Local scale groundwater modeling in a landslide. The case of Super-Sauze mudslide (Alpes-de-Haute-Provence, France) Debieche T.-H., Marc V., Emblanch C., Cognard-Plancq A.-L. & Garel E. UMR 1114 INRA-UAPV (EMMAH), Université d’Avignon et des Pays de Vaucluse, Faculté des Sciences Exactes et Naturelles, 33 rue Louis Pasteur, 84000 Avignon, France
Bogaard T.A. Water Resources Section, Faculty of Civil Engineering and Geosciences, Delft University of Technology, PO Box 5048, 2600 GA Delft, The Netherlands
Malet J.-P. CNRS UMR 7516, School and Observatory of Earth Sciences, University Louis Pasteur, Strasbourg, France
Introduction: the Super-Sauze mudslide is a typical complex landslide with huge soil heterogeneity, Objective:
identifying and modelling the impacts of preferential flows on groundwater variations in landslides developed in black marls.
matrix and preferential flow and spatial differences in landslide dynamics. The quantification and modelling of the hydrological behaviour is challenging as well as very relevant because hydrology determines to a large extent the landslide dynamics and forecasting or planning of mitigation need thorough understanding of underlying physics. To achieve this, we have setup a large scale infiltration (100 m2) and tracing experiment. In this poster we will present the experimental setup and we will discuss preliminary results of the unsaturated-saturated zone modelling of the experiment.
Situation and experimental setup: The Super-Sauze mudslide is located in the South East France. The elevation ranges from 1740 to 2105 m and the area is 17 ha. The geology is mostly Callovian-Oxfordian black marls. Artificial rainfall was applied to a 100 m2 plot, where slope averaged 20°. The instrumentation comprised of 6 sprinklers, 15 standard rain gauges, 12 tensiometers located between 0.2 and 0.7 m deep, 7 soil moisture sensors and 38 piezometers for water level measurements 1, 2 and 3 m deep. Artificial rainfall was applied over a period of 14 days (10-23/07/2007). KBr was used as tracer during the first week (10-16/07/2007) whereas KCl was used during the second week (17-23/07/2007). The mean rainfall intensity was 8.5 mm.h-1 with a mean tracer concentration of 100 mg.l-1 (for both Cl- and Br -). Down slope BI-L BI-23 BI-21 BI-K BI-20 BI-22 BI-24 BI-19 BI-J Ar-3
236680
Z max = 2105 m
37 piezometers (1, 2 et 3 m)
BI-13 Pluvio-13
236678 BI-14 BI-G
Pluvio-14 Pluvio-15
BI-12
BI-F
BI-11
South French Alps Department of Alpesde-Haute-Provence (04, France)
BI-17 BI-18 BI-15 BI-I BI-H BI-16 Ar-4
Limit of the infiltration area
BI-9
236676
BI-E BI-8 BI-D
BI-10 Pluvio-11
Pluvio-10
BI-7
Pluvio-12
236674
135 m
5 82
Pluvio-7
Pluvio-8
Ar-2
m
Pluvio-9 Ar-5
236672
7m
Pluvio-4
Super-Sauze mudslide
BI-1
BI-A BI-2
236670
14 m
BI-5 BI-C BI-6
Pluvio-5
Pluvio-6
BI-3 BI-4 BI-B
BV-24
Experimental area
Pluvio-1
236668
15 rains gauges
Pluvio-2 Pluvio-3
6 sprinklers 236666 Ar-1 Ar-6 945840
945842
945844
two sections were chosen parallel to the flow direction for the model purpose: one on the left side (West) of the plot (apparent fissured part, characterized by several fissures of different length (up to 2 m) and depth (10 at 25 cm)) and the other on the right side (East, no apparent fissured part). The features of these sections are 1) there was no lateral flow (from west and east) and 2) there were several piezometers equipped with automatic recorders of water head.
hydrodynamic behaviours (large, medium and low variation of groundwater levels) in the subsurface piezometers. The simulation period was the second week of the experiment, because the first week had less reliable rainfall data.
BI-L BI-23 BI-21 BI-K BI-20 BI-22 BI-24 BI-19 BI-J Ar-3
236680
BI-17 BI-18 BI-15
2 RG-10
Cl (100 mg.l ) 236674
1902
0
RG-8
RG-4
BI-6 BI-9 Steady flow
1893
RG-9 Ar-5
30
Unsteady flow
BI-18 BI-20 Modeling period
1892 10/07
20
Experimental area (fine grid resolution)
50 m
BI-C
BI-3 BI-4 BI-B
BI-2 RG-5 RG-6
236668
50 m
RG-1
BI-6
RG-2 RG-3
236666 Ar-1
BI-9
Ar-6
Near BI-1/BI-2
945840
945842
945844
945846
945848
BI-18
945850
Up slope
7m
BI-2
1895 1894
RG-12
BI-C
BV-24
-1
BI-C
236670
Average rain (mm.h ) tt
BI-1
Large variation
1896
10
Low variation Medium variation
1900
BI-A
BI-8 BI-D BI-7
BI-6 BI-5
BI-1
1897
RG-11
236672
1901
Altitude of groundwater level (m) lll
RG-7 Ar-2
on 1
-1
θr : 0.13 θs : 0.34 Alpha : 2 n : 1.4 Ks : 0.6 m.h-1 I : 0.5
BI-E
BI-10
Sec ti
Br (100 mg.l )
BI-9
S ec tion
-
Near BI-17/BI-18
RG-15
BI-F
BI-11 236676
-1
RG-14
BI-12
Near BI-19/BI-20 -
Ar-4
RG-13
236678 BI-14 BI-G
BI-H BI-16
BI-I
boundary conditions of the section consisted upslope of a constant groundwater flux. The lower boundary at the contact limit with the substratum was set to a no flow boundary condition. An atmospheric boundary condition consisting of rainfall and evaporation and surface runoff at the soil surface was chosen at the surface. The section limits were situated at 50 m upslope and downslope from the experimental area, so that the boundary conditions had negligible impact on the groundwater flow.
Zone with apparent fisssures
Down slope
Zone with no apparent fissures
BI-13
1898
945850
Characteristics and boundary conditions of the model: the
Modeling sections:
Water level variations and choice of the simulation period: the hydrodynamic monitoring showed different
1899
945848
Up slope
Z min = 1740 m
BI-A
945846
20°
40
Atmospheric
Constant flux (0.28 mm.h-1)
Seepage face 50
11/07
12/07
13/07
14/07
15/07
16/07
17/07
18/07
19/07
20/07
21/07
22/07
23/07
Piezometer
No flux
Groundwater flow
Time (day)
Where : θr : residual soil water content; θs : saturated soil water content; alpha : parameter in the soil water retention function [L-1]; n: parameter in the soil water retention function; Ks: saturated hydraulic conductivity, Ks [LT-1]
Near BI-6
Results
125
150
0
25
50
Time (h)
75 100 Time (h)
125
Water head (m)
2.5 2.0
5 10 15
1.5
20 25
1.0
Intensive sampling
30
0.5
35
Piezometer in no apparent fissure part 0.0
40 0
25
50
75
100
Time (h)
125
150
Average rain (mm.h-1)
H mesured BI-20 H simulated BI-20 Average rainfull
35
Piezometer in apparent fissure part 0
25
50
75
100
2
12
1898.6
1
BI-C (m) 16
BI-A (m)
1898.4
40 125
Rainfull (mm/h)
GWL (m)
-1
-1
0.0
Rainfall
150
1898.2 0:00
Time (h)
150
3) the case of piezometers characterised by large water level variations and with intensive sampling (1 hours sampling time step): the model fitted well with the observation over the starting and recession periods but not during the sampling period. Most probably this error is due to the uncertainty in sampling discharge. The impact of sampling was not correctly simulated because the nodal discharge was kept constant for all the period whereas water sampling resulted in an intermittent process 3.0 0 of water extraction.
30
8
12:00
0:00
20 0:00
12:00
1
BI-C
2
BI-C
3
BI-C
4) the case of piezometers with low variations and “normal sampling”: the model performed poorly (BI-2 and BI-9). This could be due to the calibration criteria, which was focussed on optimising high dynamic groundwater behaviour and not matrix flow. 3.0 H mesured BI-2 H modelised BI-2 Average rain
2.5 2.0
0
2.0
5
1.8
10
20
1.0
25
0.8
30
35
0.6
35
40
0.4
30 Piezometer in no apparent fissure part
Piezometer in apparent fissure part
0.0 0
25
50
75 Time (h)
10
1.2
25
0.5
1.6
5
15
20
1.0
H mesured BI-9 H modelised BI-9 Average rain
1.4
15
1.5
0
-1
100
Macro-pore connection effect
0.2
40
0.0
25
1898.8
3
Average rain (mm.h )
75
0.4
Macro-pore connection
Macro-pore connection
50
35
Piezometer in no apparent fissure part
20
1899
Water head (m)
25
30 0.2
40 0
0.4
0.6
4
-1
35
Piezometer in apparent fissure part
25
15
1899.2
Average rain (mm.h )
30
20
0.6
10
Average rain (mm.h )
25
15
0.8
5
1m
20
0.8
10
H mesured BI-C H simulated BI-C Average rainfull
0
1899.4
0
1.0
Water head (m)
15
1.0
5
Water head (m)
10
H mesured BI-1 H simulated BI-1 Average rain
1.2
Average rain (mm.h )
5
Water head (m)
H mesured BI-18 H simulated BI-18 Average rain
2) the case of piezometers characterised by large water level variation, “normal” sampling (3 or 6 hours sampling time step) and macropore connection (piezometer BI-C-3): the model simulated fairly well the normal groundwater level evolution, but was unable to simulate the water table variation due to the macropore connection (soil structure variation over the time of the experiment).
0
1.2
0 -1
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6
Average rain (mm.h )
Water head (m)
1) the case of piezometers characterised by large water level variation and “normal” sampling (3 or 6 hours sampling time step): the model simulated fairly well the groundwater level (BI-1 and BI-18). The small difference between measured and simulated values can mainly be attributed to the error in the initial conditions.
100
125
150
40 0
25
50
75 100 Time (h)
125
150
Conclusion: the results of the flow simulations indicated that the large water levels variations were well estimated by the model, but the low groundwater variations were over-estimated. This is a consequence of the models calibration chosen to simulate fissure flow dynamics rather than matrix flow dynamics. This also resulted in a large difference between the hydraulic conductivities used in the models (10-4 m.s-1) and measured in the field (10-6 to 10-7 m.s-1). In locations where soil macropore connectivity has been detected (from artificial tracing results), the models performed badly. These results showed the limitations of the traditional groundwater modelling approach in such environment. They showed the need to have a stepwise modeling approach with progressive complexity to fit the system heterogeneity. Assimilation of geotechnics data and information on mudslide movement in the models is also requested to improve the simulation of the flow processes. Landslide Processes: From geomorphologic mapping to dynamic modelling. 6 - 7 February 2009 (Strasbourg, France)
