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Integrated Flood Forecasting,Warning and Response System

3

48

3.1 Defining an Integrated System

Establishing a viable flood forecasting andwarning system for communities at riskrequires the combination of data, forecasttools, and trained forecasters. A flood-forecast system must provide sufficient leadtime for communities to respond.Increasing lead time increases the potentialto lower the level of damages and loss of life.Forecasts must be sufficiently accurate topromote confidence so that communities willrespond when warned. If forecasts areinaccurate, then credibility of theprogramme will be questioned and noresponse actions will occur.

Flood-warning systems must be reliable anddesigned to operate during the most severefloods. The greatest benefits for an effectiveflood-warning programme occur whenflooding is severe, widespread, and/orsudden, and when communities andorganizations are prepared to mitigateimpacts.

The implementation of an end-to-end floodforecast, warning and response systemconsists of many components. Thesecomponents must be linked for successfuloperation. The interaction of components ofthe integrated flood forecast system orprogramme could be represented as a chain

composed of many links. Each link must bepresent and functional if benefits are to beachieved. Figure 12 shows a schematicrepresentation of the system as links in achain.

The essential links or components of theintegrated flood forecasting, warning andresponse system consist of a Data Source,Communications, Forecasts, DecisionSupport, Notification (often referred to asdissemination), Coordination, and Actions(or responses). A flood forecast and warningprogramme should be designed to mitigatefloods, and, as such, it is an asset to overallwater management. To achieve this, it isimportant that all of the components of thesystem be functional. If any component isdysfunctional, then this weak link couldbreak the chain, resulting in an ineffectivewarning and response process. For example,if critical rainfall or streamflow data areunavailable or if the data are not relayed to aforecast centre for use in forecasting, thenthe critical lead time required to makedecisions, coordinate activities, warncitizens, and take actions is not possible. Ifa perfect flood forecast is generated but doesnot reach the population at risk, then thewarning system is useless. Equally, shouldthe population at risk receive the warning

Figure 12Integrated flood forecasting, warning and response system within IWRM

GIS T oo lsMa th ema tic a l Mode ls

Data Communication Forecast DecisionSupport Notification Coordination Actions

Sense WaterAvailability

Get Datawhere

Needed

FutureWater

Availability

Water Mgmt.&

Flood ControlDecisions

AppropriateIndividuals& Groups

Tasks fromResponse

PlansOrganisationsCivil society

RelocationSand baggingOther responses

Flood and Water Resources Management: a Critical Chain of Events and Actions

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Guidelines for Reducing Flood Lossesbut not know what actions should be taken,then the system again would not haveaccomplished its purpose.

In the overall design of the integratedsystem, there are many factors that shouldbe considered. The remainder of thischapter reviews some of these factors.

Basin characteristics

The physical characteristics of the basin,such as surface area, topography, geology,and land surface cover, will help todetermine the nature of potential floodingand the basin's susceptibility to relatedhazards such as landslides and mudflows.

The hydrological response of the basin canbe impacted upon by changes in land useassociated with urbanization, forestry,agriculture, drainage, or channelmodifications. A record of such changesover time is useful in establishing thedynamic relationship between rainfall andrunoff. The following also contribute to anunderstanding of flood hazards: records ofclimate norms and trends for parameters,such as precipitation and evapotranspiration;and information on the usual effects ofENSO events and extreme events, fromsynoptic to mesoscale.

Population centres often are adjacent torivers, and flood plains can be richagricultural resources. Identification ofpopulations and economic activities at riskshould be carried out early in the process, asthis will shape the eventual forecast output.

Flood history

Flood history, also known as paleohydrology,can be inferred from study of sedimentdeposits, tree ring analysis, and examinationof a number of other biological indicators.Such analyses will not lead to adetermination of flood volumes, but may

help put a recent flood into context. Often,such records are of value in defining theflood history in a river basin, particularlywhen combined with stream gauge recordsthat exist for contemporary periods. Othermore recent historical information can bedrawn from newspapers, journals and oralhistories. Usually there is a perception levelassociated with flooding at a given location;large events are noted, smaller events arenot.

The flood history will identify the portionsof a basin subject to flooding, whetherflooding is urban or rural or both, seasonalcharacteristics of flooding, and the feasiblewarning time. The type of flooding andassociated hazards may be significantlydifferent on tributaries compared to themain stem of the river. Knowledge of thefactors contributing to or causing theflooding such as meteorological andantecedent conditions should be establishedfor each flood event.

Lake flooding as well as flooding fromocean surge and tsunamis may poseproblems quite different from river flooding.These guidelines are oriented to riverflooding, but many principles containedherein are also applicable to varying degreesto other water-related disasters.

Environmental factors

Floods can induce major changes in rivermorphology, mobilize nutrients andcontaminants in the soil, release othercontaminants from storage depots, anddischarge effluents to the river.Deforestation, fires and erosion of materialscombined with saturated soils can lead tolandslides, mudflows and other threats tohuman settlements. Sometimes floods areaccompanied by strong winds that also canpose threats to human life and property. Ananalysis of potential environmental risks willhelp determine flood forecasting and

warning needs. This can help to shapefuture approaches to flood plainmanagement and regulation, and can assistin the design and establishment of responseactions.

Economic factors

A flood forecasting, warning and responsesystem comprises an important element ofintegrated water resources management.The benefits of river forecasts for powergeneration, navigation or irrigatedagriculture make implementation of such asystem more cost effective and sustainable.Even then, maintaining a system in a state ofreadiness between floods may be difficult.

An examination of past damages and thepotential for future damages will helpdetermine priority areas for floodforecasting, warning and response.Rigorous analysis would call for statisticalanalysis of flood peaks and the calculation ofthe present value of costs and benefits offlood forecasting and warning. In mostcases, however, the benefits of floodforecasting, warning and response arevirtually self-evident. The real questions arethe affordability of various options and thedesire of society to invoke a more pro-activestance to reducing flood losses.

Communities at risk

While flood losses in rural districts can bedevastating to those areas, the mostsignificant losses are usually in urbancommunities because of the concentration ofpeople and related socio-economicinvestments. The basin characteristics andflood history of individual communities,combined with damage estimates fromprevious floods, will give some indication ofthe type of flood forecasting and warningsystem that may be most suitable foreffective warnings. Once the system isdefined, consideration should be given as tohow it could benefit rural areas as well.

System identification

Depending on the nature of the basin andthe type of event causing the flooding,potential warning times could vary fromhours to several days to weeks.Communities subject to flash floodingrequire warnings of meteorologicalconditions that, when combined withantecedent basin conditions, could lead toflooding. This represents a special case offlood forecasting and warning. Thechallenge in such cases is rapid depiction ofcritical flood thresholds and theirsubsequent communication and emergencyresponse. An analysis of historical rainfallrecords, including storm transposition andthe resulting streamflow would help toidentify areas of concern.

When warning times are longer than a fewhours, full-fledged forecast systems shouldbe contemplated. The degree of desiredautomation and sophistication must beconsidered in light of current needs andcapabilities. Automation needs can beconsidered in sub-systems: data acquisitionand transmission; data processing; forecast

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Guidelines for Reducing Flood Lossespreparation; and forecast distribution.Different levels of automation may berequired as the overall system develops andexpands, and as financial resources becomeavailable. Systems may vary from thoseusing largely manual observations, graphsand tables to highly automated multi-modelsystems running on computer workstations.

Benefit-Cost analysis

An analysis of the cost of floods and thepotential benefits may help determine thetype of forecast and warning system andresponse mechanisms that would be mostcost effective. Costs resulting from floodingcan be estimated for various magnitudes ofevents for various centres. Damage statisticsfrom previous floods are also valuable inestablishing the costs associated with suchevents. Judgement is needed to estimate thebenefit of flood forecasting and warning inreducing damages and loss of life.Governments and financial institutionsrequire such information on costs andbenefits to help understand whereexpenditures will reap the largest rewards.Studies and analyses have shown thatdamage reduction due to forecastimprovements can range from a fewpercentage points to as much as 35% ofaverage annual flood damages.

A standard set of flood damage categoriesrelevant to the basin should be developed.When loss of life is a threat, this tooshould be identified, even though it isdifficult or impossible to quantify ineconomic terms. Other damage categoriescould include residential buildings;commercial, institutional and industrialbuildings; agricultural lands; andinfrastructure. Additional costs includetemporary relocation and flood-fightingcosts. Floods can have an effect on thepopulation and economy of an entirecountry, and business losses should also beincluded in the analysis. Developingstandard damage categories allows

damages to be more accurately estimatedfor various levels of flooding.

A rigorous cost-benefit analysis wouldrequire determining a flood frequencydistribution so that the present value offuture benefits can be determined. In theabsence of sufficient data or analysis, a morerudimentary presentation of costs andbenefits may be sufficient to determine thesize of the investment that is justified forflood forecasting, warning and response.

Evaluating existing capabilities

Most countries have basic networks ofmeteorological and hydrometric stations thatare necessary for flood forecasting andwarning. It is likely that the operators of theexisting networks may be in many differentagencies. In many cases, the networks maynot have been designed to acquire dataduring extreme events, or they may notprovide data in real-time or to commonstandards. Also, networks may not providedata for key urban centres where forecastsare required or for areas where the majorinputs to the flooding are occurring.Identifying the existing network, itsoperators, and the existing approaches andcapabilities are necessary steps in theevolution of a flood forecast system.

Another important element is anexamination of existing communicationscapability. Given that important data areavailable at a remote site, how can these databe reliably transmitted to a flood-forecastingcentre? Will telephone or radio links -manual or automated - function during aflood emergency?

Some agencies may have developedhydrological mathematical rainfall-runoffmodels and flow routing models for theirown purposes. These models may be usefulas flood forecasting models. An inventoryof existing models will also help definecurrent capability within individual agencies.

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52

Identification of key users andcollaborators

Establishment of a successful floodforecasting, warning and response systemdepends on a thorough analysis of existingcapabilities, identification of key users forthe system, and a good understanding of theinteragency arrangements needed in aneffective system. Considering these factorswill lead to forecasts that meet user needsand that are more likely to be acted uponduring an emergency. The ultimate goal ofsuch a system is to ensure the safety andsecurity of the public and to protect propertyand the environment. To achieve this result,however, means that the public must receiveand understand forecasts, and the myriad ofagencies having responsibility for emergencyaction and response also must receive theforecasts, have response strategies in place,and act upon the forecasts accordingly.

Key users typically include: civil agencies atthe national, provincial/state, and local level;military organizations; corporations,especially those which operate structures;volunteer emergency response organizations;and the media. A user analysis, and close

ties and interaction with these groups shouldbe considered to help establish overall floodforecast needs and response measures. Therole of the media in informing the publiccannot be underestimated. It is critical thatthe media receive timely and authoritativeforecasts and warnings. Mediacommunications should encourage theappropriate public response and should notlead to counterproductive speculation.

Many of the world's river basins have atransboundary component. In some casestransboundary basins are covered by treaty,international agreements, or otherinstitutional arrangements. Sucharrangements may or may not include riveror flood forecasting. Shared basins imply ashared responsibility; an analysis of userneeds should include users in othercountries.

Often the mandate and capabilities ofgovernmental organizations are not entirelyclear. An institutional analysis of eachagency's mandate, needs, capabilities andlegal responsibility during a floodemergency will help shape an emergencypreparedness and response plan. Overall,one agency should be assigned the leadresponsibility (and accountability) for anend-to-end system, with the system itselfpotentially being operated by a number oforganizations.

In the evaluation of the existing system,agencies that operate data collectionnetworks or models, or that can contributeto a forecast system in other ways, will havebeen identified. In some cases the potentialrole of a specific agency in a flood forecastsystem may be relatively clear, while in othercases that role may have to be identified andnegotiated.P

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Guidelines for Reducing Flood LossesA fundamental question is that of hydrologicaland meteorological coordination. In manycountries one or more agencies operatemeteorological forecast and climate networks,while hydrological networks may be theresponsibility of entirely different agencies ordepartments. Coordination among thesebodies is essential because development of aflood forecasting system may require theaddition of new sensors or telemetry equipmentfunded by one agency being installed at a siteoperated by another one. A successful forecastsystem will depend upon cooperation amongmeteorological and hydrological agencies andcould involve financial transactions amongthem.

Similarly there may be a number of agencieswith responsibility for operation of structuresfor water management and flood control.These could include hydroelectric generationfacilities, irrigation headworks, water supplyreservoirs, and so forth. Individual structuresmay have an established operating plan, but anintegrated plan for operations during extremeevents is needed to provide optimal floodcontrol benefits and to avoid structural failure.Interagency co-ordination and cooperation isrequired to ensure the integrity of the entirewater management system during extremeevents. A flood forecasting and warningsystem provides the information necessary toimprove decision support for the operation ofstructures.

Some agencies may have arrangements fortechnical support or financial assistance withinternational organizations or with othercountries. These may prove beneficial indeveloping or improving a forecast systemthrough training and in general strengtheningof much needed organizational infrastructure.

A logical agency to lead a country'sflood forecasting and warning effort mayemerge from this analysis. Theidentified agency will require technicalsupport and leadership from severalagencies. More importantly, there is aneed for long-term political support forthe endeavour. Flood forecasting andwarning will have to compete with othernational priorities, and resources andfinancial support can atrophy,particularly in the absence of flooding.

There is a need to establish a flood-forecasting centre having a legalmandate to issue authoritative forecastsand warnings on the river basin or at theprovincial/state or national level. Theseforecasts must be understood byagencies having the responsibility foremergency response and by the generalpublic. Such agencies, civil organizationsand the general public must be aware oftheir roles, have response mechanisms inplace, and know what actions to takeunder various circumstances.

Determination of specificforecast system requirements

Analysis of the basin characteristics, floodhistory, flood damages, and the existingdatabases will give some indication of thetype of forecast system that is achievableand affordable. It is likely that the systemwill be based on enhancing existingnetworks and agency capabilities.Establishing a new system implies phaseddevelopment from the existing system tothe new one. Establishing a long-termplan with specific milestones is critical tofuture success.

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3.2 The Hydrometeorological Network for Forecasting

The hydrometeorological network is the keyrequirement for most flood forecasting. Inparticular, precipitation and streamflow dataare needed. If snowmelt is a factor inflooding, then measurements of snow waterequivalent, extent of snow cover, and airtemperature are also important. In manycases agencies other than the forecast agencyhave useful data. Rather than duplicatenetworks, it is preferable to developcooperative arrangements. In some respects,it is preferable that the network serves manypurposes as this may result in its broaderfinancial support.

In most cases data network operationalperformance is the weakest link within theintegrated system. Operational datanetworks must be examined. Are the rainfalland stream gauge (hydrometric) datanetworks satisfactory in sampling rainfall(intensity and spatial distribution) andstreamflow response for the river basin? Arestream gauges operating properly, and arethey providing accurate conditions of waterlevel and streamflow? Are datacommunicated reliably between the gaugesites and the forecast centre? How often areobservations taken, and how long does ittake for observations to be transmitted to theforecast centre? Are data available to userswho need the information for decisionmaking? Are the data archived for futureuse? Are the data collected to knownstandards, is the equipment properlymaintained and calibrated, and are the dataquality controlled?

Network design

It is not possible to manage water or forecastfloods without data. Various types andsources of data are needed to monitor the

environment, conduct a water balance orprovide input to hydrological models thatestimate streamflow from rainfall. Operatinga real-time hydrometeorological network isessential, as data provide the foundation forestablishing the potential for flooding. In-situ observations of meteorological andhydrological parameters are required asinputs to the hydrological prediction system.

An analysis of the existing network should beundertaken. Tables and maps should beavailable providing details on monitoringlocations, parameters, sensors, recorders,telemetry equipment and other related data.In addition, monitoring sites in adjacentbasins should be inventoried. In low reliefbasins, data from those sites could be veryuseful. Analysis should be performed toidentify sub-basins that are hydrologically ormeteorologically similar.

Based on forecast needs, the adequacy ofnetworks can be determined and requiredmodifications can be noted. These couldinclude new stream gauges, rain gauges andpossibly other sensors in the headwaters, oradditional telemetry equipment. In somecases, network sites may not be well suitedfor obtaining flow measurements or otherdata under extreme conditions. Costlystructural alterations may be needed.Interagency agreements may be needed formaintenance and operation of the network.

A key variable to be established is the timestep needed to adequately forecast a flood fora given location. If the time step is, say, sixhours then data must be collected every threehours or even more frequently. In manycases, supplementing a manual observernetwork with some automated gauges mayprovide an adequate operational network.

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Guidelines for Reducing Flood LossesData acquisition

Generally the design and operation of datanetworks have a large influence on forecastsystem accuracy and in the ability of thesystem to provide the necessary lead-time toissue warnings so that response actions canbe taken. It should be underscored that thedesign of reliable real-time operationalobserving networks is critical to the successof a forecast, warning and responseprogramme. In order to be effective duringextreme conditions, sensor installations mayhave to be "hardened" to withstand extremesin wind, rain or flood stage.

The advent of remotely sensed data hassignificantly improved the ability ofoperational hydrology to infer watershedconditions in data-sparse regions. Theapplication of radar-derived precipitationestimates serves as the principle tool inforecasting floods and flash floods in manycountries. The use of geostationary andpolar orbiting satellites to derive largevolumes of meteorological and hydrologicalproducts is rapidly advancing. Remotelysensed data can now be used to provideestimates of precipitation, snowpack extent,vegetation type, land use,evapotranspiration, soil moisture and floodinundation. This information is becomingincreasingly useful in data-sparse regions ofthe world where water availability and floodforecasts are needed.

Many countries use the GlobalTelecommunications System (GTS) of theWorld Meteorological Organization(WMO) for the transfer of real-timemeteorological data. More recently, somehydrological data from a number of projectswere added to the system. Even with theseadvances in remotely sensed data and theiruse, inadequacy of data remains the biggestweakness in establishing a viable floodforecast programme for a river basin or for acountry.

Data communications

For data to be useful to the forecast centre,point data observed at remote locations mustbe converted to digital formats. This mayrequire changing the sensor itself or simplyadding another component to an existingsystem. The format for the digital datamust be specified. Remotely sensed imagesused in forecasts are usually already in aspecified digital format. If the sensor outputis film, arrangements must be made to makethe conversion to a specified digital formatwithin the required time.

Once data have been observed or collectedat sites throughout the river basin orcountry, the data must be transmitted tolocations where they can be stored, accessed,and used. The value of data increases withthe speed of transmission and processing,from their initial observation to where theyare used. Meteorological and hydrologicaldata are needed almost instantaneously sothat the hydrological forecast system canproduce up-to-date and reliable forecasts.More importantly, this allows the system toprovide the critical warning times needed forusers to take actions. This is especially truefor issuing warnings of flash flood eventsand of potentially hazardous mudslideconditions.

There are many types of communicationtechnologies that can be applied to transmitdata from sites in remote locations to theforecast centres. The most common form ofdata communication is by telephone.However, telephone lines frequently failduring severe flood events. More reliablebut potentially more expensive forms of datacommunications are satellite, line of siteradio, cellular radio and meteor bursts.These also have their strengths andweaknesses. An evaluation should beperformed to establish the most suitable,reliable and cost-effective form ofcommunication for the local situation.

Data may be transmitted by dedicated satellitelinks, radio links, by commercial telephonelinks or other shared services. In some casesthe data link could simply be a voicetelephone communication. The forecastcentre may be required to poll sitesindividually, interrogate a third-party system,or use the Internet to obtain data. Datatransmission links must be identified, andtheir reliability and speed should bedetermined. If image products are to play arole in the real time forecasts, bandwidth ofthe transmission system and speed of theprocessing system should be examined.

In many cases, national, provincial/state, localgovernments and the private sector operatereal-time data networks to support theirindividual needs. In most cases, these data arenot shared, and each organization is limited toits own data. Coordination and data sharingcan significantly increase the amount of dataavailable for all organizations. Theseadditional data, possibly complemented withnew sites, will help increase forecast accuracyat the least cost.

Network operation

Often times the current operator of a site willnot have had a need for, or experience with, realtime data acquisition. Intensive staff trainingmay be required to ensure that data areavailable when needed and are of a suitablequality.

Long-term maintenance is a major requirementin operational forecasting. The forecastnetwork may be in operation only seasonally orless frequently. Keeping the network in a stateof readiness though necessitates major changesin operating philosophy. The development ofwater management operational forecasts by theforecast centre, as well as flood forecasts,enhances the usefulness of the data network andcommunications systems, as well as maintaininga state of readiness.

Funding of alterations to existing networks andfor future maintenance presents a majorchallenge. Negotiations among operating orfunding agencies may require abandoningentrenched positions if success is to be achieved.

3.3 Meteorological Support

Given the importance of meteorological dataand forecasts to the production of floodforecasts, it is very important that there beclose collaboration between nationalmeteorological and hydrological services.This collaboration could take several formsand should focus on increasing the accuracyand utility of knowledge of existingconditions and forecasted states. Twoimportant products are optimal quantitativeprecipitation estimates - where, when andhow much precipitation has actually fallen -and quantitative precipitation forecasts -where, when, and how much precipitationwill actually fall. Other parameters ofinterest include wind speed and direction,surface temperature, and relative humidity.

Quantitative Precipitation Estimation(QPE)

Optimal estimates of existing precipitationconditions provide the hydrologist with themost accurate estimates of what are termed"antecedent conditions". These are extremelyimportant for hydrological process modelling.Much work has been done to increase theaccuracy of the estimates through increasingthe density of in-situ stations, implementingground-based surface radar, in processing ofsatellite based data, and in merging varioussources of data.

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57

Guidelines for Reducing Flood LossesQuantitative PrecipitationForecasting (QPF)

The ultimate goal of flood forecasting is toprovide accurate forecasts of hydrologicalconditions. Currently, deterministicquantitative precipitation forecasts and otherforecasted meteorological parameters can beapplied as input to hydrological models inorder to derive hydrological forecasts usingnumerical modelling methods.

It is typical that the hydrological forecasterreceives single "best effort" meteorologicalproducts such as QPF, wind speeds anddirection, temperature, and pressure. Theseproducts are based on numerical weatherprediction model output and are modifiedusing expert forecaster judgement. Forecastmodels are typically run once or twice dailydepending on the operational practices ofthe national meteorological service. Theuseful forecast horizon of such products istypically about five days, with accuracydecreasing rapidly towards that of long-termclimatology. The usefulness of QPFproducts derived from such modelling isusually constrained to one to two days dueto poor performance beyond these limits.

When very short forecast horizons on theorder of six hours or less could provebeneficial, extrapolative and trend-basedmeteorological techniques are used. Theuse of these techniques is referred to as"nowcasting", resulting in short-range QPF.These shorter time horizons associated withnowcasting are particularly useful for flash-flood forecasting. Beyond this horizon,numerical weather prediction modelscombined with expert judgement providemore accurate estimates of futuremeteorological conditions such as QPF.

Work is currently proceeding on thecoupling of mesoscale numerical weather

prediction models with high-resolutionhydrological process models. Questions stillexist on how to best incorporate expertjudgement into this process in order toprovide a single "best effort" estimate of futurehydrological conditions.

One forecast methodology that can be appliedto both meteorological and hydrologicalforecasting is the "Ensemble Technique",wherein multiple forecast scenarios aregenerated by the execution of several modelruns, each with slightly varied initial states.The magnitude and degree of the uncertaintyassociated with the forecast ensemble provide aprobabilistic view of the potential futuremeteorological and hydrological states.Although more study and further developmentare needed before this becomes more broadlyused in operational practice, the techniqueholds much promise.

Once a flood forecasting centre has been inoperation for a period of time and closecollaboration exists with meteorologicalcounterparts, weaknesses in bothmeteorological and hydrological forecastproducts may become evident. Sometimes theweaknesses can be overcome by improving thedatabase used for the forecast. In other cases,there will be a need to improve understandingof the underlying hydrological processesinvolved in the production of hydrologicalforecasts.

Other parameters of interest

Estimation of other parameters is importantfor flood forecasting and for assessingantecedent basin conditions. These includeantecedent temperature, humidity, andevapotranspiration, all of which are veryimportant in assessing soil moisture conditionsand water deficits prior to the onset ofprecipitation.

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3.4 The Forecast Centre

The flood forecast centre must be identifiableto agencies and to the public as theauthoritative source of flood forecasts andwarnings. The forecasts produced by thecentre must be to the highest achievabletechnical standard and be released to thepublic unfiltered by agency or politicalinterests. The long-term stability of the centreis dependent on the credibility and utility of itsforecasts.

Administratively the centre can be part of oneagency or it could be a new entity supportedby several agencies. The forecast centre couldbe self-contained or, more likely, will dependon other agencies for support.

Establishment of the centre

Analysis of existing conditions and needs willdetermine whether a forecast centre will beestablished by strengthening an existingfacility or by creating a completely newenterprise. The decision should be based onpolitical, administrative and technicalleadership of candidate agencies, as well as theability of the selected agency to work withothers.

A project initiation team drawn from severalnational agencies or consultants, with supportfrom international organizations and workingto agreed-upon terms of reference, couldexamine the issues identified in theseGuidelines and make recommendationsconcerning the development of a ForecastCentre. Their report should identify technicalissues, personnel needed, administrative issues,costs, and timelines.

Interagency agreements will be needed forprovision of data, operation of structures,weather forecasts, technical and administrativesupport, and other tasks. Depending on thebasin, it is possible that such agreements

might already exist. They may include clearlyarticulated roles and responsibilities, clearspecifications of work, performance measures,and provisions for financial arrangements.International agreements for provision of data,use of satellite technology, and other supportactivities may also be required.

The Centre needs financing over both theshort and longer term. Short-term financingwill be capital intensive as funding may benecessary for network improvements,construction, the acquisition of computers andsoftware, and many other items. These couldbest be funded by special nationalappropriations or international support.There may also be a local market forspecialized forecast products, which could bepaid for by users.

Long-term financing will be needed to operatethe Centre, pay staff, upgrade computersystems, and make improvements in theforecast methodologies. This operation willrequire on-going national support even wherespecific improvements are fundedinternationally. If the mandate of the Centrewere expanded to include river forecasts foroperational water management purposes,financial support could be made available fromagencies using the river forecasts. Thepossibilities include revenue from sale of waterlicenses, other water use charges, or feesassessed to discourage development within theflood plain.

To operate effectively the Centre will need keypersonnel. Aside from technical skills, theCentre will need people capable of workingcollaboratively with other agencies and whocan communicate effectively. A significanttraining programme will be needed at theonset of the Centre, and the costs of on-goingtraining should be built into the budget. Inthe early stages, forecast procedures may have

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Guidelines for Reducing Flood Lossesto be tailored to the ability of existing staff, anda training and development plan should beestablished to upgrade skills and techniques toimprove the accuracy and utility of the forecast.

There will be an early need to gather basicdata, calibrate and verify models, and establishworking arrangements with other agencies.Visiting experts, or placing key staff in otherForecast Centres for training, could aid thisprocess.

One approach in the early stages ofdevelopment would be to concentrate efforts ona key basin or one of its sub-basins. Such apilot project could help verify the suitability ofmodels selected and the capabilities of staff.This would give funding agencies a level ofcomfort. In the very earliest stages ofdevelopment, the Centre could simply analyzeweather forecasts and provide warnings ofpotential high flow conditions.

Data processing

Although data may have been pre-processedelsewhere, the Forecast Centre will require in-house data processing capability. Althoughsome work could be done manually, computersystems should have uninterruptible powersupplies. At the very least, emergency powersystems should be available.

If Geographic Information Systems and imageproducts are expected to be used for real-timeforecasts, computer memory and speed mustbe taken into account. Overall computingsystem architecture and design should beplanned for as part of the future developmentof the Centre.

Data processing needs will also depend uponthe selected forecast models and their dataprocessing requirements. In the absence ofother requirements, data should be digitallyavailable and easily converted to formats usedby commercially-available spreadsheetprogrammes.

Arrangements must also be made toelectronically archive data so that they areavailable for use in subsequent years. Somedata will have to be brought forwardfrequently for use, while other data will only beused on occasion.

Forecast Centre operation

It is necessary to establish basic operatingprocedures and assign staff responsibilitiesearly in the operation of the Centre. Part ofthis is the assignment of responsibilities for on-going maintenance of systems.

Capable staff are the key to producing goodforecasts and maintaining the Centre'scredibility. Capable staff will also be attractedto positions elsewhere so some staff turnovercan be expected. A systematic plan for stafftraining and development and the assignmentof challenging work will help reduce turnoverrates. Some contingency planning will beneeded to ensure that the Centre will continueto operate when key staff members leave.Efforts should be made to develop anoperations manual in order to reduce theCentre's vulnerability to loss of staff or otherunanticipated events. The manual shouldcover all aspects of the Centre's operation andmaintenance, and it should include lists of keycontacts beyond the Centre.

Once the Centre has completed its firstsignificant flood forecast season, an end-to-endreview of all aspects of the forecast should beconducted to identify what went well andwhere improvement might be necessary. Thereview should include interviews with personsfrom other agencies and forecast users. Theresults of such a review should be used tomodify procedures. Periodic audits of forecastprocedures and Centre operations should becarried out, perhaps involving staff from otherForecast Centres.

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Forecast models

There are a large number of public domainand proprietary models available for use inflood forecasting. Sometimes the model canbe simply a statistical rainfall-runoff relationwith a routing equation, while other modelscan be much more complex. Hydrologicalmodels can be classified as lumped, semi-distributed or distributed, and as being singleevent or continuous. Probabilistic models thattake data uncertainties into account are alsoavailable. Model selection will depend onavailable data, basin characteristics, and theneeds of the local user community.

A lumped model treats the watershed as asingle unit for inputting data and calculatingrunoff. The calculations are statistically basedand relate to the underlying hydrologicalprocesses as a spatially averaged process.Models based on scaling unit hydrographswould fall into this category. Some lumpedmodels allow the watershed to be subdividedor for some parameters to be physicallyestimated and modelled. When subdivisionsof a basin are combined to produce a forecast,this modelling approach is termed semi-distributed. Depending on forecast needs andthe characteristics of the watershed, a lumpedmodel may be all that is required.

A distributed model simulates the keyhydrological processes that occur in awatershed using distributed data inputs andprocesses. For forecasting purposes thesecommonly include precipitation, interception,infiltration, interflow, and baseflow. Overlandflow and channel routing may be incorporatedinto the model or calculated in a hydraulicmodel. Distributed models require muchmore data and knowledge of watershedprocesses than lumped models. When themodel is first established, precipitation andland cover characteristics may be the onlydistributed features.

Hydraulic models used in channel routingcalculate the travel time of the flood wave and

its attenuation. These models use thestandard equations of unsteady, non-uniformflow with various simplifications dependingon the channel characteristics, available dataand accuracy requirements. Storage-flowrelations are often incorporated intohydrological models. One-dimensionalunsteady flow hydraulic models can be usedto route flows through multiple channels orin situations where overland flow is a seriousconcern.

Probabilistic forecasts are typically derivedusing hydrological process models whereinstatistical distributions are used to describethe uncertainty of input data and basinconditions such as precipitation data, soilmoisture and snow pack conditions. A largenumber of model projections are producedthat can be statistically analyzed to allow fora better understanding of the uncertainty ofthe forecasted future water conditions. Thisapproach is rapidly gaining popularity, as itprovides the decision-maker with theprobability of an extreme event to occur, notjust that it might occur.

Simplified probabilistic methodologies thatprovide a range of possible forecasts haveexisted for some time. This is achieved bythe forecaster making assumptionsconcerning future precipitation to determinerunoff under normal, lower, or upper decileconditions. More modern approaches,which tend to be in the pilot testing stage,attempt to better quantify the uncertaintyassociated with the forecasted meteorologicalconditions and to directly link thisuncertainty to the uncertainty of the floodestimate.

Sophistication of hydrologicalforecasting

Essentially, the prevailing geomorphologicalconditions of the river basin and theinteraction of communities at risk with theriver system dictate the level of sophisticationof the modelling solution. The performance

Guidelines for Reducing Flood Lossesof existing models or forecast procedures shouldbe evaluated. Whether the forecast processinvolves use of simple graphs or tables or arobust integrated modelling system, evaluationof forecast accuracy versus lead-time should bedetermined. Does the system perform well whenadequate data are available? Are modelparameters up-to-date? Do model parametersreflect land use changes that have occurredwithin the basin? Can the model or itsparameters be easily modified to reflect pendingland-use changes within the basin? Does themodelling system reflect flood control structuresand their operations within the basin? Are thereimportant hydrological processes occurring inthe basin that are not reflected in the existingforecast model? Does the forecast systemperform well for flooding but is inadequate tomeet routine or low flow forecast requirements?Is there a need to convert hydrological forecaststo water level (stage) using a hydraulic model?In general, is the existing modelling systemappropriate and sufficient to meet userrequirements?

Hydrological forecasting knowledge

Highly trained hydrologists produce reliablehydrological forecasts. Forecasters use real-timedata, knowledge of hydrology, knowledge of thehydrological modelling system and experiencein producing forecasts and warnings. Indetermining the operational readiness orhydrological forecast capability of a forecastcentre, the education, knowledge and skills ofthe forecasters are as important as the tools theyuse. Is the number of forecasters availablesufficient to handle the flooding situation? Dothe forecasters have sufficient education inhydrology and meteorology to appropriatelyapply the tools? Are they properly trained, anddo they understand the limitations of themodelling system being used? Do theforecasters know the users, how to contactthem, and what information they require forflood response actions? An assessment of theadequacy of the operational forecaster capabilityis important in determining how to improveflood forecast operations.

Forecasts

In order to produce a flood forecast forthe communities and locations at risk,there must be a hydrological modellingcapability that uses meteorological andhydrological data. Hydrological modelsuse real-time precipitation and streamflowdata. The models translate observedconditions into future stream conditions.Hydrological models or procedures varyin complexity, accuracy and ease of use.Simple hydrological models consist oftables, graphs or empirically derivedrelationships. More sophisticatedhydrological modelling systems use in-situdata, remotely sensed data, and multiplehydrological models that are integrated toproduce very accurate hydrologicalforecasts. Due to advances in GeographicInformation Systems and the availabilityof geo-referenced data, parameters ofsome hydrological models can now beestimated without having to relyexclusively on historical hydrological datafor model calibration. The evolution ofpersonal computer technology has pavedthe way for quite complex modellingsystems to be run on them. These systemsare easier to use, are easier to maintain,and are more affordable.

Current hydrological forecast systems arequite affordable and powerful. Thedegree of success associated with thesesystems is dependent on the amount oftraining received by the hydrologistsusing them. These systems are capable ofproducing a broad range of forecasts ofstream conditions that will occur in a fewhours to seasonal probabilistic outlookstargeted to months in advance for largerrivers. Model system selection dependson the amount of data available,complexity of hydrological processes tobe modelled, accuracy and reliabilityrequired, lead-time required, type andfrequency of floods that occur, and userrequirements.

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Hydraulic models are often used to translatehydrological model-derived streamflow towater-level conditions. Hydraulic models arealso valuable in forecasting the streamflowconditions of large rivers where sufficientlead-time is accorded through translation ofupstream water levels to downstreamcommunities at risk. Such models can beinterfaced with geographical informationsystems to provide dynamic water levelconditions on maps of communities. Thesetypes of forecast products can be invaluable tocommunities and emergency organizations, asthey provide very precise information aboutareas that will be inundated and when.

Decision support

Hydrometeorological data and accurateforecasts are of no value if the forecasts donot reach users and if decisions are not madeas to the appropriate actions required.Hydrological forecasts and hydraulicconditions must be disseminated so thatdecisions can be made and actions taken toreduce the impact of the pending event.Decision support refers to everything fromforecasts reaching decision makers such as amayor of a flood-prone community to theoperator of a flood-control structure. Fordecision support to be effective, advancedplanning must define prescribed actionslinked to forecasted values.

Decision support systems vary from rules orprocedures that must be followed underprescribed conditions to mathematicaloptimization programmes. Such approachesdefine actions to be taken based on tradeoffsamong various options for water allocation.

Forecast output

Typically, calculations used in preparing aforecast are based on units of discharge,although some simplified systems correlateupstream with downstream water levels.

However, decision makers and the public aremost often concerned with water levels andvelocities at specific points, usually urbancentres. Forecasted flows can be converted towater levels and velocities using stage-discharge and stage-velocity relations,hydraulic models, or other techniques.Decision makers and emergency workersshould be consulted on their specific forecastrequirements.

In many cases, forecast water levels are givenaccording to a local vertical datum. This maybe convenient for some purposes but mayintroduce potential for confusion. With theadvent of global positioning systems, it ispossible to provide a geodetic datum at anylocation and this should be done.

The forecast water levels released to thepublic could be in several formats: tabular,hydrographs, or inundation maps. It is veryuseful to provide comparisons with theprevious year or with previous major floods.Inundation maps linked to databases canprovide information on individual propertiesand can be most useful for public awarenessand emergency services.

The forecast should be formally released andmade available to key agencies as well as themedia. The forecast should also be placed onInternet websites for easy access.

The points for which the forecast appliesshould be communicated very clearly. Forexample, the name of a city in the basinshould be identified, or in the case of verylarge cities, well-known points within the cityshould be specified.

Uncertainty in the forecast should berepresented accurately, but in non-technicallanguage. Phrases such as "if presentconditions continue…" are useful as are thosethat require simple statistical knowledge suchas "there is an 80% chance that…"

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Guidelines for Reducing Flood LossesDissemination of forecastsand warnings

Forecast and warning dissemination isextremely important. Frequently, the lack ofability to disseminate warnings to thepopulation at risk is the weakest link in theintegrated system. Forecasts and warningsmust reach users without delay and withsufficient lead-time to permit response actionsto take place. Dissemination of forecasts andwarnings can be achieved through a variety ofcommunication methods. An inventory of thevarious communications media used by theforecast system will reveal the competency ofthe dissemination process. How are warningstransmitted to the public, to the flood controlagencies, to the emergency services and civilprotection organizations? Are communicationsystems reliable? What types ofcommunication modes are used (such assatellite, radio, meteor bursts, telephone orinternet)? How are communications linesmaintained? Are there backup modes ofcommunication? In what format arewarnings transmitted? Do users understandthe content of the warnings? These are a fewof the questions that need to be answered inassessing the performance of disseminationsystems.

Users

Who are the users? Understanding theneeds of users is fundamental to achieving asuccessful flood forecast, warning andresponse programme. What kinds of dataand information do they need? Where arethey? How do you reach them during theday or late at night or during a nationalholiday? How do they make decisions?

Establishing a user group association or aninventory of users is important for a wellfunctioning and effective forecasting system.The sophistication and needs of users canvary considerably. For example, thehydropower industry needs high-resolution

data, detailed hydrological forecasts for short-term as well as for longer time horizons.Emergency services groups and mediaorganizations need clearly stated warninginformation that defines the hazard and spellsout what steps the public must take tominimize their risk.

Notification and action

The entire process of establishing a floodforecast is of no value unless data andforecasts reach users. An effective fail-safeforecast dissemination system must beestablished to allow forecasts and warnings toreach users, wherever they are. Notifyingusers is often problematic because manycountries do not have communication systemsthat reach rural villages and othercommunities at risk. Internet is an effectiveworldwide communication system, but theuser must have access and be vigilant in orderto receive the warning. There are manyexamples of effective dissemination systemsbased on high-speed telecommunications thatuse satellite, microwave, radio, or meteorburst technology.

An effective flood-warning programme mustalso be linked to the media to reachpopulations inhabiting the areas at risk. Inmany cases, the media can provide aneffective means of re-broadcasting warningsand assist in response-orientedcommunications.

The payoff from a successful end-to-endflood forecast and response programme iswhen actions are taken to reduce the impactof the impending flood. Actions can be assimple as moving contents from the first floorof a person's house to the second floor,evacuating the flood plain, blocking roadsthat will be flooded, or closing floodgates of alevee system that protects a city. If nomitigation actions result from flood forecastsand warnings, society is simply paying moneyfor ineffective results.