travel takes several days, reaches may be a few hundreds of kilometres in length. In smaller catch- ments, where flood passage takes one to two days, suitable reach length will be in the order of tens of kilometres. A reach may be identified by a flood warning gauge at its upstream and downstream extremities. This approach is only useful when there is knowledge of how riparian areas may be affected by the flood, or if there are any flooding
“hotspots” along the reach.
Within England and Wales, the Environment Agency is moving away from a reach-based struc- ture for warnings, to concentrate attention on specific areas where risk is high and floods would have the most impact. In this approach, flood warn- ings are concentrated on rivers passing through urban areas or key points where communication links or other key infrastructure may be at risk.
The areal definition of warnings must also be suit- able from the point of view of incident management and operations. This allows for a certain amount of hierarchy for organizational areas, the most local being those in which warnings and on-the-spot actions can be coordinated, for example with emer- gency labour teams, and liaison with communities and police. For strategic management of resources and coordination of larger events, several of the smaller operational areas may be combined. Above this, there may be a national organization, which can coordinate and report at a high level during a major event.
8.3 LEAD TIME OF METEOROLOGICAL AND FLOOD WARNINGS
8.3.1 General considerations
There are no hard and fast rules regarding the provi- sion of lead time for warnings. The requirement depends on specific operational need and rests on a number of considerations, principally:
(a) The size of the catchment and nature of flood- ing: large catchments with extensive flood plains are slow to respond, while, conversely, headwater catchments in steep hilly areas afford little potential to provide advance warn- ing of flooding;
(b) The nature of the risk and impacts, and whether or not evacuation or physical protection (for example sandbagging, embankment strength- ening) needs to be provided;
(c) Whether or not staged alerts and warnings are used.
Lead times are dependent not only on the appropri- ate action related to the flood warning, but also to
the type of information available. The discussion in the subsections following considers the role of meteorological warnings and flood (hydrological) warnings.
8.3.2 Meteorological warning
Meteorological warnings for flood warning purposes are derived from more general weather forecasts.
Figure 8.1 illustrates the relationship of the lead time of various meteorological forecast types to the response action. This is based on practices in the United Kingdom coordinated between the Met Office and the Environment Agency flood warning services, but the principles are similar, whatever the scale or country.
Long-range forecasts, that is, those relating to peri- ods of several weeks or months, are not sufficiently detailed or accurate to be considered as part of the flood warning process, despite the considerable efforts of research and development to improve their accuracy. There could be benefits where an area is under a regular annual flood regime, for example Monsoon Asia or the African inter-tropical convergence zone (ITCZ), where an advance fore- cast could enhance preparedness actions. In South Africa, for example, the forecast of an El Niño event is used in advance planning by the Department of Water Affairs and Forestry.
Forecasts within the range of 5 to 10 days now have a high level of confidence in terms of major mete- orological features, for example storm tracks. These forecasts are most useful for large basins or areas as an indicator of a possible widespread event, partic- ularly if the catchments are currently experiencing high water levels or there is a risk of snowmelt. The benefit of the forecast is primarily to assist preparation.
Under five days, forecasts can provide more focused information on the locality and severity of an event, though still insufficient to provide specific local warnings. Some broad quantitative rainfall information can be provided, but timings are still only in outline. It is for lead times of under two days that meteorological forecasts can introduce benefits to the flood warning process. In major catchments a two-day lead-time forecast of heavy rainfall is important during periods when river levels are already high, and further increases could affect transport and areas known to be at risk of flooding. This is the case, for example, for the Ganges–Brahmaputra delta in India and Bangladesh during the monsoon period, and for major European catchments such as those of the Elbe and Rhine.
Even in catchments with a response time of less than one day, the forecast can aid the flood warn- ing authority to make initial decisions on manning
CHAPTER 8. STRUCTURE AND ORGANIZATION OF FLOOD WARNINGS 8-3
levels and preparedness of local flood defence activ- ities, possibly to the level of issuing an alert or early stage warning to the public.
Once an event is imminent, either through a devel- oping situation (a convection breaking out or a depression approaching), or if a severe event is affecting adjacent catchments or basins, then detailed weather forecasts provide a significant tool to help flood warning decisions and actions.
Particular catchments and hence localities at risk within them can now receive an advance warning.
Within the sub-daily framework, and particularly for lead times of 6 to 12 hours, numerical forecasts can be of sufficient detail to provide quantitative inputs into flood forecasting models. In the United Kingdom, STEPS progressively assimilates satellite and radar data as the forecast time approaches zero, and this information is produced on a grid basis and so is highly suitable for input into distributed models. As the Environment Agency has a standard of service to provide a minimum lead time for flood warning of two hours, it is feasible that in some rapidly responding catchments a warning could be issued on the basis of a meteorological forecast, but to date this has not been done. In the United States flash-flood alerts are issued from knowledge of meteorological conditions.
Weather radar is particularly suited to providing short-lead-time information to update and provide more detail in a critical situation. The viewing of a sequence of radar images over intervals of 30 minutes or one hour will provide a good qualita- tive definition of storm movement and development and can therefore be used to provide warning to
catchments that the storm is approaching. The lead time depends on the nature of the system that will produce the rain. The rainfall echoes associated with a front or depression can be tracked for some hours, whereas the rapid development of convec- tive cells and their localized movement may be more variable, and hence will only provide warn- ings of one to three hours, in line with the life cycle of convective cells.
8.3.3 Flood warning
Hydrologically based flood warning relies on known (observed) river conditions and projections. The process can involve varying degrees of sophistica- tion, from the use of simple correlation techniques between upstream and downstream sites, to complex hydrological or hydraulic model-based warning systems. As with meteorological warning, the lead time for the in-river situation is based on size and response time, but it is a prerequisite that the river system should already be responding to the event before projections can be made. On major rivers, for example the Nile, Indus and de la Plata, the progress of severe conditions downstream should allow warnings to be provided several days in advance. At the lower end of the scale, a mini- mum warning lead time can be established that reflects the capacity for receiving timely data and forecasts and the time to implement necessary response actions. This lower limit is particularly relevant to steep catchments or those in urban areas.
Figure 8.2 represents the relationship between information, lead time and response in a similar Figure 8.1. Schematic of flood-forecasting lead time
GENERAL CIRCULATION MODEL FORECAST SOURCE
FORECAST TYPE
EVENT RESPONSE ACTIONS
SITUATION REVIEW AND MONITORING
FLOOD WATCH FLOOD WARNING HEAVY-RAINFALL WARNING
STANDBY MOBILIZATION
FULL MOBILIZATION. 24-HR INCIDENT ROOM STAFFING. ESTABLISH LINK WITH EMERGENCY SERVICES.
OUTLOOK DAILY GENERAL FORECAST
QPF: SUB-DAILY PERIODS NIMROD
FORECAST NIMROD ACTUALS TELEMETRY RAINGAUGES FORECAST PERIOD
10 days 5 days 2 days 24 hrs 12 hrs 6 hrs ZERO
NWP MESOSCALE MODEL HIGH-RESOLUTION MODEL/
RADAR
way to the forecast process in Figure 8.1.
Throughout the process, comparison between observed and forecast information must be main- tained to allow response actions to be controlled.
This is particularly relevant to liaison with emer- gency services and other agencies outside the flood monitoring organization, to give sufficient time to escalate levels of response or downgrade the status.
8.4 SELECTION OF WARNING STAGES Flood warning relies on “triggers” relating to criti- cal river levels or rainfall amounts that are indicative of flood states approaching or worsening. The trig- gers initiate certain actions or provision of information to external users. They are used to decide when to undertake certain actions during a flood event and should be designed to give enough time to undertake the response action. For exam- ple, if a river water level reaches a certain trigger level it might mean that an area or community will flood in a few hours and the response action could be to evacuate the village. Triggers relating to rain- fall include:
(a) Accumulations exceeding a threshold in a given time period, for example 100 millimetres in 12 hours or less; this threshold may need to be changed according to season;
(b) Rainfall accumulation and catchment wetness conditions;
(c) Rainfall intensity exceeding a given rate; this is particularly important in urban areas where drainage capacity can be exceeded and flash floods can occur.
Triggers relating to water level include:
(a) The river level rising to within a set warn- ing level, for example 1 metre below danger level;
(b) The rate of rise of level is faster than a threshold level, for example 25 centimetres per hour.
Triggers have to be established by careful study of local conditions. The advice and knowledge of the local community, if available, is therefore impor- tant. Triggers must not be arbitrary, or standard within an organization, for example within 1 metre of “danger” level, but linked to local conditions and features of risk. Where levels are concerned, these need to relate to significant happenings, for example:
(a) The level at which water flows out of a channel onto the flood plain;
(b) The level of water that submerges areas of land used for livestock, or at which low lying roads become flooded;
(c) The level at which major areas, includ- ing residential and business properties, and communications are affected;
(d) The level at which depth and velocity combined pose a threat of structural damage and danger of loss of life.
In assigning suitable forecast triggers for urban flooding, the criteria used could be based on a fore- cast that would result in a high risk (at least a 30 per cent occurrence) of actual flooding taking place.
The general arrangement for flood warning stages is illustrated in Figure 8.3. A schematic of the forecast and warning relationship from rainfall forecast to downgrading of a warning is illustrated in Figure 8.4.
Figure 8.2. Flood warnings and responses
INFORMATION SOURCES INFORMATION SOURCES
TELEMETRY RIVER GAUGES AND RIVER
GAUGES METEOROLOGICAL
FORECASTS
RAINFALL
RADAR LOCAL
OBSERVERS
INFRASTRUCTURE DAMAGE REPORT OPERATIONS
CENTRE
River rise
commences Out of bank
flow Extensive
inundation Damage and
threat to life Decline of river levels ALERTING FACILITIES
FIRST STAGE RIVER LEVEL TRIGGERS
RESPONSE ACTIONS FLOOD STATUS
HIGHER STAGE TRIGGERS
ALERT
INITIATED FLOOD
WARNING SEVERE
FLOOD WARNING
EMERGENCY
MOBILIZATION DOWNGRADING OF WARNINGS RAINFALL
TRIGGERS
CHAPTER 8. STRUCTURE AND ORGANIZATION OF FLOOD WARNINGS 8-5
Triggers for flood warnings should be set so that they are not reached too frequently, so creating unneces- sary responses or disruption. Frequency can also lead to a negligent attitude by operators and public, where warnings are not heeded. A similar problem exists if too many false alarms are given. There is a crucial balance to be struck between a precautionary approach and an unwillingness to issue warnings for fear of these being “wrong”. A phased-alert warning system goes some way to overcoming these problems, as long as the various stages and their implications are clearly understood. The system should also allow for the alert-warning situation to be downgraded if conditions improve or forecasts change.
“False” alarms are inevitable given the nature of meteorological and hydrological events. Over time, they should be a small proportion, for example 20 per cent of the forecasts that are issued, or of the number of correct forecasts. Similarly, the number of times that alerts and warnings are issued should not be excessive. As experience of both operators and users develops, triggers can be modified to reflect the balance of achieved forecasts and false alarms. An indicative number of no more than five warning events per year in a given location, averaged over a period of years, is considered to be of the correct order where flooding is an irregular occurrence.
8.5 PRESENTATION OF WARNINGS TO USERS
8.5.1 Flood warning symbols
Serious flooding occurred in England and Wales in 1998, and concern was expressed at the lack of
effectiveness of warnings. The high-level govern- ment review that followed highlighted the need for better communication with the public on flooding, especially with regards to providing clearer flood warnings. These would replace the colour codes, which the report showed were confusing and misunderstood “by nearly all who received them”. Colour codes (for example yellow, amber, red) or symbol codes based on other mete- orological warnings (for example cone or flag signals used for cyclone warning in Bangladesh) tend to be understood only by professionals directly involved, and even emergency service personnel are often not clear on the meaning or implication of the code.
The system that has been introduced into the United Kingdom as a result is a combination of a symbol using a simple illustration, to which specific instructions are related. Awareness and informa- tion on the symbols and instructions was disseminated through a comprehensive publicity campaign covering poster and television advertis- ing, leafleting and public information centres (libraries, council offices). The symbols are now used in television weather forecasts when warning situations arise. The symbols and explanatory instructions are shown in the table below.
These types of symbols can be considered as generic, and so use typical representations of local houses.
In Bangladesh, Mozambique and other countries, more visual representations, meant to convey the message to non-literate people, have been developed.
Figure 8.4. Flood forecasting and warning schematic
Figure 8.3. Arrangement of trigger levels for various flood warning stages
Time 6
5
4
3
2
1
0
Water level in river (m)
Trigger level for severe flood warning
Trigger level for flood warning
Trigger level for flood watch
Figure 8.3
FORECAST – RAIN
RAINFALL
FORECAST UPDATE – NO RAIN FORECAST
UPDATE – INCREASED RAIN
SEVERE FLOOD TRIGGER FLOOD WARNING TRIGGER
RIVER LEVEL
ALERT WARNING SEVERE
WARNING WARNING DOWNGRADED
Time
River level
8.5.2 Examples of flood warning information outputs
A wide range of information formats are used by national meteorogical and hydrological services (NMHSs) across the world. In order for a newly
developing flood forecasting and warning service to appreciate the range of information and presenta- tion formats in use, a selection of flood forecasting and warning outputs available from national flood warning service Websites are given in 8.5.2.1–
8.5.2.3 and Boxes 8.1–8.5.
United Kingdom flood warning symbols and the instructions that accompany them