Rapid Early Warning System for Small Scale Flooding Events and ...

Rapid Early Warning System for Small Scale
Flooding Events and Sediment Transport
Manfred Spatzierer, Anton Henle, Christian Rachoy
11 July 2008

Introduction
> Flash flooding is a common phenomenon in the
Alpine region during the summer. It is often caused
by convective systems that hit a small catchment.
> The rate of precipitation often exceeds 40-60 mm/h,
the total amount of precipitation can in rare cases
exceed 300-600 mm (e.g. Pitten catchment, Aug.
2nd 1999)
> Such extreme events are caused by slow moving
thunderstorms over complex terrain.
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Introduction
> Flash floodings are an immanent threat to live,
goods and infrastructure in the Alpine region.
Especially roads and railways may be severely
damaged by thunderstorm-related flooding
> The Austrian Federal Railways are in need of an
early flash flooding warning system, to take
measures to warrant security for passengers, staff
and goods.
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Introduction
Requirements of an early warning system
> The time scale of reaction to excessive amounts of
rainfall within a small catchment ranges from 15 to 60
min.
> An early warning system for flash flooding in Alpine
areas has to be very close to a real time system.
> Decisions that are based on the warnings have to be
made within a couple of minutes.
> It is also obvious that methods that involve excessive
calculations and computer power are bound to fail.
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Study area
Gumpenbach catchment
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Study Area
Lowest radar beams
from Zirbitzkogel
and Feldkirchen
radar stations.
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Method – 1 radar calibration
> Weather radar data can be used to calculate the
amount and rate of precipitation.
> semi-empiric correlation between the reflectivity
of particles in the atmosphere to microwave
radiation and the rate of precipitation.
> reflectivity of hydrometeors depends on their size
and state
> assumptions of these parameters have led to the Z-
R relation:
1.6
Z = 200⋅R
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Method – 1 radar calibration
> depending on vertical temperature profile and type of
precipitation (stratiform or convective), errors up to
50% are possible using this estimation.
> Normally one has to calibrate the derived analysis of
precipitation using rain gauge data (weather stations).
> Calibration of radar information may be carried out by
means of a conditional merging technique: use of
calibration field, which is computed through deviation between a
radar gauge interpolation field and an interpolation field of radar
pixel values at rain gauge sites. The deviation field is applied on
the raw radar data, resulting in a precipitation field that
preserves the spatial structure of the radar field while following
the main field of the rain gauge information.
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Method - 1 radar calibration
Rain amounts (mm) between 19 June 20:00 and 20 June 02:00 (local time)
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Method - 2 synoptic situation
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Method - 2 synoptic situation
Radar reflectivity representing the rate of precipitation
at 5:00 pm on June 19th 2006
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Method - 2 synoptic situation
Course of the thunderstorm over the Gumpenbach catchment
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Method - Definition of Warning Levels
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Summary and Outlook
- overall good performance of fast responding model for
analysing flash flooding in a small catchment.
- By using nowcasting methods to predict the movement
and the development of a thunderstorm, we expect to
retrieve fairly reliable precipitation forecasts, which
makes it possible to use the model in a predictive sense.
Further work:
- Investigation of more events in different catchments
- Taking into account prediction of development and
movement of convective cells as well as obvious
uncertainties of radar data
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Thank you for your attention.
mspatzierer@meteomedia.at