chapter 22: morphodynamics of recirculating and feed flumes

1

1D SEDIMENT TRANSPORT MORPHODYNAMICSwith applications to

RIVERS AND TURBIDITY CURRENTS© Gary Parker November, 2004

CHAPTER 22:MORPHODYNAMICS OF RECIRCULATING AND FEED FLUMES

Laboratory flumes have proved to be valuable tools in the study of sediment transport and morphodynamics. Here the case of flumes with vertical, inerodible walls are considered. There are two basic types of such flumes:

• Recirculating flumes and

• Feed flumes

In addition, there are several variant types, one of which is discussed in a succeeding slide.

Recirculating flume in the Netherlands used by A. Blom and M. Kleinhans to study vertical sediment sorting by dunes. Image courtesy A. Blom.

2



THE MOBILE-BED EQUILIBRIUM STATE

The flume considered here is of the simplest type; it has a bed of erodible alluvium, constant width B and vertical, inerodible walls. The bed sediment is covered by water from wall to wall. One useful feature of such a flume is that if it is run long enough, it will eventually approach a mobile-bed equilibrium, as discussed in Chapter 14. When this state is reached, all quantities such as water discharge Q (or qw = Q/B), total volume bed material sediment transport rate Qt (or qt = Qt/B), bed slope S, flow depth H etc. become spatially constant in space (except in entrance and exit regions) and time. (The parameters in question are averaged over bedforms such as dunes and bars if they are present.)

pump

sediment

waterH

S

Neglect entrance and exit regions

3



Friction relations:

where kc is a composite bed roughness which may include the effect of bedforms (if present).

THE EQUILIBRIUM STATE: REVIEW OF MATERIAL FROM CHAPTER 14

The hydraulics of the equilibrium state are those of normal flow. Here the case of a plane bed (no bedforms) is considered as an example. The bed consists of uniform material with size D. The governing equations are (Chapter 5):

UHqw Water conservation: gHSb

2fb UC )StricklerManning(

k

HCor)Chezy(constC

6/1

cr

2/1ff

Momentum conservation:

Generic transport relation of the form of Meyer-Peter and Müller for total bed material load: where t and nt are dimensionless constants:

tn

cb

tt

RgDDRgD

q

4



THE EQUILIBRIUM STATE: REVIEW contd.

In the case of the Chezy resistance relation, for example, the equations governing the normal state reduce to:

3/12wf

gS

qCH

tn

c

3/23/12wf

tt RD

S

g

qCDRgDq

In the case of the Manning-Stickler resistance relation, the equationsgoverning the normal state reduce with to:

10/3

2r

2w

3/1c

gS

qkH

tn

c

10/710/3

2r

2w

3/1c

tt RD

S

g

qkDRgDq

Let D, kc and R be given. In either case above, there are two equations for four parameters at equilibrium; water discharge per unit width qw, volume sediment discharge per unit width qt, bed slope S and flow depth H. If any two of the set (qw, qt, S and H) are specified, the other two can be computed.

5



THE RECIRCULATING FLUME

pump

sediment

water

In a recirculating flume all the water and all the sediment are recirculated through a pump. The total amounts of water and sediment in the system are conserved. In addition to the sediment itself, the operator is free to specify two parameters in operating the flume: the water discharge per unit width qw and the flow depth H.

The water discharge (and thus the discharge per unit width qw) is set by the pump setting. (More properly, what are specified are the head-discharge relation of the pump and the setting of the valve on the return line, but in many recirculating systems flow discharge itself can be set with relative ease and accuracy.)

The constant flow depth H reached at equilibrium is set by the total amount of water in the system, which is conserved. Increasing the total amount of water in the system increases the depth reached at final mobile-bed equilibrium.

Thus in a recirculating flume, equilibrium qw and H are set by the flume operator, and total volume sediment transport rate per unit width qt and bed slope S evolve to equilibrium accordingly.

6



THE FEED FLUME

In a feed flume all the water and all the sediment are fed in at the upstream and allowed to wash out at the downstream end. Water is introduced (usually pumped) into the channel at the desired rate, and sediment is fed into the channel using e.g. a screw-type feeder at the desired rate. In addition to the sediment itself, the operator is thus free to specify two parameters in the operation of the flume: the water discharge per unit width qw and the total volume sediment discharge per unit width qt reached at final equilibrium, which must be equal to the feed rate qtf.

Thus in a feed flume, equilibrium qw and qt are set, by the flume operator, and equilibrium flow depth H and bed slope S evolve accordingly.

pump

sediment

water

sediment feeder

tailgate

7



A HYBRID TYPE: THE SEDIMENT-RECIRCULATING, WATER-FEED FLUME

In the sediment-recirculating, water-feed flume the sediment and water are separated at the downstream end. Nearly all the water overflows from a collecting tank. The sediment settles to the bottom of the collecting tank, and is recirculated with a small amount of water as a slurry. The water discharge per unit width qw is thus set by the operator (up to the small fraction of water discharge in the recirculation line). The total amount of sediment in the flume is conserved. In addition, a downstream weir controls the downstream elevation of the bed. The combination of these two conditions constrains the bed slope S at mobile-bed equilibrium. Adding more sediment to the flume increases the equilibrium bed slope.

Thus in a sediment-recirculating, water-feed flume, qw and S are set by the flume operator and qt and H evolve toward equilibrium accordingly.

slurry of sediment + small fraction of water discharge

weir

sediment-free waterpump

sediment

water

recirculated sediment

slurry pump

water

8



The final mobile-bed equilibrium state of a flume is usually not precisely known in advance. Flow is thus commenced from some arbitrary initial state and allowed to approach equilibrium. This motivates the following two questions:

• How long should one wait in order to reach mobile-bed equilibrium?• What is the path by which mobile-bed equilibrium is reached?It might be expected that the answer to these questions depends on the type of flume

under consideration. Here two types of flumes are considered: a) a pure feed flume and b) a pure recirculating flume.

In performing the analysis, the following simplifying assumptions (which can easily be relaxed) are made:

1. The flow is always assumed to be subcritical in the sense that Fr < 1.2. The channel is assumed to have a sufficiently large aspect ration B/H that

sidewall effects can be neglected.3. Bed resistance is approximated in terms of a constant resistance coefficient Cf, so that the details of bedform mechanics are neglected. 4. The sediment has uniform size D.

The analysis presented here is based on Parker (2003).

MORPHODYNAMICS OF APPROACH TO EQUILIBRIUM

9



pump

THE LEGEND OF SEDIMENT LUMPS IN RECIRCULATING FLUMES

In the world of sediment flumes, there is a persistent legend concerning recirculating flumes that is rarely documented in the literature. That is, these flumes are said to develop sediment “lumps” that recirculate round and round, either without dissipating or with only slow dissipation. The author of this e-book has heard this story from T. Maddock, V. Vanoni and N. Brooks. One of the author’s graduate students encountered these lumps in a recirculating, meandering flume and showed them to the author (Hills, 1987).

10



pump



11



pump



12



pump



13



pump



14



x = streamwise coordinatet = timeH = H(x, t) = flow depthU = U(x, t) = depth-averaged flow

velocity = (x, t) = bed elevationS = - /x = bed slopeg = gravitational accelerationqt = volume bed material sediment

transport rate per unitwidth

qw = UH = water discharge per unitwidth

b = boundary shear stress at bedL = flume lengthB = flume widthD = sediment sizep = porosity of bed deposit of sediment

H

B

PARAMETERS

pump

sediment

water

L

pump

sediment

water

sediment feeder

tailgate

L

15



pump

sediment

water

L

pump

sediment

water

sediment feeder

tailgate

L

KEY APPROXIMATIONS AND ASSUMPTIONS

H

B• Flume has constant width B.• Sediment is of uniform size D.• H/B << 1: flume is wide and sidewall

effects can be neglected.• Flume is sufficiently long so that entrance and exit regions can be neglected.• Flow in the flume is always Froude-

subcritical: Fr = U/(gH)1/2 < 1.• qt/qw << 1: volume transport rate of

sediment is always much lower than that of water.

• Resistance coefficient Cf is approximatedas constant.

16



GOVERNING EQUATIONS: 1D FLOW

Flow mass balance

b22

xgH

x

Hg

2

1

x

HU

t

UH

0x

UH

t

H

Flow momentum balance

Closure relation for shear stress: Cf = dimensionless bed friction coefficient

x

q

t)1( t

p

Sediment mass balance

2fb UC

The condition qt/qw << 1 allows the use of the quasi-steady approximation introduced in Chapter 13, according to which the time-dependent terms in the equations of flow mass and momentum balance can be neglected.

17



REDUCTION TO BACKWATER FORM

2f

22

w

w

UCx

gHx

Hg

2

1

x

HU,qUh

qUH0x

UH

or thus

3

2w

3

2w

f gHq

1gHq

Cxx

H

gH

U

gH

q,CS

2

3

2w22

ff FrFr2f

1

SS

x

H

Fr

where

The equations of flow mass and momentum balance reduce to the standard backwater equation introduced in Chapter 5.

18



SEDIMENT TRANSPORT RELATION

D = grain size (uniform)s = sediment density = water densityR = (s/ ) – 1 1.65

DRgD

qq t

t

RgD

bEinstein number Shields number

cn

ct

ct

if)(

if0q

t

Sediment transport is characterized in terms of the same generic sediment transport relation as used in Chapter 20, except that the parameter s is set equal to unity. Thus where

19



CONSTRAINTS ON A RECIRCULATING FLUME

L

0 2Cdx

)L(q)0(qdxdt

d)1( tt

L

0p

)L(q)0(q tt

L

0 1CHdx

The total amount of water in the flume is conserved.With constant width, constant storage in the return line and negligible storage in the entrance and exit regions (L sufficiently large), the constraint is (where C1 is a constant):

At final equilibrium, when H = Ho, the constraint reduces to HoL = C1, according to which Ho is set by the total amount of water.

The total amount of sediment in the flume is conserved. Neglecting storage in the return line and the head box, the constraint is (where C2 is another constant):

Integrate the equation of sediment mass conservation x

q

t)1( t

p

But from above L

00dx

dt

d So a cyclic boundary condition is obtained:

Water discharge qw is set by the pump.

to get

20



CONSTRAINTS ON A FEED FLUME

Three constraints:

tf0xt qq

dLxH

Water discharge qw is set by the pump.

The upstream sediment discharge is set by the feeder. Where qtf is the sediment feed rate:

Let = + H denote water surface elevation.The downstream water surface elevation d is set by the tailgate:

The long-term equilibrium approached in a recirculating flume (without lumps) should be dynamically equivalent to that obtained in a sediment-feed flume (e.g. Parker and Wilcock, 1993).

21



The sediment transport relation reduces to the form:

which applies whether or not mobile-bed equilibrium is reached.

MOBILE-BED EQUILIBRIUM

wqUH

H

UC

xg

x

Hg

x

UU

2

f gHSUC 2f

xS

c

2f

N

c

2f

L

c

2f

t

RgD

UCif

RgD

UC

RgD

UCif0

DRgD

qL

Let R, g, D, t, nt, c* and Cf be specified. The equations in the boxes definethree equations in five parameters Uo, Ho, qto, qw and So at mobile-bed equilibrium.

At mobile-equilibrium the equation of momentum balance reduce to the relation for normal flow introduced in Chapter 5:

The equation for water conservation reduces to:

22



MOBILE-BED EQUILIBRIUM contd.

Recirculating flume:

Water discharge/width qw is set by pump.

Total amount of water Vw in system is conserved. Assuming constant storage in return line and neglecting entrance and exit storage, Vw = HLB depth Ho is set.

Solve three equations for qto, U0, So.

Feed flume:

Water discharge/width qw is set by pump.

Sediment discharge/width qto is set by feeder.

Solve three equations for uo, Ho, So.

The subscript “o” denotes mobile-bed equilibrium conditions.

pump

sediment

water

L

pump

sediment

water

sediment feeder

tailgate

L

23



APPROACH TO EQUILIBRIUM: NON-DIMENSIONAL FORMULATION

tq

LS)1(txLxq~qqH

~HH

to

2o

ptoto

Dimensionless parameters describing the approach to equilibrium (denoted by the tilde or downward cup) are formed using the values Ho, qto, So corresponding to normal equilibrium.

Bed elevation is decomposed into into a spatially averaged value and a deviation from this d(x, t), so that by definition

)t(

L

0 dd 0dx,

)t,x(LS,)t̂(~H dodao

The above two parameters are made dimensionless as follows:;

From the above relations,ado

~,LS Fl

LS

H

o

oFl = the dimensionless flume numberwhere

24



APPROACH TO EQUILIBRIUM contd.

The sediment transport relation is :

where

The backwater relation is:

2/1

3o

2w

o32o

3d

gH

q,

H~

1

H~

xx

H~

FrFr

Fl

1

r2

N

1

r

1

r2

1

r2

H~

if1

H~

H~

if0

q~L

2o

2wf

2of

oc

or gDHR

qC

gDR

UC,

1x0x

d

1x0x

a

q~q~x

q~

t

q~q~td

~d

FlThe relation for sediment conservation decomposes into two parts:

The dimensionless relations governing the approach to equilibrium are thus as follows; where Fro and denote the Froude and Shields numbers at mobile-bed equilibrium, respectively,

o

25



The boundary condition on the backwater equation is

where d is a constant downstream elevation set by a tailgate. This condition must hold at all flows, including the final mobile-bed equilibrium. Now the datum for elevation is set (arbitrarily but conveniently) to be equal to the bed elevation at the center of the flume (x = 0.5 L) at equilibrium, so that ao = 0 and

Between the above two relations and the nondimensionalizations, it is found that

APPROACH TO EQUILIBRIUM IN A FEED FLUMEIn a feed flume, the boundary condition on the sediment transport rate at the upstream end is , or in dimensionless variables,

Thus the relations for sediment conservation of the previous slide reduce to

totf0xt qqq

doo HxS2

1

1xda1x 2

11~1H~

FL

1x

d

1x

a q~1x

q~

t,q~1

td

~d

Fl

1q~0x

dLxH

26



APPROACH TO EQUILIBRIUM IN A FEED FLUME: SUMMARYThe equations

1xda1x 2

11~1H~

FL

1x

d

1x

a q~1x

q~

t,q~1

td

~d

Fl

,1q~0x

,H~

1

H~

xx

H~

32o

3d

FrFl

must be solved with the sediment transport relation and boundary conditions:

,

H~

if1

H~

H~

if0

q~ 1

r2

N

1

r

1

r2

1

r2

L

and a suitable initial condition, e.g. where SI is an initial bed slope and aI is an initial value for flume-averaged bed elevation, , a = aI, d = SIL[0.5 – (x/L)], or in dimensionless terms.

x

2

1

S

S,~~

o

I0tdaI0ta

Note furthermore that aI ( ) and SI must be chosen so as to yield subcritical flowin the flume at t = 0 ( ).

aI~

0t

27



APPROACH TO EQUILIBRIUM IN A FEED FLUME: FLOW OF THE CALCULATIONIn the case of a feed flume, the calculation flows directly with no iteration. At any time (e.g. ) the bed elevation profile, e,g, the parameters and , are known. The backwater equation

is then solved for using the standard step method of Chapter 5 upstream from the downstream end, where the boundary condition is:

Once is known everywhere, is obtained everywhere from the sediment transport relation of the previous slide. The bed elevation one time step later is determined from a discretized version of

1xda1x 2

11~1H~

FL

1x

d

1x

a q~1x

q~

t,q~1

td

~d

Fl

1q~0x

,H~

1

H~

xx

H~

32o

3d

FrFl

where the second equation above is solved subject to the boundary condition

H~

0t

a~ d

H~ q~

28



The total amount of water in the flume is conserved. Evaluating the constant in the equation at the top right of Slide 19 from the final equilibrium, then,

In dimensionless form, this constraint becomes

APPROACH TO EQUILIBRIUM IN A RECIRCULATING FLUME

As shown at the bottom of Slide 19, the sediment transport in a recirculating flume must satisfy a cyclic boundary condition. In dimensionless terms, this becomes

As a result, the relations for sediment conservation of Slide 24 reduce to0x1x

q~q~

x

q~q~q~

x

q~

t,0q~q~

td

~d1x0x

d

1x0x

a

Fl

L

0 oLHHdx

1

01xdH

~

29



APPROACH TO EQUILIBRIUM IN A RECIRCULATING FLUME: SUMMARY

The equation for can be dropped because flume-averaged bed elevation cannot change in a recirculating flume. The remaining backwater and sediment conservation relations

x

q~

td

,q~q~1x0x

,H~

1

H~

xx

H~

32o

3d

FrFl

must be solved with the sediment transport relation and constraints:

,

H~

if1

H~

H~

if0

q~ 1

r2

N

1

r

1

r2

1

r2

L

x

2

1

S

S

o

I0td

where the initial slope SI must be chosen so as to yield subcritical flow everywhere.

a~

1

01xdH

~

The initial condition is

30



APPROACH TO EQUILIBRIUM IN A RECIRCULATING FLUME: FLOW OF THE CALCULATION

The method of solution is similar to that for the feed flume with one crucial difference: iteration is required to solve the backwater equation over a known bed

subject to the integral condition

That is, for any guess

32o

3d

H~

1

H~

xx

H~

FrFl

it is possible to solve the backwater equation and test to see if the integral condition is satisfied. The value of necessary to satisfy the backwater equation can be found by trial and error, or as shown below, by a more systematic set of methods.

1

01xdH

~

1xd H

~H~

dH~

31



x

q

initial bed profile

ultimate bed profile

initial water surface profile (M2)

initial profile of sediment transport

RESPONSE OF A RECIRCULATING FLUME TO AN INITIAL BED SLOPE THAT IS BELOW THE EQUILIBRIUM VALUE

The low initial bed slope causes the depth to be too high upstream. Total water mass in the flume can be conserved only by constructing an M2 water surface profile. The result is a shear stress, and thus sediment transport rate at the downstream end that is higher than the upstream end. This sediment is immediately recirculated upstream, where it cannot be carried, resulting in bed aggration there.

32



RECIRCULATING FLUME: ITERATIVE SOLUTION OF BACKWATER EQUATION

The shooting method is combined with the Newton-Raphson method to devise a scheme to solve the backwater equation iteratively. Now for each guess it is possible to solve the backwater equation

for , so that in general the solution can be written as

Now define the function such that

The correct value of is the one for which the integral constraint is satisfied, i.e.

dH~

32o

3d

H~

1

H~

xx

H~

FrFl

H~

)x,H~

(H~

H~

d

1

0 dd 1xd)x,H~

(H~

)H~

(

)H~

( d

01xd)x,H~

(H~

)H~

(1

0 dd

dH~

33



ITERATIVE SOLUTION OF BACKWATER EQUATION contd.

Now according to the Newton Raphson method, if is an estimate of the solution for , then a better estimate (where w = 1, 2, 3… is an iteration index) is given as

Reducing this relation with the definition of ,

results in the iteration scheme

This iterative scheme requires knowledge of the parameter .

)w(dH

~

dH~

1

0

)w(d

d

1

0

)w(d)w(

d)1w(

d

xd)x,H~

(H~H~

1xd)x,H~

(H~

HH~

1

0 dd 1xd)x,H~

(H~

)H~

(

)1w(dH

~

)H(dHd

)H(HH

~

)w(d

d

)w(d)w(

d)1w(

d

dH~

/H~

34



Taking the derivative of both equations with respect to results in the variational equation and boundary condition below;


Now define the variational parameter Hv as

dH~

d

dvH~H~

)x,H~

(H

The governing equation and boundary condition of the iterative scheme are

d1x32

o

3d

H~

H~

,H~

1

H~

xx

H~

Fr

Fl

1)1(H,H]H~

1

H~

x1[H~

1

H~

3

x

Hvv3

3d

3

4v

2o

2o2

o FrFr

FrFl

35




For any guess , then, the following two equations and boundary conditions can be used to find and for all values of .

)w(dH

~

)w(d

1x

)w(

3)w(2o

3)w(d)w(

H~

H~

,)H

~(1

)H~

(x

x

H~

Fr

Fl

1)1(HH])H

~(1

)H~

(x1[

)H~

(1

)H~

(3

x

H )w(v

)w(v3)w

3)w(d

3)w(

4)w()w(v

2o

2o2

o FrFr

FrFl

)w(H~ )w(

vH x

The improved guess is then given as)1w(dH

~

1

0

)w(v

1

0

)w(

)w(d

)1w(d

xdH

1xdH~

HH~

The iteration scheme is continued until convergence is obtained.

36



NUMERICAL IMPLEMENTATION

Numerical implementations of the previous formulation for both recirculating and feed flumes are given in RTe-bookRecircFeed.xls. The GUI for the case of recirculation is given in worksheet “Recirc”, and the GUI for the case of feed is given in worksheet “Feed”. The corresponding codes are in Module 1 and Module 2.

Both these formulations use a) a predictor-corrector method to compute backwater curves, and b) pure upwinding to compute spatial derivatives of qt in the various Exner equations of sediment conservation. The discretization given below is identical to that used in Chapter 20.

The ghost node for sediment feed is not used, however, in the implementationfor the recirculating flume.

1

i=1 2 3 M -1 i = M+1 M+1

ghost M

M

1x

1M..1i,x)1i(x i

37



NUMERICAL IMPLEMENTATION contd.

The backwater equation is solved using a predictor-corrector scheme. The solution proceeds upstream from the downstream node M+1, where for a feed flume

and for a recirculating flume is obtained from the iteration scheme using the Newton-Raphson and shooting techniques. The discretized backwater forms for dimensionless depth are:

In the case of a recirculating flume, the variational parameter Hv must also be computed subject to the boundary condition Hv,M+1 = 1. The corresponding forms are

x]}H~

1

)H~

x

ˆˆ(

1[H~

1

HH~

3]

H~

1

)H~

x

ˆˆ(

1[H~

1

HH~

3{

2

1HH

x]H~

1

)H~

x

ˆˆ(

1[H~

1

HH~

31HH

31i

2o

31i

i,d1i,d

2o3

1i2o

pred,v41i

3i

2o

3i

i,d1i,d

2o3

i2o

i,v4

ii,v1i,v

3i

2o

3i

i,d1i,d

2o3

i2o

i,v4

ii,vpred,v

FrFr

FrFrFr

FrFl

FrFr

FrFl

1M,da1M 2

11~1H~

FL1MH

~

x]H~

1

H~

xH~

1

H~

x[2

1H~

H~

,xH~

1

H~

x

ˆ1

H~

H~

3pred

2o

3pred

i,d1i,d

3i

2o

3i

i1i,d

i1i3i

2o

3i

i,d1i,d

ipred

FrFrFlFrFl

H~

38



NUMERICAL IMPLEMENTATION contd.

In the case of a feed flume, the discretized forms for sediment conservation are

In the case of a recirculating flume, the corresponding form is

1M..2i,tq̂1x

t)q~q~(

1i,tq~1x

t)q~1(

tq~11~~

1Mi1iti,d

1Miti,d

tti,d

1Mtatta

Fl

1M..2i,x

t)q~q~(

1i,x

t)q~q~(

i1iti,d

i1Mti,d

tti,d

39



CRITERION FOR EQUILIBRIUMIt is assumed that equilibrium has been reached when the bed slope is everywhere within a given tolerance of the equilibrium slope So. A normalized bed slope SN that everywhere equals unity at equilibrium is defined as

For the purpose of testing for convergence, the normalized bed slope SN,i at the ith node is defined as

At equilibrium, then, SN,i should everywhere be equal to unity. The error i between the bed slope and the equilibrium bed slope at the ith node is

Convergence is realized when

where t is a tolerance. In RTe-bookRecircFeed.xls t has been set equal to0.01 (parameter “epslope” in a Const statement).

xxS

1

S

SS d

ooN

1M..2i,x

S i1ii,N

2/)1S(

1S

i,N

i,Ni

ti)max(

40



PHASE PLANE INTERPRETATION

One way to interpret the results of the analysis is in terms of a phase plane. Let Sup denote the bed slope at the upstream end of the flume, and Sdown denote it at the downstream end, where

Further define the normalized slope SN as equal to S/So, so that

The initial normalized slope is denoted as SNI. At mobile-bed equilibrium slope S is everywhere equal to the equilibrium value So, so that SN = 1 everywhere. That is, one indicator of mobile-bed equilibrium is the equality

In a phase plane interpretation, SN,up is plotted against SN, down at every time step, and the approach toward (1, 1) is visualized. This equilibrium point (1, 1) is called the fixed point of the phase problem.

xS

xSS,

xS

xSS 1M,dM,d

o1x

dodown

2,d1,do

0x

doup

o

downdown,N

o

upup,N S

SS,

S

SS

)1,1()S,S( down,Nup,N

41



PHASE PLANE INTERPRETATION contd.

The initial condition for the bed profile is S = SI (SN = SNI) everywhere, where SI is not necessarily equal to the mobile-bed equilibrium value So (SNI is not necessarily equal to 1) . For example, in the case SI = 0.5 So, (SN,up, SNdown) begin with the values (0.5, 0.5) and gradually approach (1, 1).

An approach in the form of a spiral is usually indicative of damped sediment waves, or lumps.

0 1 20

1

2

fixed point (equilibrium)

initial point

up,NS

down,NS

42



INPUT PARAMETERS USED IN THE CODES

Input for recirculating flume

Input for feed flume

LS

H

o

oFl

o

INI S

SS

SI = initial bed slopeSo = equilibrium bed slope

43



0

0.5

1

1.5

2

0 0.5 1 1.5 2

SNdown

SN

up

This calculation requires a dimensionless time of 11.94 in order to reach mobile-bed equilibrium. The spiralling is indicative of a damped sediment wave or lump. This is shown in more detail in the next slide.

initial

final equilibrium

SAMPLE CALCULATION WITH RTe-bookRecircFeed.xls: RECIRCULATING FLUME

Fro Fro 0.4

nt nt 1.5Fl Fl 10

o*/c* taur 3

SNI SNI 0.5M M 50t dt 0.0025

Nstep 300Ntimes 10

t

44



BED EVOLUTION IN RECIRCULATING FLUME

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Dimensionless distance

Dim

ensi

on

less

ele

vati

on

Profiles from 0t

to 75.0t

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1


Dim

ensi

on

less

ele

vati

on Profiles from 75.0t

to 50.1t

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1


Dim

ensi

on

less

ele

vati

on

Profiles from 50.1t

to 25.2t

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1


Dim

ensi

on

less

ele

vati

on Profiles from 25.2t

to 00.3t

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1


Dim

ensi

on

less

ele

vati

on

Profiles from 00.3t

to 75.3t

45



0

0.5

1

1.5

2

0 0.5 1 1.5 2

SNdown

SN

up

The input parameters are comparable to that of the previous case of recirculation. Mobile-bed equilibrium is reached in a dimensionless time of only 4.33. The phase diagram shows no spiralling.

initial

final equilibrium

SAMPLE CALCULATION WITH RTe-bookRecircFeed.xls: FEED FLUME

Fro Fro 0.4

nt nt 1.5Fl Fl 10

etaatil 0

o*/c* taur 3

SNI SNI 0.5M M 50t dt 0.0025

Nstep 100Ntimes 10

aI~

t

46



BED EVOLUTION IN FEED FLUME

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0 0.2 0.4 0.6 0.8 1


Dim

ensi

on

less

ele

vati

on

Profiles from 0t

to 75.0t

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0 0.2 0.4 0.6 0.8 1


Dim

ensi

on

less

ele

vati

on

Profiles from 75.0t

to 50.1t

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0 0.2 0.4 0.6 0.8 1


Dim

en

sio

nle

ss

ele

va

tio

n

Profiles from 50.1t

to 25.2t

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0 0.2 0.4 0.6 0.8 1


Dim

ensi

on

less

ele

vati

on

Profiles from 25.2t

to 00.3t

47



APPLICATION EXAMPLE

The dimensionless numbers of the previous two calculations, i.e. Fro = 0.4, nt = 1.5, Fl = 10, = 3, SNI = 0.5 and = 0 (feed case only) are now converted to dimensioned numbers for a sample case, for whichD = 1 mm, R = 1.65ks = 2.5 DHo = 0.2 m (equilibrium depth)The sediment is assumed to move exclusively as bedload. The assumed bedload relation, given below, is from Chapter 6.

The assumed resistance relation, given below, is from Chapter 5.

From Ho, ks and the above resistance relation, Cf = 0.00354From Ho and Fro = 0.4, qw = 0.560 m2/sFrom = 3, o* = 0.148From o*, D, R and the load relation, qto = 1.57 x 10-5

m2/sFrom o* = HoSo/(RD), Ho, R and D, So = 0.00122From Fl = Ho/(SoL), Ho and So, L = 16.3 m

0495.0,)(97.3qq c2/3

cbt

c0 /aI

~

6/1s

2/1f kH1.8CCz

c0 /

48



APPLICATION EXAMPLE contd.

In the example, then, the equilibrium parameters areqw = 0.560 m2/sHo = 0.2 mSo = 0.00122qto = 1.57 x 10-5 m2/s (= 1250 grams/minute for a flume width B = 0.5 m

and the flume length isL = 16.3 m

The time to equilibrium tequil is related to the corresponding dimensionless parameter as

Assuming the value for bed porosity p of 0.4, the time to equilibrium for the rercirculating case of Slide 43 and the feed case of Slide 45 are then

Recirculating flume: tequil = 41.3 hoursFeed flume: tequil = 15.0 hours

equilt

equilto

2o

pequil tq

LS)1(t

49



SOME CONCLUSIONS

1. Cyclic lumps do occur in recirculating flumes.

2. These lumps do eventually dissipate.

3. Similar cyclic lumps are not manifested in sediment-feed flumes.

4. Feed flumes reach mobile-bed equilibrium faster than recirculating flumes.

5. Both flume types eventually reach the same mobile-bed equilibrium.

The following tentative conclusions can be reached concerning sediment lumps in flumes.

50



REFERENCES FOR CHAPTER 22

Hills, R., 1987, Sediment sorting in meandering rivers, M.S. thesis, University of Minnesota, 73 p. + figures.

Parker, G., 2003, Persistence of sediment lumps in approach to equilibrium in sediment-recirculating flumes, Proceedings, XXX Congress, International Association of Hydraulic Research, Thessaloniki, Greece, August 24-29, downloadable at http://cee.uiuc.edu/people/parkerg/conference_reprints.htm .

Parker, G. and Wilcock, P., 1993, Sediment feed and recirculating flumes: a fundamental difference, Journal of Hydraulic Engineering, 119(11), 1192‑1204.


RIVERS AND TURBIDITY CURRENTS

chapter 22: morphodynamics of recirculating and feed flumes

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