URBAN TORRENTS - THE INFLUENCE OF SETTLEMENTS ON RUNOFF AND FLOOD
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URBAN TORRENTS – THE INFLUENCE OF
SETTLEMENTS ON RUNOFF AND FLOOD
PROPAGATION
M. CHIARI*(1), G. ANGELMAIER (1) and H. HÜBL (1)
* Michael.Chiari@boku.ac.at
(1) Institute of Mountain Risk Engineering, University of Natural Resources and Life
Sciences, Vienna
ABSTRACT: The surroundings of major cities are very popular places for living
especially if the city is encircled by hills or mountains. Human activities in these areas
may have an influence on the hydrology of the catchments. Due to surface sealing and
changes of the roughness the discharge may be increased. To investigate these
phenomena several torrents in the city of Linz are investigated. One of these torrents is
the Höllmühlbach. To estimate the effects of intensive settlement a comparison of the
runoff event between a historical state and the today’s current state is made. Therefore
the catchment of the Höllmühlbach has been modelled with the hydrologic model
ZEMOKOST. In order to obtain the affected areas of the potential design event,
several two-dimensional hydraulic simulation models haven been tested for their
suitability for modelling floodplains of steep torrential catchments. The programs
MIKE FLOOD and RiverFLO-2D were applied. Due to unsatisfactory results and
unsolvable problems with the above-mentioned programs at steep slopes, the approved
program FLUMEN was used to simulate different flood scenarios. The results show
that the discharge is significantly increased due to the settlement activities and the time
of concentration is decreased. Therefore the flooded area and the associated energy
heights of the flow are increased compared to the historical state. Buildings serve as
obstacles whereas some streets can be regarded as new waterways by transferring
discharge to areas that were not affected in the historical state.
Key words: rainfall-runoff modeling, urbanization, 2D-flood modeling
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1. INTRODUCTION
Due to increased population in mountainous regions the land consumption has been
expanded. The surroundings of major cities became very popular places for living. In
hilly or mountainous regions these areas may be part of torrential catchments.
There are view investigations on the influence of soil sealing on the discharge of
torrential events available, whereas the influence of forest stands on runoff
generation in small catchments has been discussed by many authors (e.g.
Kostadinov and Mitrovic 1994; Burch et al. 1996; Hegg et al.2004). Some tools
have been developed to estimate the effects of soil sealing (Kleebinder et al. 2007,
2008).
To show the influence of the increased settlement and changed landuse on flood events
in torrential catchments, a hydrologic and a hydraulic metallization has been carried
out for the actual and a historical state of the Höllmühlbach, a torrential catchment in
the city of Linz..
2. THE HÖLLMÜHLBACH CATCHMENT
2.1 General description
The catchment of the Höllmühlbach is situated in the surroundings of Linz, the capital
of Upper Austria. The catchment area is 8.58 km² and the highest point in the
catchment is at 732 m.a.s.l. At the outlet the Höllmühlbach flows into the Pulverbach at
255 m.a.s.l. The major part of the catchment is used for agriculture, forest stands make
about 30 % of the catchments area. Nowadays there are a lot of settlements in the
lower part of the catchment.
For the catchment of the Höllmühlbach a historical map from the year 1826 is available
(Franziszeischer Kataster). The historical state and the actual state of the catchment
can be compared in Figure 1, where the catchment area and the sub catchments for the
hydrologic modelling are shown.
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Figure 1: Overview of the catchment Höllmühlbach of the historical (left)
and actual (right) state.
2.2 Hydrology
For the hydrologic part of this study the model ZEMOKOST (STEPANEK et al.,
2004) has been applied. The required surface runoff class (Table 1) and roughness
class (Table 2) were mapped in the field after Markart et al. (2004) for the actual state.
For the historical state, these data have been reconstructed according the historical
map. A comparison between the historical and actual roughness classification is shown
in Figure 2. Due to settlement the runoff class 6 (sealed areas) increased. Very high
runoff is also produced by agricultural areas, where maize is cultivated. On the other
hand the condition of the forest stands became better. Therefore there are some areas
with lower runoff coefficients nowadays compared to the historical state. But in
general, the mean runoff class of the catchment increased from 2.8 in the historical
state to 3.3 in the actual state.
Table 1: Runoff classes
after Markart et al.
(2004)
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Table 2: Roughness classes after Markart et al. (2004)
Roughness Roughness Roughness
description class coefficient (c)
Very smooth RKL 1 < 0.02
Rather smooth RKL 2 0.04-0.04
Slightly smooth RKL 3 0.04-0.06
Slightly rough RKL 4 0.06-0.08
Rather rough RKL 5 0.08-0.10
Very rough RKL 6 0.10-0.12
Figure 2: Runoff coefficient of the historical (left) and actual (right) state.
Figure 3 shows a comparison of the roughness distribution in the catchment for
the two different times. Again, major change of the surface roughness can be
found in the settlements. Sealed areas are very smooth and where fields used to
be are nowadays gardens. For the whole catchment the mean roughness
decreased from 3.7 in the historical state to 2.7 in the actual state.
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Figure 3:Roughness classification of the historical (left) and actual (right) state.
2.2.3 Simulation results hydrology
The design event for a flood with a recurrence period of 150 years has been modelled
with ZEMOKOST. Design rainfall data are available for the whole country of Austria
with a resolution of 6 km x 6 km and durations from 5 min to 6 days (eHYD 2009).
The simulation results for the design event of the Höllmühlbach are presented in Table
3. In the historical state the critical rainfall duration is longer and therefore the related
rainfall intensity is lower. The time to reach the peak discharge is longer for the
historical state. The biggest changes can be found when comparing the peak discharge
between the historical and actual state of the catchment. The design event peak
discharge is increased from 34 to nearly 53 m³/s.
Table 3: Comparison of the design event for the historical and actual state.
historical actual
Critical duration 46 41 min
Rainfall intensity 82 88 mm/h
Time to peak 104 93 min
Peak discharge HQ150 34.0 52.6 m3/s
Total water volume 218 000 252 000 m3
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2.3 Hydraulic simulations
For the hydraulic simulation a LiDAR based elevation model with 1 m x 1 m
resolution was available. The geometry of the main channel was defined by reference
cross-sections for appropriate representation of the channel flow.
To investigate the flooded area the programs MIKE FLOOD (DHI 2007) and
RiverFLO-2D (FLO-2D 2009) were applied. Due to unsatisfactory results and
unsolvable problems with the above-mentioned programs at steep slopes, the approved
program FLUMEN (Beffa, 2005) was used to simulate different flood scenarios. The
FLUMEN model is a two dimensional simulation model calculating flow and
morphologic changes on triangulated irregular networks. The flooded area and the
maximum flow depth are shown for the actual state in Figure 4. For the simulation all
buildings were considered. When the channel leaves its bed, streets have a big
influence on the flow paths. Due to this effect parts of the discharge are transported to
another channel (orographic right side of the channel).
Figure 4: Maximum flow depth for the actual state simulation
For the historical state the digital elevation model of the actual state has been used with
some modifications: only at that time existing houses were considered and different
roughness values were used. The flooded area and the related maximum flow depths
can be regarded in Figure 5. Unfortunately some streets still influence the flow paths,
but the flooded area is different compared to the actual state. Because of the reduced
discharge the affected area is much smaller.
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Figure 5: Maximum flow depth for the historical state simulation
3 DISCUSSION
Not only climate change can increase the flood risk. Changes in the catchment may
have a relevant influence on the design event. Soil sealing and changes in land use can
increase the peak discharge and the total water volume of an event. On the other hand
the time of concentration can decrease. Streets may serve as new waterways. The
example of the Höllmühlbach catchment shows, that the discharge is partly transferred
into another hydrologic catchment due to the overland flow on existing streets. In this
study the design event for the Höllmühlbach was modelled without considering flow in
the neighbouring catchments. For a more realistic flood scenario the bordering
catchments should also be considered in the hydrologic and hydraulic simulation to
show the influence of new waterways on the flood risk in a human influenced
environment.
4 CONCLUSION
The effect of settlements and change of land use on discharge and flooded area is
shown by the combination of a precipitation discharge simulation and a hydraulic
simulation. An increase of the peak discharge and the total water volume results in an
increased flooded area. This study shows, that changes in the catchment have to be
considered for the design event.
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REFERENCES:
Beffa, C. (2005), 2D-Simulation der Sohlenreaktion in einer Flussverzweigung,
Österreichische Wasser- und Abfallwirtschaft 42(1-2), 1–6.
Burch, H.; Forster, F.; Schleppi, P. (1996), Zum Einfluss des Waldes auf die
Hydrologie der Flysch-Einzugsgebiete des Alptals. Schweiz. Z. Forstwes. 147, 12:
925–937.
DHI Software (2007), MIKE FLOOD 1D-2D Modelling User Manual, DHI Software.
eHYD (2009), eHYD – das Portal für hydrographische Daten Österreichs im Internet,
Ein Service der Abteilung VII/3 – Wasserhaushalt,
http://gis.lebensministerium.at/eHYD
FLO-2D Software, Inc. (2009), RiverFLO-2D Two-Dimensional Finite-Element River
Dynamics Model - USER'S MANUAL, Nutrioso, AZ.; Pembroke Pines, Fl. USA.
Hegg, C.; Badoux, A.; Lüscher, P.; Witzig, J. (2004), Zur Schutzwirkung des Waldes
gegen Hochwasser. Schutzwald und Naturgefahren, Forum für Wissen 2004: 15-20.
Klebinder K., Bitterlich W., Kohl B., Schiffer M., Pirkl H., Markart G. (2007),
Wege zur Quantifizierung der Auswirkungen von Versiegelungen auf den Abfluss bei
konvektiven Starkregen für Siedlungsräume des oberösterreichischen
Salzkammergutes, Wildbach- und Lawinenverbau, Zell am See, (156), 164-170
Klebinder K., Bitterlich W., Kohl B., Schiffer M., Pirkl H., Markart G. (2008),
Evaluation and quantitative estimation of the effect of Surface sealing on discharge In:
Conference Proceedings - Internationales Symposion Interprävent 2008, Dornbirn,
(Extended Abstracts): 224-225
Kostadinov, S. C., Mitrovic, S. S. (1994), Effect of forest cover on the stream flow
from small watersheds, Journal of Soil and Water Conservation 49(4), 382-386.
Markart, G., Sotier, B., Schauer, T., Bunza, G., and Stern, R. (2004). Provisorische
Geländeanleitung zur Abschätzung des Oberflächenabflussbeiwertes auf alpinen
Boden-/Vegetationseinheiten bei konvetktiven Starkregen (Version 1.0). Wien: BFW-
Dokumentation; Schriftenreihe des Bundesamtes und Forschungszentrums für Wald.
Stepanek, L., Kohl, B., & Markart, G. (2004), Von der Starkregensimulation zum
Spitzenabfluss, Riva/Trient: Internationales Symposium INTERPRAEVENT 2004 -
Riva/Trient.
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