The extreme bad weather of October 2000 in the Simplon area The Gondo Spitting mudflow (Hangmure - Ovaille) J.-D. Rouiller 1, Ch.-L. Joris 2 On 14 th October 2000 at 10.15, the rain which had been falling incessantly in the area around the village of Gondo for 48 hours triggered off a 10’000 m 3 spitting mudflow. The flow composed of mud and blocks caused 14 deaths and destroyed a dozen buildings including the 400 year old Stockalper tower. Geographical and geological situation Gondo is a village on the border between Switzerland and Italy, situated on the Simplon international road, in the extreme east of Valais [fig. 1]. It is built on the scree of an imposing cliff [fig. 2] at the confluence of the river Doveria and the Grosses Wasser which brings water from the Zwischbergen valley. The Italian-Swiss border is at an altitude of 900 m on the southern side of the Simplon Pass (2002 m alt.); this area is, in fact, the only catchment area in Valais flowing into the Po river. Geologically speaking, the valley axis coincides with an important east-west thrust which has affected the Antigorio nappe gneisses. 1 Cantonal geologist [[email protected]] 2 geologist, member of the O. Schmid office in Brig [[email protected]] Figure 1: Geographical situation of Gondo - Extract of the 1: 25'000 federal topographic map [www.crealp.ch ] Gondo danger map Gondo has always been exposed to the dangers of stonefall. This situation led the commune to erect an imposing rock boulder trap above the village in 1988. Eight years later, in 1996, a structural study of the cliff carried out with the help of the MATTEROCK 3 method confirmed the danger of rockfalls and justified the presence of the rock boulder trap. The confluence of the Doveria et Grosses Wasser streams created a risk of erosion of the riverbanks in the lower part of the village each autumn. The day of the event some of the homes were evacuated. Regarding the risk of slipping of the scree on which the village has been situated for more than 400 years, there has been no historical indication nor was there during the construction of the buildings, some of which have six storeys [fig. 7]. The geotechnical study which preceded the rock boulder trap concluded that the scree was stable. Drillings had certainly not detected the presence of Figure 2: Cliff dominating the village subterranean water. of Gondo [www.crealp.ch]. The rock boulder trap can be seen above the houses Meteorological Context of the event General situation The meteorological situation which prevailed during the events of 14 th and 15 th Octobre 2000 [fig. 3] is similar to that which led to the Brig-Saastal flooding in 1993, with a front coming from the Mediterranean rising to 9000 m. altitude. This resulted in it easily crossing the Valaisan Alps with a strongly penetrating into the Rhone valley. 3 method of cliff auscultation and potential rockslide detection developed by the Research Centre of the Alpine Environment (see website www.crealp.ch) Figure 3: General situation indicating the origin of the October bad weather according to MeteoSuisse (H,T: high/low pressure; warm fronts are in red) [www.crealp.ch] Intensity of the pluviometric event As the isohyet map below [fig. 4] shows, the Gondo region is the one which received most of the rainfall between 11 th and 15 th October with 678 mm at the Simplon-Dorf weather station (7 km to the north west of Gondo) compared with 366 mm in 1993. The highest level was recorded at Lago Paione (5 km east of Gondo) with 809 mm, 603 mm of which fell on 13 th and 14 th (495 mm in Simplon-Dorf). As will be explained later, the natural cul-de-sac at whose end Gondo is situated certainly favoured a quantity of rainfall superior even to that even at Lago Paione. More than 50 mm / day of precipitation fell from 1 a.m. on 12 th October. Between 1 a.m. on 13 th October and 1 a.m. on 14 th October it reached 245 mm / day. At the moment of the spitting mudflow (10.15 a.m. on 14 th ) there was still 200 mm / day. Precipitation ceased during the morning of 16 th October. In 5 days, the north west of Ticcino and the Simplon region had recorded an accumulation of rainfall 35% to 45% higher than that over six days in September 1993. Once again, the behaviour pattern of the 0°C isotherm was the key parameter: at first risi ng to above 3000 m of altitude before dropping to 2000 m from the morning of 15 th . In fact, the return of snow to low altitudes saved the canton of Valais from an even greater catastrophe. Figure 4: Isohyet map drawn according to statistics provided by MeteoSvizzera Regione Piemonte, Regione Autonoma Aosta, Grande Dixence and Crealp [www.crealp.ch] Flooding of the Rhône and torrential lava As the table below shows, this meteorological event provoked a virtual hundred-year frequency flood over 120 km (80 miles) of the section of the Rhone from the town of Brig to Lake Geneva. As was the case during the bad weather in 1987 and 1993, the presence of hydroelectric dams at high altitude enabled the flood level of the Rhone to be lowered (0.5 m between Sion and Martigny), which avoided even more serious breaches of the dykes. Table 1: Observed water flows and rises in water levels in 1987, 1993, 1994 et 2000 [Crealp/ CONCRU 2000] . August 1987 September 1993 September 1994 October 2000 Basin slope Qobs. Qnat. Qobs. Qnat. Qobs. Qobs. Qnat. m3/s m3/s m3/s m3/s m3/s m3/s m3/s Vispa at Visp 285 450 330 494 161 196 257 Rhône at Sion 775 1045 830 1054 546 910 1035 Rhône at Branson 820 1029 930 1091 614 980 1080 natural = without the retention action of dams Throughout the canton of Valais, below 2000m altitude, torrential material from major river beds and banks of numerous streams produced human victims and severe damage. The 0°C isotherm - which fortunately during p revious summers (and especially in 2000) had rarely been above 3500 m - meant that the permafrost on the periglacial moraines had not sufficiently thawed to provide torrential material such as in 1987 and 1993. Setting off of the spitting mudflow Coming up against an orogenic boundary, which closes the Zwischbergen valley to the north, the disturbance deposited phenomenal quantities of water over Gondo. The first geologists who arrived on the scene observed that 5 hours after the start of the spitting mudflow the runoff flow was still forming an important trench of laminar discharge at the surface of the rock face which overlooks the village. According to the most probable scenario [fig. 5 and after], the water penetrated the ground directly at the foot of the cliff and saturated the 3-5 m of scree (K = 10-5) which covers an impermeable moraine base at this point which, itself, covers 15 m high gneissic rocks. Through solifluction, a mass of 10,000 m3 of mud and blocks obstructed the rock boulder trap and in a few minutes formed a dam for the water to flowing into it. Figure 5: Solifluction of the soil and start of the spitting mudflow. The Stockalper Tower is in the centre. [www.crealp.ch] The water accumulated behind the boulder traps infiltrated it at depth and undermined the foundations of the rock boulder trap in less than thirty minutes. The wall - constructed in a way to be able to form a sufficiently high screen for rock boulders despite the steep slope - was composed of Cyclopean type sections of concrete (dim. 2.5 x 8 x 15 m), each weighing 500 tons and linked to one another by simple expanding joints. These were designed to absorb any dilations and thus avoid damage which could be caused by the considerable seasonal temperature variations at this altitude. Once the foundations were soliflucted, the three sections on which the mass of mud was pressing toppled backwards (the leverage effect was estimated at 150-300 T/m’). Figure 6: Brutal rupture of the rock boulder trap In less than 20 seconds the mudflow cuts the village in two. NOTE: The 8 computer graphics illustrating the start of the spitting mudflow are available on [www.crealp.ch] Two of the boulder traps rode on the mudflow, adding to the destruction of the houses. The first embedded itself in the historic tower, whilst the other continued on its way as far as the edge of the Doveria [figs. 6 and 7]. In the space of about twenty seconds, the village was completely sliced in two - with a 50 m gap - by a wave travelling at 15-30 m/s. Figure 7: Trench formed in the village of Gondo. We can notice the rock boulder trap, the three wall sections (encircled), the Doveria and the Simplon international road which crosses the village from west to east [www.crealp.ch] Period of frequency of the phenomenon In September 1993 the same type of meterological disturbance originating from the south divided into two main axes as it entered Valais: the Simplon and the Saas valley (the valley parallel to that of Zermatt), causing the now well-known damage in Brig. The Entremont valleys (Grand-St-Bernard) in the extreme west of the canton were much less affected. In October 2000, the larger part of the meterological disturbance invaded the Simplon area and the north west of Ticcino resulting in a far more widespread penetration of the Valais than in 1993.