Scenario modeling in hydrogeological risk involve a multidisciplinary approach in order to analyze, forecast and define the evolution of landslides and flood hazards. The first part of the work which focuses on the application of geology in engineering practice mainly for landslide evaluation and erosion evaluation.
4. General Overview
Sondrio is situated in Valmalenco, close to the confluence between Mallero and Adda river.
Sondrio is exposed to different geological risk:
โ Spriana landslide;
โ Debris flow could occur locally in sub-basins of Valmalenco;
โ Erosion in Valmalenco basin.
6. Geological profile
By the data coming from:
โข Boreholes
โข Inclinometers
Itโs possible to know:
โข The soil layers
โข The water level
โข The surfaces of rupture
Geological profile of
Spriana slope
(section 1 and section 2)
7. Section 1:
โข Surface debris layer
โข Deeper layers of brecciated and strongly brecciated gneiss
โข Water level at -85m in B113 and -68m in B109. Spring at 730 m asl
โข Higher scarp at 1440m and lower scarp at 1160m
โข Foot of the landslide at spring (730 m)
โข 4 possible surfaces of rupture:
โ Debris โ higher scarp
โ Debris โ lower scarp
โ SBG โ higher scarp
โ SBG โ lower scarp
Geological profile
9. Limit equilibrium analysis
โข Simplification of the real profile (neglected lens of brecciateed gneiss and SBG)
โข Variation of the water level
Slope software
Limit equilibrium analysis
Estimation of the safety factor FS
according to the different
water levels
10. Limit equilibrium analysis
โข Variation of the water level (higher increasing in the foot of the landslide than in the
bed rock)
11. Limit equilibrium analysis
โข Assumption: Limit equilibrium at 1.05 (conservative estimation)!
โข Cylindrical surface (Debris): incorrect, the landslide is highly unstable and
probably is happening!
โข Free surface:
Lower scarp - Debris is unstable considering an increasing of water of 25m
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
0 5 10 15 20 25 30 35 40
Fs
Increase of water level [m]
SAFETY FACTOR
Higher scarp- Debris Higher scarp - SBG Lower scarp - Debris Lower scarp - SBG Cylindrical Limit equlibrium
12. Emergency phases
LEVEL OF
SAFETY
LEVEL
OF
ALARM
DESCRIPTION
๐ญ๐บ โฅ ๐. ๐๐ High 0 The slope of Spriana is safe, no incipient landslide is going to occur.
๐. ๐๐ โค ๐ญ๐บ < ๐. ๐๐ Normal 1
The entire stability of Spriana is safe, but the occurrence of small movements of
the soil due to increasing of saturation is possible. In any cases, these movements
are negligible and are not able to cause damage to people or infrastructure.
๐. ๐๐ โค ๐ญ๐บ < ๐. ๐๐ Moderate 2
PRE-ALARM: in this stage, the state of the slope is studied with much more details
in order to understand the future slope conditions. Possible small movements can
occur, as before. The increasing of ground water level, decreases the stability of
the slope, so new investigations are performed in order to understand the
behavior of slope.
๐. ๐๐ โค ๐ญ๐บ < ๐. ๐๐ Low 3
ALARM: one or more incipient landslides are prone to occur. Small movements are
often. A constant monitoring is active on the slope. Based on the analysis, the
decision to evacuate the people close to Spriana could be taken.
๐ญ๐บ < ๐. ๐๐ Emergency 4 The landslide is highly instable. All the actions aim to protect people.
13. Volume estimation
The four possible landslides could be considered extremely large.
Layer Volume [m3]
Higher scarp
Debris 10,694,502
SBG 45,409,996
Lower scarp
Debris 6,436,480
SBG 21,770,490
15. Volume estimation
Parameters:
- Slope channel ๐บ ๐ [%]
- Slope of the fan ๐บ ๐ [%]
- Area of the basin ๐จ [km2]
- Length of the river channel ๐ณ [m]
- Geological index ๐ฎ๐ฐ
Solid volume estimation is 45%
of the total volume:
โข Takei [m3] = 22,974
Total volume [m3] = 51,053
โข Rickemann [m3] = 288,113
Total volume [m3] = 640,251
โข DโAgostino [m3] = 108,861
Total volume [m3] = 241,914
โข DโAgostino โ Marchi [m3] = 80,796
Total volume [m3] = 179,547
16. Debris flow simulation
Parameters:
โข DEM
โข Starting point
โข Total volume
โข Mobility factor Kb (critical parameter)
Outputs:
โข Deposition area
โข Deposition height
โข Probability of inundation
Volume [m3] ๐ฒ ๐ [-]
Deposition Area
[m2]
Avg. Dep.
Height [m]
Sim_Takei 51,053 31 42,662 1.2
Sim_DโAgo 241,914 31 120,356 2
Sim_marchi 179,547 31 98,661 1.8
20. Long term analysis
Goal: Estimation of the annual sediment volume
eroded over Valmalenco basin and transported by the Mallero river up
to the closing section G [m3/y].
Gavrilovic approach
G = W R
โWโ [m3/y] Sediment production
due to erosion
โข Mean yearly temperature
โข Mean yearly rainfall
โข Area of the basin
โข Erosion coefficient
โRโ Routing coefficient
โข Length of the main river
โข Length of minor rivers
โข Basin perimeter
โข Mean height of basin
21. Long term analysis
Subdivision of the
main basin into
different sub-basins
Computation of the sediment
produced by erosion W for
each sub-basin
Computation of the routing
coefficient R regarding the
whole basin
ฮฃW Total sediment
amount is the sum of
contributions of each
sub-basin
ยซGยป Sediment
crossing the
closing section
23. Critical parameter: Erosion coefficient. It depends on:
โข Soil resistance
โข Soil cover
โข Extension of erosion
Soil resistance and soil cover coefficient affect sediments production ยซWยป (and consequently the
sediments volume ยซGยป crossing the closing section) more than the Soil type & extension of erosion
50,000
70,000
90,000
110,000
130,000
150,000
170,000
190,000
0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4
G[m3/y]
Amplification
SENSITIVITY ANALYSIS
Soil resistence
Soil cover
Extension of erosion
Long term analysis
24. Short term analysis
Accumulation of sediments over the years
๐ ๐ = ๐ฅ 1 โ ๐ ๐ ๐๐ ร # ๐๐ ๐ฆ๐๐๐๐ ๐๐๐ก๐ค๐๐๐ ๐๐๐ก๐๐๐ ๐ ๐๐ฃ๐๐๐ก๐
Sediment yield event
related over the main basin
๐ฎ๐๐๐๐๐๐๐ ๐๐๐๐๐
Estimation of the available sediment at the closing section due to an intense
event
๐๐ = ๐ฅ 1 โ ๐ ๐ ร # ๐๐ ๐ฆ๐๐๐๐ ๐๐๐ก๐ค๐๐๐ ๐๐๐ก๐๐๐ ๐ ๐๐ฃ๐๐๐ก๐ + ๐บ๐๐๐ก๐๐๐ ๐ ๐๐ฃ๐๐๐ก
Critical event:
high return period
25. Short term analysis
Assumption:
โข Event-related sediment yield
โ Critical event of 10 years can erode and transport sediment volume typical of 2 months
โ Critical event of 40 years can erode and transport sediment volume typical of 1 year
โข Sediment accumulation
โ Not all the sediments accumulated due to erosion can be transported to the closing section
๏ only the accumulated sediment in sub-basin before Sondrio๏ differential approach.
โ Contribution of the lateral sub-basin
โ Return period of the event: 10 and 40 years (similar to Sondrio flooding in 1987)
๐บ ๐ [m3/y] ๐ฎ ๐๐๐๐๐๐๐ ๐๐๐๐๐ [m3] ๐ป [year] ๐ธ ๐ [m3]
50,921
20,117 2mths 10 529,337
120,704 1year 40 2,157,584
Hydraulic Model