- Recently, radar has been used in broadly two ways in snow avalanche research:
- (a) To derive a general picture of avalanche movement by installing the radar at some
xxxxxxdistance from the flow,
- (b) To measure the avalanche velocities near the ground by installing radar in the track.
- An important advance that has occurred in the last decade is the use of pulsed Doppler radar for measuring the velocity remotely. This type of system, which will be used in the SATSIE project, operates on two separate frequencies.
- This allows measurement of the velocity characteristics of the powder cloud as well as the denser layer of snow beneath it. The existing radar of this type is able to provide velocity estimates averaged over 50m sections of the avalanche path.
- More detail on the pulsed Doppler radar system can be found by clicking [ here ].
- By installing pairs of radar within the avalanche track itself that point upwards into the flow it is possible to use the correlation in the signal between the radar to measure avalanche velocities at the bed.
- Furthermore, the depth of snow before, after and during the avalanche can also be determined. These data are extremely important because many of the processes occurring within an avalanche (erosion of the underlying snow, the nature of grain-to-grain collisions) are related to behaviour near the bed.
Video recording of avalanches
- Load plates can be used to determine both normal and tangential stresses. This means that they can be installed on instrument towers and within the avalanche track.
- Plates that are installed to measure tangential forces must have similar roughness characteristics to the rest of the path to ensure that the stresses are transmitted from the avalanche to the sensor in an appropriate manner.
- Electronic sensoring equipment means that stresses can be measured at high frequency. Thus, the possibility arises that the nature of collisions within the avalanche can be inferred from the plates. However, because snow becomes squashed against the sensor, interpreting these results (which are also sensitive to the area of the plate) is rather difficult.
xxxxxxLoad plates on an instrument tower at Ryggfonn.
- For some images showing installation of load plates at Ryggfonn, which will be used in the SATSIE project click [ here ].
- For some images showing load plate measurements from the Großer Gröben avalanche in Austria, click
[ here ].
Seismic studies of avalanche behaviour
- If there is good visibility this simple method can provide useful information on the dynamics of the avalanche front and of the powder cloud. Stereo-imaging can help to provide accurate surface velocity information.
- However, the techniques used to analyse this video data seem to need improving.
- To see some videos of avalanche events, please click [ here ]
- Much of the recent work that uses seismic methods to determine avalanche flow properties has been undertaken by the Department of Geodynamics and Geophysics at the University of Barcelona [ link ].
- Attempts have been made to interpret such measurements with respect to the processes occurring within an avalanche, although such research is hampered by the damping effect that the snowcover has on the passage of seismic waves.
- Hence, research is needed into optimising this method for avalanche analysis.
- These sensors measure changes in pressure and can be used to infer velocity. They have been used effectively when making measurements around ping-pong ball avalanches, for the research homepage click [ here ].
- They have the potential to enhance our understanding of the dynamics of the powder cloud, but cannot be used for making measurements within the denser layers of the flow.
- This technique has been successfully implemented at both Ryggfonn [ link ] and at Montana State University's Revolving Door site [ Research home page ].
- It is another technique that determines velocity by correlating signals; in this case the light emitted from LEDs and reflected by the snow is recorded at neighbouring photocells. These sensors are mounted on a tower in the avalanche path (or at Revolving Door are protected by a large boulder in the path).
- The problem with using simple pairs of sensors is that vertical fluctuations can result in decorrelation of the signal. However, these sensors are cheap to construct and thus, have some real potential.
- The main restriction with this type of sensor is that the devices are highly sensitive to the presence of free-water, which makes their use in wet-snow avalanches highly problematic.
- Velocities can be derived from these probes using a similar correlation procedure to that for the LEDs, although due to the higher price of these probes, this will not usually be feasible.
- However, the main advantage of the capacitance probes is that they have the potential to provide measurements of the density of the flowing snow.