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.

1. Hydrogeological Risk in Mountain Area
Geological Assessment

2. Contents
• Context
• Spriana lanslides
– Geological profile
– Limit equilibrium analysis
– Emergency phases
– Volume estimation
• Debris flow
– Volume estimation
– Debris flow simulation
• Erosion
– Long term analysis
– Short term analysis

3. 𝑸 𝑸 𝒔
𝒛 𝒃𝒆𝒅
𝑸 𝒐𝒖𝒕
𝒉 𝑽
Overall Framework

4. General Overview
Sondrio is situated in Valmalenco, close to the confluence between Mallero and Adda river.
Sondrio is exposed to different geological risk:
– Spriana landslide;
– Debris flow could occur locally in sub-basins of Valmalenco;
– Erosion in Valmalenco basin.

5. Spriana landslide

6. Geological profile
By the data coming from:
• Boreholes
• Inclinometers
It’s possible to know:
• The soil layers
• The water level
• The surfaces of rupture
Geological profile of
Spriana slope
(section 1 and section 2)

7. Section 1:
• Surface debris layer
• Deeper layers of brecciated and strongly brecciated gneiss
• Water level at -85m in B113 and -68m in B109. Spring at 730 m asl
• Higher scarp at 1440m and lower scarp at 1160m
• Foot of the landslide at spring (730 m)
• 4 possible surfaces of rupture:
– Debris – higher scarp
– Debris – lower scarp
– SBG – higher scarp
– SBG – lower scarp
Geological profile

8. Geological profile
Section 2:
• Surface debris layer
• Depper layers of brecciated and strongly brecciated gneiss

9. Limit equilibrium analysis
• Simplification of the real profile (neglected lens of brecciateed gneiss and SBG)
• 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
• Variation of the water level (higher increasing in the foot of the landslide than in the
bed rock)

11. Limit equilibrium analysis
• Assumption: Limit equilibrium at 1.05 (conservative estimation)!
• Cylindrical surface (Debris): incorrect, the landslide is highly unstable and
probably is happening!
• 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
𝑭𝑺 ≥ 𝟏. 𝟑𝟎 High 0 The slope of Spriana is safe, no incipient landslide is going to occur.
𝟏. 𝟏𝟎 ≤ 𝑭𝑺 < 𝟏. 𝟑𝟎 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.
𝟏. 𝟎𝟕 ≤ 𝑭𝑺 < 𝟏. 𝟏𝟎 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.
𝟏. 𝟎𝟓 ≤ 𝑭𝑺 < 𝟏. 𝟎𝟕 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.
𝑭𝑺 < 𝟏. 𝟎𝟓 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

14. Debris flow

15. Volume estimation
Parameters:
- Slope channel 𝑺 𝒄 [%]
- Slope of the fan 𝑺 𝒇 [%]
- Area of the basin 𝑨 [km2]
- Length of the river channel 𝑳 [m]
- Geological index 𝑮𝑰
Solid volume estimation is 45%
of the total volume:
• Takei [m3] = 22,974
Total volume [m3] = 51,053
• Rickemann [m3] = 288,113
Total volume [m3] = 640,251
• D’Agostino [m3] = 108,861
Total volume [m3] = 241,914
• D’Agostino – Marchi [m3] = 80,796
Total volume [m3] = 179,547

16. Debris flow simulation
Parameters:
• DEM
• Starting point
• Total volume
• Mobility factor Kb (critical parameter)
Outputs:
• Deposition area
• Deposition height
• Probability of inundation
Volume [m3] 𝑲 𝒃 [-]
Deposition Area
[m2]
Avg. Dep.
Height [m]
Sim_Takei 51,053 31 42,662 1.2
Sim_D’Ago 241,914 31 120,356 2
Sim_marchi 179,547 31 98,661 1.8

17. Debris flow simulation
Area of inundation Area and height of deposition

18. Debris flow simulation
Sensitivity analysis on kb:
The sensitivity analysis shows that by increasing the mobility coefficient, the
deposition area increases while the deposition height decreases.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
15 20 25 30 35 40 45
Depositionheight[m]
Kb [-]
DEPOSITION HEIGHT
D'Agostino
Takei
0
20
40
60
80
100
120
140
160
180
15 20 25 30 35 40 45Depositionarea[m2]x1000
Kb [-]
DEPOSITION AREA
D'Agostino
Takei

19. Erosion

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
“W” [m3/y] Sediment production
due to erosion
• Mean yearly temperature
• Mean yearly rainfall
• Area of the basin
• Erosion coefficient
“R” Routing coefficient
• Length of the main river
• Length of minor rivers
• Basin perimeter
• 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
ΣW Total sediment
amount is the sum of
contributions of each
sub-basin
«G» Sediment
crossing the
closing section

22. Bacin_ID H [mm/y] F [km2] t [°] T I [%] Ξ Π Φ Z W [m3/y]
184 1,122 46 2 0.5 0.52 0.81 1.15 0.65 1.27 127,789
183 1,249 42 2 0.5 0.60 0.78 1.15 0.65 1.28 132,795
177 1,314 53 8 0.9 0.62 0.50 1.55 0.55 1.04 220,979
178 1,500 14 2 0.5 0.66 0.74 1.15 0.40 1.03 38,355
181 1,417 13 2 0.5 0.70 0.79 1.15 0.40 1.13 38,396
179 1,434 0.2 2 0.5 0.42 0.35 1.55 0.40 0.57 191
180 1,362 11 2 0.5 0.73 0.81 1.15 0.65 1.40 43,357
175 1,155 25 8 0.9 0.57 0.59 1.30 0.85 1.23 117,288
176 1,140 25 8 0.9 0.60 0.47 1.55 0.85 1.18 111,633
182 966 26 8 0.9 0.63 0.40 1.55 0.65 0.89 64,217
174 966 23 8 0.9 0.73 0.21 1.55 0.65 0.49 22,749
173 925 29 8 0.9 0.73 0.56 1.30 0.55 1.02 83,694
172 972 11 12.1 1.1 0.50 0.21 1.55 0.85 0.50 13,416
l [km] li [km] D [km] O [km] R
195.5 364.9 2.2 88.9 0.119
Σ W [m3/y] R G [m3/y]
1,016,722 0.119 120,704
Long term analysis

23. Critical parameter: Erosion coefficient. It depends on:
• Soil resistance
• Soil cover
• Extension of erosion
Soil resistance and soil cover coefficient affect sediments production «W» (and consequently the
sediments volume «G» 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
𝑆 𝑎 = 𝛥 1 − 𝑅 𝑛 𝑊𝑛 × # 𝑜𝑓 𝑦𝑒𝑎𝑟𝑠 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑖𝑛𝑡𝑒𝑛𝑠𝑒 𝑒𝑣𝑒𝑛𝑡𝑠
Sediment yield event
related over the main basin
𝑮𝒊𝒏𝒕𝒆𝒏𝒔𝒆 𝒆𝒗𝒆𝒏𝒕
Estimation of the available sediment at the closing section due to an intense
event
𝑄𝑠 = 𝛥 1 − 𝑅 𝑊 × # 𝑜𝑓 𝑦𝑒𝑎𝑟𝑠 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑖𝑛𝑡𝑒𝑛𝑠𝑒 𝑒𝑣𝑒𝑛𝑡𝑠 + 𝐺𝑖𝑛𝑡𝑒𝑛𝑠𝑒 𝑒𝑣𝑒𝑛𝑡
Critical event:
high return period

25. Short term analysis
Assumption:
• Event-related sediment yield
– Critical event of 10 years can erode and transport sediment volume typical of 2 months
– Critical event of 40 years can erode and transport sediment volume typical of 1 year
• Sediment accumulation
– 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.
– Contribution of the lateral sub-basin
– Return period of the event: 10 and 40 years (similar to Sondrio flooding in 1987)
𝑺 𝒂 [m3/y] 𝑮 𝒆𝒙𝒕𝒓𝒆𝒎𝒆 𝒆𝒗𝒆𝒏𝒕 [m3] 𝑻 [year] 𝑸 𝒔 [m3]
50,921
20,117 2mths 10 529,337
120,704 1year 40 2,157,584
Hydraulic Model

26. Thank you