Hydrological impact of an extreme precipitation event a rain on snow flood

The extreme events discussed above have all been different in the patterns of precipitation and the scale of their impacts but have been the result of rainfalls alone. In other parts of the world, particularly in mountain areas, some of the most extreme flood events are caused by the melting of snow, which may be exacerbated when a warm weather system brings rain to a snow pack that is already close to melting. The additional heat provided by turbulent advection of warm air over the cold pack, and by the infiltration of warmer rain into the snowpack can result in rapid melting.

However, the analysis of such events can be complex, particularly where the temperatures are such that at higher elevations there may be snow during the event while rain is falling at lower elevations and where the snow is not yet close to zero degrees so that heat is required to raise its temperature as well as providing the latent heat of melting. In addition, the ground may or may not be frozen; or may be frozen where there was pasture but not frozen under forest cover. Thus, the runoff generation during such events can be difficult to predict.

This is illustrated by an event that occurred in the Vallée des Ormonts, a tributary to the upper Rhone in the Valaisian Alps, Switzerland (Schoeneich, 1992). On the 10-20 February 1990, a major precipitation event ocurred that initially fell as 30-40 mm of snow but which was followed by 300-350 mm falling as rain at lower altitudes but with further snowfall at higher altitudes (Fig. 9.6). The total precipitation input was estimated as expected only once in about 500 years. In addition, the soil was mostly frozen to depths of 80 cm, resulting in decreased infiltration into the soil surface. This event resulted in a significant, but not extreme flood discharge at Aigle at the outlet of the valley (Fig. 9.6). From the statistics, it was only the tenth highest flood peak recorded at this site. It seems that, in this case, although the inputs to the catchment were extreme, the limited magnitude of the flood peak was a result of the effective area contributing to the runoff generation being relatively small. Only 15 per cent of the rain (or 13 per cent of the total of the initial snowfall plus the rain) contributed to the volume of discharge at Aigle. The rest either fell as snow at higher altitude or was retained in the snow pack without melting the snow or infiltrated into the soil where it was not totally frozen. There were 'slush flows' of rain-saturated snow that moved downslope in the catchment, but without contributing to the discharge peak. It is clear that, in this case, under slightly warmer conditions, the peak discharges could have been much higher.

Fig. 9.6 Rain-on-snow event, Vallée des Ormonts, Switzerland, 10-21 February 1990 (redrawn from Schoeneich, 1992).

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