Illgraben
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Testsite Illgraben
The Illgraben catchment, in southwestern Switzerland, extends from the summit of the Illhorn mountain (elevation 2716 m a. s. l.) to the confluence of the Illbach river and the Rhone River (610 m a. s. l.).
The catchment has an area of 8.9 km2, but the subcatchment susceptible to debris flow activity occupies 4.6 km2. The geology is heterogeneous, composed of bands of quartzite, conglomerates, and calcareous sedimentary rocks on the southeast valley wall and massive cliff-forming dolomites on the northwest wall. Debris flows generally occur from May through October following convective rain storms. Flow initiation is probably related to sediment mobilization in areas where steep bedrock channels deliver large discharges of water on sediment deposits, similar to other debris flow-prone areas in the Alps.
Most of the large blocks transported by debris flows, quartzite boulders up to several meters in diameter, are covered with percussion marks indicating vigorous collisions between particles.
The channel on the alluvial fan has a U-shaped crosssection with base-widths of 5–10 m. Twenty-nine check dams are present over the distal 4.8 km of the channel and cause step-like vertical drops of up to several meters along the channel bed. The slope of the distal 2 km of channel decreases from 10 to 8% with local variations on the order of 7–18% persisting for 50 m-long reaches.
Measurements
Instrumentation includes devices to measure front velocity, flow depth, and a force plate on the bed of the channel to measure normal and shear forces and fluid pressure. The average front velocity of a debris flow is determined using the travel time between a geophone sensor installed on a concrete check dam 460 m upstream of the force plate and a geophone mounted on the force plate. Flow depth is determined using a laser distance-measuring device mounted above the force plate. While the elevation of the surface of a debris flow may decrease somewhat, depending on the Froude number as the flow approaches the brink of the check dam, the bed cannot be eroded and little deposition occurs, thereby increasing the accuracy in comparison with less constrained reaches of the channel. Assuming that no sediment is deposited on the force plate, the flow depth is determined using the distance from the surface of the flow to the top of the force plate.
Force Plate
The force plate is mounted horizontally at the crest of a concrete check dam with a trapezoidal shape (base width = 4.8 m). The force plate, installed flush with the channel bed, consists of a 2 m long (in the flow direction), 4 m wide, and 0.015 m thick steel plate, attached to an underlying steel frame which in turn rests on four corner-mounted vertical load cells. Two horizontal load cells are attached at the upstream end. The frame structure is acoustically insulated from the underlying check dam with stiff elastomer elements which provide overload protection for the sensors. The gap between the plate and surrounding concrete is a few mm wide and is sealed with a flexible silicon bead. Similar four-cell corner-mounted arrangements are commonly deployed for structural stability in industrial applications even though generally upon loading only three transducers respond at any given time. The transducer signals are sampled at 4 Hz, and the mean values per second are stored on a data logger.
Basal pore fluid pressure
Basal pore fluid pressure is measured near the center of the force plate. A pressure transducer is mounted at the top of a closed sedimentation reservoir under the plate and is connected to the base of the flow via a short fiberreinforced tube which is connected to a hollow mounting screw with an internal diameter of 8 mm; a steel plate is welded to the top of the mounting screw with a 2 mm diameter opening which is in contact with the base of the debris flow. The two-diameter reservoir entrance remained unclogged during the debris flow, unlike constant-diameter prototypes tested in 2004 or wire-mesh filters.
Geophone
A geophone measures vertical accelerations related to particle collisions on the force plate. We simplify the signal by recording the number of times per second that the voltage signal exceeds a small positive threshold value, thereby eliminating background noise. Under conditions of relatively low transport rate, as in gravel-bed rivers, this reduced signal corresponds approximately to the number of times that particles with a diameter larger than a few cm land on the sensor.
(Text from “Field observations of basal forces and fluid pore pressure in a debris flow”, Brian McArdell et al., 2007, GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L07406, doi:10.1029/2006GL029183, 2007)