Aim of the project was to identify Alpine catchments that might react unexpectedly strong to increased rainfall. Such catchments store large amounts of water and therefore have a delayed runoff reaction. However, if substantial drainage of these storages takes place within the timescale of flood formation, runoff reactions become much stronger.
In the Schächen catchment, several springs were monitored. Further, the hydrologic behaviour of hill slopes and talus areas were examined. Thereby we identified areas that react with varying degree of damping to rainfall. It was found that steep slopes with deep soils as well as talus areas can have a considerably delayed runoff response. Measurements revealed large differences of the depth to bedrock between sites, with up to 100 meter depth at a talus site.
In addition, there have been measurements on a steep creeping landmass slope. This slope was associated to one of the observed springs which has its peak discharge only after the rain has stopped. Boreholes for groundwater measurements were drilled into the bedrock and soil moisture sensors were installed. The behaviour of the slope during extreme rainfall occurring naturally only every 100 years was studied with a sprinkling experiment whereby more than 500 mm of artificial rain was applied. The experiment confirmed the high storage capacity of such steep slopes. To understand the mechanisms responsible for the damped reaction, a numerical model of the slope was set up with HydroGeoSphere. The results exposed the roles of the soil and groundwater body for runoff delay. The soil serves as an initial buffer and smoothes the infiltration to the groundwater from where it drains only slowly. The interaction of these processes is responsible for the delay of the peak discharge.
The gained knowledge was incorporated into a mapping procedure. The developed maps differentiate between areas with different storage and drainage characteristics. This allows evaluating the differences in flood runoff response between the investigated catchments. The maps serve as an input to the developed Qarea+ model. This model considers the different storages and explicitly takes into account slow drainage processes. The discharge reactions of the Schächen and the Hinterrhein were modelled. It was found that the mapping and modelling tools could well explain the damped flood runoff response in the Schächen and the strong runoff response in the Hinterrhein.
The project identified three groups of catchments: (1) Catchments like the Hinterrhein have a large fraction of fast reacting areas which dominate the flood peak response. Those catchments react foreseeable. (2) Catchments like the Schächen have a large fraction of delayed reacting areas that start to contribute to the flood runoff after a substantial amount of rainfall. A significant increase in runoff has to be expected for extremer rainfall events rainfall. (3)Catchments like the Dischma: A small fraction of fast reacting areas is complemented by a large fraction of extremely damped reacting areas. Their flood response is always strongly damped and therefore not very sensitive to extremer rainfall.
The project contributed to understand relevant flood runoff formation processes and therefore helps to identify catchments of type 2, which are critical in times of increased rainfall.
Susceptibility of alpine catchment flood runoff to changes in meteorological boundary conditions (SAC-FLOOD)