Shallow interflow – an overriding factor in flood formation during persistent rainfallFlooding and damage caused by long term precipitation (persistent rainfall) has grown in importance since the end of the 20th century. Whereas during heavy convective rainfall the surface runoff contributes especially to floodwater formation, the proportion of near surface interflow and return flow in the area runoff increases with increasing precipitation duration.
Academy of Sciences – ProjectTherefore within the framework of the from the Austrian Academy of Sciences and Austrian National Hydrology Committee (IHP) supported project “Shallow Interflow”, in a cooperation between the BFW’s Institute for Natural Hazards, the Geological Survey of Austria (Technical Department of Geophysics) and the Technical Office for Geology of Dr. Pirkl, extensive methodical tests and studies were carried out in five test areas : Bromberg, Bucklige Welt (Lower Austria), Brixenbach (Tyrol), Truppenübungsplatz Wattener Lizum (Tyrol), Längental (Tyrol), and Ruggbach (Vorarlberg). These tests had the following aims:
- Development, adaptation and testing of methods to measure the interflow in various substrates
- Derivation of the bandwidths of flow velocities for various geological substrates (via literature analysis and field measurements)
- Testing of approaches to extrapolate this information obtained at a small scale onto alpine catchment areas and the empirical application of such punctual information during regionalisation.
|Figure 1: Permanent irrigation at Möls Hochleger/Hinteren Wattental (Military training ground, Federal Armed Forces, Tirol). Pre-irrigation with more than 100 mm in two days on an area of 600 m², addition of LiCl as tracer (50 m² in 1 h with the irrigation construction in the middle of the picture) and post-irrigation with additional 400 mm in three days.|
The punctual measurements and measurements at hillslope-scale respectively on selected regionally representative sites encompass:
- Precipitation simulations with tracer-feeding, tracking of the water and the tracer in soil and underground via TDR and Geoelectric analysis (Figure 1).
- Punctual tracer-feeding in colluvium with temperature or conductivity measurements on underlying receiving waters.
- Measurement of further Geoelectric profiles and supplementary aerogeophysical measurements to characterise the below-ground situation in the testing areas.
- Temperature or conductivity measurements with occasional runoff volume measurements on selected regionally representative springs and confluence points in order to differentiate the dominant runoff components (surface runoff, near surface interflow, deep delayed runoff) and to derive bandwidths from flow velocities.
- Physical soil analysis at reference points.
| Figure 2: Dissemination of the salt tracer within the permanent irrigation area at Möls Hochleger|
These results, obtained on small slope segments (points) or on slopes, shall be extrapolated on the drainage basin using available basic maps and basic maps obtained in the course of the project (substrate maps, geomorphological maps, surface-runoff-coefficient maps, maps of the dominant runoff processes) and will be used to derive maps of the relevant areas during persistent rainfall (Figure 3).
|Figure 3: Area contributing to the interflow at Mölstal, hinteres Wattental (Military training ground Lizum/Walchen, Federal Armed Forces, Tirol|
The regionalised approaches shall be tested in sub-basins, in order to derive and better understand total reaction patterns of the catchment areas. These approaches enabled for the first time an implementation of the measurement data for analogue terrain-situations (Regionalisation).
The results of the field measurements obtained in Brixenbach and Ruggbach and the hydrological modelling are included in the revision of the present Hazard zone plans through the WLV and the office of the Vorarlberg provincial government.
Markart G., B. Kohl, B. Sotier, K. Klebinder, T. Schauer, G. Bunza, H. Pirkl and R. Stern (2011): A Simple Code of Practice for the Assessment of Surface Runoff Coefficients for Alpine Soil-/Vegetation Units in Torrential Rain (Version 2.0). Report in the frame of the Interreg-SEE-Project CC-WaterS (WP7 - Water Supply Management Measures, Act 7.1).
Markart G., A. Römer, G. Bieber, H. Pirkl, K. Klebinder, C. Hörfarter, A. Ahl, A. Ita, B. Jochum, B. Kohl, G. Meissl, K. Motschka, D. Ottowitz, I. Schattauer, B. Sotier, M. Strasser, K. Suntinger und E. Winkler (2013): Abschätzung der Bandbreiten von Fließgeschwindigkeiten des oberflächennahen Zwischenabflusses in alpinen Einzugsgebieten. Endbericht - 3. Projektjahr an die Österreichische Akademie der Wissenschaften (ÖAW), Nationalkomitee Hydrologie Österreichs (IHP), ISBN-Online: 978-3-7001-7392-2; doi:10.1553/Shallow