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Floods are natural disasters whereas damage by floods are not!
Only the coincidence of floods and man-made damage potential results in
catastrophes. To eliminate the risk would require a complete drawback from
hazard regions. This is often not possible due to demographic and economic
constraints. However, appropriate area planning and landscape development and
the design of protection devices may reduce the magnitude of damage in the case
of extreme events. For the planning and implementation of protection measures
it is of utmost importance to have as much information as possible on the scope
and frequency of disaster events.
The Division of Torrent Hydrology is
addressing this problem. Through the implementation of experimental watersheds
which are equipped with a wide range of measuring devices and instruments the
necessary data are recorded to solve the problem. Field surveys provide
additional information on the development of natural phenomenons. The results
of the analyses provide the basis for the planning of protection measures by
the Forest Technical Service for Torrent and Avalanche Control (WLV). Annual
expenditure of public authorities amounts to more than 100 Mio EUR for the
protection of human lifes, properties, industrial infrastructure, trade and
traffic underlining the importance of this research topic.
A key task of the Division of Torrent
Hydrology is to improve the state-of-the art on precipitation and run-off in
torrential watersheds. For this purpose several "Torrent Research Areas"
are operated, in which precipitation and run-off are mesured with high
resolution during a period of several years and, according to the risk,
additional parameters such as perched water level, snow depths, air temperature
are recorded. These data are to be used either to directly support the
protection measures taken by the Forest Technical Service of the Torrent and
Avalanche Control on the spot or to provide supporting documentation for
further research projects (e.g. testing of simulation models)
Fig: Location
of the Torrent Research Areas operated by the BFW (hitherto
FBVA).
The lack of validated data about extreme
events in small watersheds continues to be a main obstacle to the planning of
torrent control measures. The recording and analysis of flood episodes
contributes to an improvement of flood protection, to reduce the negative
impact of extreme events and to reduce the cost for protection structures. The
special conditions prevailing in torrents such as the sudden increase of the
amount of water, accompanied by large quantities of bedload, require a
continuous development of measuring points which have to face enormous
pressures. As for the research into the causes of extreme events it is not
enough to record the flow alone, also precipitation, air moisture and
temperature are measured at regular intervals. Single investigations (e.g. snow
measurements, heavy rain simulation) complete the measuring programme. Even if
the recording of fundamental data for the investigation of flood events seems
to be exaggerated there is still a great need for research activities in view
of the total construction volume of the Torrent and Avalanche Control which
amounts to 100 Mio. Euro annually.
Figure: Documentation of a 100 years' flood
event ( Oselitzenbach/ Carinthia). (13.-14.10.1993)
Precipitation forecast is very important
for the planning of control measures in torrent watersheds. The maximum
rainfall and intensity during short, heavy and small-scale thunderstorms must
be identified because they can lead to a sudden increase of torrents with
bottom and bank erosion, transport and deposition of material until reaching
the size of a debris flow. Also abundant rainfall for longer periods which
result in a water saturation of the soil involves this danger and may,
especially in combination with snow melt, trigger disasters such as landslides,
etc. The special conditions prevailing in torrential watersheds, such as big
altitudinal differences in the area, the steepness of the terrain and the often
extreme weather conditions require much energy and money for the establishment
and operation of measuring points. Considerable wind influence and temperature
variations between the mountain and the valley may impede the measurements and
make the interpretation of the results difficult. Especially during winter it
is very difficult to get meaningful results. Precipitation in the form of snow
may easily be blown, before it can be recorded by an instrument. Heating
systems designed to melt the snow should not cause evaporation which might
falsify the measuring results. Due to its Torrent Research Areas, the
Department of Avalanche and Torrent Research has a long experience in operating
meteorological observation networks at exposed Alpine locations. For these
areas and for the surrounding regions and based on long-term high-resolution
measuring series, information is available on heavy rain intensity of a given
occurrence probability or duration-frequency-relationship of
precipitations..
Figure: Precipitation quantities / annual
diagramme for the measuring point at Sonnalm in the watershed of the
Schmittenbach (Zell am See/ Salzburg)
In an Alpine country like Austria snow is a determining factor
for the development of the natural environment. The storage of precipitation in
the snow cover plays a major role in the water balance. In the field of torrent
control, the assessment of melt water quantities is very important for the
planning of flood protection measures. The water-logged slopes due to snow melt
often cause slope movements which cannot be dominated easily. The lack of
validated data is the biggest obstacle for problem solution. Even the best
possible mathematical abstractions to model natural processes cannot renounce
to careful, long-term observation and measurements. As a remedial measure, in
Carinthia, in two torrential watersheds alignment bases have been established,
both in the forest and in the open field. On these plots at an interval of
approx. 100 a.s.l., snow depths and water equivalents are being recorded
continuously. Meanwhile, the observation period extends to more than 10 years
now. The gained data allow for the improved assessment of the melt water
quantities being useful also for the verification and plausibility control of
snow melt run-off models.
Figure: Comparison of the water equivalent (water quantity
contained in the snow cover) between forest and open field by the example of
the Gradenbach/Berchtoldhang
The Austrian Federal Office and Research Centre for Forests
(BFW), under their torrential measuring programme conducts comprehensive
investigations on one of the most dangerous slope creeps of Austria. This is an
approx. 2 km large area at the outlet of the Gradental/Carinthia which played a
major role during the disaster events of 1965 and 1966. Large rainfall episodes
caused floods which eroded the toe of the slope in the area of a 900 m long
ravine. The thereby caused slope movement triggered enormous debris mass
transport, repeated debris flows of the village of Putschall and, in a chain
reaction through deviation of the river Möll and the creek Zirknitzbach
affected also the village of Döllach. Comprehensive control measures, both
in the stream bed and at the slope, were undertaken to interrupt the vicious
cercle between erosion of the toe of the slope and subsequent mass movement. As
the slope is strongly water-logged it was drained by means of a sophisticated
drainage system. The effect of these measures as well as the interaction
between precipitation, run-off and slope movement are verified and investigated
under the measuring programme of the Department for Avalanche and Torrent
Research. For this purpose, the BFW operates a number of measuring points in
order to register perched water levels and spring discharges, in addition to
observation points for precipitation and run-off. The practical relevance and
the benefit for the national economy is that the data can be used for the
design of protective devices on the spot. The transfer of knowledge to other
watersheds with similar problems is guaranteed by the close cooperation with
the Forest Technical Service for Torrent and Avalanche Control (WLV).
During flood events the problem of bedload transport is of
great importance. Contrary to this the ways to measure the transport of
sediment load are still limited.
Therefore, it is planned to develop new techniques for the
measurement of bedload transport in torrents and to improve the existing ones.
In a first step, suitable structures and prototypes of measuring instruments
should be developed under laboratory conditions. Secondly, the promising
structures, with the support of the Provincial Office of the Torrent and
Avalanche Control for Flach and Tennengau, are to be tested in the field and
assessed as to their suitability for practical application. The project will be
implemented in collaboration with the University of Agricultural Sciences
(Institute for Water Management and Water Engineering), the University of
Innsbruck (Institute for Water Engineering), the Austrian Federal Office and
Research Centre for Forests (BFW) and the Federal Ministry for Agriculture and
Forestry, Environment and Water Management.
Precipitation:
At the moment, the measuring service of the BFW operates
omrographs which transport the precipitation by means of a collector to a
dipping bucket gauge. As the tipping bucket gauge moves, the hour, quantity and
intensity of the precipitation are being recorded. The omrographs of the whole
year round operated base in the Torrent Research Areas is equipped with several
heating cycles which allow to record also solid precipitation. The heating
system avoids both the freezing of the measuring instrument and snow
accumulation in the collector. In addition, measuring points without heating
are used during the summer months which are less costly and allow for an
assessment of the precipitation distribution in the catchment area. For the
control and the competion of the measuring values stemming from those
self-writing devices also ombrometers are used. Thus, it is possible to record
values daily at the same hour by local observers. These persons report any
disturbance to the Department and carry out simple maintenance works at the
measuring points.
The instruments equipped with heating reduce the options for
the selection of a site as the high energy consumption can only be satisfied by
connecting it with the local energy supply network. Solar panels cannot be
used. The tuning of the heating is also rather difficult - if it is too low the
tipping bucket gauge will freeze, if it is to high the precipitation might
evaporate before the measurement takes place.
Figure: Measuring point of the BFW with
ombrometer, ombrograph and thermohydrograph
Precipitation can be measured using a weigh which has proved
to be useful in extreme locations. The intensity of precipitation can be
calculated with this equipment from a change in the weigh per time unit. Low
energy consumption suggests the use of solar panels.
Run-off:
The design of the measuring instruments
is important in order to get accurate measurement results. Our experience with
run-off measurements in torrents suggests as follows:
- The measuring profile should be
trapezoid in torrents. This design allows for an accurate measurement of low
water level recording at the same time peak floods many times over this value.
The non-structured transverse profile allows for a simple determination of the
gauge key as the velocity distribution in the channel is balanced which does
not apply to structure transverse profiles (low flow channel....).
-
A stilling basin should be upstream in order to reduce the
influence of the washing of the vagues.
-
At the end of the measuring channel should be a transverse
dike with complete overfall.
-
The establishment and calibration of the gauge key curve
(which is used for the conversion of the measured water levels in terms of flow
quantities) should, if possible, foresee continued measuring of the rate of
flow. The usual random sample measurement of the rate of flow is often
impossible in torrents. High water levels during floods which are of short
duration consist a problem as nobody can foresee them to pick the right moment.
- The mesurement of the flow velocity should take place by
radar as in case of a flood episode bedload and wooden logs cause measurement
errors and may even destroy the instruments.
Figure: Run-off weir Oselitzenbach
(Carinthia)
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Austrian Federal Office and Research Centre for
Forests | Natural Hazards 
