Operational European Flood Forecasting - Environmental Modelling: Finding Simplicity in Complexity - page 401

1% Flood (Q1)

Severe warning

Warning

Bankfull

Figure 25.1 Schematic example of a river
discharge hydrograph for an extreme flood
event. Four critical discharge thresholds are
shown: (1) bankfull, (2) warning (e.g. a few
houses underwater), (3) severe warning (e.g.
large area flooded) and (4) 1% flood or Q1
(often referred to as the '1 in 100 year' flood).

Day 1 Day 2

Time

Day 3

data), which means that critical values cannot be defined
for every location in Europe, though this definition is
what an operational flood forecast demands (Thielen
et al ., 2009a). In order to overcome these limitations
numerical modelling techniques can be used to derive a
'proxy' observed river-discharge data set (Thielen et al .,
2009a). First, observed meteorological data are used to
drive the hydrological model. Then, long river-discharge
time series are calculated by the model for every grid cell.
The model parameters that are used should be the same
as those used when the model runs in forecast mode. This
proxy river discharge dataset is also known as the 'model
climatology'. River-discharge (flood) return periods can
then be computed for every grid cell from the time series

and the thresholds defined from them (for example, for
the 1% flood).
In Table 25.1, an example of the four critical thresholds
used in the EFAS is shown. These thresholds are generated
from the model climatology of a 16-year run (1990-2006)
driven by observed meteorological data and run on a
daily time step. They are coded by different colours and
associated with a specific flood-hazard description. In
Figure 25.2a, an example flood alert using these thresholds
is shown for the River Danube catchment. Grid cells
in which the river is forecast to be above a particular
threshold are coloured according to Table 25.1.
Themajor advantage of this approach is that it accounts
for the relative difference of simulated river discharges

Table 25.1 EFAS thresholds, their colour codes and associated hazard class.

EFAS threshold Colour Hazard description

S (severe) Pink Very high probability of flooding, potentially severe flooding expected. Ten-year return

period as derived from 20 year simulation (1990-2009)

H (high) Red High possibility of flooding, bankfull conditions or higher expected. Five-year return

period as derived from 20 year simulation (1990-2009)

M (medium) Yellow Water levels high but no flooding expected. Two-year return period as derived from 20

year simulation (1990-2009)

L (low) Green Water levels higher than normal but no flooding expected. One-year return period as

derived from 20 year simulation (1990-2009)

Source: Reproduced with permission from Thielen et al ., 2009a Thielen J., Bartholmes J., Ramos M.-H and de Roo A., (2009a) The European
Flood Alert System - Part 1: Concept and development. Hydrology and Earth System Sciences , 13, 125-40.