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3 HISTORICAL CONTEXT

3.1 INTRODUCTION

This section provides a summary of the historical information available in relation to the weather events that are the subject of this study and outlines how this information relates to the design and operation of the road network. Further details of the data sources relating to the information presented here are included in tabular form at the end of this section.

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.

To assist in the assessment of impact of climate change average regional temperature series have been developed. The best known of these is the Central England Temperature ( CET) series, which reaches back to 1659 and is the longest monthly series in the world (Parker et al, 1992). CET has been widely used as a measure of thermal conditions over the British Isles. However, Jones and Lister (2004) have developed two specifically Scottish temperature series, covering the periods between 1861 and 2000. One of these is for mainland Scotland ( SMT) and the other is for the north and north-western isles ( SIT). Both series show long-term statistically significant seasonal warming, except for the winter (Dec-Feb) season. For mean annual temperatures the change, expressed as a linear trend, over the period is an increase of 0.69 ^oC for SMT and 0.64 ^oC for SIT. This compares closely with the 0.75 ^oC increase for CET.

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<</b>

3.3.1 General

Rain is one of the most important factors affecting the design and operation of the road network. It affects the design of drainage systems to collect and discharge surface water, these systems being designed to accommodate the 1 in 1 year storm without system storage being required and to store the 1 in 5 year storm without flooding of the road surface. It also affects the sizing of river bridges/culverts, which are designed to accommodate a much larger rainfall event, typically the 1 in 100 year storm for the catchment concerned. Rain also creates a hazard to road users when it is not shed sufficiently quickly from the carriageway, resulting in loss of visibility and skid resistance, both of which are frequent contributing factors in road accidents.

Rain also has the potential to cause significant landslide events, for example those witnessed in August 2004. These occur through large volumes of surface water eroding the land surface and/or 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 of rainfall events, and therefore a particular concern is that if rain is considered likely to increase, these records may no longer correctly describe the design storm event.

3.3.2 Rainfall

Recent changes in Scottish rainfall are evident. 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. The increase in the winter summer ratio is also suggested by the UKCIP02 projections.

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.

Many researchers have recognised that the recent past has seen a shift in the spatial and temporal patterns of precipitation across the UK. Marsh (1996) notes that there has been a tendency towards wetter winters and drier summers, with rainfall totals increasing, particularly in the north and west of the UK. Smith (1995) concludes that the increased precipitation in Scotland in the 1980s and 1990s can be regarded as the largest sustained anomaly within the period of record 1757-1992. Foster et al (1997) observed marked differences in annual rainfall trends between the west and east of Scotland over the period 1861-1994, with the west appearing to become significantly wetter while the east remained more or less the same. They also showed that during the recent past (1970-1994) the rainfall gradient across Scotland, which shows a wetter west and relatively drier east, has intensified with much of the additional rain being recorded during the winter in the west. The UKCIP02 projections of future precipitation suggest a reversal of this trend.

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. Their work does not, however, extend to an examination of the rarest of events that are of key interest to this report. They postulated that changes in winter weather types may have contributed to the increase in the proportion of precipitation provided by heavy events.

This change in rainfall as recorded by observed data is also illustrated in Figure 3.2, which shows the trends in rainfall amount from 1961 to 2000 (paper in preparation, Perry et al).

Figure 3.2 - Average trend in rainfall amount, 1961 to 2000. Source: Met Office, Paper in preparation.

3.3.3 River Flooding

The historical change in river flooding, which has a direct link with rainfall, has been the subject of research for some time. The most thorough search for long-term trends in the UK flood record has been conducted by CEH Wallingford (Robson et al 1997, Robson et al 1998). Their work focused on changes that have occurred at the national scale, making use of flood records from 890 river flow stations around the UK. This information included detailed records for the period 1941-1990, with more limited data for the period 1870-1995.

The analysis of this information, which included seasonal records, found that no significant long-term trends in national flood behaviour could be detected. In addition, a simple north-west/south-east divide was investigated using the Tees-Exe line as the dividing axis. Again no significant regional long-term trends for either flood counts or annual maxima could be detected. Although single site analysis indicated that in north and western Scotland some sites showed a trend which is probably of climatic origin, no significant trend could be found when data was pooled across the region.

In the context of climate change the authors (Robson et al, 1998) concluded that although their work could provide no conclusive proof that climate change had affected national flood behaviour, it did not mean that climatic effects could be disregarded. On the basis of their research the case remained unproven.

Robinson et al (1998) also showed a clear link between annual rainfall and the numbers of floods. This indicted that years of higher than average rainfall tend to have a higher than average number of floods. They suggest that this relationship could provide a baseline estimate for the degree to which flood frequency may change under various climate change scenarios. However no link between flood magnitude and annual rainfall was reported.

Figure 3.3 - River flooding affecting the A82 at Drumnadrochit, 1993. Photograph courtesy Glen Urquhart Landuse Partnership.

Several authors have specifically studied Scottish flood behaviour using the records from the last few decades. In particular, Black (1996) found that many rivers in western Scotland registered new maximum floods during the late 1980s and early 1990s, along with increases in event frequency. This behaviour was not apparent in the flood records for rivers in western England and Wales, which may suggest a link either to changes in regional precipitation characteristics, or perhaps as a result of reduced snowfall in Scotland. Black also noted the similarity of increased flood activity in other countries, such as southern Norway, southern Finland and Estonia, which are exposed to westerly weather systems and suggests the reason may be a common climatic link.

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.

3.3.4 Groundwater/Soil Moisture

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 3.4 shows observed changes in the length of the snow season for western Scotland (paper in preparation, Perry et al).

Figure 3.4 - 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. Once again this result is preliminary but it is consistent with predicted changes by climate models.

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 3.5, 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 3.5 - 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. It 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. Such trends cannot be reliably identified within the available records when due allowance is made for the effects of natural variability. 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, although these observations are insufficient to prove a long-term sustained trend (Bacon and Carter, 1991 and Price and McKenna, 2003).

3.8 SUMMARY OF WEATHER EVENTS AND THEIR IMPACT ON THE DESIGN AND OPERATION OF THE ROAD NETWORK

Tables 3.1 to 3.6 present a summary of how the key climate variables can potentially impact on the design or operation of the road network, and what data is available in relation to these impacts. The tables are numbered as follows:

• Table 3.1 - Temperature
• Table 3.2 - Rain
• Table 3.3 - Snow
• Table 3.4 - Wind
• Table 3.5 - Fog
• Table 3.6 - Coastal Flooding

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