Geoscience Reference
In-Depth Information
Characteristics of lake surroundings
Mass movements like rock fall or ice avalanches into a lake are important
triggering mechanisms for an outburst. Hence, an analysis of the
surrounding topography is of high importance. A simple but robust model
for this purpose is the Modifi ed Single-Flow Model (MSF, Huggel et al.
2003). The model is a modifi ed D8 fl ow direction algorithm and calculates
the likelihood that a raster cell will be affected by such a mass movement.
Similar methods to model rock and ice avalanches were applied by Allen
et al. (2009) and Salzmann et al. (2004).
If no detailed ground information is available, it might be appropriate
to model the probability of a rock fall based on Kaibori et al. (1988), who
presented detailed statistics for the slope at the detachment zone and for
the angle of friction. The angle of friction is defi ned by the average slope
between the starting and endpoints of the mass movement (Hsü 1975).
In contrast to Kaibori et al. (1988) who chose average values, one might,
however, use minimum values so that 90% of all occurred events are
included.
For ice avalanches, based on the empirical work of Alean (1985),
thresholds of 45° for the slope of the detachment zone for cold glaciers and
25° resp. 17° for warm glaciers may be used (cf. Bolch et al. 2011a). Although
these values are based on studies in the Alps and high mountains in Japan,
these values seem to be reasonable for a fi rst estimation as they represent
a worst-case scenario. Van der Woerd et al. (2004) estimated a gradient of
about 45° for the origin of ice avalanches for cold glaciers in the Central
Asian Kunlun Range. An example for a modelling of ice avalanches is
shown in Fig. 14.5.
The probability of a dam failure depends mainly on the characteristics
of the lake dam itself. Most of the lake dams consist of moraine-ice material.
The width and height of a dam as well as the freeboard between the lake
level and the crest can be visually determined by means of high-resolution
DTMs, nowadays possibly derived from satellite imagery. SRTM data can
provide a hint but the resolution is too coarse for detailed investigations
(Fujita et al. 2008). Since 2013 the TanDEM-X interferometric SAR mission
of DLR acquires global DTM data with a planimetric resolution of 12 metres
and an accuracy of better than two metres which seems to be well suited
for a baseline dataset.
A dam can become unstable if it contains permafrost or buried ice which
thaws or will thaw due to changing temperature conditions (Richardson
and Reynolds 2000). Here, a comparison of multi-temporal high resolution
DTMs can give a hint for the thawing of the ice content and the extent of
the lowering of the dam can also be detected (Fujita et al. 2008).
Search WWH ::
Custom Search