish annual precipitation anomalies from the start of the 18 ^th century to 2001. The smoothed Scottish curve, represented by the blue line, shows a significant increase in annual rainfall during the 1980's and 1990's. There also appears to be a trend in seasonality, which is the ratio of winter to summer precipitation and is represented by the purple line, with the ratio increasing.

Figure 3.1 - Annual precipitation in over Scotland for the period 1800-2001. The bars denote annual variations from the 1961-1990 mean (1430mm).

Source: UKCIP02 Scientific Report.

Trends in storm event rainfall have been the subject of less research. However, Osborn et al. (2000) found evidence that the intensity distribution of daily precipitation across Scotland has changed over the relatively short period 1961-1995. For 26 stations across Scotland they showed that the majority have recorded a general shift from light and medium events to heavier events in the winter and, to a lesser extent, also in the spring and autumn. The reverse was found to be true in the summer. They suggested that changes in winter weather types may have contributed to the increase in the proportion of precipitation provided by heavy events.

The historical change in river flooding, which has a direct link with rainfall, has been the subject of research for some time. CEH Wallingford conducted research on long-term trends in the UK flood record (Robson et al 1997, Robson et al 1998). This found that no significant long-term trends in national flood behaviour could be detected. In a study of Scottish flood behaviour Werritty (1998) found that there had been little change in flood magnitudes during the period 1970-1996 for 44 river flow stations across Scotland. However, a greater increase in flood frequency was found in the same study. Werritty suggests that the significance of the increase in flood frequency should not be overstated as, although there have been a number of major catastrophic floods since 1989, there appears to be no consistent increase in the size of moderately high flood events across Scotland.

Following the extreme flooding in England and Wales during Autumn 2000, the Met Office and CEH Wallingford were commissioned to assess whether the floods and rainfall could be linked to climate change (Met Office & CEH Wallingford, 2001). They concluded that although the events were consistent with model predictions of how human-induced climate change affects rainfall, it was not yet possible to say how far rainfall and flooding events can be attributed to climate change, as opposed to natural variability.

Historic evidence on changes in Scottish groundwater levels is not available. In addition, very limited information exists on historical Scottish soil moisture conditions and so no evidence of sustained long-term trends can be identified.

3.4 SNOW AND ICE

Snow and ice are significant factors affecting the operation of the road network. Measures such as gritting are usually implemented to try to prevent ice forming or snow deposits remaining on the road surface. Snow clearing is required where heavier falls occur. Snow and ice also create a hazard to road users and are contributing factors in some road accidents. In addition, snowmelt has the potential to increase catchment runoff by releasing volumes of surface water previously held in a frozen state.

The last two decades of the twentieth century experienced relatively low amounts of snow in Scotland. The on-going analysis of the Met Office's observational data archive has included preliminary investigation into snowfall changes. Figure 3.2 shows observed changes in the length of the snow season for western Scotland (paper in preparation, Perry et al).

Figure 3.2 - Number of days each year upon which snow lying on the ground was reported in Western Scotland, 1960 to 2003. Source: Met Office, paper in preparation.

While there are years such as 1995 in which a high number of days of snow lying are recorded, it is apparent that the tendency is to fewer such days during each season.

3.5 WIND

Wind is a design consideration for structures such as bridges and roadside furniture, which can include signs, streetlights, gantries, variable message signs and CCTV cameras. Design of these elements must include for the physical effect of wind on the elements concerned to ensure their durability and robustness. Items of roadside furniture all require consideration of the design event wind loading when designs of these items and their foundations are being developed. In addition, high winds also create a hazard to road users, particularly high-sided vehicles, and are a contributing factor in some road accidents. Consideration may therefore be given to the need for measures to prevent wind affecting safe operation of the road network, such as the inclusion of wind barriers on major bridges in exposed areas.

An analysis of observed storminess across the UK is being conducted by the Hadley Centre. Observed pressure data from meteorological stations across the country, including stations in Scotland, is being used to estimate the number of storms crossing the UK in a year. The research is on-going but preliminary results show a significant increase in the number of wintertime storms over recent decades (Alexander et al, paper in preparation). This is shown in Figure 3.3, which indicates the number of 'storms' observed each year divided by the number of observing stations reporting in that year. The red line indicates a significant trend at the 5% level.

Figure 3.3 - Number of UK winter (January to March) storms, 1949-2001.
Preliminary results from an analysis by the Hadley Centre.

Source: Met Office, paper in preparation.

Although the identified trend over the last fifty years is statistically significant the number of storms observed in a season is highly variable. A longer period of consideration is required to properly assess whether the apparent trend is part of a long-term sustained change in climate.

3.6 FOG

Fog is a factor that affects the operation of the road network, creating a hazard to road users, and being a contributing factor in some road accidents. However, historic evidence on changes to the occurrence of fog in Scotland has not been researched, and therefore no assessment of any change is possible at this time.

3.7 COASTAL FLOODING

Coastal flooding is a factor that affects the design of the road network by influencing the location of roads. In can affect the operation of the existing road network by creating a hazard to road users and being a contributing factor in some road accidents.

The Proudman Oceanographic Laboratory has reviewed historical records for sea-level around the coast of Britain. They estimate that both the mean and the extreme sea-levels are increasing at a rate of 1.0 to 1.3mm per year, disregarding the effect of any vertical land movement (Dixon and Tawn, 1995). However, Scotland continues to rise following removal of the weight of ice that formed during the ice age. This rebound is of a similar magnitude to the mean sea-level rise, and to a large degree negates its effect.

Although long-term trends in sea-level change are apparent, long term trends in tidal surge and the size of coastal waves are less apparent. The search for trends in this area is made difficult by the short period for which records exist. However, it may be noted that some increase in wave height in both the North Sea and the North Atlantic Ocean have been observed (Bacon and Carter, 1991 and Price and McKenna, 2003).

4.1 INTRODUCTION

This section of the report discusses the predicted trends in climate change identified in this study and summaries of these are highlighted in shaded boxes. The full technical report provides greater details on these trends and also includes the outputs obtained from the climate change modelling for the weather events considered.

4.2 TEMPERATURE

Over the next few decades daily mean temperatures are predicted to rise by up to 1°C over all of Scotland for all emissions scenarios. Figure 4.1 shows the summer maximums and winter minimums for the Medium-High emissions scenario.

Figure 4.1 - Change in daily maximum temperatures during the summer (June to August), left, and daily minimum temperatures during the winter (December to February), right, (°C) by the 2020's, for the Medium-High emissions scenario.

Source: Met Office.

An analysis has been carried out for this study of the probability of specific summer daily maximum temperatures and winter daily minimum temperatures occurring, selecting two locations. The first of these is an area around Glasgow, representing a lowland urban area and the second is an area around Aviemore, representing a highland rural area.

The results of this analysis are presented below. It should be noted that the winter analysis has been based on air temperatures, not ground temperatures. As ground frost can occur when air temperatures are above 0°C, the comments on the number of days when the temperatures fall below freezing should be taken as indicative of the type of changes that are likely to occur.

Temperature also affects the growing season, which is expected to extend. It is likely that southern Scotland will experience slightly larger increases than northern Scotland.

4.3 RAIN

There is little predicted change in annual mean precipitation over Scotland over the next few decades, with any changes occurring being within the range of natural variability. However, the predictions suggest a seasonal pattern of change, with winter precipitation being expected to increase in eastern Scotland and summer precipitation predicted to decrease with only the far northwest seeing little change. Spring and autumn precipitation is not expected to change significantly. Figure 4.2 illustrates the changes predicted by the 2020's for winter and summer, for the Medium-High emissions scenario.

Figure 4.2 - Precipitation change (%) by the 2020's for the Medium-High emissions scenario. Winter (December to February) is at the top and summer (June to August) is at the bottom. Results are presented for 50km and 25km grids. Source: Met Office.

Some validation of extreme rainfall events simulated by the regional climate model was completed for UKCIP02. It was shown that the HadRM3 model has a tendency to overestimate the heaviest rainfall events when compared to current climate, particularly in winter. Despite this bias the model does successfully reproduce aspects of daily precipitation climatology. Analysis of daily precipitation was completed for the 2080's, however, it is reasonable to take the predicted changes as indicative of the type rather than the size of change that may occur in coming decades.

River and burn flooding is not covered by the Met Office and UKCIP climate change scenarios. However, several research studies (Werritty el al, 2002, Price and McKeena, 2003, Kay et al, 2003, Reynard et al, 2004) have been undertaken to transfer the predicted changes in storm event rainfall and the preceding wetness condition of the area receiving the rainfall into predictions of how peak resulting runoff flows will change. In Scotland these are generally predicted to progressively increase during the 21 ^st century.

Soil moisture is also expected to show seasonal variation, as shown in Figure 4.3.

Figure 4.3 - Average soil moisture content change (%) by the 2020's for the Medium-High emissions scenario. Winter is top and summer is bottom. 50km HadRM3 ensemble mean is left and BIC 25km model is right. Source: Met Office.

Average soil moisture is likely to increase during winter and decrease during summer months. Although winter precipitation is likely to increase, higher temperatures and reductions in relative humidity mean that evaporation will increase. This may be the reason why soil moisture levels across Scotland are likely to increase less than may have otherwise been expected. Changes in autumn soil moisture levels are predicted to be very similar to those in summer. This is due to the long time taken to recover soil moisture levels following hotter and drier summers.

4.4 SNOW

It is predicted that as the climate warms both snowfall totals and the length of the snow season in Scotland will decrease. Figure 4.4 shows the predicted average change in snowfall for winter by the 2020's, under the Medium-High emissions scenario.

Figure 4.4 - Change in average winter snowfall (%) by the 2020's for the Medium-High emissions scenario for the 2020's. The 50km HADRM3 model result is left and the 25km BIC result is right. Source: Met Office.

Changes in wind strength and the frequency of storms are a topic of much uncertainty in climate model simulations. Any changes predicted are very model dependent and therefore little confidence can be placed in them.

4.6 FOG

Predicted changes in fog are not routinely output by current climate models. For UKCIP02 an estimate of the change in the number of days of fog was calculated from changes in relative humidity simulated by the regional climate model. The calculation was based upon a relationship derived from weather forecasting.

4.7 COASTAL FLOODING

Sea level rise is an important consequence of climate change and global warming, mainly arising through thermal expansion of ocean water and through the melting of mountain glaciers. The extremes of sea level rise are the 'storm surges', high water levels generated by storm or cyclone activity, especially over relatively shallow water, or in estuaries where a 'funnelling' effect can act to increase surge heights even further. When storm surges coincide with high tide the most extreme high water levels can be achieved. Figure 4.5 shows predicted global mean sea-level changes until the year 2100, relative to 1860, as simulated by the Met Office climate model under the four different emissions scenarios.

Figure 4.5 - Global mean sea-level changes, relative to 1860, in metres, simulated by Met Office climate model under four different emissions scenarios. Source: Met Office.

All scenarios show a rise by the end of the century. It should be noted that it is not until after about 2050 that the lines for different scenarios diverge.

The possible change in coastal flooding depends on the interaction between the modest sea level rise predicted over the next few decades, the continuing isostatic rebound and rise of Scotland, and the relatively low risk of storm surge which exists. However, it should be noted that changes in wind patterns are uncertain, as are possible changes in wave climatology. Thus the risk of on-shore winds and increased wave heights, which in combination would cause increased coastal flooding, cannot, at this time, be described.

5.1 INTRODUCTION

This section presents a summary of the likely impacts on the road network of the predicted trends in climate change discussed previously and outlines recommendations, highlighted in shaded boxes, in relation to these impacts. In doing so it also takes account of network management issues identified through consultations with Operating Companies. The full technical report provides greater details of these impacts.

In connection with all of the assessments presented here, it is noted that the next update of the UK Climate Impacts Programme, UKCIP06, is understood to be due in 2006.

5.2 TEMPERATURE

Four main effects of temperature on the road network are considered in the following sub-sections and the contribution of low temperatures to the formation of ice is discussed later under the heading of 'Snow'.

Effects of High Temperatures on Bituminous Pavements

While higher temperatures may affect the long-term durability of pavements, the current predictions do not suggest that this is likely to be a significant concern. The recent introduction of stiffer pavement materials should result in effects due to high temperatures on recently constructed major roads being very limited. Older sections of road, and much of the local road network, which is largely composed of less stiff materials, or materials of unknown stiffness, may be more susceptible to failure.

Effects of Freeze-Thaw Action on Bituminous Pavements

During the winter months moisture in certain types of road surfacing material may freeze and thaw in cycles. This cycle of freezing and thawing can cause a volume change within the material, with changing stresses resulting in a loosening effect. This may be exacerbated by vehicles displacing or disturbing surfacing materials, allowing more moisture to ingress.

Surface dressing in particular reacts poorly to freeze/thaw conditions as it has a highly exposed binder. A combination of this, its susceptibility to freeze/thaw action and its thickness may result in embrittlement of the surface. This is then no longer able to provide the function of sealing the upper layers of the underlying bituminous carriageway.

The Managing Agents considered that temperature changes observed at present tend to result in a greater extent of thawing and refreezing, possibly following a day/night pattern, than observed previously. If so, this would suggest that greater damage may occur from freeze-thaw action in the future.

Growing Season

Landscaping of roads may entail both significant planting that matures over time to meet visual and ecological objectives, and seeding of verges and side slopes to provide a simple finished form. In order to maintain an appropriate appearance of the soft estate and to maintain a safe road it is essential that cyclic maintenance is undertaken to ensure that vegetation does not obscure signage or visibility splays.

Figure 5.1 - Example of grass-cutting in verges.

Air Quality

The predicted average temperature increase in Scotland of 1°C suggests that the future temperature regime in Scotland will not change to be significantly different from that observed elsewhere in the UK at the present time. As the air quality assessment guidance is used throughout the UK, it is therefore not considered necessary at this time to recommend any change to current practices of air quality assessment and prediction.

5.3 RAIN

The main effects of rain on the road network are considered in the following five sub-sections.

Road Surface Drainage

Road drainage design has two major objectives:

• the rapid removal of surface water to provide safety and minimum nuisance for the road user
• provision of effective sub-surface drainage to maximise longevity of the pavement and protect its associated earthworks

A number of locations on the trunk road network have been identified as particularly at risk of drainage 'failure' and inspections by Managing Agent Area Teams are consequently prioritised during periods of heavy rainfall. The purpose of these inspections is to ensure unimpaired drainage. The most common cause of flooding in areas where drainage is present is due to detritus being washed into the system, resulting in partial or complete blockage.

Pavement Deterioration as a Result of Wet Conditions

The durability of a mixed material depends on either its ability to keep the weather out if it is intended to be an impermeable material, or the ability of its components to resist the weather, if it is permeable. Certain elements of bituminous pavements can be permeable, and the pavement will deteriorate if moisture remains within the bound or sub-grade layers. It is therefore essential for pavement durability that effective drainage is present to remove both surface and sub-surface water.

The Managing Agents noted that recent summers appear to have been wetter than average and this has coincided with the need for more routine trunk road maintenance than in previous years. It may be that the level of deterioration experienced is as a result of the high levels of this precipitation.

Figure 5.2 - Damage to A9 Raigmore Slip Road, Inverness, 2002, following heavy rainfall. Photograph courtesy of BEAR Scotland Ltd.

Watercourse Flooding

Whether major river or minor watercourse, flooding from catchment response to storm events is a significant risk, with the potential to impact on the safe operation of the road network. Examples of the issues that may arise include:

• Bridge/culvert capacities exceeded, causing upstream flooding to occur
• Overtopping and scouring problems to structures
• Roads and any properties on flood plains becoming inundated

Culverts are generally considered by the Managing Agents to represent a concern, with flood events regularly affecting particular locations. This is due, in part, to the culverts at these locations being unable to accommodate flood-borne detritus, which then reduces the available capacity of the culvert and hence exacerbates the impact of the flood event. To address this there are programmes for culvert inspections, focussing attention in specific months, when areas of particular concern are identified and monitored by the Managing Agent for attention.

Ground Water

Ground water is one of the critical elements affecting the design of cutting slopes. Parameters used in design include the height of ground water and the degree of movement to which it is susceptible. Changes in these parameters can materially affect the design or operational effectiveness of the cuttings concerned. The presence of effective surface and sub-surface drainage previously discussed for the road pavement, together with well maintained pre-earthworks drainage at the top of slopes, also enables cuttings to remain stable. In some instances counter-forte slope drainage is required to maintain slope stability.

Soil Moisture

Soil moisture is one of the factors that affects catchment response, with soils holding greater moisture contributing to an increased surface water runoff.

The Managing Agents noted that some landslide events occurred when a period of intense rainfall followed a longer period of general rainfall, which could be a demonstration of the implications of increasing soil moisture producing greater catchment runoff.

5.4 SNOW

Snow has two significant effects on the road network. The contribution of snowmelt to catchment runoff has been discussed previously. The other effect, discussed here, is the impact of winter weather conditions on the operation of the road network. This includes the implications of predicted climate change trends on both snowfall and ice formation.

Figure 5.3 - A90, South of Aberdeen, 2004.

Photograph courtesy of Performance Audit Group.

Maintaining availability, reliability and safety of the road network is a key objective of winter maintenance. Snow and ice on the road causes hazardous driving conditions and can result in damage to the fabric of the road pavement. Therefore, effective winter maintenance makes important contributions towards road safety and the minimisation of whole life costs.

5.5 WIND

The two main impacts wind has on the road network are considered in the following sub-sections.

Effect on Structural Elements/Roadside Furniture

Many elements of the road network require evaluation of their performance under extreme wind events as part of their design process. The achievement of satisfactory operational performance is dependent on the results of this evaluation. Hence, extreme winds may affect the built environment, for example traffic signs. In addition, they may also affect landscaping adjacent to the road network and significant disruption may result as a consequence of damage to large elements, such as trees.

Effects on Operation of the Road Network

Extreme winds can disrupt operation of the road network through impacts on high sided vehicles, which can become increasingly unstable in gusts of over 20 m/s (45mph). To maintain stability, drivers need to slow down when experiencing high winds. At some sites, such as major bridges, closure of the road to high sided vehicles is required to prevent their exposure to these winds. This can result in such traffic, generally heavy goods vehicles, being diverted from major roads to less suitable local roads.

The Managing Agents noted that where vehicles are blown off the carriageway, it is usually necessary to temporarily close some or all lanes to recover the vehicles. This creates additional disruption to road users. In addition, the Managing Agents expressed concerns that insufficient measures exist for advance signing and the provision of parking/turning areas for high-sided vehicles when sections of the road network are closed.

5.6 FOG

Fog impacts upon the safe operation of the road network through reducing visibility and thus creating a road safety hazard.

The concerns in respect of safe operation of the road network in fog conditions are capable of being addressed through improved road user behaviour, and this is discussed more fully in a later part of this section, under the heading 'Road User Behaviour'.

5.7 COASTAL FLOODING

Coastal flooding, including the effects of storm surge, has the potential to affect low lying roads in coastal areas. This may result in damage to the road, road closure or road safety hazards, as seen following the severe winter storm of January 2005. It should be noted that the roads at risk are predominantly part of the local, rather than the trunk, road network.

Figure 5.4 - Coastal Damage in South Uist, 2005.

Photograph courtesy Mr D I Caimbeul.

5.8 ROAD USER BEHAVIOUR

In addition to the impact of severe weather on performance of the road network discussed previously, road users are also affected in other ways by severe weather events, such as:

• Heavy rainfall and/or poor surface water drainage, which can result in excessive spray, reducing visibility, and wet pavements providing poorer skidding resistance
• Flooding, whether catchment or coastal in origin, which can create areas of deep ponding that may not be apparent to road users
• Winter conditions, which can result in poorer skidding resistance
• High winds, which can result in unexpected forces being applied to vehicles, affecting driving behaviour
• Fog, which can reduce visibility

While the occurrence of some of these events, such as winter conditions and fog, may reduce in the future, instances of others are expected to increase. Given the level of natural variability in weather events, it is not possible to eliminate all of these potential effects completely through design. Therefore, there is the potential for road safety hazards to continue to occur. Although effective management of the road network can provide additional information to road users, the avoidance of these hazards is largely dependent on road users modifying their behaviour in response to this information. It is considered that ongoing road user education is an essential component in raising the awareness of the need to modify behaviour during severe weather events. It is also considered that the provision of relevant information to road users in respect of such events would assist in encouraging modified behaviour.

Introduction

From the recommendations identified in this study and highlighted in the previous section, six are considered to be priority recommendations. These are summarised below, categorised by types, that is:

• Design Issues, where changes in the design of the road network are proposed
• Operational Issues, where changes in the operation of the road network are proposed
• Research Issues, where detailed recommendations are not possible at this time and further research is required
• Policy Issues, where recommendations would have an impact on current policies

The full technical report assigns this categorisation to all of the recommendations, and sub-divides those in addition to the priority recommendations as being of short or long-term significance.

Priority Recommendations

Design Issues

• Revise the parameters for the design storm for surface water drainage performance. This could be achieved by continuing the use of historical information but for an increased return period, or by basing the approach on further research on rainfall changes arising from climate change.
• Revise the parameters for the design storm for watercourse structures. This could be achieved by continuing the use of historical information but for an increased return period, or by basing the approach on further research on rainfall changes arising from climate change.

Operational Issues

• Identify locations where flooding of the road network has occurred, and develop potential solutions for evaluation on a cost/benefit basis, prioritising those areas where repeated flooding has occurred.
• Pre-emptively clear detritus from channels/watercourse structures in known areas of flooding risk in response to predicted heavy rainfall.

Research Issues

• Undertake further research in respect of catchment runoff estimation parameters, and provide guidance on possible risk-based design approaches, including evaluation of alternative solutions on a cost/benefit basis

Policy Issues

• Consider using the VMS network to provide a greater level of locally relevant information to road users on predicted severe weather events, expressed in terms of probability of occurrence.

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