The wind hazard in the British Isles and its effects on transportation

Allen Perry and Leslie Symons Dqxpcrrtnwnt oj Geogrqhy, Unir~ersit~ College SWY~~UCN. Sir&tor~ Park. S~~wlsec~. SA.? HPP. UK

The impact of severe wind episodes on both the travelling public and the transport network operator is reviewed. The greatest impact is on road, air and sea transport and the examples of storm-force winds in 1987 and 1990 reveals the scale and nature of the disruption. Improvements to the format of weather forecasts and warnings is considered to offer some prospect of reducing the hazard, but a databank of wind-related transport accidents is also needed for planning and preliminary operational statistical analysis.

While the effect of strong wind and gales on the building fabric is well documented (by the Building Research Establishment) the same is not true of transportation. Although severe wind events. such as the Burns’ Day Storm of 35 January 1990, can cause many deaths. enormous insurance claims and can disrupt transport networks. such events are only the most noteworthy of more frequent and common events which occur each year. Hardly a winter season passes without gales causing overturning of vulnerable vehicles at some time. The scale of the problem is, however. little known and there is no databank in existence from which lessons could be learnt about susceptible vehicles and geographical locations. Such a databank could be of use in road planning. highway design and in providing factual evidence to the police contemplating highway closures during severe gale episodes. Until WC have a nominated body to collect information on wind- related accidents. most of our information on how wind can impact on transport networks comes from studying incidents during the well-documented severe gale episodes like those of October lYX7 and January 19YO.

Figures on the impact of wind on transport arc inevitably highly influenced by events like the January 1990 gales and the storm of October 1987 in south-east England. Data on the average number of events per annum based on a period of years before these major events are given in Ttrhlc 1. They suggest that on average wind is a less important climatic variable on the roads than ice. snow or rain. and its major disruptive influence is on sea and air transport. These average figures do. however. throw into relief the exceptional nature ot the 1987 and IYYO windstorms and the close temporal proximity of

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Table I. Average number of events per annum in which major climatic variables affect UK transport system

these two events inevitably raises the question as to whether our climate is getting windier.

To attempt to answer this question it has been found that annual windiness across the UK has in fact been in decline since about 1950 but that random fluctuations in the long-term wind climate ol the UK are to be expected (Hammond, IYYI). The occurrence of X-day windy periods comparable to January/February I990 was noted in winter/spring lY67. lY74-75 and 198243. although the 1YYO wind\ which had return periods in excess of 300 years in parts of southern England were. at many locationx. the strongest on record. In these circumstance? It is hardly surprising that the storm death toll reached 46. the majority being killed in transport-rclatcc1~ accidents. Motor insurance weather damage claim\ totalled 55 000. in JanuarviFcbruary l’s90 (Thornes, 1991). Most motorways ‘in southern Britain were closed for some hours. mainly hccausc high-sided vehicles had been blown over. Over one million trees were blown down.

The impact of wind on transport can be divided into two components:

(1) the effect on land-based termini and structures such as seaports and airports, where operations may be impeded or actual damage caused to infrastructure and transport media, eg aircraft. Design criteria, better operating procedures and safety standard legislation as well as better forecasting of wind velocities and. especially, gusts, strength and duration may help to reduce the impact at such points;

(2) in-transit delays and damage to the means of transport itself. These essentially operational impacts affect most modes of transport.

In the remaining sections of the paper, the authors intend to discuss the impact of the wind hazard on various modes of transportation. The quantity of research that has been undertaken has largely dictated the length accorded to each mode of transport.

Road transport

The authors have already published jointly reviews of the wind hazard (Perry and Symons, 1991; Symons and Perry, 1990) and this work will now be summarized.

There are three main categories of wind hazards:

(1)

(2)

(3)

direct interference with a vehicle through the force of the wind, as a minimum making steering difficult but, with sufficient wind strength, overturning the vehicle or pushing it off the road or into the path of another vehicle; causing obstruction by blowing snow. sand or other material into the highway, blowing down trees, parts of buildings and other debris: and indirect effects such as causing build-up of snow on lee slopes, creating conditions for avalanches, danger to bridges, etc.

The British Transport and Road Research Labor- atory (TRRL. 1975) put the threshold wind speed for danger to road vehicles generally at I5 m/s (53.6 km/h), gusting to 22 m/s (78.4 km/h). Winds in excess of this average speed are not uncommon in Britain. especially on high ground. Fortunately, gusts of over 90 km/h are not common. However, the force exerted by wind on a vehicle is propor- tional to the square of the wind speed and to the area of vehicles presented to the wind so that /zig/z-sided t&i&s are in much greater danger than others because of risk of loss of balance. Stability of all vehicles in motion is a complex problem in dynamics because of the sideways overturning moment, oscillatory forces at the rear of the vehicle and turbulent nature of low-level airflow and the eddies induced by the traffic itself. The sudden gusts induced by the moving traffic may exacerbate the

situation (Telionis, 1984). Overturning accidents are the most common type of wind-induced accidents and in the 1990 storm 66% of accidents involved high-sided lorries or vans. whilst only 27% involved cars.

At the interface between the atmosphere and the ground surface, friction reduces wind speeds and makes the air turbulent, showing itself in sharp fluctuations in wind speed (gusts and lulls) and changes in wind direction. Added to all these hazards, the sharp transitions in velocity which occur at tunnel mouths, bridges, etc. result in frequent risks to the stability of high-sided lorries, double- decker buses, caravans and motor-cycles. Further studies are required of both driver behaviour and driver perception of the high wind hazard.

Trees. walls, fencing panels and parts of buildings may be blown directly on to vehicles or in front of them in the roadway. In Britain, the most common obstructions caused by the wind are fallen trees. Most roads, other than motorways and major routes of near-motorway status. are fringed by large numbers of trees and many country by-roads and lanes are overhung by them. Trees are among the principal causes of the attractiveness of the British landscape but they do cause problems in stormy conditions. Branches and brushwood are commonly torn off trees by winds of 4G50 km/h. which are common in Britain. while winds of 70 km/h upwards threaten whole trees, especially shallow-rooted and old ones.

The highway engineer will be concerned with the effect of wind on traffic using bridges. Wind-induced accidents must be minimized because of the danger to the occupants of vehicles, the bridge itself and the traffic flow.

The Severn Bridge, the principal road link between England and South Wales, illustrates the problems. Records of lane closure on the Severn Bridge indicate that, in the 1980s:

(1)

(2)

(3)

(4)

lanes were closed because of high winds on average some 130 hours per annum on 20 days annually; the lane closures occur almost entirely in autumn and winter, with December and January accounting for 65% of the closures: the average duration of lane closure is about seven hours, although some have exceeded 20 hours but nearly half are of three hours or less; and three-quarters of the closures occur between 0600 and 2300 hours.

In addition, high-sided vehicles are barred from the bridge for about 2&25 hours annually.

In 1983 consulting engineers examined corrosion in the bridge, reported that it might not survive a wind speed of 160 km/h and recommended that traffic should be stopped when winds of 112 km/h are forecast. The original designers of the bridge

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admitted that it was designed for a maximum wind speed of 160 km/h at carriageway level. This figure represents the maximum three-second gust expected in the area with a 50-year return period.

Exposure in a given spot is a function of the local

topography and wind field and its assessment is difficult (Baker. 1984). Anemographs can be pro- duced only by bulky and expensive equipment and widespread installation of such equipment cannot be expected and would not be cost-effective. particularly bearing in mind the varying tracks of storms. Following experiments in the 1960s and 1970s with Doppler radar, new commercial wind-profiling radar offers important possibilities for analysing wind patterns on the mesoscale, complementing radio- sondes for meteorological work, but it is unlikely that there will be early applications of revolutionary technology at the level of the individual road in the near future (Fleming and Hayenga, 1987).

For dedicated studies for new roads, realignments, etc, studies of models in wind tunnels may be useful though expensive. an example being in the design of the M62 motorway across the north of England, with the resulting provision of a windbreak (Rutter, 1968). A much less expensive method is to use tatter flags. pioneered in Britain by the Forestry Commis- sion. which uses them to define planting limits in upland regions. The amount of areal loss or tatter of these flags has been shown to be directly related to the square of the mean wind speed at the site.

For operational purposes it is essential to have constant readings, preferably with automatic record- ing and warning devices. Cost is again, of course, the problem, but there is now an opportunity to utilize remote sensing stations installed for ice warning purposes. Present practice in relation to measuring wind at such stations varies - in many cases wind data are not recorded or transmitted to the central processing stations. In the case of the Findlay Irvine network established in Wales, wind speeds are measured and recorded at fixed times when data are collected and made available to the Meteorological Office for use in their road weather forecasts. and can be accessed at any time. Wind data from these stations have probably been used in the past mainly in relation to the likelihood of disturbed air reducing the risk of freezing of the road surface but during the 1989-90 winter observers at the University College of Swansea found some interesting material. Immense variation was noted, with recorded wind speeds at the same time on 25 February varying from around 8 km/h to over 70 km/h with no relationship to relief but entirely related to the passage of the storm.

Speeds of up to 90 km/h have been observed but. unfortunately, the readings are taken only at intervals of 2.5 minutes and averaged over 30 seconds, thus three-second gusts. as used in engineering and construction standards, are not observable, though this could be altered by software changes at moder-

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ate cost. As this lack of information on the develop- ment of gusts clearly limits the usefulness of the information available at present, we should like to see improvements made to the software for this purpose.

Measures to rcducc the wind hazard, apart from those of routeing. etc, included at the design stage, may be divided into the following categories.

Fixctl or perrm~rzerzt yrec~udom These include artificial wind breaks of all kinds such as slatted fence5 which can achieve 50% reductions of wind- speed across a road, tree screens and solid parapets to bridges (Hay, 1971).

Wind breaks can have undesirable effects, including funnelling and consequent increase 01 winds from some directions, snow drifting, sudden changes from protected and unprotected sections 01 road and increase in the risk of icing from shadow- ing. They have. however. had considerable success and have an additional advantage of reducing the spread of motorway noise.

It~formatiot~ am1 rvartzit~gs. Fixed sign5 may be valuable a5 warnings but tend to become ignored over time. Electric signs have the advantage that they need appear only when required but the disadvantage that interruptions to signals, such as occurred on some British motorway5 during the storms of 1990, result in their complete disappearance.

Radio broadcasts, as with all other road informa- tion, have a particular value but riced to be sent out on all relevant channels and not just restricted to local radio which does not interest manv drivers and is. in any case. difficult to tune to. Dedicated traffic broadcasts, which an appropriately equipped radio will automatically monitor and tune to when messages for drivers are broadcast. are needed. Such systems have been used successfully in continental Europe for many years hut in the UK their adoption ha5 been much slower.

Closure of roads to all or certairl c~1u.s.se.s of ~~chiclcs. As already noted. bridges such as the Scvern, Tay and others in Britain may be closed to all or to high- sided traffic in high winds. A two-level control system has been suggested (Baker, 1987): gusts of over 63 km/h should result in warning signs being activated and high-sided vehicles restricted to 35 km/h (22 mph), and with wind gusting over 80 km/h (50 mph) all vehicles should be stopped. Such firm. some would say Draconian controls, arc not likely to be adopted in Britain. there being virtually no effort by the police who complained that during the high winds in January and February 1990 lorry drivers were not observing their advice to slow down, or. in the case of high-sided vehicles. stop, until winds abated to a less dangerous level.

Information to assist in planning routes may bc of

- Motorways

--*s__ Gust speed (knots).Once in 50 year return period

N

I ,’

-. - --._ e ‘CT

I 100 km I

Figure 1. The impact of the wind hazard on the road network. exposed sites, once in 50 year gust speeds and the tracks of the 1987 and 1990 storms

value to drivers in enabling them to avoid hazardous stretches of road. British Road Services has produced a map to indicate the roads and stretches of road in Britain considered to be subject to particular winter hazards ~ mist or fog, ice, surface water, snow, drifting snow and cross-winds. Although on a small scale (1:1,140.480), enabling the whole country to be shown on one easily manageable sheet. the attempt to highlight these hazards is to be commended. Figure I shows the wind clement from this map with the addition of the generalized tracks of the storms referred to above, and the gust speed over Great Britain with a recurrence period of SO years. The map probably underestimates the locations liable to severe winds. Similarly hazardous are the mountain passes of Scotland, Wales and northern England, and some roads in Cornwall and other areas with marked exposure.

Further field assessment of accident sites is needed. Baker (198X) has developed a computer programme. BLOWOVER, to predict accident wind speeds at specific locations. Used in conjunction with wind exposure surveys it should be possible to assess dangerous wind speeds at particular points.

Between 1962 and 1980 over 100 people were killed and nearly 700 injured on Britain’s roads as a result of strong winds. Such figures seem alarming initially yet. when compared with the overall causalty rate, they account for fewer than 0. I %J of the total. In individual western and northern counties which have the highest frequencies of strong winds over the period 198(&90 wind-related accidents accounted for -l-6%, of the overall accident total (Edwards, 1994).

Air transport

In no other field of transport is wind as critical and potentially dangerous as in aviation. A sudden change in wind direction or gustiness when an aircraft is taking off or landing may precipitate an accident which may be fatal, and a change of wind at altitude may also have serious consequences. A sudden cessation of wind may also result in a catastrophic accident. Less dramatically, the direc- tion and strength of winds influence the range of aircraft and an increased headwind may lead to unexpected diminution of fuel reserves and the need to seek a diversionary airport which introduces other hazards, such as landing in unfamiliar and ill- equipped airports or in adverse weather conditions.

It is in the take-off and landing operations that the greatest hazards occur. This is found to be true when all causes of accidents are examined but it is most of all true in the case of weather-related accidents. Wind hazards range from a moderate gust across the runway, or the addition of a four- or five-knot tailwind component, to a ‘microburst’ associated with a thunderstorm. ‘Windshear’ is one of the most dreaded words in the pilot’s vocabulary.

JLIIIC~‘.Y Acrosoprrce Dictiowry defines windshear

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as ‘Exceptionally large local wind gradient recognized as an extremely dangerous phenomenon because encountered chiefly at low altitude (in squall or local front systems) in approach configura- tion at a speed where [it] makes a sudden and potentially disastrous difference to airspeed and thus lift’ (Gunston. 1980).

When the pilot encounters windshear everything depends on his/her reactions within a second or two. Changes occur so rapidly that rcpcatcd adjustment\ to the controls may be needed within the space of a few seconds. One false move or wrong interpretation of the situation may be disastrous. All pilots need training to deal with these problems. Simulators provide valuable opportunities for airline and military crews to cope with these. as with other emcrgcncics.

Pilots have been aware of the hazards ol strong and changeable winds and gusts from the very earliest days of flying but the hazards of extreme windshear have only been recognized since the late 1970s when accident investigators were seaching for causal factors to explain several spectacular cata- strophes, notably in the USA. It has become widely recognized that modern jet aircraft are particularly vulnerable to windshear accidents because the quest for shorter take-off and landing distances has left aircraft with less reserve power to cope with sudden changes, such as diminution of headwind during take-off. or an unexpected tailwind component during landing. If there is a sudden change of direction from headwind to tailwind on either take- off or landing there must be immediate acceleration to restore the airspeed, otherwise lift diminishes and the aircraft will fly into the ground despite application of other control measures.

The most extreme type of windshcar is associated with thunderstorms which produce violent down- draughts near their centre, to which the name ‘microburst’ has been given. Following extensive research in the USA an advisory bulletin for pilots. meteorologists. air traffic controllers and flight operations officers was produced by ICAO (Inter- national Civil Aviation Organisation).

It is recognized that the time taken to discern windshear and take action is usually no more than five to IS seconds. Convective activity, thunder- storms. heavy rain and blowing dust arc all conducive to windshear.

At 150 knots, the transit time of a microburst some 4 km across is only about 45 seconds and in that time an aircraft may be subjected to multiple wind changes of 6tMO knot velocities from headwind to tailwind associated with violent downdraughts of up to 60 knots (Figure 2).

There are a number of measures that can bc taken to reduce the dangers from windshear. At some major airports Low Level Windshear Alert systems have been installed. Basically these consist of a number of anemometers placed around the airport area providing data which are analysed by a computer

Aircraft starts losing headwlnd

and plcklng up downdraft

Aqxed starts to decrease

and pllot starts to lower nose

Aircraft hlts core of mcroburst

Verttcal speed drops rapIdly

Pilot lowers nose further to

to fmprove Pilot rotates Aircraft leaves dow

talwlnd Vertical s d starts to lnclease

Core downburst

6000 7000 fpm (60kt)

Mawmum gusts 70. loon (22 O-33 5 m) above ground

Normal rotation and Mtoff

Takeoff prior to gust front

PIlot attempts to malntaln V2+10

and when a pre-set level of differences between their readings is attained, a warning is issued and passed to pilots about to land or take off (Taylor, 19X8).

Research carried out by a group of aerospace cornpanics and consultants in the USA aims to produce more sophisticated and. especially. better airborne warning systems for dangerous wind condi- tions (Kupcis. 1987; Higgins, IYM). Doppler radar, which distinguishes intensity of rainfall, rather than merrly its presence, is one tool for both airborne and ground use. If the picture is of very high precipita- tion. sudden development of heavy rainfall, or of the highest levels of precipitation facing the aircraft on approach, the pilot must be prepared for windshear (Collins. IYM). Even so, radar has the capability of only limited penetration of storms; its accuracy falls off sharply with distance into the storm and it may present highly inaccurate information about the other side of the storm or of another storm further away. Doppler radar has the advantage of measuring the movement of particles toward or away from the radar site, indicating where the wind is shifting and when windshear may occur. This still does not overcome the problems caused by the rapidity of change in and around storms. A review of current developments in this research and a summary of the views expressed by the Civil Aviation Authority has been provided by Goold and Daly (1993).

Windshear Alerting Services are available at some airports in the UK. Forecasters for London (Heath-

row) and Belfast (Aldergrove) airports review conditions hourly and monitor reports of windshear experienced on approach or climb-out. Where a potential low-level windshear condition exists an alert is issued based on one or more of the following criteria:

(3)

mean wind speed of at least 20 knots; where the magnitude of vector difference between the mean surface wind and the gradient wind (6 km feet) is at least 40 knots; the presence of thunderstorm(s) or heavy shower(s) within approximately five miles of the airport (AA/B Bulletin. 3/90, p. 9).

An example of an accident in which windshear appeared to play a significant part was that to a Shorts SD 3-60-100 landing at Bristol Airport on 20 December 1989. During the approach the crew were warned of adverse conditions including thunder- storm activity to the east of the airport. The aircraft commander. because of the severe turbulence that was being experienced, intended to cross the runway threshold at 10 knots above the selected target threshold speed of 98 knots. On touch-down the aircraft bounced and on the second touch-down the landing gear suffered damage. The aircraft was brought to a stop and there were no injuries. The report by the Air Accidents Investigation Board noted that Bristol Airport did not have a Windshear Alert Service. Had there been one an alert would

127

probably have been issued prior to this accident

(AAIR Rullctin 3190, pp. 5-9). Large multi-engined aircraft are generally safer

than small aircraft and are more comfortable in

rough weather. There have been bery few cases of

large aircraft being destroyed by flying through had

weather. though this should be avoided when

possible. Small aircraft should ncvcr be flown into

storms if at all avoidable. Nevertheless, despite the

sophisticated design and great power of modern jet

transport\ they are not necessarily better than light

aircraft at surviving hostile wind patterns. This i$

because lighter types may have better reserva of

power available for sudden acceleration. as long as

full power is available. Collins (1986) argues that

twin-engincd aircraft. designed to be able to continue

to climb with one failed engine, are likely to be more

fitted to survive in such condition\ than machines

with four engines. This i\ because four-engined

aircraft also are designed to be capable of climbing

with one engine out (retaining three-quarter\ of

their power). They therefore require Icss excess (or

rescrvc) power with all engincs operating than a

twin-engined aircraft which loses half its power with

one engine failed. This docs not, of course, alter the

fact that four engina confer other obvious safety

advantages.

In general, light aircraft suffer many more

accidents than do large aircraft. especially jets. This

is because of the higher level of crew training,

qualifications and maintenance reyuired for jet

aircraft, especially those in airline service. All

known accidents to aircraft in the UK arc detailed in

the Air Accidrrtt Itz~wtiptiott Btwtdt Rullclitl. divided into weight categories. From these reports

an attempt has been made to show how many

accidents wcrc at least partly caused by weather and

in how many it appears that wind was a major factor.

An examination of all the accidents reported in

the AA/R Rulleh issued between January 1990 and

October 1997 revealed a total of 717 accidents to

fixed-wing aircraft and X9 to rotorcraft (essentially

helicopters). It should be emphasized that in man\i

of the events listed as accidents there w;i\ little

damage and no injury to persons. In aviation any

damage and many other incidents qualify for an

‘accident’ report,

Dealing first with light aircraft. although these

were not for the most part engaged in true transport

operation\. there were 691 accidents (63-l to acre-

plancs and 74 to rotorcraft). Weather W;I\ at Icat ;I

partial cause in IO-1 (16.6%) of these accidents to

aeroplanes and Gx (S. I ‘!L) of tho\c to rotorcralt. and

wind wa\ the major identifiable factor in 73 of the

1 O-1 weather-related accidents. ie 70%.

For the much smaller number ot accidents to

passenger and cargo transport (including corporate

flying) the incidence of weather cause\ is shown in

T~rhlc 2 It will be seen that wind, including

turbulence, was the major identifiable factor. import-

ant in scvcn of the ten weather-related accident\.

Many pilots of light aircraft have not received the

training ncccssary to cope safely with cxtrcmc ot

cvcn quite moderate but unexpected gusts. or other

rough weather. and relatively few are qualified to fly

in cloud. which demands instrument flying qualifica-

tions. A new book has recently been written which

should bc especially valuable for pilots of limited

experience (Wickson. 1993).

Rail transport

Probably the most famous effect of wind on a

railway structure was the failure of the Tay Bridge

on 28 December IX79 as a result of an engineering

failure in the structure to allow for stray gusts

exerting loading on the bridge spans. Today the

likelihood of extensive disruption to rail service4

from high winds arises principally from two sources:

(1) Overhead cabling systems will be disrupted by

trees blown on them. Trees may also block the

track itself. The cost of removing all tree\ that

could fall on British Rail lines is high and would

7‘ahle 2. Accidents to transport aircraft in the Ilh: 1990-92 (summer): relationship to wind and other weather elements

(2)

result in a local amenity loss. On Southern Region alone the bill for felling 60 000 trees is currently put at 212 million; British Rail estim- ates that there are 75 000 acres of lineside vegetation along the rail network and in autumn high winds can sweep leaves onto the track and cause skidding and disruption to trains. Heavy spray on coastal routes can deposit salt on insulators, or make uses of coastal sections of line such as those in south Devon and eastern Scotland hazardous.

Wind alarm systems have been installed at several points on Scot Rail and trigger alarms which can result in necessary actions like speed restrictions or the checking of wagon sheet security covers. Severe gales may disrupt signalling and thus be hazardous to train operations. On 2 January 1976, the driver and second man of a diesel locomotive were killed when they crashed into the back of a stationary parcel train near Worcester as a result of a signal fault.

New lightweight trains and trams could be over- turned or derailed by extreme wind gusts and the development of such systems in some British cities has required a knowledge of the aerodynamic characteristics of the train and the effect of cuttings and embankments in modifying the wind field.

Blowing snow leading to drifting and the formation of deep drifts can immobilize train services and, in the most severe episodes. bury trains. In Great Britain these events are most frequent in Scotland and, as a result of several trains being trapped by drifts in both 1978 and 1985 (Symons and Perry, 1980; Perry and Symons. 1985). the installation of cab radio and the carrying of emergency supplies has been initiated on lines where the threat appears to be significant. More recently. during very cold weather in February 1991 in south-east England, fine, powdery snow was blown into mechanical and electrical equipment causing damage to over 400 electric train motors. A report on railway perform- ance at the time (Department of Transport. 1991) led to the implementation of a number of technical and operational procedures that might alleviate the problem in the future, although it is clear that the level of protection against such conditions has to be based on an assessment of the likely return period of such adverse conditions.

Sea transport

At sea the hazard posed by high winds, gales and storms is at its most obvious. Small vessels are constantly at risk and quite large vessels face dangers in very bad weather even in open sea. Substantial ferries may be endangered when entering and leaving harbour in high seas and deck personnel are constantly at risk. Vessels are particularly likely to get into trouble if cargo shifts, if engine rooms are flooded or if steering gear is damaged. Once a ship is

drifting without power in inclement conditions there is often little that can be done, short of a tow. to rescue it.

Britain’s coasts are littered with shipwrecks and it was this problem which led to the setting up of a meteorological service in the UK in 3854. The actions of the Royal National Lifeboat Institution and the use of aircraft and helicopters in inshore waters have helped to reduce the toll of lives from merchant ships in trouble in coastal waters. New types of craft such as wave-piercing catamarans and jetfoils. now being introduced on high-density routes like the English Channel, generally become inoperative in high wind speed situations with significant seas, as do hovercraft. Drive-on. drive-off passenger ferries have been involved in a number of notable storm-related disasters. During the severe gale of 31 January 1953 the Stranraer-Larne ferry Princess Victoria had its car-deck doors damaged and the vessel was lost with 133 fatalities. This tragedy led to the introduction of new safety measures to allow for greater resistance to wind and storm hazards.

Conclusions

The preceding sections serve to highlight the con- siderable influence that high winds can have on the UK transport system and its component parts. In most years only a few days will be affected and only very occasionally is the scale of damage and disrup- tion likely to be severe. Projected global warming may well be accompanied by changes in the frequency of storms. although it is by no means clear at present whether more or less severe gales might be expected. Although the January 1990 storm remains fresh in many people’s memory and clearly had a long return period in southern and central England, it would be imprudent to believe that a more severe storm could not occur. The great storm of December 1703 probably exceeded in severity that of 1990 and in the short period from 1664-1720 at least four exception- ally severe windstorms occurred.

Since the poor forecasts of the 1987 storm by the Meteorological Office. the quality and accuracy of forecasts has improved notably. A new Meteoro- logical Office National Severe Weather Warning Service (NSWWS) has now been established (Hymas, 1993). There are two tiers of warnings: Tier I, warning of severe or exceptionally severe weather; Tier 2, warning of hazardous conditions (with the severity of the weather less severe than in Tier 1). Criteria for the issue of wind warnings are given in Tdde 3.

From April 1992 Emergency Flash Messages have been introduced when exceptionally severe condi- tions are expected to occur over a wide area. The messages are intended to receive prominence from the national media.

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Table 3. Criteria for the issue of wind warnings

Categor? Tier

50 mph gu\ts 7 (72 rnk)

60 mph gu\t\ 2 (77 Ill/\)

71) mph gu\t\ I (31 Ill/\)

so mph gu\t\ I

(ih m/s)

Given the infrequency with which damaging high- wind episodes occur. the importance of providing informative, easily understood forecasts for both travellers and transport network operatives cannot be over-emphasized. Many accidents occur because there is a failure to foresee the possible consequences of conditions which themselves may have heen accurately forecast. The issuing of appropriate warnings and guidance for expected events in a manner and format that the transportation industry and its users can best make use of is at least as important as the ability to forecast the event accurately.

Acknowledgement

This research has been supported by a fellowship granted to Professor Symons by the Leverhulme Foundation.

I30