Environmental Engineering Reference
In-Depth Information
1 minute
1 hour
1 day
- Temperature
- Pressure
- Topography
- Water
distribution
Initial
conditions
Nowcasting
ensemble
Issue
Flood
Warning
NWP
ensemble
Above Threshold
Flood
Probability
Hydrologic Model
ensembles
QPF ensemble
Below threshold
No
Warning
Fig. 10.2
Combining the advantages of nowcasting with Numerical Weather Prediction for improved flood decision
support systems. QPF, quantitative precipitation forecasting.
ensemble is dictated by how long it takes to run
the NWP and hydrological models. Beven
et al. (2005) describe a process for constraining
and propagating the uncertainty in a cascade of
rainfall-runoff and flood routing models in an
application to the River Severn in the UK.
Flood forecasting systems are used in areas
where advanced flood warnings could prevent
loss of life or reduce damages. The desired
lead time corresponds to the time it takes to evac-
uate; themore time required, themore important it
is to use a flood forecasting system that projects
into the future with some degree of accuracy. In
areas with poor drainage due to topography and/or
poor flood conveyance or flood prevention infra-
structure, there may be a very short period of time
available between the formation of rainfall and
surface inundation. As the rainfall event materi-
alizes and it becomes evident that flooding will
occur, it becomes more important to estimate the
spatial extent of inundation areas.
The traditional approach to flood forecasting is
purely deterministic, where observed rainfall or a
single rainfall forecast is input to a hydrological
model and decisions are made based on the resul-
tant flood hydrograph - specificallywhether or not
an areawill flood andwhether or not to evacuate or
take other emergency action. Where the likely
damage caused by a flood event is high, real-time
streamgauges and water level monitoring equip-
ment can be used to corroborate the model. How-
ever, usually the decision whether or not to take
action is a judgment call by the emergency re-
sponse team, who may or may not have good
knowledge of the performance and reliability of
the floodwarning system. Aflood forecast that has
an associated probability of occurrence provides a
framework for making more informed decisions.
For example, an emergency management team
may decide to call for the evacuation of part of a
city if the probability of inundation is greater than
20%. By contrast, in a situation where a loss of life
is likely, such as a dam or levee failure, evacua-
tions may be called for if the probability of occur-
rence is much less.
The use of improved NWP models in hydrolog-
ical applications is widespread, as is the use of
nowcasting. Combined QPFs, where
the
