Horlach valley

The Horlach valley is located in the Stubai Alps and is a right-hand side tributary of the Oetz valley. It is dominated by E-W striking gneisses and mica schists of the Oetztal massif (BÖGEL & SCHMIDT 1976). Three large tributary valleys (Zwieselbachtal, Larstigtal, Grastal) branch off to the south from the main valley, which also runs in an E-W direction, and the Finstertal valley follows to the north.

View from Zwieselbach valley in the downvalley direction towards Finstertal (Photo: Betz)

The valley was formed by Pleistocene glaciers (HEUBERGER 1966), while today only a small part (4.9%; BECHT 1995) of the area is glaciated, the proportion constantly decreasing, so that currently the landscape is dominantly formed by flowing water and mass movements. The area has a topographic relief of 1811 m (from 1476 to 3287 m a.s.l.). In the lower altitudes extensive pasture farming is practiced, the timberline is currently at about 2200 m above sea level; the forest below this area consists of larch, spruce and Swiss stone pine stands. Above the timberline, dwarf shrub heath, alpine meadows and pioneer vegetation are found up to the vegetation-free scree and rocky region. Cambisols, podsols and raw soils are found in the study area (BECHT 1995). The average annual precipitation is about 1000 mm (TIWAG, station Niederthai). The annual runoff regime in the Horlach creek is characterized by snowmelt runoff in early summer, with additional peaks caused by summer rainfall events (BECHT 1995); the contribution of glacier melt is reflected in diurnal and seasonal variations of runoff. Heavy rainfall events in the summer months have led to increased geomorphological activity in large parts of the study area in recent decades (e.g. 1991, 2005) (numerous landslides and debris flows, floods with strong bedload transport and overbank debris flows, partial outburst floods from lake Grastal).

Slope-type debris flows leave typical traces (incised channels accompanied by levées) on the talus cones at Zwieselbachtal (Photo: Betz)

The TIWAG (Tyrolean waterpower company) has monitored river discharge and sediment transfer of the main channel at gauges near Horlachalm and at Niederthai since more than 30 years ago. The SEHAG project has installed additional streamgauges and a meteorological station.