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Channel Routing

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The method does not explicitly allow for separation of the main channel and the overbanks. ... K is estimated to be the travel time through the reach. ... – PowerPoint PPT presentation

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Title: Channel Routing


1
Channel Routing
  • Simulate the movement of water through a channel
  • Used to predict the magnitudes, volumes, and
    temporal patterns of the flow (often a flood
    wave) as it translates down a channel.
  • 2 types of routing hydrologic and hydraulic.
  • both of these methods use some form of the
    continuity equation.

Continuity equation Hydrologic Routing Hydraulic
Routing Momentum Equation
2
Continuity Equation
Continuity equation Hydrologic Routing Hydraulic
Routing Momentum Equation
  • The change in storage (dS) equals the difference
    between inflow (I) and outflow (O) or
  • For open channel flow, the continuity equation is
    also often written as

A the cross-sectional area, Q channel flow,
and q lateral inflow
3
Hydrologic Routing
Continuity equation Hydrologic Routing Hydraulic
Routing Momentum Equation
  • Methods combine the continuity equation with some
    relationship between storage, outflow, and
    possibly inflow.
  • These relationships are usually assumed,
    empirical, or analytical in nature.
  • An of example of such a relationship might be a
    stage-discharge relationship.

4
Use of Manning Equation
Continuity equation Hydrologic Routing Hydraulic
Routing Momentum Equation
  • Stage is also related to the outflow via a
    relationship such as Manning's equation

5
Hydraulic Routing
  • Hydraulic routing methods combine the continuity
    equation with some more physical relationship
    describing the actual physics of the movement of
    the water.
  • The momentum equation is the common relationship
    employed.
  • In hydraulic routing analysis, it is intended
    that the dynamics of the water or flood wave
    movement be more accurately described

Continuity equation Hydrologic Routing Hydraulic
Routing Momentum Equation
6
Momentum Equation
Continuity equation Hydrologic Routing Hydraulic
Routing Momentum Equation
  • Expressed by considering the external forces
    acting on a control section of water as it moves
    down a channel
  • Henderson (1966) expressed the momentum equation
    as

7
Combinations of Equations
  • Simplified Versions

Continuity equation Hydrologic Routing Hydraulic
Routing Momentum Equation
Unsteady -Nonuniform
Steady - Nonuniform
Diffusion or noninertial
Kinematic
Sf So
8
Routing Methods
  • Kinematic Wave
  • Muskingum
  • Muskingum-Cunge
  • Dynamic

Kinematic Wave Muskingum Muskingum-Cunge Dynamic
Modeling Notes
9
Kinematic Wave
  • Kinematic wave channel routing is probably the
    most basic form of hydraulic routing.
  • This method combines the continuity equation with
    a very simplified form of the St. Venant
    equations.
  • Kinematic wave routing assumes that the friction
    slope is equal to the bed slope.
  • Additionally, the kinematic wave form of the
    momentum equation assumes a simple
    stage-discharge relationship.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
10
Kinematic Wave Basic Equations
Q aAm
Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
  • An explicit finite difference scheme in a
    space-time grid domain is often used for the
    solution of the kinematic wave procedure.

11
Working Equation
Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
12
Wave Speed TOO Fast?
When the average celerity, c, is greater than the
ratio ?x/?t, a conservative form of these
equations is applied. In this conservative form,
the spatial and temporal derivatives are only
estimated at the previous time step and previous
location.
Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
13
Kinematic Wave Assumptions
  • The method does not explicitly allow for
    separation of the main channel and the overbanks.
  • Strictly speaking, the kinematic method does not
    allow for attenuation of a flood wave. Only
    translation is accomplished. There is, however,
    a certain amount of attenuation which results
    from the finite difference approximation used to
    solve the governing equations.The hydrostatic
    pressure distribution is assumed to be
    applicable, thus neglecting any vertical
    accelerations.
  • No lateral, secondary circulations may be
    present, i.e. - the channel is represented by a
    straight line.
  • The channel is stable with no lateral migration,
    degradation, and aggradation.
  • Flow resistance may be estimated via Manning's
    equation or the Chezy equation.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
14
Muskingum Method
Sp K O
Prism Storage
Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
Sw K(I - O)X
Wedge Storage
Combined
S KXI (1-X)O
15
Muskingum, cont...
Substitute storage equation, S into the S in
the continuity equation yields
Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
S KXI (1-X)O
O2 C0 I2 C1 I1 C2 O1
16
Muskingum Notes
  • The method assumes a single stage-discharge
    relationship.
  • In other words, for any given discharge, Q, there
    can be only one stage height.
  • This assumption may not be entirely valid for
    certain flow situations.
  • For instance, the friction slope on the rising
    side of a hydrograph for a given flow, Q, may be
    quite different than for the recession side of
    the hydrograph for the same given flow, Q.
  • This causes an effect known as hysteresis, which
    can introduce errors into the storage assumptions
    of this method.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
17
Estimating K
  • K is estimated to be the travel time through the
    reach.
  • This may pose somewhat of a difficulty, as the
    travel time will obviously change with flow.
  • The question may arise as to whether the travel
    time should be estimated using the average flow,
    the peak flow, or some other flow.
  • The travel time may be estimated using the
    kinematic travel time or a travel time based on
    Manning's equation.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
18
Estimating X
  • The value of X must be between 0.0 and 0.5.
  • The parameter X may be thought of as a weighting
    coefficient for inflow and outflow.
  • As inflow becomes less important, the value of X
    decreases.
  • The lower limit of X is 0.0 and this would be
    indicative of a situation where inflow, I, has
    little or no effect on the storage.
  • A reservoir is an example of this situation and
    it should be noted that attenuation would be the
    dominant process compared to translation.
  • Values of X 0.2 to 0.3 are the most common for
    natural streams however, values of 0.4 to 0.5
    may be calibrated for streams with little or no
    flood plains or storage effects.
  • A value of X 0.5 would represent equal
    weighting between inflow and outflow and would
    produce translation with little or no attenuation.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
19
More Notes - Muskingum
  • The Handbook of Hydrology (Maidment, 1992)
    includes additional cautions or limitations in
    the Muskingum method.
  • The method may produce negative flows in the
    initial portion of the hydrograph.
  • Additionally, it is recommended that the method
    be limited to moderate to slow rising hydrographs
    being routed through mild to steep sloping
    channels.
  • The method is not applicable to steeply rising
    hydrographs such as dam breaks.
  • Finally, this method also neglects variable
    backwater effects such as downstream dams,
    constrictions, bridges, and tidal influences.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
20
Muskingum Example Problem
  • A portion of the inflow hydrograph to a reach of
    channel is given below. If the travel time is
    K1 unit and the weighting factor is X0.30, then
    find the outflow from the reach for the period
    shown below

21
Muskingum Example Problem
  • The first step is to determine the coefficients
    in this problem.
  • The calculations for each of the coefficients is
    given below

C0 - ((10.30) - (0.51)) / ((1-(10.30)
(0.51)) 0.167
C1 ((10.30) (0.51)) / ((1-(10.30)
(0.51)) 0.667
22
Muskingum Example Problem
C2 (1- (10.30) - (0.51)) / ((1-(10.30)
(0.51)) 0.167
  • Therefore the coefficients in this problem are
  • C0 0.167
  • C1 0.667
  • C2 0.167

23
Muskingum Example Problem
  • The three columns now can be calculated.
  • C0I2 0.167 5 0.835
  • C1I1 0.667 3 2.00
  • C2O1 0.167 3 0.501

24
Muskingum Example Problem
  • Next the three columns are added to determine the
    outflow at time equal 1 hour.
  • 0.835 2.00 0.501 3.34

25
Muskingum Example Problem
  • This can be repeated until the table is complete
    and the outflow at each time step is known.

26
Muskingum-Cunge
  • Muskingum-Cunge formulation is similar to the
    Muskingum type formulation
  • The Muskingum-Cunge derivation begins with the
    continuity equation and includes the diffusion
    form of the momentum equation.
  • These equations are combined and linearized,

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
27
Muskingum-Cungeworking equation
Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
  • where
  • Q discharge
  • t time
  • x distance along channel
  • qx lateral inflow
  • c wave celerity
  • m hydraulic diffusivity

28
Muskingum-Cunge, cont...
  • Method attempts to account for diffusion by
    taking into account channel and flow
    characteristics.
  • Hydraulic diffusivity is found to be

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
  • The Wave celerity in the x-direction is

29
Solution of Muskingum-Cunge
  • Solution of the Muskingum is accomplished by
    discretizing the equations on an x-t plane.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
30
Calculation of K X
Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
Estimation of K X is more physically based
and should be able to reflect the changing
conditions - better.
31
Muskingum-Cunge - NOTES
  • Muskingum-Cunge formulation is actually
    considered an approximate solution of the
    advection diffusion equation.
  • As such it may account for wave attenuation, but
    not for reverse flow and backwater effects and
    not for fast rising hydrographs.
  • Properly applied, the method is non-linear in
    that the flow properties and routing coefficients
    are re-calculated at each time and distance step
  • Often, an iterative 4-point scheme is used for
    the solution.
  • Care should be taken when choosing the
    computation interval, as the computation interval
    may be longer than the time it takes for the wave
    to travel the reach distance.
  • Internal computational times are used to account
    for the possibility of this occurring.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
32
Muskingum-Cunge - NOTES
  • Muskingum-Cunge may also be used distributed
    modeling
  • The data inputs needed are
  • Control parameters
  • Hydrologic Inflow hydrographs
  • Physical system channel geometry
    (cross-sections and channel profile)
  • Data outputs Method will sum and route discharge
    hydrographs to overall basin outlet.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
33
Muskingum-Cunge Example
  • The hydrograph at the upstream end of a river is
    given in the following table. The reach of
    interest is 18 km long. Using a subreach length
    Dx of 6 km, determine the hydrograph at the end
    of the reach using the Muskingum-Cunge method.
    Assume c 2m/s, B 25.3 m, So 0.001m and no
    lateral flow.

34
Muskingum-Cunge Example
  • First, K must be determined.
  • K is equal to
  • Dx 6 km, while c 2 m/s

35
Muskingum-Cunge Example
  • The next step is to determine x.
  • All the variables are known, with B 25.3 m, So
    0.001 and Dx 6000 m, and the peak Q taken from
    the table.

36
Muskingum-Cunge Example
  • A curve for Dx/cDt is then needed to determine Dt.
  • For x 0.253, Dx/(cDt) lt 0.82

37
Muskingum-Cunge Example
  • Therefore, Dt can be found.

38
Muskingum-Cunge Example
  • The coefficients of the Muskingum-Cunge method
    can now be determined.

39
Muskingum-Cunge Example
  • The coefficients of the Muskingum-Cunge method
    can now be determined.

40
Muskingum-Cunge Example
  • The coefficients of the Muskingum-Cunge method
    can now be determined.

41
Muskingum-Cunge Example
  • The coefficients of the Muskingum-Cunge method
    can now be determined.

42
Muskingum-Cunge Example
  • Then a simplification of the original formula can
    be made.
  • Since there is not lateral flow, QL 0. The
    simplified formula is the following

43
Muskingum-Cunge Example
  • A table can then be created in 2 hour time steps
    similar to the one below

44
Muskingum-Cunge Example
  • It is assumed at time zero, the flow is 10 m3/s
    at each distance.

45
Muskingum-Cunge Example
  • Next, zero is substituted into for each letter to
    solve the equation.

46
Muskingum-Cunge Example
  • Using the table, the variables can be determined.

10 18 10
47
Muskingum-Cunge Example
  • Therefore, the equation can be solved.

48
Muskingum-Cunge Example
  • Therefore, the equation can be solved.

49
Muskingum-Cunge Example
  • This is repeated for the rest of the columns and
    the subsequent columns to produce the following
    table. Note that when you change rows, n
    changes. When you change columns, j changes.

50
Full Dynamic Wave Equations
  • The solution of the St. Venant equations is known
    as dynamic routing.
  • Dynamic routing is generally the standard to
    which other methods are measured or compared.
  • The solution of the St. Venant equations is
    generally accomplished via one of two methods
    1) the method of characteristics and 2) direct
    methods (implicit and explicit).
  • It may be fair to say that regardless of the
    method of solution, a computer is absolutely
    necessary as the solutions are quite time
    consuming.
  • J. J. Stoker (1953, 1957) is generally credited
    for initially attempting to solve the St. Venant
    equations using a high speed computer.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
51
Dynamic Wave Solutions
  • Characteristics, Explicit, Implicit
  • The most popular method of applying the implicit
    technique is to use a four point weighted finite
    difference scheme.
  • Some computer programs utilize a finite element
    solution technique however, these tend to be
    more complex in nature and thus a finite
    difference technique is most often employed.
  • It should be noted that most of the models using
    the finite difference technique are
    one-dimensional and that two and
    three-dimensional solution schemes often revert
    to a finite element solution.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
52
Dynamic Wave Solutions
  • Dynamic routing allows for a higher degree of
    accuracy when modeling flood situations because
    it includes parameters that other methods
    neglect.
  • Dynamic routing, when compared to other modeling
    techniques, relies less on previous flood data
    and more on the physical properties of the storm.
    This is extremely important when record
    rainfalls occur or other extreme events.
  • Dynamic routing also provides more hydraulic
    information about the event, which can be used to
    determine the transportation of sediment along
    the waterway.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
53
Courant Condition?
  • If the wave or hydrograph can travel through the
    subreach (of length ?x) in a time less than the
    computational interval, ?t, then computational
    instabilities may evolve.
  • The condition to satisfy here is known as the
    Courant condition and is expressed as

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
54
Some DISadvantages
  • Geometric simplification - some models are
    designed to use very simplistic representations
    of the cross-sectional geometry. This may be
    valid for large dam breaks where very large flows
    are encountered and width to depth ratios are
    large however, this may not be applicable to
    smaller dam breaks where channel geometry would
    be more critical.
  • Model simulation input requirements - dynamic
    routing techniques generally require boundary
    conditions at one or more locations in the
    domain, such as the upstream and downstream
    sections. These boundary conditions may in the
    form of known or constant water surfaces,
    hydrographs, or assumed stage-discharge
    relationships.
  • Stability - As previously noted, the very complex
    nature of these methods often leads to numeric
    instability. Also, convergence may be a problem
    in some solution schemes. For these reasons as
    well as others, there tends to be a stability
    problem in some programs. Often times it is very
    difficult to obtain a "clean" model run in a cost
    efficient manner.

Modified Puls Kinematic Wave Muskingum Muskingum-C
unge Dynamic Modeling Notes
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