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1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
1
CHAPTER 5:
REVIEW OF 1D OPEN CHANNEL HYDRAULICS
Dam at Hiram Falls on the Saco River near Hiram, Maine, USA
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
2
TOPICS REVIEWED
This e-book is not intended to include a full treatment of open channel flow. It is
assumed that the reader has had a course in open channel flow, or has access
to texts that cover the field. Nearly all undergraduate texts in fluid mechanics for
civil engineers have sections on open channel flow (e.g. Crowe et al., 2001).
Three texts that specifically focus on open channel flow are those by Henderson
(1966), Chaudhry (1993) and Jain (2000).
Topics treated here include:
• Relations for boundary resistance
• Normal (steady, uniform) flow
• St. Venant shallow water equations
• Gradually varied flow
• Froude number: subcritical, critical and supercritical flow
• Classification of backwater curves
• Numerical calculation of backwater curves
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
3
SIMPLIFICATION OF CHANNEL CROSS-SECTIONAL SHAPE
River channel cross sections have complicated shapes. In a 1D analysis, it is
appropriate to approximate the shape as a rectangle, so that B denotes channel
width and H denotes channel depth (reflecting the cross-sectionally averaged depth
of the actual cross-section). As was seen in Chapter 3, natural channels are
generally wide in the sense that Hbf/Bbf << 1, where the subscript “bf” denotes
“bankfull”. As a result the hydraulic radius Rh is usually approximated accurately by
the average depth. In terms of a rectangular channel,
H
B
channel floodplain
floodplain
H
B
H
2
1
H
H
2
B
HB
Rh 










1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
4
THE SHIELDS NUMBER:
A KEY DIMENSIONLESS PARAMETER QUANTIFYING SEDIMENT MOBILITY
gD
R
b




3
c
2
b
2
g
3
4
~










D
R
D
b = boundary shear stress at the bed (= bed drag force acting on the flow per unit
bed area) [M/L/T2]
c = Coulomb coefficient of resistance of a granule on a granular bed [1]
Recalling that R = (s/) – 1, the Shields Number * is defined as
It can be interpreted as a ratio scaling the ratio impelling force of flow drag acting on
a particle to the Coulomb force resisting motion acting on the same particle, so that
The characterization of bed mobility thus requires a quantification of boundary shear
stress at the bed.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
5
QUANTIFICATION OF BOUNDARY SHEAR STRESS AT THE BED
2
/
1
f
C
u
U
Cz 



U = cross-sectionally averaged flow velocity ( depth-averaged
flow velocity in the wide channels studied here) [L/T]
u* = shear velocity [L/T]
Cf = dimensionless bed resistance coefficient [1]
Cz = dimensionless Chezy resistance coefficient [1]
2
b
f
U
C



BH
Q
U 




b
u
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
6
RESISTANCE RELATIONS FOR HYDRAULICALLY ROUGH FLOW
6
/
1
s
r
2
/
1
f
k
H
C
u
U
Cz 










 

Keulegan (1938) formulation:
90
s
k
s D
n
k 











 
 s
2
/
1
f
k
H
11
n
1
C
u
U
Cz 
where  = 0.4 denotes the dimensionless Karman constant and ks = a roughness
height characterizing the bumpiness of the bed [L].
Manning-Strickler formulation:
where r is a dimensionless constant between 8 and 9. Parker (1991) suggested
a value of r of 8.1 for gravel-bed streams.
Roughness height over a flat bed (no bedforms):
where Ds90 denotes the surface sediment size such that 90 percent of the
surface material is finer, and nk is a dimensionless number between 1.5 and 3.
For example, Kamphuis (1974) evaluated nk as equal to 2.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
7
COMPARISION OF KEULEGAN AND MANNING-STRICKLER RELATIONS
r = 8.1
Note that Cz does not
vary strongly with depth.
It is often approximated
as a constant in broad-
brush calculations.
1
10
100
1 10 100 1000
H/ks
Cz
Keulegan
Parker Version of Manning-
Strickler
6
/
1
s
k
H
1
.
8
Cz 








1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
8
BED RESISTANCE RELATION FOR MOBILE-BED FLUME EXPERIMENTS
Sediment transport relations for rivers have traditionally been
determined using a simplified analog: a straight, rectangular
flume with smooth, vertical sidewalls. Meyer-Peter and Müller
(1948) used two famous early data sets of flume data on
sediment transport to determine their famous sediment transport
relation (introduced later). These are a) a subset of the data of
Gilbert (1914) collected at Berkeley, California (D50 = 3.17 mm,
4.94 mm and 7.01 mm) and the set due to Meyer-Peter et al.
(1934) collected at E.T.H., Zurich, Switzerland (D50 = 5.21 mm
and 28.65 mm).
Bedforms such as dunes were present in many of the
experiments in these two sets. In the case of 116 experiments
of Gilbert and 52 experiments of Meyer-Peter et al., it was
reported that no bedforms were present and that sediment was
transported under flat-bed conditions. Wong (2003) used this
data set to study bed resistance over a mobile bed without
bedforms.
Flume at Tsukuba
University, Japan
(flow turned off).
Image courtesy H.
Ikeda. Note that
bedforms known
as linguoid bars
cover the bed.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
9
BED RESISTANCE RELATION FOR MOBILE-BED FLUME EXPERIMENTS
contd.
Most laboratory flumes are not wide enough to prevent sidewall effects. Vanoni
(1975), however, reports a method by which sidewall effects can be removed from
the data. As a result, depth H is replaced by the hydraulic radius of the bed region
Rb. (Not to worry, Rb  H as H/B  0). Wong (2003) used this procedure to
remove sidewall effects from the previously-mentioned data of Gilbert (1914) and
Meyer-Peter et al. (1934).
The material used in all the experiments in question was quite well-sorted. Wong
(2003) estimated a value of D90 from the experiments using the given values of
median size D50 and geometric standard deviation g, and the following relation for
a log-normal grain size distribution;
Wong then estimated ks as equal to 2D90 in accordance with the result of
Kamphuis (1974), and s in the Manning-Strickler resistance relation as 8.1 in
accordance with Parker (1991). The excellent agreement with the data is
shown on the next page.
28
.
1
g
50
90 D
D 

1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
10
1.00
10.00
100.00
1.00 10.00 100.00
Rb/ks
Cz
ETH 52
Gilbert 116
Parker Version of Manning-
Strickler
6
/
1
s
b
k
R
1
.
8
Cz 








TEST OF RESISTANCE RELATION AGAINST MOBILE-BED DATA WITHOUT
BEDFORMS FROM LABORATORY FLUMES
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
11
NORMAL FLOW
Normal flow is an equilibrium state defined by a perfect balance between the
downstream gravitational impelling force and resistive bed force. The
resulting flow is constant in time and in the downstream, or x direction.
Parameters:
x = downstream coordinate [L]
H = flow depth [L]
U = flow velocity [L/T]
qw = water discharge per unit width [L2T-1]
B = width [L]
Qw = qwB = water discharge [L3/T]
g = acceleration of gravity [L/T2]
 = bed angle [1]
b = bed boundary shear stress [M/L/T2]
S = tan = streamwise bed slope [1]
(cos   1; sin   tan   S)
 = water density [M/L3]
As can be seen from Chapter 3, the
bed slope angle  of the great
majority of alluvial rivers is sufficiently
small to allow the approximations
1
cos
,
S
tan
sin 






x
B
x
gHxBS
bBx
H
1D SEDIMENT TRANSPORT MORPHODYNAMICS
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© Gary Parker November, 2004
12
UHB
B
q
Q
UH
q w
w
w 


Conservation of downstream momentum:
Impelling force (downstream component of weight of water) = resistive force
x
B
xS
gHB
sin
x
gHB b 








gHS
b 


Reduce to obtain depth-slope
product rule for normal flow:
NORMAL FLOW contd.
Conservation of water mass (= conservation of water volume as water can be
treated as incompressible):

x
B
x
gHxBS
bBx
H
gHS
u 

1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
13
ESTIMATED CHEZY RESISTANCE COEFFICIENTS FOR BANKFULL FLOW
BASED ON NORMAL FLOW ASSUMPTION FOR u*
The plot below is from Chapter 3
1
10
100
1 10 100 1000 10000 100000
Grav Brit
Grav Alta
Grav Ida
Sand Mult
Sand Sing
bf
Cz
Ĥ
50
bf
bf
bf
bf
bf
bankfull
bf
D
H
Ĥ
,
S
gH
H
B
Q
u
U
Cz 











1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
14
Relation for Shields stress  at normal equilibrium:
(for sediment mobility calculations)
gHS
U
C 2
f 


RELATION BETWEEN qw, S and H AT NORMAL EQUILIBRIUM
D
R
HS
gD
R
b





D
R
S
g
q
C 3
/
2
3
/
1
2
w
f










3
/
1
2
w
f
gS
q
C
H 








2
/
1
2
/
1
z
2
/
1
2
/
1
f
S
H
g
C
S
H
C
g
U 

or
Reduce the relation for momentum conservation b = gHS with the resistance form
b = CfU2:
Generalized Chezy
velocity relation
Further eliminating U with the relation for water mass conservation qw = UH and
solving for flow depth:
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
15
ESTIMATED SHIELDS NUMBERS FOR BANKFULL FLOW
BASED ON NORMAL FLOW ASSUMPTION FOR b
The plot below is from Chapter 3
2
50
50
bf
50
bf
50
b
50
bf
D
gD
Q
Q̂
,
D
R
S
H
gD
R






1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02 1.E+04 1.E+06 1.E+08 1.E+10 1.E+12 1.E+14
Grav Brit
Grav Alta
Sand Mult
Sand Sing
Grav Ida
Q̂

 50
bf
1D SEDIMENT TRANSPORT MORPHODYNAMICS
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© Gary Parker November, 2004
16
RELATIONS AT NORMAL EQUILIBRIUM WITH MANNING-STRICKLER
RESISTANCE FORMULATION
6
/
1
s
r
2
/
1
f
3
/
1
2
w
f
k
H
C
gS
q
C
H 

















 
RD
S
g
q
k 10
/
7
10
/
3
2
r
2
w
3
/
1
s











Relation for Shields stress  at normal equilibrium:
(for sediment mobility calculations)
10
/
3
2
r
2
w
3
/
1
s
gS
q
k
H 









2
/
1
3
/
2
6
/
1
s
r
2
/
1
2
/
1
f
S
H
k
g
S
H
C
g
U 

 6
/
1
s
r
2
/
1
3
/
2
k
g
n
1
,
S
H
n
1
U 


Manning-Strickler velocity relation
(n = Manning’s “n”)
Solve for H
to find
Solve for U
to find
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
17
BUT NOT ALL OPEN-CHANNEL FLOWS ARE AT OR CLOSE TO EQUILIBRIUM!
Flow into standing water (lake or
reservoir) usually takes the form
of an M1 curve.
Flow over a free overfall
(waterfall) usually takes the form
of an M2 curve.
A key dimensionless parameter describing the way
in which open-channel flow can deviate from
normal equilibrium is the Froude number Fr: gH
U

Fr
And therefore the calculation of bed shear stress as b = gHS is not always
accurate. In such cases it is necessary to compute the disquilibrium (e.g.
gradually varied) flow and calculate the bed shear stress from the relation
2
f
b U
C



1D SEDIMENT TRANSPORT MORPHODYNAMICS
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RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
18
NON-STEADY, NON-UNIFORM 1D OPEN CHANNEL FLOWS:
St. Venant Shallow Water Equations
0
x
UH
t
H






Relation for water mass conservation
(continuity):
Relation for momentum conservation:
2
f
2
2
U
C
x
gH
x
H
g
2
1
x
H
U
t
UH














x = boundary (bed) attached nearly horizontal coordinate [L]
y = upward normal coordinate [L]
 = bed elevation [L]
S = tan  - /x [1]
H = normal (nearly vertical) flow depth [L]
Here “normal” means “perpendicular to the bed” and has
nothing to do with normal flow in the sense of equilibrium.
x
y

H
Bed and water surface slopes
exaggerated below for clarity.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
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© Gary Parker November, 2004
19
DERIVATION: EQUATION OF CONSERVATION OF OF WATER MASS
Q = UHB = volume water discharge [L3/T]
Q = Mass water discharge = UHB [M/T]
/t(Mass in control volume) = Net mass inflow rate
x
x
UH
B
UHB
UHB
t
x
HB
x
x
x
















0
x
UH
t
H






Reducing under assumption of
constant B:
H
B
x
Q
Q
1D SEDIMENT TRANSPORT MORPHODYNAMICS
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© Gary Parker November, 2004
20
STREAMWISE MOMENTUM DISCHARGE
Momentum flows!
Qm = U2HB = streamwise discharge of streamwise momentum [ML/T2]. The
derivation follows below.
Momentum crossing left face in time t = (HBU2t) = mass x velocity
Qm = momentum crossing per unit time, = (Momentum crossing in t)/ t = U2HB
HB
U
Q 2
m 

Ut
(HBUt)(U)
U
Note that the streamwise momentum discharge has the same units as force,
and is often referred to as the streamwise inertial force.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
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21
The flow is assumed to be gradually varying, i.e. the spatial scale Lx of variation in
the streamwise direction satisfies the condition H/Lx << 1. Under this assumption the
pressure p can be approximated as hydrostatic. Where z = an upward normal
coordinate from the bed,
STREAMWISE PRESSURE FORCE
 


H
0
2
p BgH
2
1
pdz
B
F
Fp = pressure force [ML/T2]
g
z
p





Integrate and evaluate the constant of integration under the condition of zero (gage)
pressure at the water surface, where y = H, to get:
 
z
H
g
p 


p = pressure (normal stress) [M/L/T2]
Integrate the above relation over the cross-sectional area
to find the streamwise pressure force Fp:
p
H
Fp
B
1D SEDIMENT TRANSPORT MORPHODYNAMICS
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22
DERIVATION: EQUATION OF CONSERVATION OF STREAMWISE MOMENTUM
/t(Momentum in control volume) = net momentum inflow rate + sum of forces
Sum of forces = downstream gravitational force – resistive force + pressure force
at x – pressure force at x + x
xB
U
C
x
x
gHB
B
gH
2
1
B
gH
2
1
HB
U
HB
U
t
xU
HB 2
f
x
x
2
x
2
x
x
2
x
2

























2
f
2
U
C
x
gH
x
H
gH
x
H
U
t
HU














x
Qm
B
H
Qm
Fp
Fp
gHBxS
bBx
or reducing,
1D SEDIMENT TRANSPORT MORPHODYNAMICS
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© Gary Parker November, 2004
23
CASE OF STEADY, GRADUALLY VARIED FLOW
0
x
UH
t
H






2
f
2
2
U
C
x
gH
x
H
g
2
1
x
H
U
t
UH
















 w
q
UH
H
U
C
dx
d
g
dx
dH
g
dx
dU
U
2
f





Reduce equation of water mass
conservation and integrate:
dx
dH
H
q
dx
dU
H
q
U 2
w
w




Thus:
Reduce equation of streamwise
momentum conservation:
But with water conservation:
dx
dU
UH
dx
dUH
U
dx
H
dU2


So that momentum
conservation reduces to:
constant
1D SEDIMENT TRANSPORT MORPHODYNAMICS
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© Gary Parker November, 2004
24
THE BACKWATER EQUATION
to get the backwater equation:
2
f
f
2
3
2
w
2
C
S
,
gH
U
gH
q
,
x
S Fr
Fr 







2
f
1
S
S
dx
dH
Fr



H
U
C
x
g
x
H
g
dx
dU
U
2
f









Reduce
with
dx
dH
H
q
dx
dU
,
H
q
U 2
w
w



where
Here Fr denotes the Froude number of the flow and Sf denotes the friction slope.
For steady flow over a fixed bed, bed slope S (which can be a function of x) and
constant water discharge per unit width qw are specified, so that the backwater
equation specified a first-order differential equation in H, requiring a specified
value of H at some point as a boundary condition.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
25
2
f
1
S
S
dx
dH
Fr



3
/
1
2
w
f
n
3
n
2
w
f
f
gS
q
C
H
gH
q
C
S
S 











Consider the case of constant bed slope S. Setting the numerator of the right-hand
side backwater equation = zero, so that S = Sf (friction slope equals bed slope)
recovers the condition of normal equilibrium, at which normal depth Hn prevails:
Setting the denominator of the right-hand side of the backwater equation = zero yields
the condition of Froude-critical flow, at which Fr = 1 and depth = the critical value Hc:
3
/
1
2
w
c
3
c
2
w
2
g
q
H
gH
q
1 










 Fr
NORMAL AND CRITICAL DEPTH
At any given point in a gradually varied flow the depth H may differ from both Hn and
Hc. If Fr = qw/(gH3)1/2 < 1 the flow slow and deep and is termed subcritical; if on the
other hand Fr > 1 the flow is swift and shallow and is termed supercritical. The
great majority of flows in alluvial rivers are subcritical, but supercritical flows
do occur. Supercritical flows are common during floods in steep bedrock rivers.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
26
COMPUTATION OF BACKWATER CURVES
The case of constant bed slope S is considered as an example.
Let water discharge qw and bed slope S be given.
In the case of constant bed friction coefficient Cf, let Cf be given.
In the case of Cf specified by the Manning-Strickler relation, let r and ks be given.
Compute Hc:
Compute Hn
:
If Hn > Hc then (Fr)n < 1: normal flow is subcritical, defining a “mild” bed slope.
If Hn < Hc then (Fr)n > 1: normal flow is supercritical, defining a “steep” bed slope.
,
)
H
(
1
)
H
(
S
S
dx
dH
2
f
Fr



Requires 1 b.c. for
unique solution:
1
x
H
H
1

3
/
1
2
w
c
g
q
H 








3
/
1
2
w
f
n
gS
q
C
H 








10
/
3
2
r
2
w
3
/
1
s
n
gS
q
k
H 









or
3
2
w
3
/
1
s
2
r
f
3
2
w
f
f
3
2
w
2
gH
q
k
H
)
H
(
S
or
gH
q
C
)
H
(
S
,
gH
q
)
H
(














Fr
where x1 is a starting point. Integrate upstream
if the flow at the starting point is subcritical,
and integrate downstream if it is supercritical.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
27
COMPUTATION OF BACKWATER CURVES contd.
Flow at a point relative to critical flow: note that
It follows that 1 – Fr2(H) < 0 if H < Hc, and 1 – Fr2 > 0 if H > Hc.
3
c
2
w
3
2
w
2
gH
q
1
,
gH
q
)
H
( 

Fr
Flow at a point relative to normal flow: note that for the case of constant Cf
3
n
2
w
f
3
2
w
f
f
gH
q
C
S
,
gH
q
C
)
H
(
S 

3
n
2
w
3
/
1
s
n
2
r
3
2
w
3
/
1
s
2
r
f
gH
q
k
H
S
,
gH
q
k
H
)
H
(
S
























and for the case of the Manning-Strickler relation
It follows in either case that S – Sf(H) < 0 if H < Hn, and S – Sf(H) > 0
if H > Hn.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
28
MILD BACKWATER CURVES M1, M2 AND M3
M1: H1 > Hn > Hc
Again the case of constant bed slope S is considered. Recall that
M2: Hn > H1 > Hc
M3: Hn > Hc > H1






)
H
(
1
)
H
(
S
S
dx
dH
1
2
1
f
x1
Fr
Depth increases downstream,
decreases upstream
Depth increases downstream,
decreases upstream
Depth decreases downstream,
increases upstream
A bed slope is considered mild if Hn > Hc. This is the most common case in
alluvial rivers. There are three possible cases.
3
2
w
3
/
1
s
2
r
f
3
2
w
f
f
3
2
w
2
gH
q
k
H
)
H
(
S
or
gH
q
C
)
H
(
S
,
gH
q
)
H
(














Fr






)
H
(
1
)
H
(
S
S
dx
dH
1
2
1
f
x1
Fr






)
H
(
1
)
H
(
S
S
dx
dH
1
2
1
f
x1
Fr
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
29
M1 CURVE
M1: H1 > Hn > Hc






)
H
(
1
)
H
(
S
S
dx
dH
2
f
Fr
3
2
w
2
3
2
w
f
f
gH
q
,
gH
q
C
S 
 Fr
Water surface elevation  =  + H (remember H is measured normal to the bed, but
is nearly vertical as long as S << 1). Note that Fr < 1 at x1: integrate upstream.
Starting and normal (equilibrium) flows are subcritical.
As H increases downstream, both Sf and Fr decrease toward 0.
Far downstream, dH/dx = S  d/dx = d/dx(H + ) = constant: ponded water
As H decreases upstream, Sf approaches S while Fr remains < 1.
Far upstream, normal flow is approached.
Bed slope has
been exaggerated
for clarity.
H
Hc Hn
H1


The M1 curve
describes subcritical
flow into ponded water.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
30
M2 CURVE
M1: Hn > H1 > Hc






)
H
(
1
)
H
(
S
S
dx
dH
2
f
Fr
3
2
w
2
3
2
w
f
f
gH
q
,
gH
q
C
S 
 Fr
Note that Fr < 1 at x1; integrate upstream. Starting and normal (equilibrium) flows
are subcritical.
As H decreases downstream, both Sf and Fr increase, and Fr increases toward 1.
At some point downstream, Fr = 1 and dH/dx = - : free overfall (waterfall).
As H increases upstream, Sf approaches S while Fr remains < 1.
Far upstream, normal flow is approached.
Bed slope has
been exaggerated
for clarity.
The M2 curve describes
subcritical flow over a free
overfall.
Hc Hn


1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
31
M3 CURVE
M1: Hn > Hc > H1






)
H
(
1
)
H
(
S
S
dx
dH
2
f
Fr
3
2
w
2
3
2
w
f
f
gH
q
,
gH
q
C
S 
 Fr
Note that Fr > 1 at x1; integrate downstream. The starting flow is supercritical, but
the equilibrium (normal) flow is subcritical, requiring an intervening hydraulic jump.
As H increases downstream, both Sf and Fr decrease, and Fr decreases toward 1.
At the point where Fr would equal 1, dH/dx would equal . Before this state is
reached, however, the flow must undergo a hydraulic jump to subcritical flow.
Subcritical flow can make the transition to supercritical flow without a hydraulic
jump; supercritical flow cannot make the transition to subcritical flow without one.
Hydraulic jumps are discussed in more detail in Chapter 23.
Bed slope has
been exaggerated
for clarity.
The M3 curve describes
supercritical flow from a
sluice gate.
H1
Hc Hn


Hydraulic jump
M3 curve
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
32
HYDRAULIC JUMP
subcritical
flow
supercritical
In addition to M1, M2, and M3 curves, there is also the family of steep S1, S2 and S3
curves corresponding to the case for which Hc > Hn (normal flow is supercritical).
These curves tend to be very short, and are not covered in detail here.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
33
CALCULATION OF BACKWATER CURVES
Here the case of subcritical flow is considered, so that the direction of integration is
upstream. Let x1 be the starting point where H1 is given, and let x denote the step
length, so that xn+1 = xn - x. (Note that xn+1 is upstream of xn.) Furthermore, denote
the function [S-Sf(H)]/(1 – Fr2(H)] as F(H). In an Euler step scheme,
or thus
)
H
(
x
H
H
dx
dH
n
1
n
n
F



 
x
)
H
(
H
H n
n
1
n 


 F
A better scheme is a predictor-corrector scheme, according to which
x
)
H
(
H
H n
n
1
n
,
p 


 F
  x
)
H
(
)
H
(
2
1
H
H 1
n
,
p
n
n
1
n 


 
 F
F
A predictor-corrector scheme is used in the spreadsheet
RTe-bookBackwater.xls. This spreadsheet is used in the calculations of the
next few slides.
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
34
BACKWATER MEDIATES THE UPSTREAM EFFECT OF BASE LEVEL
(ELEVATION OF STANDING WATER)
A WORKED EXAMPLE (constant Cz):
S = 0.00025
Cz = 22
qw = 5.7 m2/s
D = 0.6 mm
R = 1.65
H1 = 30 m H1 > Hn > Hc
so M1 curve
m
01
.
3
gS
q
C
H
3
/
1
2
w
f
n 









m
49
.
1
g
q
H
3
/
1
2
w
c 









H
Hc Hn
H1


Example: calculate the variation in H and b = CfU2 in x upstream of x1
(here set equal to 0) until H is within 1 percent of Hn
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
35
0
5
10
15
20
25
30
35
-140000 -120000 -100000 -80000 -60000 -40000 -20000 0
x, m
H
(m),

b
(N/m
2
),
U
(m/s)
H
U
tb
RESULTS OF CALCULATION: PROFILES OF DEPTH H, BED SHEAR STRESS b
AND FLOW VELOCITY U
b
U
H
1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
36
RESULTS OF CALCULATION: PROFILES OF BED ELEVATION  AND WATER
SURFACE ELEVATION 
0
5
10
15
20
25
30
35
40
-140000 -120000 -100000 -80000 -60000 -40000 -20000 0
x m
bed
(

)
and
water
surface
(

)
elevations
m
h
x


1D SEDIMENT TRANSPORT MORPHODYNAMICS
with applications to
RIVERS AND TURBIDITY CURRENTS
© Gary Parker November, 2004
37
REFERENCES FOR CHAPTER 5
Chaudhry, M. H., 1993, Open-Channel Flow, Prentice-Hall, Englewood Cliffs, 483 p.
Crowe, C. T., Elger, D. F. and Robertson, J. A., 2001, Engineering Fluid Mechanics, John Wiley
and sons, New York, 7th Edition, 714 p.
Gilbert, G.K., 1914, Transportation of Debris by Running Water, Professional Paper 86, U.S.
Geological Survey.
Jain, S. C., 2000, Open-Channel Flow, John Wiley and Sons, New York, 344 p.
Kamphuis, J. W., 1974, Determination of sand roughness for fixed beds, Journal of Hydraulic
Research, 12(2): 193-202.
Keulegan, G. H., 1938, Laws of turbulent flow in open channels, National Bureau of Standards
Research Paper RP 1151, USA.
Henderson, F. M., 1966, Open Channel Flow, Macmillan, New York, 522 p.
Meyer-Peter, E., Favre, H. and Einstein, H.A., 1934, Neuere Versuchsresultate über den
Geschiebetrieb, Schweizerische Bauzeitung, E.T.H., 103(13), Zurich, Switzerland.
Meyer-Peter, E. and Müller, R., 1948, Formulas for Bed-Load Transport, Proceedings, 2nd
Congress, International Association of Hydraulic Research, Stockholm: 39-64.
Parker, G., 1991, Selective sorting and abrasion of river gravel. II: Applications, Journal of
Hydraulic Engineering, 117(2): 150-171.
Vanoni, V.A., 1975, Sedimentation Engineering, ASCE Manuals and Reports on Engineering
Practice No. 54, American Society of Civil Engineers (ASCE), New York.
Wong, M., 2003, Does the bedload equation of Meyer-Peter and Müller fit its own data?,
Proceedings, 30th Congress, International Association of Hydraulic Research, Thessaloniki,
J.F.K. Competition Volume: 73-80.

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RTe-bookCh5Hydraulics.ppt

  • 1. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 1 CHAPTER 5: REVIEW OF 1D OPEN CHANNEL HYDRAULICS Dam at Hiram Falls on the Saco River near Hiram, Maine, USA
  • 2. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 2 TOPICS REVIEWED This e-book is not intended to include a full treatment of open channel flow. It is assumed that the reader has had a course in open channel flow, or has access to texts that cover the field. Nearly all undergraduate texts in fluid mechanics for civil engineers have sections on open channel flow (e.g. Crowe et al., 2001). Three texts that specifically focus on open channel flow are those by Henderson (1966), Chaudhry (1993) and Jain (2000). Topics treated here include: • Relations for boundary resistance • Normal (steady, uniform) flow • St. Venant shallow water equations • Gradually varied flow • Froude number: subcritical, critical and supercritical flow • Classification of backwater curves • Numerical calculation of backwater curves
  • 3. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 3 SIMPLIFICATION OF CHANNEL CROSS-SECTIONAL SHAPE River channel cross sections have complicated shapes. In a 1D analysis, it is appropriate to approximate the shape as a rectangle, so that B denotes channel width and H denotes channel depth (reflecting the cross-sectionally averaged depth of the actual cross-section). As was seen in Chapter 3, natural channels are generally wide in the sense that Hbf/Bbf << 1, where the subscript “bf” denotes “bankfull”. As a result the hydraulic radius Rh is usually approximated accurately by the average depth. In terms of a rectangular channel, H B channel floodplain floodplain H B H 2 1 H H 2 B HB Rh           
  • 4. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 4 THE SHIELDS NUMBER: A KEY DIMENSIONLESS PARAMETER QUANTIFYING SEDIMENT MOBILITY gD R b     3 c 2 b 2 g 3 4 ~           D R D b = boundary shear stress at the bed (= bed drag force acting on the flow per unit bed area) [M/L/T2] c = Coulomb coefficient of resistance of a granule on a granular bed [1] Recalling that R = (s/) – 1, the Shields Number * is defined as It can be interpreted as a ratio scaling the ratio impelling force of flow drag acting on a particle to the Coulomb force resisting motion acting on the same particle, so that The characterization of bed mobility thus requires a quantification of boundary shear stress at the bed.
  • 5. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 5 QUANTIFICATION OF BOUNDARY SHEAR STRESS AT THE BED 2 / 1 f C u U Cz     U = cross-sectionally averaged flow velocity ( depth-averaged flow velocity in the wide channels studied here) [L/T] u* = shear velocity [L/T] Cf = dimensionless bed resistance coefficient [1] Cz = dimensionless Chezy resistance coefficient [1] 2 b f U C    BH Q U      b u
  • 6. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 6 RESISTANCE RELATIONS FOR HYDRAULICALLY ROUGH FLOW 6 / 1 s r 2 / 1 f k H C u U Cz               Keulegan (1938) formulation: 90 s k s D n k                s 2 / 1 f k H 11 n 1 C u U Cz  where  = 0.4 denotes the dimensionless Karman constant and ks = a roughness height characterizing the bumpiness of the bed [L]. Manning-Strickler formulation: where r is a dimensionless constant between 8 and 9. Parker (1991) suggested a value of r of 8.1 for gravel-bed streams. Roughness height over a flat bed (no bedforms): where Ds90 denotes the surface sediment size such that 90 percent of the surface material is finer, and nk is a dimensionless number between 1.5 and 3. For example, Kamphuis (1974) evaluated nk as equal to 2.
  • 7. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 7 COMPARISION OF KEULEGAN AND MANNING-STRICKLER RELATIONS r = 8.1 Note that Cz does not vary strongly with depth. It is often approximated as a constant in broad- brush calculations. 1 10 100 1 10 100 1000 H/ks Cz Keulegan Parker Version of Manning- Strickler 6 / 1 s k H 1 . 8 Cz         
  • 8. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 8 BED RESISTANCE RELATION FOR MOBILE-BED FLUME EXPERIMENTS Sediment transport relations for rivers have traditionally been determined using a simplified analog: a straight, rectangular flume with smooth, vertical sidewalls. Meyer-Peter and Müller (1948) used two famous early data sets of flume data on sediment transport to determine their famous sediment transport relation (introduced later). These are a) a subset of the data of Gilbert (1914) collected at Berkeley, California (D50 = 3.17 mm, 4.94 mm and 7.01 mm) and the set due to Meyer-Peter et al. (1934) collected at E.T.H., Zurich, Switzerland (D50 = 5.21 mm and 28.65 mm). Bedforms such as dunes were present in many of the experiments in these two sets. In the case of 116 experiments of Gilbert and 52 experiments of Meyer-Peter et al., it was reported that no bedforms were present and that sediment was transported under flat-bed conditions. Wong (2003) used this data set to study bed resistance over a mobile bed without bedforms. Flume at Tsukuba University, Japan (flow turned off). Image courtesy H. Ikeda. Note that bedforms known as linguoid bars cover the bed.
  • 9. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 9 BED RESISTANCE RELATION FOR MOBILE-BED FLUME EXPERIMENTS contd. Most laboratory flumes are not wide enough to prevent sidewall effects. Vanoni (1975), however, reports a method by which sidewall effects can be removed from the data. As a result, depth H is replaced by the hydraulic radius of the bed region Rb. (Not to worry, Rb  H as H/B  0). Wong (2003) used this procedure to remove sidewall effects from the previously-mentioned data of Gilbert (1914) and Meyer-Peter et al. (1934). The material used in all the experiments in question was quite well-sorted. Wong (2003) estimated a value of D90 from the experiments using the given values of median size D50 and geometric standard deviation g, and the following relation for a log-normal grain size distribution; Wong then estimated ks as equal to 2D90 in accordance with the result of Kamphuis (1974), and s in the Manning-Strickler resistance relation as 8.1 in accordance with Parker (1991). The excellent agreement with the data is shown on the next page. 28 . 1 g 50 90 D D  
  • 10. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 10 1.00 10.00 100.00 1.00 10.00 100.00 Rb/ks Cz ETH 52 Gilbert 116 Parker Version of Manning- Strickler 6 / 1 s b k R 1 . 8 Cz          TEST OF RESISTANCE RELATION AGAINST MOBILE-BED DATA WITHOUT BEDFORMS FROM LABORATORY FLUMES
  • 11. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 11 NORMAL FLOW Normal flow is an equilibrium state defined by a perfect balance between the downstream gravitational impelling force and resistive bed force. The resulting flow is constant in time and in the downstream, or x direction. Parameters: x = downstream coordinate [L] H = flow depth [L] U = flow velocity [L/T] qw = water discharge per unit width [L2T-1] B = width [L] Qw = qwB = water discharge [L3/T] g = acceleration of gravity [L/T2]  = bed angle [1] b = bed boundary shear stress [M/L/T2] S = tan = streamwise bed slope [1] (cos   1; sin   tan   S)  = water density [M/L3] As can be seen from Chapter 3, the bed slope angle  of the great majority of alluvial rivers is sufficiently small to allow the approximations 1 cos , S tan sin        x B x gHxBS bBx H
  • 12. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 12 UHB B q Q UH q w w w    Conservation of downstream momentum: Impelling force (downstream component of weight of water) = resistive force x B xS gHB sin x gHB b          gHS b    Reduce to obtain depth-slope product rule for normal flow: NORMAL FLOW contd. Conservation of water mass (= conservation of water volume as water can be treated as incompressible):  x B x gHxBS bBx H gHS u  
  • 13. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 13 ESTIMATED CHEZY RESISTANCE COEFFICIENTS FOR BANKFULL FLOW BASED ON NORMAL FLOW ASSUMPTION FOR u* The plot below is from Chapter 3 1 10 100 1 10 100 1000 10000 100000 Grav Brit Grav Alta Grav Ida Sand Mult Sand Sing bf Cz Ĥ 50 bf bf bf bf bf bankfull bf D H Ĥ , S gH H B Q u U Cz            
  • 14. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 14 Relation for Shields stress  at normal equilibrium: (for sediment mobility calculations) gHS U C 2 f    RELATION BETWEEN qw, S and H AT NORMAL EQUILIBRIUM D R HS gD R b      D R S g q C 3 / 2 3 / 1 2 w f           3 / 1 2 w f gS q C H          2 / 1 2 / 1 z 2 / 1 2 / 1 f S H g C S H C g U   or Reduce the relation for momentum conservation b = gHS with the resistance form b = CfU2: Generalized Chezy velocity relation Further eliminating U with the relation for water mass conservation qw = UH and solving for flow depth:
  • 15. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 15 ESTIMATED SHIELDS NUMBERS FOR BANKFULL FLOW BASED ON NORMAL FLOW ASSUMPTION FOR b The plot below is from Chapter 3 2 50 50 bf 50 bf 50 b 50 bf D gD Q Q̂ , D R S H gD R       1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+04 1.E+06 1.E+08 1.E+10 1.E+12 1.E+14 Grav Brit Grav Alta Sand Mult Sand Sing Grav Ida Q̂   50 bf
  • 16. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 16 RELATIONS AT NORMAL EQUILIBRIUM WITH MANNING-STRICKLER RESISTANCE FORMULATION 6 / 1 s r 2 / 1 f 3 / 1 2 w f k H C gS q C H                     RD S g q k 10 / 7 10 / 3 2 r 2 w 3 / 1 s            Relation for Shields stress  at normal equilibrium: (for sediment mobility calculations) 10 / 3 2 r 2 w 3 / 1 s gS q k H           2 / 1 3 / 2 6 / 1 s r 2 / 1 2 / 1 f S H k g S H C g U    6 / 1 s r 2 / 1 3 / 2 k g n 1 , S H n 1 U    Manning-Strickler velocity relation (n = Manning’s “n”) Solve for H to find Solve for U to find
  • 17. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 17 BUT NOT ALL OPEN-CHANNEL FLOWS ARE AT OR CLOSE TO EQUILIBRIUM! Flow into standing water (lake or reservoir) usually takes the form of an M1 curve. Flow over a free overfall (waterfall) usually takes the form of an M2 curve. A key dimensionless parameter describing the way in which open-channel flow can deviate from normal equilibrium is the Froude number Fr: gH U  Fr And therefore the calculation of bed shear stress as b = gHS is not always accurate. In such cases it is necessary to compute the disquilibrium (e.g. gradually varied) flow and calculate the bed shear stress from the relation 2 f b U C   
  • 18. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 18 NON-STEADY, NON-UNIFORM 1D OPEN CHANNEL FLOWS: St. Venant Shallow Water Equations 0 x UH t H       Relation for water mass conservation (continuity): Relation for momentum conservation: 2 f 2 2 U C x gH x H g 2 1 x H U t UH               x = boundary (bed) attached nearly horizontal coordinate [L] y = upward normal coordinate [L]  = bed elevation [L] S = tan  - /x [1] H = normal (nearly vertical) flow depth [L] Here “normal” means “perpendicular to the bed” and has nothing to do with normal flow in the sense of equilibrium. x y  H Bed and water surface slopes exaggerated below for clarity.
  • 19. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 19 DERIVATION: EQUATION OF CONSERVATION OF OF WATER MASS Q = UHB = volume water discharge [L3/T] Q = Mass water discharge = UHB [M/T] /t(Mass in control volume) = Net mass inflow rate x x UH B UHB UHB t x HB x x x                 0 x UH t H       Reducing under assumption of constant B: H B x Q Q
  • 20. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 20 STREAMWISE MOMENTUM DISCHARGE Momentum flows! Qm = U2HB = streamwise discharge of streamwise momentum [ML/T2]. The derivation follows below. Momentum crossing left face in time t = (HBU2t) = mass x velocity Qm = momentum crossing per unit time, = (Momentum crossing in t)/ t = U2HB HB U Q 2 m   Ut (HBUt)(U) U Note that the streamwise momentum discharge has the same units as force, and is often referred to as the streamwise inertial force.
  • 21. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 21 The flow is assumed to be gradually varying, i.e. the spatial scale Lx of variation in the streamwise direction satisfies the condition H/Lx << 1. Under this assumption the pressure p can be approximated as hydrostatic. Where z = an upward normal coordinate from the bed, STREAMWISE PRESSURE FORCE     H 0 2 p BgH 2 1 pdz B F Fp = pressure force [ML/T2] g z p      Integrate and evaluate the constant of integration under the condition of zero (gage) pressure at the water surface, where y = H, to get:   z H g p    p = pressure (normal stress) [M/L/T2] Integrate the above relation over the cross-sectional area to find the streamwise pressure force Fp: p H Fp B
  • 22. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 22 DERIVATION: EQUATION OF CONSERVATION OF STREAMWISE MOMENTUM /t(Momentum in control volume) = net momentum inflow rate + sum of forces Sum of forces = downstream gravitational force – resistive force + pressure force at x – pressure force at x + x xB U C x x gHB B gH 2 1 B gH 2 1 HB U HB U t xU HB 2 f x x 2 x 2 x x 2 x 2                          2 f 2 U C x gH x H gH x H U t HU               x Qm B H Qm Fp Fp gHBxS bBx or reducing,
  • 23. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 23 CASE OF STEADY, GRADUALLY VARIED FLOW 0 x UH t H       2 f 2 2 U C x gH x H g 2 1 x H U t UH                  w q UH H U C dx d g dx dH g dx dU U 2 f      Reduce equation of water mass conservation and integrate: dx dH H q dx dU H q U 2 w w     Thus: Reduce equation of streamwise momentum conservation: But with water conservation: dx dU UH dx dUH U dx H dU2   So that momentum conservation reduces to: constant
  • 24. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 24 THE BACKWATER EQUATION to get the backwater equation: 2 f f 2 3 2 w 2 C S , gH U gH q , x S Fr Fr         2 f 1 S S dx dH Fr    H U C x g x H g dx dU U 2 f          Reduce with dx dH H q dx dU , H q U 2 w w    where Here Fr denotes the Froude number of the flow and Sf denotes the friction slope. For steady flow over a fixed bed, bed slope S (which can be a function of x) and constant water discharge per unit width qw are specified, so that the backwater equation specified a first-order differential equation in H, requiring a specified value of H at some point as a boundary condition.
  • 25. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 25 2 f 1 S S dx dH Fr    3 / 1 2 w f n 3 n 2 w f f gS q C H gH q C S S             Consider the case of constant bed slope S. Setting the numerator of the right-hand side backwater equation = zero, so that S = Sf (friction slope equals bed slope) recovers the condition of normal equilibrium, at which normal depth Hn prevails: Setting the denominator of the right-hand side of the backwater equation = zero yields the condition of Froude-critical flow, at which Fr = 1 and depth = the critical value Hc: 3 / 1 2 w c 3 c 2 w 2 g q H gH q 1             Fr NORMAL AND CRITICAL DEPTH At any given point in a gradually varied flow the depth H may differ from both Hn and Hc. If Fr = qw/(gH3)1/2 < 1 the flow slow and deep and is termed subcritical; if on the other hand Fr > 1 the flow is swift and shallow and is termed supercritical. The great majority of flows in alluvial rivers are subcritical, but supercritical flows do occur. Supercritical flows are common during floods in steep bedrock rivers.
  • 26. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 26 COMPUTATION OF BACKWATER CURVES The case of constant bed slope S is considered as an example. Let water discharge qw and bed slope S be given. In the case of constant bed friction coefficient Cf, let Cf be given. In the case of Cf specified by the Manning-Strickler relation, let r and ks be given. Compute Hc: Compute Hn : If Hn > Hc then (Fr)n < 1: normal flow is subcritical, defining a “mild” bed slope. If Hn < Hc then (Fr)n > 1: normal flow is supercritical, defining a “steep” bed slope. , ) H ( 1 ) H ( S S dx dH 2 f Fr    Requires 1 b.c. for unique solution: 1 x H H 1  3 / 1 2 w c g q H          3 / 1 2 w f n gS q C H          10 / 3 2 r 2 w 3 / 1 s n gS q k H           or 3 2 w 3 / 1 s 2 r f 3 2 w f f 3 2 w 2 gH q k H ) H ( S or gH q C ) H ( S , gH q ) H (               Fr where x1 is a starting point. Integrate upstream if the flow at the starting point is subcritical, and integrate downstream if it is supercritical.
  • 27. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 27 COMPUTATION OF BACKWATER CURVES contd. Flow at a point relative to critical flow: note that It follows that 1 – Fr2(H) < 0 if H < Hc, and 1 – Fr2 > 0 if H > Hc. 3 c 2 w 3 2 w 2 gH q 1 , gH q ) H (   Fr Flow at a point relative to normal flow: note that for the case of constant Cf 3 n 2 w f 3 2 w f f gH q C S , gH q C ) H ( S   3 n 2 w 3 / 1 s n 2 r 3 2 w 3 / 1 s 2 r f gH q k H S , gH q k H ) H ( S                         and for the case of the Manning-Strickler relation It follows in either case that S – Sf(H) < 0 if H < Hn, and S – Sf(H) > 0 if H > Hn.
  • 28. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 28 MILD BACKWATER CURVES M1, M2 AND M3 M1: H1 > Hn > Hc Again the case of constant bed slope S is considered. Recall that M2: Hn > H1 > Hc M3: Hn > Hc > H1       ) H ( 1 ) H ( S S dx dH 1 2 1 f x1 Fr Depth increases downstream, decreases upstream Depth increases downstream, decreases upstream Depth decreases downstream, increases upstream A bed slope is considered mild if Hn > Hc. This is the most common case in alluvial rivers. There are three possible cases. 3 2 w 3 / 1 s 2 r f 3 2 w f f 3 2 w 2 gH q k H ) H ( S or gH q C ) H ( S , gH q ) H (               Fr       ) H ( 1 ) H ( S S dx dH 1 2 1 f x1 Fr       ) H ( 1 ) H ( S S dx dH 1 2 1 f x1 Fr
  • 29. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 29 M1 CURVE M1: H1 > Hn > Hc       ) H ( 1 ) H ( S S dx dH 2 f Fr 3 2 w 2 3 2 w f f gH q , gH q C S   Fr Water surface elevation  =  + H (remember H is measured normal to the bed, but is nearly vertical as long as S << 1). Note that Fr < 1 at x1: integrate upstream. Starting and normal (equilibrium) flows are subcritical. As H increases downstream, both Sf and Fr decrease toward 0. Far downstream, dH/dx = S  d/dx = d/dx(H + ) = constant: ponded water As H decreases upstream, Sf approaches S while Fr remains < 1. Far upstream, normal flow is approached. Bed slope has been exaggerated for clarity. H Hc Hn H1   The M1 curve describes subcritical flow into ponded water.
  • 30. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 30 M2 CURVE M1: Hn > H1 > Hc       ) H ( 1 ) H ( S S dx dH 2 f Fr 3 2 w 2 3 2 w f f gH q , gH q C S   Fr Note that Fr < 1 at x1; integrate upstream. Starting and normal (equilibrium) flows are subcritical. As H decreases downstream, both Sf and Fr increase, and Fr increases toward 1. At some point downstream, Fr = 1 and dH/dx = - : free overfall (waterfall). As H increases upstream, Sf approaches S while Fr remains < 1. Far upstream, normal flow is approached. Bed slope has been exaggerated for clarity. The M2 curve describes subcritical flow over a free overfall. Hc Hn  
  • 31. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 31 M3 CURVE M1: Hn > Hc > H1       ) H ( 1 ) H ( S S dx dH 2 f Fr 3 2 w 2 3 2 w f f gH q , gH q C S   Fr Note that Fr > 1 at x1; integrate downstream. The starting flow is supercritical, but the equilibrium (normal) flow is subcritical, requiring an intervening hydraulic jump. As H increases downstream, both Sf and Fr decrease, and Fr decreases toward 1. At the point where Fr would equal 1, dH/dx would equal . Before this state is reached, however, the flow must undergo a hydraulic jump to subcritical flow. Subcritical flow can make the transition to supercritical flow without a hydraulic jump; supercritical flow cannot make the transition to subcritical flow without one. Hydraulic jumps are discussed in more detail in Chapter 23. Bed slope has been exaggerated for clarity. The M3 curve describes supercritical flow from a sluice gate. H1 Hc Hn   Hydraulic jump M3 curve
  • 32. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 32 HYDRAULIC JUMP subcritical flow supercritical In addition to M1, M2, and M3 curves, there is also the family of steep S1, S2 and S3 curves corresponding to the case for which Hc > Hn (normal flow is supercritical). These curves tend to be very short, and are not covered in detail here.
  • 33. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 33 CALCULATION OF BACKWATER CURVES Here the case of subcritical flow is considered, so that the direction of integration is upstream. Let x1 be the starting point where H1 is given, and let x denote the step length, so that xn+1 = xn - x. (Note that xn+1 is upstream of xn.) Furthermore, denote the function [S-Sf(H)]/(1 – Fr2(H)] as F(H). In an Euler step scheme, or thus ) H ( x H H dx dH n 1 n n F      x ) H ( H H n n 1 n     F A better scheme is a predictor-corrector scheme, according to which x ) H ( H H n n 1 n , p     F   x ) H ( ) H ( 2 1 H H 1 n , p n n 1 n       F F A predictor-corrector scheme is used in the spreadsheet RTe-bookBackwater.xls. This spreadsheet is used in the calculations of the next few slides.
  • 34. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 34 BACKWATER MEDIATES THE UPSTREAM EFFECT OF BASE LEVEL (ELEVATION OF STANDING WATER) A WORKED EXAMPLE (constant Cz): S = 0.00025 Cz = 22 qw = 5.7 m2/s D = 0.6 mm R = 1.65 H1 = 30 m H1 > Hn > Hc so M1 curve m 01 . 3 gS q C H 3 / 1 2 w f n           m 49 . 1 g q H 3 / 1 2 w c           H Hc Hn H1   Example: calculate the variation in H and b = CfU2 in x upstream of x1 (here set equal to 0) until H is within 1 percent of Hn
  • 35. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 35 0 5 10 15 20 25 30 35 -140000 -120000 -100000 -80000 -60000 -40000 -20000 0 x, m H (m),  b (N/m 2 ), U (m/s) H U tb RESULTS OF CALCULATION: PROFILES OF DEPTH H, BED SHEAR STRESS b AND FLOW VELOCITY U b U H
  • 36. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 36 RESULTS OF CALCULATION: PROFILES OF BED ELEVATION  AND WATER SURFACE ELEVATION  0 5 10 15 20 25 30 35 40 -140000 -120000 -100000 -80000 -60000 -40000 -20000 0 x m bed (  ) and water surface (  ) elevations m h x  
  • 37. 1D SEDIMENT TRANSPORT MORPHODYNAMICS with applications to RIVERS AND TURBIDITY CURRENTS © Gary Parker November, 2004 37 REFERENCES FOR CHAPTER 5 Chaudhry, M. H., 1993, Open-Channel Flow, Prentice-Hall, Englewood Cliffs, 483 p. Crowe, C. T., Elger, D. F. and Robertson, J. A., 2001, Engineering Fluid Mechanics, John Wiley and sons, New York, 7th Edition, 714 p. Gilbert, G.K., 1914, Transportation of Debris by Running Water, Professional Paper 86, U.S. Geological Survey. Jain, S. C., 2000, Open-Channel Flow, John Wiley and Sons, New York, 344 p. Kamphuis, J. W., 1974, Determination of sand roughness for fixed beds, Journal of Hydraulic Research, 12(2): 193-202. Keulegan, G. H., 1938, Laws of turbulent flow in open channels, National Bureau of Standards Research Paper RP 1151, USA. Henderson, F. M., 1966, Open Channel Flow, Macmillan, New York, 522 p. Meyer-Peter, E., Favre, H. and Einstein, H.A., 1934, Neuere Versuchsresultate über den Geschiebetrieb, Schweizerische Bauzeitung, E.T.H., 103(13), Zurich, Switzerland. Meyer-Peter, E. and Müller, R., 1948, Formulas for Bed-Load Transport, Proceedings, 2nd Congress, International Association of Hydraulic Research, Stockholm: 39-64. Parker, G., 1991, Selective sorting and abrasion of river gravel. II: Applications, Journal of Hydraulic Engineering, 117(2): 150-171. Vanoni, V.A., 1975, Sedimentation Engineering, ASCE Manuals and Reports on Engineering Practice No. 54, American Society of Civil Engineers (ASCE), New York. Wong, M., 2003, Does the bedload equation of Meyer-Peter and Müller fit its own data?, Proceedings, 30th Congress, International Association of Hydraulic Research, Thessaloniki, J.F.K. Competition Volume: 73-80.