Lecture33 - Flood Routing

7/27/2019 Lecture33 - Flood Routing

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Module 8 (L31 L34): Storm

Water & Flood Management:Storm water management, design of drainage system, flood

,reservoir operation, case studies.

Fl R in

1111

1


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T i v r

Flood routing though channels, ReservoirFlood routing though channels, Reservoir

, ,, ,routing, Lumped flow routing, Muskingumrouting, Lumped flow routing, Muskingum

met o , t. enant equat onsmet o , t. enant equat ons

Ke words:Ke words: Flood routing, Channel, Reservoir,Flood routing, Channel, Reservoir,Hydrologic & hydraulic routing.Hydrologic & hydraulic routing.

2222Prof. T I Eldho, Department of Civi l Engineering, IIT Bombay


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What is Flood Routing?What is Flood Routing?

Watershed receives rainfall as input-

produces runoff as output outflowy rograp ers n s ape, ura on

magnitude attribute to storage properties of.

Flood (Flow) routing procedure to computeout ut h dro ra h when in ut h dro ra h & physical dimensions of the storage are known.

Used for flood forecasting, design of spillways,

reservoirs & flood protection works etc.



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Flood RoutingFlood Routing -- MotivationMotivation

1) Floods

predict flood propagation protection

warning

water conveyance systems

protective measures

hydrosystem operation

3) Water dynamics ungauged rivers

peak flow estimation

-

44

.

Prof. T I Eldho, Department of Civi l Engineering, IIT Bombay


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Flood RoutingFlood Routing -- ClassificationClassification

i) Reservoir Routing considers modulation effects on

a flood wave when it passes through a water reservoir

enlarged time bases .

Variations in reservoir elevation & outflow can be

predicted with time when relationships betweenelevation & volume are known.

of input hydrograph while flood waves pass through achannel downstream.

input hydrographs & channel characteristics areknown.



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Flood RoutingFlood Routing ProcedureProcedure

Flood routing - methods can be classified as hydraulic -

in which both continuity and dynamic equations are used- or h drolo ic which enerall uses the continuitequation alone

Hydrologic routing methods - Equation of continuity Hydraulic routing methods St. Venant equations

-Flood forecasting

-Flood protection-Reservoir design

-Design of spillway and outlet structures



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Flood Routing TechniqueFlood Routing Technique Flood routing- technique

of determining the floodh dro ra h at a section of

lag

attenuation

a river by utilizing the dataof flood flow at one or moreu stream sections

scharge

Lumped flow routing:Flow is a function of time atarticular location

Time

Acc

Distributed flow routing:Flow is a function of space

umulated

stora

ationstorage

e easefromstorage

system eS

Chow et.al 1988



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Lumped Flow Routing

Basic equation for hydrologic routing: Continuitye uation

( ) ( )I t Q tdt

Input I(t), Output Q(t) and storage S(t); KnownI(t) & unknowns Q & S

relate S, I and Q

Specific form of storage function depending onna ure o sys em.

ainchannel

ainchannel


Reservoirin

Reservoirin


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Lumped Flow Routing - MethodsBased on storage function

Level pool reservoir routing - Storage is anon near unc on o on y. =

Muskingum method for flow routing in

Linear reservoir models Storage is a linear

Effect of reservoir storage is to redistribute the

hydrograph by shifting the centroid of the inflowhydrograph to the position of that of the outflowhydrograph in time



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Lumped Flow Routing - Methods

Case 1: Invariable relationship between S & Q

Storage & outflow are functions of water surface elevation- when reservoir has a horizontal water surface elevation

S = f (Q) combination of these two functions

Peak outflow occurs at intersection of inflow hydrograph

1010



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Lumped Flow Routing - Methods

Case 2: Variable relationship between S & Q

Applies to long, narrow reservoirs & open OutChannels

Water surface profile curved due to back water

flow

ec s

Peak outflow occurs later than point of intersectiontime

Storage

Replacement of loop with dashed line when backwater effect is not significant

Out

In-flow

Based on Chow et.al(1988)


ow


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Reservoir Routing

Procedure for calculating the outflow

hydrograph from a reservoir with a annel

annel

or zon a wa er sur ace

Flow of flood waves from rivers/streams kee s on chan in the head ir

inmainch

irinmainch

of water in the reservoir h = h(t) Required to find variations of S, Q, &

Reserv

Reserv

In a small interval of time

Avera e inflow in time t Avera e

I t Q t S (1)

outflow in time t, Change in storagein t

1 2 1 22 1

I I Q Qt t S S

1212Prof. T I Eldho, Department of Civi l Engineering, IIT Bombay (2)


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Reservoir RoutingFor reservoir routing the following data should be known

Elevation vs Storage

outflow discharge

Inflow hydrograph, and

Initial values of inflow, outflow Q, and storage S attime t = 0.

t must be shorter than the time of transit of the floodwave through the reach

Variety of methods- for reservoir routing

Inflow

Outflow

Discharge

TimeBased on Chow et.al(1988)


Time

Storage


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Reservoir Routing Puls Method

Rearrangement of equation (2) as

All terms on the left hand side are

1 2 1 21 2

2 2 2t S S

-

RHS is a function of elevation h for achosen time interval t

ainchann

el

ainchann

el

reparat on o grap s or vs , vsand h versus

2

Q tS

eservoirin

eservoirin

Procedure is repeated for full inflowhydrograph



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Reservoir Routing Goodrich Method Rearrange equation is 2

Preparation of graphs for h vsand h vs S and h versus

1 21 2 1 2I I Q Qt t

2S

Flow routing through timeinterval t, all terms on the

t

Value of outflow Q forcan be read from the graph

2S Qt

ainchann

el

ainchann

el

Value ofcalculated by

-

2 SO

t

Q

22

SO O

t

QQ

eservoirin

eservoirin

Repetition of computations forsubsequent routing periods



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Channel Routing- Muskingum Method Hydrologic routing method for handling variable discharge

storage relationship. orage s a unc on o o ou ow n ow sc arges

Water surface in a channel reach is not only parallel to thechannel bottom but also varies with time

Models storage in channel-combination of wedge & prism Prism storage: Volume that would exist if uniform flow

occurre a e owns ream ep

Wedge storage : Wedge like volume formed between

actual water surface profile & top surface of prism storageInflow tochannel

Wedge storage

1616Outflow

fromchannel

Prism storage

ase on ow et.a


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Channel Routing- Muskingum Method

During the advance of flood wave, inflow exceeds outflow Positive wed e

During recession, outflow exceeds inflow Negative wedge

Assumption: Cross sectional area of the flood flow section isdirectly proportional to the discharge at the section

Volume of prism storage is equal to KO

-

K proportionality coefficient, X -weighing factor having

the range 0 < X < 0.5

Wedge storage= KX(I-O)

Based on Chow et.al 1988

Inflow tochannel

1717Prof. T I Eldho, Department of Civi l Engineering, IIT BombayPrismstorage=KO


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Total storage: ---- Muskingumstorage equation

Linear model for routing flow in streams

Value of X depends on shape of modeled wedge storage

.

wedge K- Time of travel of flood wave through channel reaches

Va ues o storage at time j an j+1 can e written as

Change in storage over time interval t is



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Channel Routing- Muskingum Method From t e continuity equation

Simplifying.----- Muskingums

Where

1919

.



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t should be so chosen that K > t > 2KX .. For best

results Ift < 2KX . Coefficient C1 will be negative

Required input for Muskingum routing

Values of K and X for the reach Value of the outflow Oj from the reach at the start

or a g ven c anne reac , are a en asconstant

K is determined empirically (eg. Clarks method:K=cL/s0.5; c constant; L length of stream,; s mean slope of channel) or graphically.

X is determined by trial and error procedure



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Muskingum Method - Procedure

Routing Procedure:

Knowing K and X, select an appropriate value oft

Calculate C1, C2, and C3

Starting rom t e initia con itions nown in ow,outflow, calculate the outflow for the next time step

Repeat the calculations for the entire inflowhydrograph



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Flood Routing by St. Venant Equations

Physically based theory of flood propagation - fromthe Saint Venant e uations for raduall var in flowin open channels.

Hydraulic routing method

Flow as 1 D flow Gradually varied flow condition

Conservation of mass- continuity equation

2222Hydrographat outlet ofwatershed


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Gov. Equation for Flow Routing

Equation of continuity

0

AQ

tx

Momentum equation

hQQ 2

xAxtf

0

q-lateral inflow; Q-discharge in the channel;A-area of flow in the channel, S0-bed slope;

-

2323

.



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Channel Flow- Diffusion &

Diffusion 0

AQ

Initial conditions

fo SSx

tx

oun ary con ons SRn

Qfh

S0=Sf 0

q

t

A

x

Q



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Solution Methodologies

Analytical method: simplified

governing equations, boundarycon ons geome ry, ana y casolutions can be obtained.

Com utational method:

0

q

t

A

x

Q

solution is obtained with the helpof some approximate methodscf

hS S

x

. ,numerical methods (FDM, FEM,FVM) are used to obtain solution

( 2/ 3) (1/ 2)1cf

Q R S An

.

Finite Element Method: ( )c ik i

f i

h hS S

L

2525

qqtQQBtACAC 11


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FEM Based Flood Routing Model

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Flood RoutingFlood Routing Case studyCase study

Catchment Area 847.52 HaElevation varies from 0.5 m to 227 m above MSL.

Rainfall event 26/07/ 2005; 15/07/2009Length of channel 5271 m

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near anne e emen s

Tidal Range 3.25 m to -1.0 m (design)


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Case Study: Flood Routing



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60

80

100

Hytograph of Kal a mboli rai nf all

m

m/hr

Hyetograph 26/07/2005

12 13 14 15 16 17 18 19 20 21 22 23 240

20

40

Rainfall

Ti me st arti ng fr om12 hr hr

2929


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15July200915July2009

.and simulated stagesat chainage 5121 m on

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ReferencesReferences

American Society of Civil Engineers and Water EnvironmentFederation (ASCE and WEF). 1998. Urban Runoff QualityManagement. WEF Manual of Practice No. 23, ASCE Manual andReport on Engineering Practice No. 87.

http://ndma.gov.in/ndma/guidelines.html

http://www.epa.gov/oaintrnt/stormwater/index.htm

-New Delhi, 294-300

Chow, V.T., Maidment, D.R., and Mays, L.W. (1988).Applied

- , , .,

Bedient, P.B.,Huber, C.W. (1988). Hydrology and Flood PlainAnalysis, Addison-Wesley Publishing Company

31313131



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Tutorials - Question!.?.

Study the various flood routing

applications of each.

What are the software available for floodWhat are the software available for floodrouting?. (routing?. (www.hec.usace.army.milwww.hec.usace.army.mil)Evaluate the applications for variousEvaluate the applications for various

problems such as reservoir routing/ channelproblems such as reservoir routing/ channel

routing.routing.

32323232



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Self Evaluation - Questions!.

What is flood routing and where it is used?

xp a n reservo r rou ng.

Differentiate between Puls method & Goodrich

.

Describe the Muskingum method of flood routing.

channel.

routing?.



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Assignment- Questions?.

What are the motivations for flood routing?.

flood routing.

.

Describe the lumped flow routing.

scuss p ys ca y ase oo rou ng n

channels by using St. Venant equations.



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Dr. T. I. EldhoDr. T. I. Eldho

Professor,Professor,

Department of Civil Engineering,Department of Civil Engineering,Indian Institute of Technology Bombay,Indian Institute of Technology Bombay,

Mumbai, India, 400 076.Mumbai, India, 400 076.

Email:Email: [email protected]@iitb.ac.in

3535

Phone: (022)Phone: (022) 25767339; Fax: 2576730225767339; Fax: 25767302http://www.http://www.civil.iitb.ac.incivil.iitb.ac.in

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Lecture33 - Flood Routing