The fresh instances in the list above show that the new estimation of dangers in contact with glacier floods cannot be produced on the basis of one to factor alone (age.g. the surface area of a glacial lake (Post and you may Mayo 1971)). As an alternative, the blend out-of release quantities, rupture components, peak-launch viewpoints, lithology and you can inclination during the reach of the glacier weight enjoys to be assessed. Outbursts out-of professional-glacial ponds with moraine dams which has had flat ice try, in any case, unsafe and you can guarantee preventive action, since they usually ensure it is heavy rain situations. For this reason the release may be superimposed to flood as a result of precipitation. Peak-release viewpoints for outbursts caused by modern enhancement of freeze channels should be calculated with the entirely empirical Clague-Mathews formula:
For hundreds of years, of several mountain villages were mainly based in which they truly are impacted because of the such as for instance situations which, toward one-hand, are particularly uncommon, and on the other, may very well features serious consequences later
where V is the outburst frequency inside the cubic yards (Clague and you may Matthews 1973). Beliefs determined that way is quite higher than the highest observed height discharge thinking (Desk II).
The formula is, therefore, admirably suitable for estimates in such cases. However, much higher peak-discharge values are expected in the event of sudden ruptures of ice barriers (Table III). For such events, with the exclusion of the minimal outburst at Glacier Bas d’Arolla, the following formula has to be applied: Q maximum (sudden break (m 3 s ?1 )) = V/tw
where V is again the outburst volume in cubic metres and tw is an empirical time constant [about 1 000 to 2 000 s, cf. Table III). In Figure 4, the average slope between the place of rupture and the outermost limit of the recorded area of damage, as a measure of the potential area of damage in the valley, is plotted as a function of the expected value of the peak-discharge. Since the spatial extent of populated areas in the Swiss Alps has not changed significantly over the centuries under consideration, the historical records of events causing damage can be used as a first approximation to appropriate standards. On the other hand, the extent of the damage in comparable cases today and in the future may well be considerably greater because of the expansion of cultivated land in the interim. The most extensive damage occurs when there is relatively small debris content. Peak-discharge values below 20 m 3 s ?1 are dangerous only for slopes over 10 to 12° and when there is, in addition, readily eroded loose material. The relationship given in Figure 4 also takes into account a few (known to the author), well-documented, non-Swiss glacier floods in the Alps, especially the catastrophe on Glacier de la Tete Rousse 1892, French Alps (sudden waterpocket rupture, many people killed in St Gervais (Mougin and Bernard 1922)), and at Vernagtferner, Austrian Alps (repeated sudden dumping of the Rofen lake, dammed-up during and after the surges of Vernagtferner, damages right into the Inntal: Reference Lanser Lanser 1959, Reference Hoinkes Hoinkes 1969). These are, however, extreme events historically. Many glacier floods take place and terminate well within the indicated extremes. These extremes have to be taken as standards, however, until the reasons are known why it is that, in some cases, the limits are not being reached.
Fig. 4. Extent away from damage along side glacier stream to own good enough recorded glacier flooding regarding the Swiss Alps, ?crit = mediocre hill between your place of rupture (usually the glacier terminus) as well as the outermost restrict of the recorded ruin. Incidents is actually labelled since the “dirt disperse” in the event St. Louis city dating that area from the weight away from debris is higher than the newest part by pounds of drinking water.
Results
Damages in connection with glacier flooding will always be to get requested on Swiss Alps as time goes on, but by sanctuary out-of glaciers while the framework off reservoirs to own energy design, it can mainly getting a question of outbursts from relatively short water masses in the easily eroded, loose sediments. To the estimate of such glacier problems, which are usually regional, one has to ask feel out of historical situations. Peak-discharge values shall be determined approximately, utilizing the Clague-Matthews algorithm, so long as the sudden break off a freeze dam is actually perhaps not thought; this may are present pursuing the surge regarding an effective glacier, a frost avalanche otherwise a temporary blockage out of an excellent subglacial launch station. Should this be the case, then launch per 2nd can be visited step one/one thousand of the outburst regularity and you can a ton revolution many yards in height get ensue. If mountain is actually high, also small outburst volumes and you will small peak-discharge opinions may result in the synthesis of risky dust circulates. During the appropriate point (20 to 31° high moraine hills, scree mountains or rock-glacier fronts), erosional channels which have a cross-sectional section of to five-hundred m dos could form. The greatest, erratic height-launch values out of glacierized portion are found concerning liquids-wallet ruptures, due to modern enhancement out of intra- and you will subglacial channels concerning heavier rain, and just have out of abrupt drinking water-pocket ruptures shortly after short-term blockage out-of launch avenues. Such as opinions is of your own order away from one hundred in order to 150 meters 3 s _step one .
Leave a Reply