Flood Forecasting and Flood Warning in the Firth of Clyde, UK

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<ul><li><p>Flood Forecasting and Flood Warning</p><p>in the Firth of Clyde, UK</p><p>YUSUF KAYAw, MICHAEL STEWART and MARC BECKER16 Carrick Drive, Mount Vernon, Glasgow G32 0RW, UK</p><p>(Received: 18 November 2003; accepted: 30 June 2004)</p><p>Abstract. Coastal ooding has caused signicant damage to a number of communities aroundthe Firth of Clyde in south-west Scotland, UK. The Firth of Clyde is an enclosed embayment</p><p>aected by storm surge generated in the Northern Atlantic and propagated through the IrishChannel. In recent years, the worst ooding occurred on 5th January 1991 with the estimateddamage of approximately 7M. On average, some 0.5M damage is caused each year by</p><p>coastal ooding. With the latest climate change predictions suggesting increased storm activityand the expected increase in mean sea levels, these damages are likely to increase. In line withthe expansion of ood warning provision in Scotland, the Scottish Environment Protection</p><p>Agency (SEPA) has developed a ood warning system to provide local authorities andemergency services with up to 24 h warning of coastal ooding within the Firth of Clyde andRiver Clyde Estuary up to Glasgow City Centre. The Firth of Clyde ood warning systemconsists of linked 1-D and 2-D mathematical models of the Firth of Clyde and Clyde Estuary,</p><p>and other software tools for data processing, viewing and generating warning messages. Thegeneral methodology adopted in its implementation was developed following extensive con-sultation with the relevant authorities, including local councils and police. The warning system</p><p>was launched in October 1999 and has performed well during four winter ood seasons. Thesystem currently makes forecasts four times a day and is the only operational coastal oodwarning system in Scotland.</p><p>This paper summarises the development of the warning system, gives a review of its oper-ation since its launch in 1999 and discusses future developments in ood warning in Scotland.</p><p>Key words: ood forecasting, ood warning, tides, surges, numerical modelling</p><p>1. Introduction</p><p>Following the ooding in January 1991 SEPA, (the then Clyde River Puri-cation Board) immediately commissioned an initial study (Townson andCollar, 1993) to assess the frequency of ooding and associated cost ofdamage at several sites within the Firth of Clyde area.</p><p>The initial study (Townson and Collar, 1993) investigated the damagecaused by ooding in four locations: Tarbert, Rothesay, Dumbarton, andSaltcoats (see Figure 1). The study estimated an average annual ood damage</p><p>w Author for correspondence. E-mail: yusuf_kaya@msn.com</p><p>Natural Hazards (2005) 36: 257271 Springer 2005</p></li><li><p>cost to property around the Firth of Clyde to be in excess of 0.45M (Townsonand Collar, 1993) (based on 1999 costs). This was based on using publisheddata on ood damage costs per unit area of property aected (FLAIR, 1990).</p><p>It was estimated that the average benet that would be gained by theprovision of a Flood Warning System, for the above locations alone wouldbe in the order of 158,000 per annum (based on 1993 gures). The con-clusion of the study was that a signicant proportion of the ood damagecost could be saved by the provision of a ood warning system. Thisprompted SEPA to proceed with a further study (Firth of Clyde ModellingStudy, 1996) to assess the practicalities of storm surge modelling in the Firthof Clyde. Although the UK Continental Shelf model covers the Firth ofClyde area, its representation of the upper parts of the Firth is crude due toits large grid size. Following the successful outcome of the initial modellingwork, SEPA decided to set up a coastal ood warning system covering theentire Firth of Clyde and part of Clyde Estuary up to Glasgow City Centrewhich is about 15 km east of Renfrew. The system was developed in 1999 andhas been operational since 1 October 1999.</p><p>Figure 1. Location plan.</p><p>YUSUF KAYA ET AL.258</p></li><li><p>The earlier work relating to the development and operation of the schemewas reported elsewhere (Townson et al., 1994; Collar et al., 1995; Becker andKaya, 2000; Becker et al., 2000; Burns et al., 2000).</p><p>2. SEPAs Role in Flood Warning</p><p>In Scotland the provision of ood warnings is a discretionary function ofSEPA. In recognition that other organizations, including local authoritiesand the various emergency services, also have key roles to play during oodevents SEPA developed a Flood Warning Strategy (Policy No. 34) which setsout the levels of service oered by SEPA (sepa.org.uk).</p><p>In November 2001, SEPA extended the successful Floodline service fromEngland and Wales to Scotland. Although not a statutory duty, oodwarning is a high prole activity for SEPA and over the years an increasingnumber of local authorities and the public have come to rely upon the serviceprovided.</p><p>Floodline allows users access, via the telephone (0845 988 1188) or website (sepa.org.uk), to general ood alerts, Flood Watch, or any locallyspecic ood warnings that may be in force. Floodline also provides infor-mation and advice on how to prepare for ooding.</p><p>The introduction of Floodline in Scotland, the setting up of the NationalFlood Warning Development Team and the appointment of a full time FloodAwareness Public Relation (PR) Ocer shows SEPAs commitment toimproving ood warning in Scotland.</p><p>3. Flood Forecasting System Development</p><p>3.1. WARNING SYSTEM</p><p>The Firth of Clyde ood warning system was developed after extensiveconsultation with various parties including the relevant Local Authorities,Police, Fire Brigade and the Maritime and Coastguard Agency. The coastalarea covered by the scheme falls within seven local authorities. These areArgyll and Bute, North Ayrshire, South Ayrshire, Inverclyde, West Dun-bartonshire, Renfrewshire, and Glasgow City.</p><p>At SEPAs request, local Councils provided threshold levels at 42 criticallocations where ooding were known to occur. These included those areaswith a known history of ooding such as Tarbert, Rothesay, Saltcoats, andDumbarton. The threshold levels were the current defence levels whenovertopped ooding of properties or public services would occur. All 42locations were included in the warning system. However, other areas, notidentied at present, maybe at risk from higher surges could be added to thesystem in future if required.</p><p>FLOOD FORECASTING AND FLOOD WARNING 259</p></li><li><p>A schematic overview of the Firth of Clyde Flood Warning System isshown in Figure 2. The main elements of the Flood Warning Scheme are:</p><p> receipt of forcing data from Met Oce (predicted surge and wind), a linked mathematical modelling system of Firth of Clyde and ClydeEstuary,</p><p> data visualisation, pager alarm, fax generation.</p><p>The predicted surge data at the North Channel and predicted wind datarepresenting the Firth of Clyde area (speed and direction) is automaticallyreceived from the Met Oce through MIST (Meteorological InformationSelf Brieng Terminal), for a xed period of 36 h commencing at 00.00 GMTand 12:00 GMT each day. This data is produced by the UK ContinentalShelf model and Met Oce atmospheric model respectively. The forecastdata, in the form of hourly values, is received by SEPA at 03:00, 09:00, 15:00and 21:00 GMT each day respectively.</p><p>Figure 2. Schematic overview of Firth of Clyde ood warning scheme.</p><p>YUSUF KAYA ET AL.260</p></li><li><p>The surge and wind data received is then automatically passed to themathematical model for forecasting of tide levels within the model bound-aries. The process is described below.</p><p>3.2. HYDRODYNAMIC MODELLING SYSTEM</p><p>The entire Firth of Clyde has been modelled using the FLOFIELD two-dimensional modelling system (a derivative of the DIVAST (Falconer, 1986)software package). DIVAST (Depth Integrated Velocity And Solute Trans-port) is widely used for modelling coastal waters where stratication isinsignicant. FLOFIELD is the hydrodynamic part of DIVAST. The two-dimensional model covers the entire Firth of Clyde and lower parts of theClyde Estuary and is based on uniform 500 m grid spacing.</p><p>A one-dimensional numerical model of the Clyde Estuary was previouslyset up for another study. The model was based on in-house river modellingsoftware package FLOODTIDE which is based on the solution of Saint-Venant equations for shallow water waves using nite dierence techniques.The average cross section spacing is 300 m.</p><p>The two models were linked together to form a single modelling systemcovering the entire Firth of Clyde and Clyde Estuary up to Glasgow (i.e.from the North Channel in the Irish Sea to Glasgow City Centre, includingthe sea lochs/fjords). The linking of the two models eliminated the need for aphysical boundary between the two models in the Greenock area.</p><p>The models overlap over a distance of about 24 km as shown in Figure 1(distance between the river model and 2-D model boundaries). This wasnecessary to allow smooth transition of water level predictions from onemodel to the other at their boundaries at each time step. Boundary condi-tions at the furthest downstream section of the river model are based on thepredicted water levels from the two-dimensional model at that point. Simi-larly, boundary conditions at the eastern open boundary of the two-dimen-sional model are based on the predicted water levels from the river model. Atthe start of each time step this information is exchanged between the twomodels and the models are run consecutively.</p><p>The linked model is driven by tidal variations (astronomical tide andsurge) at the boundary in the North Channel, uvial ow input at the tidallimit of the Clyde Estuary, and wind data (speed and direction) representingthe Firth of Clyde area.</p><p>An L-shaped open boundary was chosen at the North Channel to allowexibility during model calibration in adjusting water level variations alongboth boundaries and also allowing small phase dierences between the twoboundaries. Test runs were carried out using a number of phase dierencesand water surface slopes along both boundaries. Although no phasedierence was introduced between the two boundaries in the nal calibrated</p><p>FLOOD FORECASTING AND FLOOD WARNING 261</p></li><li><p>model, water surface elevation was adjusted along each boundary to takeCoriolis eects into account (Falconer, 1986).</p><p>The astronomical component of the tide in the North Channel is calcu-lated automatically within the modelling software using 97 harmonic con-stants representing astronomical tides in Campbeltown and Girvan (i.e. atthe two ends of the model boundary shown in Figure 1). The forecast surgelevels in the North Channel and corresponding representative wind speed anddirection data are provided by the Met Oce for a 36 h period and receivedthrough MIST. The surge data is then combined with astronomical tide andthe combined level is used as open boundary conditions along the L-shapedboundary at the North Channel. Wind data is used to calculate shear stresson the free surface.</p><p>At present the uvial data at the tidal limit upstream of Glasgow is set to aconstant average ow. There are two reasons for this: rstly there is noforecast uvial ow data available at the tidal limit, and secondly variation inuvial ows in this location would not signicantly aect the model pre-dictions within the area of interest (i.e. Glasgow city centre and downstream).However, the system has been designed in a modular way, which makes itsuitable for the inclusion of any number of forecast river ow data if suchinformation becomes available in future.</p><p>The model bathymetry was obtained from relevant Admiralty charts.Model test runs carried out subsequently indicated that model predictions arenot sensitive to bathymetry.</p><p>In order to provide data for model calibration and verication and alsofor assessing model performance, a number of tide gauges were installed inthe Firth of Clyde by SEPA. These are located at Tarbert, Rothesay,Campbeltown and Girvan. These gauges, together with the existing tidegauges at Millport, Ayr Harbour, and subsequently installed Renfrew tidegauge, provide a good coverage of water level and tidal propagation withinthe Firth of Clyde area. The gauge data at Campbeltown and Girvan allowscomparison to be made with the forecast surge data at the model boundary inthe North Channel received from the Continental Shelf Model.</p><p>Model calibration and verication was carried out against a number ofrecent surge events for which relevant data was readily available. Some of themodel results are given in Tables I and II. At all locations the average dif-ference between the predicted and recorded tide levels was well within thetarget value of 0.1 m if there were no gross errors in the boundary surgeforecast received from the Met Oce.</p><p>The model results indicated that the accuracy of model predictions isstrongly dependent on the accuracy of the surge data applied at the openboundary in the North Channel. Using recorded surge levels at Campbel-town and Girvan as boundary data produces very good match with recordeddata at other tide gauge locations within the Firth, as shown in Table I and</p><p>YUSUF KAYA ET AL.262</p></li><li><p>Figure 3a. Larger dierences were predicted when the Met Oce predictedsurge data was used to determine the conditions along the southern openboundaries as shown in Table II.</p><p>A discrepancy was observed at Renfrew between the model predictionsand recorded tide levels for some tidal events as shown in Figure 3b for theDecember 1998 event. In these limited number of cases the recorded datashowed a double peak which the model has failed to reproduce.</p><p>Further investigations were carried out and the model was subsequentlyrened. This involved the extension of the two-dimensional model furtherinto the Clyde Estuary, and also increasing the resolution of the rivermodel over the area where the two models overlap to better represent thecomplex nature of the ooding and drying processes in this area. Therened model appears to reproduce the double peak at Renfrew as shownin Figure 4.</p><p>Table I. Dierence between model predicted and recorded peak water levels and their timingsat a number of locations within Firth of Clyde for the 22 February1 March 1997 event</p><p>(boundary conditions were based on recorded tide levels at Girvan and Campbeltown)</p><p>Location Dierence in</p><p>Water level (m) Timing (min)</p><p>Ayr 0.04 15</p><p>Millport 0.06 15</p><p>Rothesay 0.05 15</p><p>Tarbert 0.05 15</p><p>Table II. Dierence between model predicted and recorded peak water levels and theirtimings at a number of locations within Firth of Clyde for three surge events (boundaryconditions were based on Met Oce forecast surge levels combined with astronomical tides)</p><p>Event Location Dierence in</p><p>Water Level (m) Timing (minutes)</p><p>January 1995 Millport 0.2 15</p><p>Rothesay 0.2 15</p><p>Tarbert )0.3 15December 1994 Millport )0.15 15</p><p>Rothesay 0.1 15</p><p>812 February 1997 Girvan 0.05 15</p><p>Rothesay 0.1 15</p><p>FLOOD FORECASTING AND FLOOD WARNING 263</p></li><li><p>Recent studies of surges within the Firth of Clyde indicate that up to onethird of the surge arriving at the upper parts of the Clyde could be generatedwithin the Firth itself. This indicates the importance of a detailed model torepresent local conditions accurately, particularly within the inner Firth,w...</p></li></ul>

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