Flood risk evaluation for Serio river, Italy

Alireza Babaee,
Maryam Izadifar,
River Hydraulics Project
Case Study: River Serio
Ahmed El-Banna,
Svilen Zlatev
Team Members:
June 2014
Prof. Alessio Radice
Eng. Gianluca Crotti
Budiwan Adi Tirta,

Table of Contents
1- Introduction
2- One Dimensional (1D) Modelling
2.1- Theoretical Background
2.2- Modelling Description
2.3- Scenarios
2.4- Ordinary Flow
2.5- Peak Flow
2.6- Unsteady Model
3- Two Dimensional (2D) Modelling
4- Sediment Transport
5- Hazard Evaluation

1- Introduction
Location :
• Serio river flows entirely through
the Lombardy region and has total
length of 125 km and area of the
basin 1250 km2.
• The model under consideration
spans the last 15 km of the river,
from Crema to the confluence with
Adda at Bocca di Serio.
• Average discharge: Q=25 m3/s
• Water used for hydropower and
irrigation.

1- Introduction
Longitudinal Profile:

1- Introduction
Goals of the study:
• The main goal is to model River Serio in 1D and 2D in order to compare
the results obtained from the two models and investigate the influence
on the results coming from each dimensional hypotheses implemented.
• Establish the level of accuracy/suitability of each model in capturing the
real conditions of the river reach.
• Apply theoretical knowledge into practice by using Hec-Ras for 1D
modeling and River 2D for 2D modelling.
• Run steady and unsteady analyses for various flow cases in 1D.
• Calculate Sediment Transport rate for steady flow.
• Identify capabilities, advantages and drawbacks for the purposes of
hazard evaluation with respect to the 1D and 2D models.

1- Introduction
Methodology:
2D
Modelling
1D
Modelling
Steady
Model
Ordinary
Flow
Manning
Sensitivity
BC
Sensitivity
Sediment
Transport
Peak Flow
Manning
Sensitivity
BC
Sensitivity
Sediment
Transport
Unsteady
Model
200 year
hydrograph
Comparison and Analysis of
Advantages/Disadvantages of Models

2- One Dimensional (1D) Modelling
HEC-RAS is an 1D modelling software is based on the Saint Venant
Equations. These equations are based on the following hypothesis:
1. Flow is one-dimensional
2. All the quantities can be described as continuous and derivable functions
of longitudinal position (s) and time (t)
3. Fluid is uncompressible
4. Flow is gradually varied
5. Bed slope is small enough to consider cross sections as vertical
6. Channel is prismatic in shape
7. Flow is fully turbulent

• Saint Venant Equations:

Boundary Conditions
• In case of steady flow, modelling is simple: a constant discharge should be
assign to the entire reach, and a boundary condition for water level which
would be at upstream for supercritical flows or at downstream for
subcritical flows.
• In case of unsteady flow, an initial condition is necessary together with an
upstream boundary condition (usually a discharge hydrograph) and a
second boundary condition which must be upstream for supercritical flows
or downstream for subcritical flow.
Depths
• The critical depth is the depth at which Specific Energy is minimum.
• Uniform depth is when uniform flow is present. This happens when the bed
slope is constant and the slope of the water surface is parallel to it, thus also
the energy grade line EGL is parallel (So=Sw=Sf).

Energy Head Loss
The change in the energy head
between adjacent sections equals
the head loss. The head loss
occurring between two cross-
sections is consisting of the sum
of the frictional losses and
expansion or contraction losses.
The energy head is given by:

Supercritical and Subcritical Flows
 When d < dc, the flow is called
supercritical (velocity larger than that
for critical flow).
 When d > dc, the flow is called
subcritical.
We may have d0 > dc or d0 < dc depending
on the bed slope.
 The channel is mild in the first case
(the uniform flow is subcritical)
 The channel is steep in second (the
uniform flow is supercritical).

 Supercritical and Subcritical Flows
Therefore increase or
decrease of profile can be
predicted (M or S profiles)
Normal Depth d0:
If no quantity varies with the longitudinal direction, the flow is called uniform, and the
momentum equation representing the process is S0= Sf. The depth for which this
happens is called the normal depth.

Reach and Cross-sections
 Modelling was initiated by defining
the main channel for each section thus
establishing the right and left banks,
and the corresponding floodplains.
 Pictures provided for the sections and
Google Earth have been utilized to
help in choosing the boundaries for
the main channel by analyzing given
widths for sections.
 In addition, significant changes in
elevation in the cross-sections have
been considered to indicate the main
channel.

 Cross-sections – Example of Banks
Section 17
Section 15

Manning Coefficient (n):
 The values of roughness for main channel, left and right banks are chosen
according to the topography and given pictures of the sections. The values
of main channel are obtained from the “Verified Roughness
Characteristics of Natural Channels” provided by USGS website.
 As a primary manning coefficient for simulation within the main channel
has been identified n=0.038 whereas for sensitivity analysis n=0.032 and
n=0.041 have been tested as possibilities.
 In peak flow analysis for the floodplains different manning values have
been considered for the three cases according to the table provided by the
software (Hec-Ras Manual).
 It should be noted that for the bridge sections the values of manning for the
left and right banks are the same as for the main channel. Because whole
cross section is considered as main channel.

Manning Coefficient (n) in Main Channel:
USGS  Moyie River at Eastport, Idaho (n = 0.038)
River Moyie
River Serio

USGS  Salt River below Stewart Mountain Dam, Arizona (n = 0.032)
River Salt River Serio

USGS  Middle Fork Flathead River near Essex, Montana (n=0.041)
River Middle Fork River Serio

Manning Coefficient (n) in Flood Plains:
Coefficients for Left and Right bank corresponding to n=0.038

Bridge Section:
 A Bridge in section 6.1 needs to be added. To do this, section to the
upstream has been added, one section to downstream and one section at the
place where the structure is located.
 The distance between this three sections is 10m from the middle section of
the bridge, and the total width of the bridge is 20m.

Limitations Considerations of 1D Modelling
1. Since Hec-Ras is a 1D modelling software, it cannot consider whether
water can move across the main channel to the flood plains or not.
Therefore, if the bed elevation at floodplain is lower than water surface,
Hec-Ras will consider water flows into the flood plains. And in this case
levee should be added to the section.
2. In ordinary flow (Q=25 m^3/sec), there is only one necessity to add levee
in Section 2 and after adding this levee water does not exist in flood plains.
3. The presence of levees is specially required for the steady peak flow and
the unsteady simulation based on 200 years hydrograph.

Adding Levee in Peak Flow (For Example: Section 12)

2.3- Scenarios
Scenario
Code
Flow
Maning
Coefficient of
Main Channel (n)
Upstream Downstream Levees
Deleting Cross
Sections
Description
Sensitivity Analysis of Boundary Condition (Ordinary Flow)
OB1
Ordinary Flow
(25 m^3/sec)
0.038 Critical Normal No No Reference scenario
OB2
Ordinary Flow
(25 m^3/sec)
0.038 Normal Critical No No
OB3
Ordinary Flow
(25 m^3/sec)
0.038 Normal Normal No No
OB4
Ordinary Flow
(25 m^3/sec)
0.038 Normal Water level = 2m No No
Sensitivity Analysis of Roughness (Ordinary Flow)
OM1
Ordinary Flow
(25 m^3/sec)
OM2
Ordinary Flow
(25 m^3/sec)
0.032 Critical Normal No No
OM3
Ordinary Flow
(25 m^3/sec)
0.041 Critical Normal No No
Adding Levees (Peak Flow)
PL1
Peak Flow
(560 m^3/sec)
PL2
Peak Flow
(560 m^3/sec)
0.038 Critical Normal Yes No

2.3- Scenarios
Scenario
Code
Flow
Maning
Coefficient of
Main Channel (n)
Upstream Downstream Levees
Deleting Cross
Sections
Description
Deleting Useless Cross Sections (Peak Flow)
PC1
Peak Flow
(560 m^3/sec)
0.038 Critical Normal Yes No Reference scenario
PC2
Peak Flow
(560 m^3/sec)
0.038 Critical Normal Yes Yes
Sensitivity Analysis of Boundary Condition (Peak Flow)
PB1
Peak Flow
(560 m^3/sec)
0.038 Critical Normal Yes Yes Reference scenario
PB2
Peak Flow
(560 m^3/sec)
0.038 Normal Critical Yes Yes
PB3
Peak Flow
(560 m^3/sec)
0.038 Normal Normal Yes Yes
PB4
Peak Flow
(560 m^3/sec)
0.038 Normal Water level = 6 m Yes Yes
Sensitivity Analysis of Roughness (Peak Flow)
PM1
Peak Flow
(560 m^3/sec)
0.038 Critical Normal Yes Yes Reference scenario
PM2
Peak Flow
(560 m^3/sec)
PM3
Peak Flow
(560 m^3/sec)

2.4- Ordinary Flow
1. The discharge for the ordinary flow is 25 m3/s.
2. The boundary conditions for the river depend on the nature of the flow. In
the case of subcritical flow, we have to input just downstream condition
and for supercritical flows, just upstream condition is needed.
3. Downstream normal depth and upstream critical depth was chosen as
reference scenario.
Results is shown in the following:
Movie name: 01-Ordinary flow sections.avi

2.4- Ordinary Flow

2.4- Ordinary Flow
3D View (No Flood):

2.4- Ordinary Flow
Bridge Section:
Flow depth variation is occurring due to contraction. Flow is subcritical
therefore water table decrease due to contraction and tends to critical depth

2.4- Ordinary Flow
Bridge Section:
 Based on the manual of Hec-Ras contraction and expansion coefficients
have been changed to 0.3 and 0.5 respectively in bridge sections.
 Then Using XS interpolation tool and adding cross sections with 10 m
space the profile became more realistic in this part.

2.4- Ordinary Flow
Sensitivity Analysis for Boundary Condition
To check the sensitivity of the results with respect to the boundary conditions, 4
sets of boundary conditions are considered and their results are compared:
1. Upstream Critical and Downstream Normal (S=0.0015) (referenced scenario)
2. Upstream Normal and Downstream Critical flow
3. Upstream Normal and Downstream Normal flow (S=0.0015)
4. Upstream Normal and Downstream fixed water surface elevation: 2 m depth

2.4- Ordinary Flow
Results:
1. Changing upstream condition (for example from critical depth to normal
depth) has no effect in water profile because water level is over critical depth
in whole reach (subcritical flow) and only in supercritical upstream could be
add as a boundary condition.
2. Therefore downstream is boundary condition in this project (subcritical
flow) and by changing the situation water level changes. Variation of water
level is started from almost 600 meters before end point.
3. In case 2 (down stream=critical depth) and case 4 (down stream = 2 m water
depth), we have same result and profile reaches to critical depth linearly
which need refinement.

2.4- Ordinary Flow
Results:
Below are shown the different cases taken into account and their relevance to the
water depth

2.4- Ordinary Flow
Results: Velocity Conditions (Hec-Ras Plot)
Dramatic increase in water velocity in cases 2 and 4 (water level tends to critical depth)

2.4- Ordinary Flow
Results: Water elevation and velocity
44
46
48
50
52
54
56
58
60
62
64
0 2000 4000 6000 8000 10000 12000 14000
DS:Normal US:Critical DS:Critical US:Normal DS:Normal US:Normal DS:Level=2m US:Normal
0
0.5
1
1.5
2
2.5
3
0 2000 4000 6000 8000 10000 12000 14000
DS:Normal US:Critical DS:Critical US:Normal DS:Normal US:Normal DS:Level 2m US:Normal
Only changes in
vicinity of last
section
(Downstream)

2.4- Ordinary Flow
Additional Note:
• M2 profile was expected in the last section when down stream is considered as
critical depth but the profile is linear.
• Based on the warning of cross section output we need to add more sections
before the last section to reach realistic profile and this is due to high variation
of velocity.

2.4- Ordinary Flow
Case 2 (Down stream = critical)
 In order to reach M2 profile XS Interpolation tool was used. To reach best
result the distance was selected equal to 10 m.
After adding section by XS Interpolation
M2 Profile
Before adding section

2.4- Ordinary Flow
Case 4 (Down stream = 2m)
 In this case M1 profile was expected. Because 2m water level is higher than
normal depth (according to water level height in last section of case 1 and 3),
but the profile is ended to critical depth.
 The reason according to HEC-RAS manual is that when a known profile
depth is selected an error in the vicinity of B.C. (downstream in this case)
happens.
 In order to solve the error in subcritical flows additional cross sections should
be added to the downstream of last section. (After last section)
 Therefore section 0 was created with same geometry of section 1 and with 200
meter distance considering the average slope (0.15%). Then using XS
Interpolation additional sections were added by 10 m distance.

2.4- Ordinary Flow
Case 4 (Down stream = 2m)
Before adding sectionAfter adding section 200 m after
downstream and XS Interpolation
Almost 2m in
Section 1

2.4- Ordinary Flow
Sensitivity Analysis for Roughness
 In applying the Manning formula the greatest difficulty lies in the
determination of the roughness coefficient n; there is no exact method of
selecting the n value. In the present study, comparison with similar system of
the other rivers has been carried out based on the database: “Verified
Roughness Characteristics of Natural Channels” provided by USGS website.
 In ordinary flow water exists only in main channel therefore the simulation is
only dependent on the manning coefficient of main channel and not left and
right banks. Therefore for flood plains same manning coefficient has been
chosen.
 To study the roughness sensitivity, the manning coefficients are once
increased from n=0.038 (reference coefficient) to n=0.041 and once
decreased from n=0.038 to 0.032
The roughness sensitivity is evaluated regarding two aspects:
1. Water surface elevation
2. Velocity

2.4- Ordinary Flow
Roughness sensitivity analysis on water surface elevation
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000
m
Sensitivity Analysis by changing the value of Manning's coefficient
b.c.: Normal depth at downstream(s=0.15%) and Q=25 m3/s (ordinary flow) at upstream
The difference in water profile by increasing n from 0.038 to 0.041
The difference in water profile by reducing n from 0.038 to 0.032
Bridge at section 6-1

2.4- Ordinary Flow
It is expected that increasing manning coefficient will reduce velocity of flow thus for
increase of n, increase in water depth is anticipated and vice versa.
In the location of bridge section, It is clear that regardless of the magnitude of the
manning coefficient, the water surface converges to the same level for all conditions.
This may indicate that in this specific location, due to the contraction caused by the
bridge piers, a critical condition has occurred. Checking the Fr =1 in this location
verifies this. The results obtained from the Hec-Ras model verifies that the flow is
subcritical before and after this location while, when the flow reaches the bridge, the
flow is critical.
In this case, the difference of water level by increasing n from 0.038 to 0.041 is 7 cm;
The difference of water level by reducing n from 0.038 to 0.032 is 15 cm;

2.4- Ordinary Flow
44
46
48
50
52
54
56
58
60
62
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000
Elevation
Kilometering of the reach
Bed level Bridge at section 6-1 Water Profile Elevation - n=0.032
Water Profile Elevation - n=0.038 Water Profile Elevation - n=0.041

2.4- Ordinary Flow
Roughness sensitivity analysis on velocity along the river
-0.25
-0.15
-0.05
0.05
0.15
0.25
0.35
0.45
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000
m/s
The difference in velocity by increasing n from 0.038 to 0.041
'Bridge at sectiob 6-1'

2.4- Ordinary Flow
Roughness sensitivity analysis on velocity along the river
• Manning coefficient has effects on velocity. Regardless of the velocity
changes along the river, it is clear that the model with the lowest manning
coefficient has the highest velocity and vice versa.
• In this case, the maximum difference of velocity by increasing n from 0.038
to 0.041 is 0.2 m/sec;
• The maximum difference of velocity by reducing n from 0.038 to 0.032 is
0.42 m/sec;
• In general, it can be seen that the difference of water level and velocity is
small.

2.5- Peak Flow
Adding Levees
• Average discharge: 560 m3/s
(Based on peak value of 200-year hydrograph)
• 13 sections need adding levees to control flood plain.
• Two different criteria were used to select the position of levees:
1. If bed elevation at floodplain is lower than water surface, Hec-Ras will
consider water flows into the floodplains. In this case levee should be
added to the section.
2. Considering the probable flood route in plan and adjusting levees
positions.

2.5- Peak Flow
Adding Levees
• Section 20
Before Adding Levee
After Adding Levee
Note: No need to add levee in section 19

2.5- Peak Flow
Adding Levees
• Section 18
Before Adding Levee
After Adding Levee
Flooded Area

2.5- Peak Flow
Adding Levees
• Section 17
Before Adding Levee
After Adding Levee
Flooded Area

2.5- Peak Flow
Adding Levees
• Section 16
Before Adding Levee
After Adding Levee
Flooded Area
Note: No need to add levee in section 15.
Note: Section 15.1 will be deleted (next part)

2.5- Peak Flow
Adding Levees
• Section 14
Before Adding Levee
After Adding Levee
Flooded Area

2.5- Peak Flow
Adding Levees
• Section 13
Before Adding Levee
After Adding Levee
Flooded Area
Note: Section 12.1 will be deleted

2.5- Peak Flow
Adding Levees
• Section 12
Before Adding Levee
After Adding Levee
Flooded Area

2.5- Peak Flow
Adding Levees
• Section 11
Before Adding Levee
After Adding Levee
Flooded Area
Note: No need to add levee in sections 10 and 9
Note: Sections 8-2 and 8-1 will be deleted

2.5- Peak Flow
Adding Levees
• There is no need to add levees in
sections 8 and 7, because these
sections are completely flooded.
• Section 6.1 is bridge and does not
need levee.

2.5- Peak Flow
Adding Levees
• Section 5
Before Adding Levee
After Adding Levee
Flooded Area
Note: No need to add levee in sections 6

2.5- Peak Flow
Adding Levees
• Section 4
Before Adding Levee
After Adding Levee

2.5- Peak Flow
Adding Levees
• Section 3
Before Adding Levee
After Adding Levee
Note: No need to add levee in sections 2.1

2.5- Peak Flow
Adding Levees
• Section 2
Before Adding Levee
After Adding Levee
Flooded Area

2.5- Peak Flow
Adding Levees
• Section 1
Before Adding Levee
After Adding Levee
Flooded Area

2.5- Peak Flow
Omitting Cross Sections
 Sections 15.1, 12.1, 8.2, 8.1 are too
narrow and flood plain is wider than
the sections, therefore these sections
shall be deleted to avoid simulating
false flood route.

2.5- Peak Flow
Sensitivity analysis in case of adding levees and omitting sections
 In the case of the 13 sections with adding levees and 4 deleted sections,
the water surface elevation and velocity at the sections upper to the
deleted sections and along the river reach are compared.
 Since the flow is subcritical, and subcritical flows need downstream
boundary conditions, the effect of changing the geometry would be on
the upper sections.

2.5- Peak Flow
44
46
48
50
52
54
56
58
60
62
64
66
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000
Elevation
Bed level
Water Profile Elevation - Complete sections- With levee
Water Profile Elevation - complete scetions without levee
Water Profile Elevation - Reduction section with levee

2.5- Peak Flow
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000
m
Effect of adding the levee,
b.c.: Normal depth at downstream(s=0.15%) and Q=560 m3/s (peak flow) at upstream
Bridge at section 6-1 Difference in water profile by adding levee

2.5- Peak Flow
-1
-0.5
0
0.5
1
1.5
2
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000
m/S
Effect of adding the levees,
'Bridge at sectiob 6-1' Difference in velocity by adding levee

2.5- Peak Flow
 The changes in water surface elevation along the channel due to adding
levees are almost notable especially in sections 19 and 18 with 40 cm rise
and in sections 5 and 2.1 with 50 cm rise.
 Deleting cross section is led to water level fall in upstream of the omitted
sections and this is due to the flow regime (subcritical).
 In the other hand there are reduction in velocity in sections 19, 5 and 2.1
and this is due to water level increasing and consistency of discharge
(steady flow)
 The maximum increasing in water velocity is occurred in section 4 and 2
with 0.7 m/s and 1.2 m/s respectively.

2.5- Peak Flow
Result of the corrected geometry (adding levees and deleting sections)
Movie name: 02-Peak flow steady sections.avi

2.5- Peak Flow
• Longitudinal Profile of the corrected geometry (adding levees and
deleting sections)

2.5- Peak Flow
• 3D View (Flooded Areas) of the corrected geometry (adding levees
and deleting sections)

2.5- Peak Flow
 To check the sensitivity of the results with respect to the boundary conditions,
4 sets of boundary conditions are considered and their results are compared:
1. Upstream critical and Downstream Normal (S=0.0015) (referenced scenario)
2. Upstream Normal and Downstream Critical flow
3. Upstream Normal and Downstream Normal flow (S=0.0015)
4. Upstream Normal and Downstream fixed water surface elevation: 6 m depth

2.5- Peak Flow
Results:
1. Changing upstream condition (for example from critical depth to normal
depth) has no effect in water profile because water level is over critical depth
in whole reach (subcritical flow) and only in supercritical upstream could be
add as a boundary condition.
2. Therefore downstream is boundary condition in this project (subcritical
flow) and by changing the situation water level changes.
3. The fluctiation of water profile and velocity have been limited to the reach
located approximately 1.5 km from the downstream section or only three
sections downstream were effected.
4. In case 2 (down stream=critical depth) and case 4 (down stream = 6 m water
depth), we have same result and profile reaches to critical depth linearly
which need refinement.
5. Refinement should be done with same way as ordinary flow by XS
Interpolation in case 2 and adding a new section after downstream in case 4.

2.5- Peak Flow
Longitudinal profile due to change of down stream boundary condition:

2.5- Peak Flow
Results: Water elevation and velocity
44
46
48
50
52
54
56
58
60
62
64
66
0 2000 4000 6000 8000 10000 12000 14000
DS:Normal US:Critical DS:Critical US:Normal DS:Normal US:Normal DS:Level=6m US:Normal
1
1.5
2
2.5
3
3.5
4
4.5
5
0 2000 4000 6000 8000 10000 12000 14000
DS:Normal US:Critical DS:Critical US:Normal DS:Normal US:Normal DS:Level 6m US:Normal
Only changes in
vicinity of last section
(Downstream)

2.5- Peak Flow
 The lower the manning coefficient is, the lower is the water surface elevation
and vice versa. And conversely occurs for velocity.
 A clear difference between this case with the one corresponding to the ordinary
flow is that, the peak flow condition is more sensitive to manning variation
when compared to the ordinary flow.
 To study the roughness sensitivity, the manning coefficients in main channel
are once increased from n=0.038 (reference coefficient) to n=0.041 and once
decreased from n=0.038 to 0.032. Also corresponding manning coefficient for
left and right banks was defined in the geometry of cross sections.
The roughness sensitivity is evaluated regarding two aspects:
1. Water surface elevation
2. Velocity

2.5- Peak Flow
44
46
48
50
52
54
56
58
60
62
64
66
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000
Elevation
Bed level Bridge at section 6-1 Water Profile Elevation - n=0.032
Water Profile Elevation - n=0.038 Water Profile Elevation - n=0.041

2.5- Peak Flow
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000
meter
b.c.: Normal depth at downstream(s=0.15%) and Q=560 m3/s (peak flow) at
upstream
The difference in water profile by increasing n from 0.038 to 0.041

2.5- Peak Flow
Roughness sensitivity analysis on velocity
-0.5
-0.3
-0.1
0.1
0.3
0.5
0.7
0.9
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000
meter/s
The difference in velocity by increasing n from 0.038 to 0.041
'Bridge at sectiob 6-1'

2.5- Peak Flow
Results:
 In this case, the difference of water level and velocity by increase n from 0.038
to 0.041 is up to 24 cm and 0.7 m/sec respectively.
 The difference of water level and velocity by reducing n from 0.038 to 0.032
is 42 cm and 0.3 m/sec respectively.
 The difference of water profile and velocity by changing n, is much more
higher in the case of peak flow, but still relative small for a flood analysis.
 In general it can be concluded the decision to use the Manning coefficient
based on the comparison of the other river systems, which have similarity to
those of river Serio is acceptable.

80
Unsteady model for 200-year Hydrograph
In unsteady modeling, all the parameters from previous models are used,
except for the levees, which modification was done in sections 2 and 3.
1. Model conditions
 Boundary condition
Upstream: 200-year hydrograph
Downstream: normal depth with slope of 0.0015
 Initial condition
Initial discharge
2.6- Unsteady Model

81
Unsteady flow data
The original dataset is interpolated with 60 minute time interval . This time
interval is small enough with respect to the whole event history.
Upstream Hydrograph
2.6- Unsteady Model

2.6- Unsteady Model
Movie name: 03-Unsteady longitudinal.avi

2.6- Unsteady Model
3D View (Flooded Areas):
Movie name 04-Unsteady 3D.avi

Comparison: Steady and unsteady elevation analaysis (at max. water profile)
b.c.: Normal depth at downstream(s=0.15%) and Q=560 m3/s (max), n=0.038
2.6- Unsteady Model

Unsteady longitudal profile after XS Interpolation:
2.6- Unsteady Model

Comparison: Steady and unsteady discharge analaysis (at max. water profile)
2.6- Unsteady Model

Comparison: Steady and unsteady velocity analaysis (at max. water profile)
2.6- Unsteady Model

• In general the difference between unsteady and steady flow analysis
becomes severe when the flood wave is very long with respect to the
profile of the channel.
• But in this case the lost of Q in unsteady flow is not so much,
approximately 6%, because the channel profile is short, i.e. water profiles
are not much different.
• For long reach, it could be also important to have information about wave
propogation considering flood hazard analysis, i.e. early warning system.
• Since we are dealing with unsteady condition in flood condition, it is
always preferabel to use unsteady analysis, but in this case results of steady
and unsteady are very similar.
• Due to the uncertainty in the boundary condition downstream, the
fluctuation of water profile has been found only 2.1 km from downstream,
which was locaten on reach where there is no exposure.
• This fluctuation in downstram was decreased using XS Interpolation.
2.6- Unsteady Model

• Theoretical Background

The numerical formulation of 2D river modelling was originated from the
analysis of shallow water. The main outputs of the 2D model are two water
velocity components and a vertical water depth for each defined node.
Basically, the output of the program is generated by the solution of the mass
conservation equation and the two momentum conservation equations.

The 2D model depth averaged, mass and momentum conservation equations are:
The bed shear stress are computed by:
The turbulent normal and shear stresses are computed according to the
Boussinesq’s assumption as:

• Benefits:
 Ability to model more complex flows including
floodplain and underground flows
 Ability to consider impact of obstructions.
 No need to force the geometry to be appropriate
for modelling
• Limitations:
 Results are limited by the accuracy of the
assumptions, input data and the computing
power of the computer program.
 Modeling complexity and precision are not a
substitute for sound engineering judgment

Comparing the results of 2-D with 1D
Since River 2D results 2 values for
velocity along the X and Y axes, and
computes the water depth at each node,
it is not possible to have single
longitudinal profiles for velocity and
water surface for the river.
Therefore, the results are compared
section by section

Comparison between 1D & 2D analysis
Steady flow : section #8
-1
1
3
5
7
9
40
45
50
55
60
65
70
75
0 100 200 300 400 500 600 700 800 900 1000
Velocity,m/s
Elevation,m
Station, m
Bed level - 2D Water Elevation -2D Bed level - 1D Water Elevation - 1D Velocity - 1D Velocity - 2D

-1
1
3
5
7
9
40
45
50
55
60
65
70
75
0 100 200 300 400 500 600 700 800 900 1000
Velocity,m/s
Elevation,m
Station, m
Bed level - 2D Water Elevation -2D Bed level - 1D
Water Elevation - 1D Velocity - 1D Velocity - 2D

-1
1
3
5
7
9
40
45
50
55
60
65
70
75
0 100 200 300 400 500 600 700 800
Velocity,m/s
Elevation,m
Station, m
Bed level - 2D Water Elevation -2D Bed level - 1D
Water Elevation - 1D Velocity - 1D Velocity - 2D

-1
1
3
5
7
9
40
45
50
55
60
65
70
75
0 100 200 300 400 500 600 700 800
Velocity,m/s
Elevation,m
Station, m

99
-1
1
3
5
7
9
40
45
50
55
60
65
70
75
-20 80 180 280 380 480 580 680 780 880 980
Velocity,m/s
Elevation,m
Station, m

-1
1
3
5
7
9
40
45
50
55
60
65
70
75
-20 80 180 280 380 480 580 680 780 880 980
Velocity,m/s
Elevation,m
Station, m

0
1
2
3
4
5
6
7
8
9
10
40
45
50
55
60
65
70
75
-50 50 150 250 350 450 550 650 750 850 950
Velocity,m/s
Elevation,m
Station, m

0
1
2
3
4
5
6
7
8
9
10
40
45
50
55
60
65
70
75
0 100 200 300 400 500 600 700 800
Velocity,m/s
Elevation,m
Station, m

0
1
2
3
4
5
6
7
8
9
10
55
57
59
61
63
65
67
0 10 20 30 40 50 60 70
Velocity,m/s
Elevation,m
Station, m
Bed level - 2D Water Elevation -2D Bed level - 1D Water Elevation - 1D
Velocity - 1D Velocity - 2D Velocity - 2D

• General Comments
The differences in the values of velocity obtained by
the two software are because:
 River2D considers two components for velocity (in
X direction and Y direction), but Hec-Ras
considers only velocity for each section along the
channel (so perpendicular to the cross sections).
 In 2D modelling, lateral stresses are also
considered while in the 1D modelling only friction
losses are considered.
 Therefore, there is only one values for velocity in
1D, however in 2D, velocity varies along the
section and usually increase in main channel and
decreases in flood plains.

4- Sediment Transport
•Waters flowing in natural streams and rivers have the ability to scour channel
beds, to carry particles (heavier than water) and to deposit materials, hence
changing the bed topography.
•We need to estimate the sediment transport rate because this phenomena has a
great impact of economy e.g. (prediction of the scouring of the bridges).
•Failure of estimating the sediment transport may cause some disasters
(bridges collapse, destruction of banks and levees)

•The term of sediment transport defines the motion of materials.
•The transported load is called sediment load, which can be divided into two
categories:
1- bed load: which defines the grains rolling along the river.
2-suspended load: which defines grains maintained in suspended by
turbulence.
Properties of Sediment Transport:

Property of a single particle and ds
Relative density:
• Utilized: s= 2.6 (ρs is taken as 2600 kg/m^3)
Sieve Diameter (ds):
• The size of the particle which passes through a square mesh
sieve of a given size not through the next smallest sieve.

Settling Velocity w0
d50= 0.015 m
d90= 0.027 m
g= 9.81 m/s2
rho= 1000 kg/m3
rhoS= 2600 kg/m3
s= 2.6
1.2
τ* crit= 0.05
Profile chosen:
0.000001007
d10= 0.004 m
Input Information Peak
Suspended Load Threshold=
Bed load Threshold for incipient motion:
kinematic viscosity of water:
Experimental Settling V for
still water (20°) and d50 (m/s)
0.47
DS:Critical-BC, n=0.038
0.62
0.24
d50= 0.015 m
d90= 0.027 m
g= 9.81 m/s2
rho= 1000 kg/m3
rhoS= 2600 kg/m3
s= 2.6
1.2
τ* crit= 0.04
Profile chosen:
0.000001007
d10= 0.004 m
Input Information Ordinary
Suspended Load Threshold=
DS:Critical-BC, n=0.038
kinematic viscosity of water:
Bed load Threshold for incipient motion:
0.47
0.62
0.24

Shields Number
• Re* is checked along the river thus for ordinary
flow = 0.04 is chosen and for peak =0.05

Bed and Suspension Load Occurrence
Bed Load Occurrence
V*/w0 > 0.2 – 2 or
Suspension Load Occurrence

Sediment Transport
Total Load: None from
our results
Bed Load:
Transport Capacity Φ
Sediment discharge

Ordinary Flow – Shields Number

0.0001
0.001
0.01
0.1
1
10
012345678910111213141516
Kilometering of Reach
Shields Number (d10,d50 and d90) - Ordinary Flow - PF2
t*(d50) t*(d10) t*(d90) t*c(min) t*c(max) t*c(chosen) Bridge
• The plot shows there is no bed load motion in most of
the sections for grain sizes d50 , d90 .
• The motion happened for the grain size of d10 almost
in all sections.
Shields Number along the River

Friction/Settling Velocity along the River
V*/w0 = 1.2

• The curve shows that there is no suspended load
because the chosen value of the ratio between v*/w0 is
larger than the actual value between v*/w0 for all of the
grain size diameter.
0.01
0.1
1
10
012345678910111213141516
Friction/Settling Velocity for Ordinary Flow - PF2
V*/w0 (d10) V*/w0 (d50) V*/w0 (d90) V*/w0(max) V*/w0 (min) V*/w0 (chosen) Bridge

0.0001
0.001
0.01
0.1
1
012345678910111213141516
Critical Diameter for Bed Load - Ordinary Flow - PF2
d_crit d_10 d_50 d_90 Bridge
No Bed Load, Below Line
Bed Load, Above Line
• According to the threshold of the bed load ds < ( ds )c ,
all of the section below the line of the grain size lines
d10 , d50 , d90 will not have bed load and all of the section
above the grain size lines will have bed load transport.
Critical Diameter for Bed and Suspended Load

0.0001
0.001
0.01
0.1
1
012345678910111213141516
Critical Diameter for Suspension Load - Ordinary Flow - PF2
No Suspension Load, Below Horizontal Lines
Suspension Load, Above Horizontal Lines
• Since all of the critical grain size diameter ( ds )c at
all sections are lower than the grain size diameter
ds > ( ds )c for all the diameters, there is no
suspended load occurring in that case.

0.0001
0.001
0.01
0.1
1
012345678910111213141516
Comparison of Critical Diameters - Ordinary Flow - PF2
d_crit_susp d_crit_bed d_10 d_50 d_90 Bridge
• In that case we can see only the occurrence
of bed load for the different sizes

• If V*/w0 is minimized to 0.3 then no physical
meaning of results is achieved as suspended
load is occurring before bed load
0.0001
0.001
0.01
0.1
1
012345678910111213141516
Comparison of Critical Diameters - Ordinary Flow - PF2

0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
012345678910111213141516
Φ - Bed Load Transport Capacity using Einstein Formula - Ordinary Flow - PF2
d10 d50 d90 Bridge
Bed Load Transport Capacity Equations
• The capacity for d10 is large in comparison to d50
and d90, so this should mean larger sediment
discharge for d10. However in most cases for d50
and d90 there is no bed load thus capacity is 0.

0
0.0005
0.001
0.0015
0.002
012345678910111213141516
qs Bed Load sediment discharge (m^2/s) using Einstein Formula - Ordinary Flow
qs_d10 qs_d50 qs_d90 Bridge
• These results support transport capacity graph,
however it can be seen that for d50 the qs is close
to d10. Because the diameter inserted in the qs
formulae where for d10 is very small in comparison
to d50.
• Again most sections for d50 and d90 have 0 value.

0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
012345678910111213141516
qs comparison between Formulae for sediment discharge (m^2/s) for d50 - Ordinary Flow
Nielsen Einstein Meyer-Peter van Rijn Bridge
• Nielsen formula yields very high results with
respect to other formulae. Also in sections with no
bed load Nielsen is close to 0.
• For results are identical to Meyer-Peter

0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
012345678910111213141516
qs Comparison between Formulae for sediment discharge (m^3/s) for d50 - Ordinary Flow
Nielsen Einstein Meyer-Peter Bridge
• Using top width of wetted channel from Hec-Ras

0.001
0.01
0.1
1
10
012345678910111213141516
Kilometering of the Reach
Shields Number (d10,d50 and d90) - Peak Flow - PF2
t*(d50) t*(d10) t*(d90) t*c(min) t*c(max) t*c(chosen) Bridge
• We can see that the bed load increased for all
sections for each grain size, sediment of grain size
d10 , almost at all of the sections.
Shields Number along the River

• No suspended load in this case
0.01
0.1
1
10
012345678910111213141516
Friction/Settling Velocity for Peak Flow - PF2
V*/w0 (d10) V*/w0 (d50) V*/w0 (d90) V*/w0(max) V*/w0 (min) V*/w0 (chosen) Bridge

0.001
0.01
0.1
1
012345678910111213141516
Critical Diameter for Bed Load - Peak Flow - PF2
Bed Load, Above Line
• According to the threshold of the bed load ds < ( ds )c , all of the
section below the line of the grain size lines d10 , d50 , d90 will not
have bed load and all of the section above the grain size lines
will have bed load transport.
• For d10 all sections have bed load and for d90 only 4 sections
have.

0.0001
0.001
0.01
0.1
1
012345678910111213141516
Critical Diameter for Suspension Load - Peak Flow - PF2
• Since all of the critical grain size diameter ( ds )c at all
sections are lower than the grain size ds > ( ds )c ,
there is no suspended load occurring in that case
apart from last section.

0.0001
0.001
0.01
0.1
1
012345678910111213141516
Comparison of Critical Diameters - Peak Flow - PF2

0.001
0.01
0.1
1
012345678910111213141516
Comparison of Critical Diameters - Peak Flow - PF2
• If V*/w0 is minimized to 0.3 then no physical
meaning of results is achieved as suspended
load is occurring before bed load

0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
012345678910111213141516
Φ Bed Load Transport Capacity using Einstein Formula - Peak Flow - PF2
d10 d50 d90 Bridge
• In this case, there is bed load for all sections
in d10 and almost all in d50. Although
capacity for d50 looks like 0 it is not.

0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
0.005
012345678910111213141516
qs Bed Load sediment discharge (m^2/s) using Einstein Formula - Peak Flow
d10 d50 d90 Bridge
• In this case same notes as for the ordinary flow
graph are valid. However, it can be seen for last
section how d50 and d90 exceed the discharge of
d10, this because of the diameter.
• But this relation is not valid in our case as the last
section of Hec-Ras has not not exact results in
terms of velocity because of boundary condition.

0
0.005
0.01
0.015
0.02
012345678910111213141516
Comparison between Formulae (d50) sediment discharge qs (m^2/s) - Peak Flow
Nielsen Einstein Meyer-Peter van Rijn Bridge
• Same relation as with ordinary flow is seen here.
Nielsen is overestimating results with respect to
other empirical formulae.
• For results are identical to Meyer-Peter

0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
012345678910111213141516
Comparison between Formulae (d50) sediment discharge (m^3/s) - Peak Flow
Nielsen Einstein Meyer-Peter Bridge
Using top width of wetted channel from Hec-Ras

This chapter is focus on discussion different capabilities, advantages and
disadvantages of HEC-RAS 1D and River2D considering the purpose of
hazard evaluation.
•(5a) Hazard definition;
•(5b) Evaluation of hazard by means of HEC RAS - Steady and Unsteady flow
and River2D in the case of River serio;
•(5c) Pros and Cons of HEC RAS- Steady /Unsteady flow and River2D;
•(5d) Application of the results of River 2D in the purpose of hazard evaluation
in the case of River serio;

(5a) Definition of hazard
 The risk is the function of hazard, vulnerability and exposure (and sometime
resilience). So hazard is one fundamental element of risk in general. Our focus is only
in Hazard as particular element of risk;
 The hazard is the probability of occurrence of a particular event within a given time
period/geographic space;
 Hazard analysis is about knowing the phenomena very well, fully and clearly. It is
important to give answers on the fundamental questions; How strong?, When?, Where?
and How long it lasts? In fact the first and second questions are about return period and
probability analysis are attached;
 In risk analysis, it is typically deterministic model when a specific scenario is
chosen. In this study the scenario of 200 years hydrograph has been concerned.

How strong?
 To define the severity of the flood, the peak discharge is considered. The consequence of
the peak discharge would be inundation due to the flood in term of water level and the
velocity of the flood. There are also other variables such as rate of rise of the flood
water, duration of the flood, sedimentation, contaminant load and also water availability
(relates to water level of course).
 In this study variables of hazard are concerned are water level, velocity and
sedimentation;
 In the following figure shows one example of the correlation of velocity and water level
to hydrodynamic load to evaluate the hazard of bridge in Italy . When the return period
is increase then the considered peak discharge is also increase, therefore the velocity and
inundations area are also expected.

When?
 Before the event
– Do we able to predict the event? For case of flood an event at giving return
period can be expected. In this study, return period of 200 years is considered. A
probability with return period of 200 year means that occurrence of an event is
once with percentage of 0.5% in terms of probably.
 Emergency time and the flood.
– It is the true time of a certain event. Its value derives from flood forecasting
which aims at anticipating the occurrence of a flood in a given area within a
certain advance .The lead time (the time span between the forecasting and the
event) depends on a lot of factors (catchments size, analysis methods, quality of
data, etc.)

Where?
 It relates to inundation, which can be shown in a map displaying areas prone to be
flooded. It represent an estimate of where the water would flow if water level
exceeds river banks;
 Inundation areas depend on the intensity of the floods that is its return period. Again:
When the return period is increase then the considered peak discharge is also
increase, therefore the velocity and inundations area are also expected.

HEC RAS 1D – Steady flow
 Strengths:
• Simple numerical model with vey simple boundary conditions, which are time independent) are required.
Therefore input data is simple. Lowest computation costs compare to the other models;
• Suit for ordinary flow analysis;
• Not so difficult to control and interpret the result of the analysis. In 1D modeling the velocity at each
section is calculated based on the main value.
 Weaknesses:
• Considering flood analysis, when the reach of the river is to long where the loss of the discharge is to
much, the result would not be so realistic. Moreover in flood, the boundary condition downstream is time
varied where it is time independend. So it is preferable to use unsteady one in this case;
• The steady analysis with peak flow can be used to calculate the maximum free water profile except in
case of lateral discharge from the channel to detention basin, diversion channel and overflow.
• No information about the propagation, therefore it is not suitable for evaluation of hazard concerning
early warning system.
 Opportunities:
• When the reach of river is short so that the loss the discharge is considered low, then it results close to
that of the unsteady analysis. It may happen due to the fact that the length of the wave is very long with
respect to the channel reach which is very short.
• It is easy to treat the output as input of the analysis to do preliminary study of sedimentation in case of
short reach;
(5b) Evaluation of Hazard using HEC RAS and River2D

HEC RAS 1D – Unsteady flow
 Strengths:
• Simple numerical model with not so complex boundary conditions are required. Only the boundary
condition at upstream is in function of time (i.e. Hydrograph) Therefore input data is simple. Low
computation costs;
• Not so difficult to control and interpret the result of the analysis. In steep slope results looks like that of
the kinematic wave. In mild slope result would look close to that of parabolic wave.
• Able to calculate the maximum free water profile instead of the unsteady analysis except in lateral
discharge from the channel to detention basin, diversion channel and overflow;
• The wave propagation can be evaluated. This model can be used for hazard evaluation concerning early
warning system.
 Weaknesses:
• Not consistent considering the theory. When we consider the case of supercritical-unsteady, the
boundary condition at downstream must be set. In this case the result would more difuse. In case of
subcritical-unsteady, the boundary condition at downstream will be useless.
• Although it ables to model the inundation appropiatly but the limitation is absence of the lateral
velocity;
 Opportunities:
• It is also possible to model appropiate inundation area by considering ‘water storage’ area in the
analysis. So the performance of the 1D analysis can be upgraded somehow close to 2D analysis.

River 2D
-Strengths:
– Able to model the lateral velocity, rate of flood and duration of
the flood which are almost not possible to do in 1D model.
Moreover in the cases like island, bypasses and also lateral mass
exchange are easy to solve and to show the real behavior for
stream lines.
–In 2D analysis, the maximum velocity occur near to the surface at
position of maximum depth at section in straight channel and not
in the case of bend channel. The following figures shows the real
velocity distribution in the cross section with different condition of
the channel as percent of maximum velocity
- Weaknesses:
- Complex numerical model with complex boundary conditions. The geometrical input data is very complex and must be
supported with adequate measurements. The highest computation costs compare to the other models;
- Difficult to control and interpret the result of the analysis, since in most case the error is very large in the area close to the
boundary;
- Opportunities:
- This model is much more demanding when the evaluation of the hazard is taking place in complex area for instance flood
in urban area, or long river with alot of bend.
- Ables to study the sedimentation in long period of time.

(5c) Pros and Cons of HEC RAS- Steady /Unsteady flow and River2D
Based on the previous subchapter, now we can able to assess the performance of the
model considering the important variable in the purpose of hazard evaluation as
shown in the following table.
Models Long and
complex river
channel,
Water
availability
analysis/Ordina
ry flow
Inundation: water
level, velocity
Rate and
duration of
flood
Sedimentation:
main channel
Sedimentation:
flood plein
HEC RAS – Steady
Flow
- ++ - - -+
(short period)
-
HEC RAS –
Unsteady Flow
+ - +- +- +
(short period)
-
River2D ++ -
(computation
cost)
+/++(for
complex
condition)
++ ++
(long period)
++
(long period)
Based on the table, it can be concluded that River2D gave more realistic results
considering the purpose of hazard evaluation. Therefore the application of hazard
evaluation of the study will be mainly based on the results of this 2D analysis

(5d) Application of the results of River 2D
Considering the output of River2D concerning
water level and velocity during the flood, some
possible hazard map can be produced for
different purposes:
1) Inundation map;
2) function availability of road during flood;
3) concerning the impact of hydrodynamic load,
scour, impact of debris as main causes of brigde
damage during flood.
Blue line: possibility to use by walking
Pink: expected to be collapse.
Green line: possibility to use by vehicles

Considering the water depth based on the output of River2D, we can define the
innundation map, which cover some area also in the flood plain as shown in the
following figure:
 (1) Inundation map

Considering the water depth and velocity we draw to find correlation between
the magnitude of hazard and the function availability of road during flood. The
yellow area overlays on the chart shows an indication of the result.
 (2) Function availability of road during flood
• Water depth in floodplain: 0-2 meter
• Velocity magnitude in floodplain: approximately 2-3 m/sec
Blue line: possibility to use by walking
Pink: expected to be collapse.
Green line: possibility to use by vehicles

Considering the water depth and velocity we can also analysis hydrodynamic load
which may occur during the flood as shown in the following. It indicates based on
the table (just for example) that the load is H4.
 (3) Hydrodynamic load
Approximation location of the bridge Approximation location of the bridge
• Velocity magnitude around the bridge and in the
channel: approximately 2-3 m/sec
• Water level around the bridge:
approximately 1-5 m and up to 5 m in the
channel

Flood risk evaluation for Serio river, Italy

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