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IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 1
Environment and Computer Laboratory HS 2011Water Resources Management
Flood Routing – Methods and ModelsExercise – Hydraulic Modelling of Open Channel Flows
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 2
Outline
Organisational Issues
Flood Routing – Methods and Models
Introduction of the Exercise
Getting to know HEC-GeoRAS
Getting to know HEC-RAS
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 3
Organisational IssuesSupervisor: Verena Maleska, Christina Schornberg, Ellen CerwinkaContact: maleska@ifu.baug.ethz.chTime table
C 29…Friday, 16. Dec
C 29…Friday, 9. Dec
C 29…Friday, 2. Dec
C 29…Friday, 25. Nov
C 29Individual work on ExerciseOffice hours: 12.45-16.30, HIL D 21.3Friday, 18. Nov
C 29Open Channel Flow – hydrologic flood routing Introduction Exercise 1 – HEC-RAS Paper assignments
Friday, 11. Nov
Room Topic Date
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 4
Organisational Issues1 Exercise + Paper Assignments
- Building up the hydraulic model with HEC-GeoRAS in ArcGIS- Hydraulic Modelling of Open Channel Flows using HEC-RAS- Transferring the results of HEC-RAS into ArcGIS
Exercises carried out in groups of 2 studentsHand in one report for the exercise and the paper assignments
- Report around 15 pagesGrading of WRM1
- HEC-RAS: 80%- Paper Assignments: 20%
Deadline for submission: 19th of December 2011
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 5
Organisational Issues
Literature
Lecture notes „Hydraulik 1“ Prof. KinzelbachBollrich: Technische Hydromechanik Band 1Manual HEC-RASwww.sciencedirect.com for paper
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 6
Flood Routing – methods and models
Lecture Outline
1 What is Flood Routing? motivationexamples
2 Flood Routinghydraulic flood routing - approximations to the St. Venant equations
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 7
Flood Routing – methods and models
1) What is flood routing?
Flood (Flow) routing isa mathematical procedure for predicting the changing in magnitude, speed and shape of a flood wave as a function of time at one or more points along a river
(Handbook of Hydrology, Fread, 1993)
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 8
• runoff production
• runoff routing
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 9
1) floodspredict flood propagationprotectionwarning
2) designwater conveyance systemsprotective measureshydrosystem operation
3) water dynamicsungauged riverspeak flow estimationriver-aquifer interaction
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 10
Risk analysis
1 [1 ( )]ntR P X x= − − ≥R = hydrologic risk of failure is given by the probability that the annual flood event x larger than xtwill occur at least once in n yearse.g. a weir with a life period of n=50 years, P(HQT=100)=1/T=1/100
5050
10011 [1 ( )] 1 1 0.4100
R P X x ⎡ ⎤= − − ≥ = − − ≈⎢ ⎥⎣ ⎦
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 11
2) Flood Routing
Flow (Flood) Routing Analysis
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F&C Lab 12
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 13
Steady Uniform Flow
dv/dt=0 dv/dx=0
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F&C Lab 14
Gradually Varied Flow dv/dt=0 dv/dx=0
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 15
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 16
Unsteady Flow: Physically Based = Hydraulic Routing
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 17
St. Venant Equations
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 18
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 19
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 20
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 21
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 22
Exercise: Hydraulic Modelling of Open Channel Flows
Objectives
Get to know- a numerical hydraulic model for flood routing- the general methodology, possibilities and limitations of numerical
flood routing- the role of different parameters and their influence on the
simulationsand not
- to reproduce a flood event as accurate as possible → no tuning exercise
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 23
Tasks
1 Steady State Simulations (d/dt = 0)1.1 Rating curve (Q-H function)1.2 Influence of roughness coefficient
2 Unsteady Flow Simulations2.1 Influence of initial conditions (=> “warm up” time)2.2 Investigation of the temporal discretization2.3 Rating curve for unsteady flow2.4 Numerical damping (influence of weighting factor Θ)2.5 Influence of downstream boundary condition
Answer questions on papers dealing with roughness in open channel flow
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 24
River Reach & Flood Events
River Reach:Thur length of about 3 km
Flood Events:Each group chooses and signs in for a flood eventFlood hydrograph: hourly mean flow for the Thur in m3/s with a duration of 30 to 50 hoursToo flashy to be realistic - but suitable for this exerciseTo be used for unsteady flow simulations
Bern
Thun
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 25
Available Data
On the webpage:Tasks Flood eventsOrthophotoElevation dataPaper and questionsLecture notes, etc.
Bern
Thun
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 26
Building up the Model for HEC-RAS Using ARGIS-Extension HEC-GeoRAS
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 27
Flood routing model: HEC-RASHEC-RAS -> Hydrologic Engineering Centers River Analysis System
developed by the US Department of Defense, Army Corps of Engineers in order to manage the rivers, harbors, and other public works
1D - hydraulic flow modela) Steady & Unsteady Flowb) Sediment Transportc) Water Temperature &
Water Quality
http://www.hec.usace.army.mil/software/hec- ras/
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 28
Flood routing model: HEC-RASHEC-RAS -> Hydrologic Engineering Centers River Analysis System
http://www.hec.usace.army.mil/software/hec- ras/
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 29
Flood routing model: HEC-RASSteady Flow
based on the solution of the one-dimensional energy equationEnergy losses evaluated by friction and contraction / expansionmomentum equation may be used in situations where water surface profile is rapidly varied (hydraulics of bridges )
Unsteady Flowfull, dynamic Saint-Venant equationsusing an implicit, finite difference method
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 30
Solutions of the Saint Venant Equations
Partial Differential Equation
Analytically for 1-Dimensional CaseClosed Integration e.g. Laplace/Fourier Transformation
Numerically for Common Caseuse partial difference approximation Solutions: finite differences
finite elementsfinite volume
Nonlinear equation system: iteration!
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 31
HEC-RAS: Implicit scheme to solve discretized equationsExplicit Schemes:
- explicit scheme: Xt+1 = F(Xt)- explicit schemes can be solved for Xt+1
- straightforward computation- stability issues for large Δt (CFL-criterion)- e.g Forward step
Implicit Schemes:- implicit scheme: Xt+1 = F(Xt and Xt+1)- generally, implicit schemes can’t be solved
explicitly for Xt+1 (Xt+1 = ….) (depending on F)→ solvable e.g. with matrix inversion
- high computational demand- less strict stability criteria
→ larger time steps possible- e.g. Trapezoidal rule
-> Exc 2.2 & 2.4
tttttt
XtXFXXFtXX
dtdX
+Δ⋅=→=Δ−
≈ ++
)()( 11
[ ])()(21 1
1tt
tt
XFXFtXX
dtdX
+=Δ−
≈ ++
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 32
Courant Friedrich Levy (CFL) - Stability Criterion
To choose Δt for a given ΔxStability Criterion for a large class of explicit advection schemes:
Time step Δt shouldn't be longer than the time a water particle needs to get from one computational node to the next (Δx)Or: The distance which a water particle moves in one time step (u*Δt) shouldn’t be larger than Δx
If Δt too long or u too high: Interpolation errors and instabilityWithin limits of stability: trade off between computation time and accuracy
-> Exc 2.2: temporal discretization
1≤ΔΔ⋅xtu
u: highest velocity in the system (⇒ flood wave speed)Δt: computational time stepΔx: spatial discretization (⇒ distance between cross sections)
uxt Δ
≤ΔtxuΔΔ
≤
Δx
tu Δ⋅
x
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 33
HEC-RAS: Preissmann-Cunge scheme
weighting factor Θ
-> Exc 2.4: weighting factor Θ
variables:
temporal derivative:
spatial derivative
how to weight “present” (t=n+1) variable / spatial derivative compared to “past” (t=n) variable / spatial derivative?numerically stable for 0.5 ≤ Θ ≤ 1 (in practice: 0.6 ≤ Θ≤ 1)
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 34
Deriving Inundation Maps Using HEC-GeoRAS
IfU Institut fürUmweltingenieurwissenschaften
F&C Lab 35
Form groups and choose a flood eventGet to know HEC-RASstart working on the paper assignmentsBathurst: What are the main elements governing flow resistance in
natural channels?Is the river roughness constant in space and time?
Wohl: How can you estimate the roughness coefficient? Whichparameters influence the roughness in a channel? How dothey influence the roughness?What is the influence of a varying roughness coefficient onthe river flow? Can you make a general statement?
Next week: working on the exercise Deadline 19th December 2011
And now…
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