Integrated hydro-geological risk for Mallero basin (Alpine Italy) – part 2: hydraulic assessment

Maryam Izadifar, Alireza Babaee,
Budiwan Adi Tirta, Ahmed Hassan El-Banna
Hydro-Geological Risks in
Mountain Area
Team Members:
Feb. 2015
Hydraulic Assessment
Professor:
Alessio Radice

Table of Contents
1- Introduction
2- River Modeling
2-1- Input Data
2-2- HEC-RAS model
2-3- BASEMENT model
3- City Modeling
3-1- Input Data
3-2- Previous Studies
3-3- Mesh Sensitivity
3-4- Steady Model
3-5- Transient Model
4- Results for Emergency Plan

1- Introduction
Area of interest
• The total area Mallero river basin is about
322 km2
• Length of the main river is 20 km
• Length of minor rivers is 36 km
• The hydraulic model starts from last 5 km
of the Sondrio’s reach

4
Flash Flood
• Flash flood event in mountains is related to the transportation of large
amounts of sediment.
• As a consequence, significant morphological changes may occur in rivers
during a single, short‐duration event, which has significant effect on the
water elevation.
1- Introduction

5
1- Introduction
Flood 1987
• A major flash flood occurred in the Mallero basin in July 1987
• The town of Sondrio was almost flooded
• Records of the event remark that a significant component of the flood
was represented by sediments which deposited along the in-town reach
due to relatively low slope
• The peak discharge of the event was estimated as almost 500 m3/s
• The sediment volume mobilized throughout the catchments was 3 × E6
m3, with 7 × E5 m3 supplied to the Mallero upstream of its last 5 km.
• Around 2 × E5 m3 of sediments were deposited in the in-town reach,
with aggradation depths of up to 5 m at the Garibaldi bridge and more
than 2 m at the Eiffel bridge.

1- Introduction
Geological
Assessment
Hydrology
River
Modeling
(1D)
Erosion
(Event 1987)
Input Sediment graph
City
Modeling
(2D)
Outflow
hydrograph
Input
hydrograph
Emergency
Plan
Flood extension,
Depth, Velocity
(Time)
Methodology
Hydraulic Assessment Part

2- River Modeling
2-1- Input Data
• Input Data:
- Geometric Data (HEC-RAS and BASEMENT Input)
- Flood Hydrograph (HEC-RAS and BASEMENT Input)
- Sediment Graph (BASEMENT Input)
• Hydraulic modelling comprises two part:
1- define some critical sections along the town reach by
means of HEC-RAS
2- evaluate morphological evolution in the reach by means
of Basement

2- River Modeling
2-1- Input Data
• Flood Hydrograph (HEC-RAS and BASEMENT Input)
• Sediment Graph (BASEMENT Input)
Hydrograph Event 1987 Sediment graph (1987)
Area below sediment graph = 700,000 m3

First Section
(Basement: Sec 36,
HEC: Sec 56)
Critical Section
(Basement: Sec 70,
HEC: Sec 22)
(Basement: Sec 91,
HEC: Sec 1 )
2-River Modeling
2-1- Input Data

Pedestrian Bridge
Garibaldi Bridge
Eiffel Bridge
2-River Modeling
2-1- Input Data

2- River Modeling
2-2- HEC-RAS Model
• A preliminary analysis was carried out using HEC-RAS
considering steady analysis with peak flow of 495 m3/s.
• From the result it was observed that along the 5 km of
the reach, three different type of water conveyances
were defined based on their capacity.

HEC-RAS (Steady analysis with peak flow Qp=495 m^3/s)
First Section (Upstream)
High Conveyance Capacity
Last Section (Downstream)
Medium Conveyance Capacity
2- River Modeling
2-2- HEC-RAS Model
(Basement: Sec 91, HEC: Sec 1 )
(Basement: Sec 37, HEC: Sec 55 )

HEC-RAS (Steady analysis with
peak flow Qp = 495 m^3/s)
Critical Sections
Low Conveyance Capacity
2- River Modeling
2-2- HEC-RAS Model
(Basement: Sec 70, HEC: Sec 22 ) (Basement: Sec 74, HEC: Sec 18 )
Garibaldi Bridge

HEC-RAS (Steady analysis with
peak flow Qp=495 m^3/s)
2- River Modeling
2-2- HEC-RAS Model
Critical Sections
Low Conveyance Capacity

Critical Sections (Looking Upstream)
2- River Modeling
2-2- HEC-RAS Model
Critical Sections (Looking Downstream)

Basement Modeling
2- River Modeling
2-3- BASEMENT Model
• Basement is a software created with the purpose to evaluate morphological
evolution in a water stream, developed by the Hydraulic Department of the
Swiss Federal Institute of Technology ETH Zürich
• As an upstream boundary condition, a flow hydrograph is required. As a
downstream boundary condition, a normal depth with a slope value is
necessary for the model

18
• upstream boundary, a sediment discharge was selected in order to
simulate sediment feed upstream.
• The downstream boundary selected (IODown) guarantees that all
sediment entering the last computational cell will leave the cell over
the downstream boundary.
• Since this boundary condition requires that the elevation of the bed in
the downstream section remain unchanged, which is obviously not
realistic, an artificial reach is attached after the last section for 2 km in
order to minimize the adverse effect of the boundary condition
Basement Modeling
2- River Modeling
2-3- BASEMENT Model

19
In downstream (city) the bank elevations are considerably lower than upstream part . So, in
the case of bed aggradation in this part, bed elevation might be higher than bank elevation.
Avoiding crash in Basement calculation we have to use elevated banks in this part of river.
2- River Modeling
2-3- BASEMENT Model
Basement Modeling- Geometry

20
• Initial condition: Dry
• Upstream BC: constant hydrograph of 20 m3/s
• Strikler Coefficient * ,Ks = 25 m1/3/s (Ks=1/n=1/0.04)
• Upstream slope=0.032
• Downstream slope= 0.001
2- River Modeling
2-3- BASEMENT Model
Basement Modeling- First Run Input data
NOTE: Ks strickler = 1/n manning. The coefficient Ks strickler varies from 20 (rough stone and
rough surface) to 80 m1/3/s (smooth concrete and cast iron).
Start with a fixed-bed model with initially dry bed, allowing enough time for a
steady condition to develop, to obtain the initial condition for the morphologic
model (second run)

21
• Initial condition is guaranteed by a constant hydrograph of 20 m3/s in the
whole length of the channel (first run)
• Upstream BC: 1987 event hydrograph
• Sediment Graph
• Strikler Coefficient * ,Ks = 25 m1/3/s (Ks=1/n=1/0.04)
• Upstream slope=0.032
• Downstream slope= 0.001
• Porosity: 0.35
• Particle Density: 2600 kg/m^3
2- River Modeling
2-3- BASEMENT Model
Basement Modeling- Second Run Input data
NOTE: Ks strickler = 1/n manning. The coefficient Ks strickler varies from 20 (rough stone and
rough surface) to 80 m1/3/s (smooth concrete and cast iron).

22
• Three different sediment size combination was used for sensitivity analysis:
Combination 1: 50mm for bed and feeding
Combination 2: 100mm for bed and feeding
Combination 3: mix 1 : 50mm (90%) and 150mm (10%) for the bed
mix 2 : 50mm (30%) and 150mm (70%) for the feeding
• Bed-load Transport Formula: mpmh (Meyer-Peter-Mueller and Hunziker)
for multiple grain classes
2- River Modeling
2-3- BASEMENT Model
Basement Modeling- Second Run Input data (continue)
Località Pendenza
S [%]
d50
[cm]
Cassandre 10 – 15 11-54
Sondrio 1 – 1.5 4-7

23
280
290
300
310
320
330
340
2000 2500 3000 3500 4000 4500
Elevation
Distance From Upstream (m)
Bed Morphology + Water Level (Peak time t=110000)
Bed Original
d=50&150
Observed Bed
Evolution
d=50mm
d=100mm
Bed aggradations in case of 50mm, 100mm and combination between (50&150mm)
at peak time of hydrograph. 50&150mm combination represents more similarity to
the observed bed evolution considering the critical sections.
2- River Modeling
2-3- BASEMENT Model
Basement Modeling- Sediment Size Sensitivity Analysis

24
As expected, the deposition of sediment is significant exactly from
section 68 to section 74 due to the decreasing slope.
295
297
299
301
303
305
307
309
311
313
315
317
319
321
323
325
327
329
2550 2650 2750 2850 2950
Elevation
Bed Morphology + Water Level (time t=130000) Sections 68 to 74
Bed Original
Left Bank
Corrected
New Bed
Water Level
Observed Bed
Evolution
2- River Modeling
2-3- BASEMENT Model
Basement Modeling- Bed Aggradation Analysis

25
295
297
299
301
303
305
307
309
311
313
315
317
319
321
323
325
327
329
2600 2620 2640 2660 2680 2700 2720 2740 2760 2780 2800
Elevation
Bed Morphology + Water Level (time t=130000) Outflow
Bed Original
Left Bank
Corrected
New Bed
Water Level
Observed Bed
Evolution
Bed aggradations and observed bed in critical section (70).
2- River Modeling
2-3- BASEMENT Model
Basement Modeling- Bed Aggradation Analysis

26
2- River Modeling
2-3- BASEMENT Model
Basement Modeling- Calculating Outflow
Q=C*A*sqrt(2*g*h)
C=0.3 (weir Coefficient)
Q=18.53 m3/s

27
2- River Modeling
2-3- BASEMENT Model
Q=C*A*sqrt(2*g*h)
Q=52.41 m3/s

28
2- River Modeling
2-3- BASEMENT Model
Q=C*A*sqrt(2*g*h)
Q=118 m3/s

29
2- River Modeling
2-3- BASEMENT Model
Q=C*A*sqrt(2*g*h)
Q=63.85 m3/s

30
2- River Modeling
2-3- BASEMENT Model
Q=C*A*sqrt(2*g*h)
Q=55.47 m3/s

31
312
313
314
315
316
317
2630 2640 2650 2660 2670 2680 2690 2700 2710 2720 2730
Elevation
Bed evolution & Water Level Out flow
CS70
Left Bank Corrected wes t=90000 wes t=110000
wes t=130000 wes t=150000 talweg t=130000
talweg t=150000
19
52
118
64
55
36
20
0
20
40
60
80
100
120
140
20 30 40 50 60
Discharge(m3/s)
Time (Hr)
Outflow Hydrograph (Section CS70)
2- River Modeling
2-3- BASEMENT Model
Basement Modeling- Outflow result
Q=C*A*sqrt(2*g*h)

32
• Peak = 118 m3/s
• Peak time = 36 hours (8 hours after the peak of inflow)
• Duration = about 40 hours
495
118
0
50
100
150
200
250
300
350
400
450
500
0 5 10 15 20 25 30 35 40 45 50 55 60
Discharge(m3/s)
Time (Hr)
Inflow and Outflow Hydrographs
Outflow in section 70
2- River Modeling
2-3- BASEMENT Model
Basement Modeling- Outflow result

2- River Modeling
2-3- BASEMENT Model
Basement Modeling- Bed and Water level Analysis in CS 70

2- River Modeling
2-3- BASEMENT Model
Basement Modeling- outflow in east part of city from section 70 or to both
sides of the city in Garibaldi bridge (section 74)

3- City Modeling
3-1- Input Data
River2D input data:
• Bed geometry
• Inflow discharge:
Steady model: 118 m^3/s
Transient model: 4-hour hydrograph
• Inlet position:
Case 1: Pedestrian bridge (section 70)
Case 2: Garibaldi bridge (section 74)
• Inflow depth = average 1 m higher than levee
• Outflow depth = 50 m lower than earth surface in order to have dry initial condition
• Groundwater Transmissivity = 0.1 (by low values actual ground water discharge is
negligible)
• Groundwater Storativity = 0.1 (Storativity is a measure of volume of water the ground will
release. The default value is 1 but for accurate transient analysis this value should be
reduced)

3- City Modeling
Scenario 1:
Q left = 50 m3/sec
Q right = 50 m3/sec
Our calculation:
Q max = 118 m3/sec
Q left = west side of the city
Q right = east side of the city

3- City Modeling
Scenario 2:
Q left = 100 m3/sec
Q right = 195 m3/sec
Our calculation:
Q max = 118 m3/sec

3- City Modeling
Scenario 3:
Q left = 125 m3/sec
Q right = 355 m3/sec
Our calculation:
Q max = 118 m3/sec

3- City Modeling
Model 1:
Mesh size: 20 m
Number of nodes: 4362
Number of elements: 7353
QI = 0.078
Model 2:
Mesh size: 40 m
QI = 0.073

3- City Modeling
Model 1: (run for 10 min flood)
Calculation time: 90 min
Lowest water depth
T = 10 min T = 10 min
Q steady = 118 m3/s

3- City Modeling
T = 10 min T = 10 min
Q steady = 118 m3/s

3- City Modeling
Model 3:
Mesh size: 80 m
QI = 0.048
Model 4:
Mesh size: 120 m
QI = 0.003

3- City Modeling
T = 10 min
T = 10 min
Q steady = 118 m3/s

3- City Modeling
T = 10 min T = 10 min
Q steady = 118 m3/s

3- City Modeling
T = 60 min
Mesh size: 120 m
T = 60 min
Q steady = 118 m3/s
Mesh size: 40 m
Almost the same extension of water

3- City Modeling
Mesh Sensitivity results
• The finer the mesh, the higher the calculation time (mesh size 20 m is very time
consuming. But mesh size 40 m has reasonable time for simulation)
• The result in terms of water extension is similar (considering all four models for 10
min run and two models for 60 min run)
• The result for water depth is different. In coarse meshes, water depth is higher.
Therefore results in very coarse meshes are overestimation.
• The result for velocity also shows significant differences. Results in small meshes
are more accurate.
• To have a decent trade-off between calculation time and accuracy, the mesh
size 40 m is recommended. This mesh size is used for next part of the study.

3- City Modeling
3-4- Steady Model
Q = 118 m3/s
T = 10 min T = 20 min
Steady Model (Water depth evolution)
Inlet in the Pedestrian bridge (critical section 70)

3- City Modeling
3-4- Steady Model
Q = 118 m3/s
T = 30 min T = 60 min

3- City Modeling
3-4- Steady Model
Q = 118 m3/s
T = 2 hr T = 3 hr

3- City Modeling
3-4- Steady Model
Q = 118 m3/s
T = 4 hr T = 6 hr

3- City Modeling
3-4- Steady Model
Q = 118 m3/s
T = 8 hr

3- City Modeling
3-4- Steady Model
Q = 118 m3/s
T = 8 hr
Steady Model (Velocity)

3- City Modeling
3-4- Steady Model
Inlet in the Garibaldi bridge position (section 74)
Q = 118 m3/s
T = 5 min T = 10 min

3- City Modeling
3-4- Steady Model
Q = 118 m3/s
T = 20 min T = 30 min

3- City Modeling
3-4- Steady Model
Q = 118 m3/s
T = 40 min T = 50 min

3- City Modeling
3-4- Steady Model
Q = 118 m3/s
T = 60 min

3- City Modeling
3-4- Steady Model
Steady Model (Velocity)
Inlet in the Garibaldi bridge position
Q = 118 m3/s
T = 60 min

3- City Modeling
3-4- Steady Model
Summary
In the case of inlet in section 70 (pedestrian bridge) half of the city will not be
inundated and results in terms of water extension is not compatible with previous
studies.
Considering the inlet position in Garibaldi bridge (Section 74), the extension of water
are more similar to previous studies.
Both cases are probable. In order to have a comparison between our study and old
versions, inlet position in Garibaldi bridge was simulated in the transient model.

3- City Modeling
Transient Model
Using 4 hours of river out-flow hydrograph
4 hours hydrograph

3- City Modeling
Transient Model (Water depth)
T = 0 min T = 5 min

3- City Modeling
T = 10 min T = 20 min

3- City Modeling
T = 30 min T = 40 min

3- City Modeling
T = 50 min T = 60 min (1 hour)

3- City Modeling
T = 120 min (2 hour)T = 90 min (1:30)
Peak discharge

3- City Modeling
T = 240 min (4 hour)T = 180 min (3 hours)

3- City Modeling
Transient Model (Velocity)
T = 240 min (4 hour)

Two scenarios are considering for the Emergency Plan
1- scenario number 3 (extreme case) with Qr = 355 m^3/s from previous studies
2- transient simulation with inlet in Garibaldi bridge with Qpeak = 118 m^3/s
• Two scenarios have significant differences in terms of water extension and depth.
• Max water depth in first scenario is 2.5 m while in the second scenario is limited to 1.5 m.
• Max velocity in first scenario is 4 m/s while in the second scenario is 3.5 m.
• Since our model has only east part of the city, the result for the west part was concluded
from second scenario of the previous studies (Qleft = 100).

Combining water depth results from River2D on the city map
T = 240 min (4 hours)
Transient model
Qpeak = 118 m^3/s

Scenario 3 in previous studies (Qr=355) Our scenario (Qr=118)

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