Figure 6.24: Cumulative and daily precipitation at Les Diablerets. The mean value between 2000 and 2100 was calculated from the MeteoSchweiz station DIB 7940. On 5 July 2007, an earth ow was triggered on the Pont Bourquin landslide.
6.9 Springs
Several springs were observed on the Pont Bourquin landslide (see Figure 6.26). They were named according to their position along the geophysical pro les ERT5 and P4 (Figure 6.11).
Their ux Q was measured and estimated during two years from summer 2009 to summer 2011.
The values for the ux reported on Figure 6.26 are mean annual estimates. Based on these uxes, the in ltration rate and the in ltration coe cient were calculated with Equations 6.1 and 6.2 (derived from the hydrological equation 4.6, see Section 4.10).
I = (P − ET ) − R (6.1)
Ci = Output
Input = Qt
(P − ET )A (6.2)
I: In ltration rate [mm/year]
P: Precipitation, 1500 mm/year ET: Evapotranspiration, 500 mm/year
R: Run-o rate ( ux measured at springs at the outlet of each catchment) [mm/year]
Ci: In ltration coe cient
Q: Flux measured at springs at the outlet of each catchment [l/min]
t: time, 1 year
A: Surface of the catchment
The aim of those calculations was to estimate the loss of water into the underground or additional water ex ltrating from the underground into the landslide. This was made for three topographic catchments in order to compare dierent zones of the landslide (the catchments are shown on Figure 6.25). The three catchments are:
6.9. SPRINGS
Figure 6.25: Hydrological map of the Pont Bourquin landslide and the surrounding area. Springs, creeks, wet zones and piezometers are shown as well as the locations of the installed meteorological station and the pluviometer, the catchment for the landslide (A) and two sub catchments of the head scarp area (B) and of Spring 32 (C).
6.9. SPRINGS
Figure 6.26: Map showing the springs on the Pont Bourquin landslide and the mean ux. The ux of spring 45R is composed of the ux from springs 49L and 49R that in ltrates and re-ex ltrates and is therefore not taken into account for the balance.
• Catchment of the Pont Bourquin landslide (A): During dry periods, a ux of 3-10 l/min was measured at the outlet of the catchment, whereas during rain events, it was estimated to be higher than 10 l/min. As a mean annual value, 10 l/min were estimated.
• Catchment of the swampy head scarp area where several springs were observed (B): A mean ux of 2-4 l/min has been measured at springs at the edge of the head scarp area.
Thus almost half of the water running of the Pont Bourquin landslide originates in this rather small area.
• Catchment of spring 32 (C): The ux of spring 32 was measured with a weir. In summer 2010, a ux between 3 and 8 l/min was measured during dry and wet periods. A mean annual value of 4 l/min was estimated.
Table 6.7 shows the calculated values for the in ltration rate and the in ltration coe cient.
Catchment A, m2 Q, l/min R, m/y I, m/y Q t, m3 (P -ET ) A, m3 Ci
A 33'000 10 0.16 0.84 5'256 33'000 0.16
B 3'500 4 0.60 0.40 2'102 3'500 0.60
C 10'000 4 0.21 0.79 2'102 10'000 0.21
Table 6.7: Table showing the in ltration rate I and the in ltration coe cient Cifor the three topographic catchments A, B and C. P-ET was assumed to be 1 m/year.
The in ltration and in ltration coe cient give information, if
• groundwater is owing into the system, for example from an aquifer in the hydrogeological catchment that is larger than the hydrological catchment. If I is small or negative and Ci is about 1 or larger, then groundwater is owing into the system (in addition to precipitation).
• water is owing out of the system, for example in ltrates into the bedrock.
6.9. SPRINGS
An in ltration rate of about 0.8 m3/(m2a) has been calculated for the catchments A and C of the Pont Bourquin landslide as well as Spring 32, whereas the calculated in ltration rate for the head scarp area B is about half of that. This could be because in this area, the top soil is less permeable and less water in ltrates. It is also possible that comparing to catchment A, area B has an additional input. It is possible that groundwater is owing into the head scarp area of the Pont Bourquin landslide through the underground from a hydrogeological catchment which is larger than the surface catchment. If it is assumed that the in ltration in catchment B is equal to A and C, then the rate of the additional input would be 0.4 m/y. It is possible that the run-o was underestimated for all three catchments.
6.9.1 Hydrograph of spring 32
The spring 32 has been trapped in a weir during two periods: 28 July - 14 August 2010 and since 15 Mars 2011. The spring was caught in a tube which conducted the water into a 1 m x 0.5 m x 0.5 m large box equipped with a Schlumberger CDT diver to measure the electric con-ductivity (EC) and water temperature and a Schlumberger micro diver sensor to measure the water pressure. The ux of the spring was calculated based on the water pressure and manual point measurements of the ux. The weir had to be completely rebuilt because it was destroyed in August 2010. Figures 6.27 and 6.28 show the curves of the rain intensity, speci c EC (Tref
25°C) and calculated ux.
Figure 6.27: Rain intensity, EC and calculated ux from spring 32 measured in summer 2010.
Dierent features which are observed in the two curves are described and interpreted in the following:
• In summer 2010, the EC was around 1.3 mS/cm and in 2011 it was about 1.7 mS/cm.
6.9. SPRINGS
Figure 6.28: Rain intensity, EC and calculated ux from spring 32 measured in spring and summer 2011.
These are values for rather high mineralised water. The variations are more meaningful than the absolute values.
• In both periods, a rapid fall in EC can be observed at the begin of rainfall with an intensity
> 5 mm/h in 2010 and > 2 mm/h in 2011. This shows the dilution with meteoric water as it was also observed in PB5.
• With a delay of 1-3 days, after the initiation of strong rain, the EC rose up for several 100 µS higher than the initial value. This indicates delayed piston ow like it was also observed in PB5. Mineralized groundwater was "pushed" by the in ltrating rain water and arrived a few days after the rain event at the spring. This could be water that was owing in fractures of the landslide. On 19 May 2011, the EC was rising for 0.45 mS/cm to a value of nearly 2 mS/cm. This was extraordinary high. A hypothesis is that the aperture of fractures due to landslide activity released water which was more mineralized than usually.
• The ux in 2010 varies between 3 and 8 litres and in 2011 it varies between 0.5 and 1 litre.
This is because the weir 2011 was built after super cial drainages were built by the canton of Vaud and it was not possible to trap the entire spring any more.
• The water ux shows daily uctuations which are more pronounced during the summer months. These uctuations could be related to evapotranspiration.
• After rain events, the ux in 2010 raised 4 l/min to a value of 8 l/min. But after the rain events 5, 12 and 15 August 2010, the measured ux seems to decrease. This could be artefacts due to tilting of the weir. In 2011, the ux was rising for 0.5 l/min after rain events, which is about 50%. The event from 15 May 2011 shows that after the ux had