12th Congress INTERPRAEVENT 2012 Grenoble / France – Extended Abstracts www.interpraevent.at
FIXING DESIGN FLOOD CONSIDERING
FLOOD EVENT CYCLE AND THRESHOLD PROCESSES
Serena Liener1, Ursin Caduff2 and Heinz Roth3
INTRODUCTION
Several flood events in Switzerland in 1999, 2005 and 2007 showed, that design floods for major rivers have been fixed much to low. Therefore the Canton Bern (CH) wanted to find a method for fixing design flood considering flood event cycles and threshold processes. The following abstract describes such a method.
HISTORICAL ANALYSIS
The procedure to fix design flood contains a historical analysis of the drainage basin. In the historical analysis, the focus is to establish a register of historical floods. The register collects information on the occurrence and the size of flood events. With this information flood cycles can be determined and showed in figures (see Fig. 1). If possible the runoff of some flood events is dedicated.
0 1 2 3 4 5 6 7 8 9
1750 1800 1850 1900 1950 2000
Frequency of flood [30 years floating sum]
Fig. 1 frequency of flood events in the Aare river at Brienzwiler (30 years floating sum of flood events)
HYDROLOGIC ANALYSIS
The hydrologic analysis is focused on the study of the biggest flood events with identification of the volume runoff coefficient. Furthermore the properties of the drainage basin such as the storage capacity of the soil and the permeability of geology are investigated. If there are hydrologic studies from the catchment area, the knowledge of these studies are part of the hydrologic analysis. The goal of the hydrologic analysis is to detect threshold process. Threshold processes are responsible for a nonlinear response of a catchment area with unexpected high discharge rates. For every catchment area, evidences for threshold processes were collected and summarized in a table. Evidences for threshold processes are historical flood events with greater discharge than in the measurement period, a big storage capacity of the soil, a high permeability of the geology, a low volume runoff coefficient and differences in the course of observed floods, which lead to a higher discharge.
1 Dr. Serena Liener, geo7, Neufeldstrasse 5 – 9, 3008 Bern, Switzerland (e-mail: serena.liener@geo7.ch)
2 Ursin Caduff, geo7, Switzerland
3 Heinz Roth, Tiefbauamt Kanton Bern, Switzerland
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SYNTHESIS
The goal of the synthesis is to use the collected knowledge of the drainage basin to ameliorate the results of the extreme value statistics.
When the historical analysis shows, that the measurement period of a catchment area covers a period with few flood events this period can be compressed according to NAEF ET AL. (2008). The compressed data is used as input for the extreme value statistics.
On the other hand, when the hydrological analysis shows, that threshold processes are possible the measured data should be amended with modelled data or historical data of runoff.
Finally for each research area several extreme value statistics are calculated (see Fig. 2). Based on the uncertainties one value will be selected as design flood.
0 100 200 300 400 500 600 700 800 900 1000 1100 1200
HQ 30 HQ 100 HQ 300 HQ 500
Runoff [m3/s]
Measurement BAFU 1905 ‐2009 (ME MLM) Measurement BAFU 1905 ‐2009 (KS) Part of serie 1954 ‐2009 (ME MLM) Part of serie 1954 ‐2009 (KS)
Part of serie 1954 ‐2009 corrected (ME MLM) Part of serie 1954 ‐2009 corrected (KS) Hazard map Aareboden
Runoff out of EHW‐Study
* *
*
* *
Fig. 2 results of different extreme value statistics for 30, 100, 300 and 500 year return period for the Aare in Brienzwiler.
REFERENCES
Geo7, IUB Ingenieur Unternehmung, Hunziker, Zarn & Partner, Emch+Berger (2007).
Extremhochwasser im Einzugsgebiet der Aare. Bau-, Verkehrs- und Energiedirektion des Kantons Bern.
Naef, F. M., Schmocker-Fackel, P., Kienzler, P., Scherrer, S. (2008). Die Häufung der Hochwasser der letzten Jahre. Umwelt Wissen Nr. 0825: 429 S.
Keywords: design flood discharge, flood frequency, extreme value statistics
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