GRADUALLY VARIED FLOW

Transcription

1 CVE 341 Water Resources Lecture Notes 5: (Chapter 14) GRADUALLY VARIED FLOW

2 FLOW CLASSIFICATION Uniform (normal) flow: Depth is constant at every section along length of channel Non-uniform (varied) flow: Depth changes along channel Rapidly-varied flow: Depth changes suddenly Gradually-varied flow: Depth changes gradually

3 FLOW CLASSIFICATION RVF: Rapidly-varied flow GVF: Gradually-varied flow

4 Figure 14.1 Examples for gradually varied flow in open channels.

5 ASSUMPTIONS FOR GRADUALLY-VARIED FLOW 1. The channel is prismatic and the flow is steady.. The bed slope, So, is relatively small. 3. The velocity distribution in the vertical section is uniform and the kinetic energy correction factor is close to unity. 4. Streamlines are parallel and the pressure distribution is hydrostatic. 5. The channel roughness is constant along its length and does not depend on the depth of flow. Copyright 007 by Nelson, a division of Thomson Canada Limited

6 ANALYSIS OF GRADUALLY-VARIED FLOW Copyright 007 by Nelson, a division of Thomson Canada Limited

7 THE EQUATIONS FOR GRADUALLY VARIED FLOW h 1 h H V g + h + z dh d V g + dh + dz dh / - S, dz / - S0 SSf: slope of EGL

8 THE EQUATIONS FOR GRADUALLY VARIED FLOW It should be noted that the slope is defined as the sine of the slope angle and that is assumed positive if it descends in the direction of flow and negative if it ascends. Hence, dh / S, dz / S0 It should be noted that the friction loss dh is always a negative quantity in the direction of flow (unless outside energy is added to the course of the flow) and that the change in the bottom elevation dz is a negative quantity when the slope descends. In the other words, they are negative because H and z decrease in the flow direction

9 THE EQUATIONS FOR GRADUALLY VARIED FLOW d V g d Q g A dh dh dh d dh Q g A - dh Q g A 3 da dh - dh Q g A B 3 dh Q B dh S S ga Q B dh 1-3 -S ga dh 1- Q S 0 - S B / g A 3 dh S S Fr General governing Equation for GVF If dh/ is positive the depth is increasing otherwise decreasing

10 dh S 1-0 Fr DERIVATION OF GVF EQUATION - S For any cross-section dh S o 1 1 y y o y c y 3 3 Wide rectangular section (Using Chezy equation for Sf) C V RS f dh y o 1 y So y c 1 y 10 / 3 3 Wide rectangular section (Using Manning s formula for Sf)

11 WATER SURFACE PROFILES For a given channel with a known Q Discharge, n Manning coefficient, and S o channel bed slope, y c critical water depth and y o uniform flow depth can be computed. There are three possible relations between y o and y c as 1) y o > y c, ) y o < y c, 3) y o y c.

12 WATER SURFACE PROFILES CLASSIFICATION For each of the five categories of channels (in previous slide), lines representing the critical depth (y c ) and normal depth (y o ) (if it exists) can be drawn in the longitudinal section. These would divide the whole flow space into three regions as: (y: non-uniform depth) Zone 1: Space above the topmost line, y> y o > y c, y > y c > y o Zone : Space between top line and the next lower line y o > y> y c, y c > y> y o Zone 3: Space between the second line and the bed. y o >y c >y, y c >y o >y

13 WATER SURFACE PROFILES CLASSIFICATION Copyright 007 by Nelson, a division of Thomson Canada Limited

14 WATER SURFACE PROFILES CLASSIFICATION For the horizontal (S o 0) and adverse slope ( S o < 0) channels, Q 1 n AR / 3 S 1/ o Horizontal channel: S o 0 Q 0 Adverse channel: S o < 0 Q cannot be computed, For the horizontal and adverse slope channels, the uniform flow depth y o does not exist.

15 WATER SURFACE PROFILES CLASSIFICATION For a given Q, n, and S o at a channel, y o Uniform flow depth, y c Critical flow depth, y Non-uniform flow depth. The depth y is measured vertically from the channel bottom, the slope of the water surface dy / is relative to this channel bottom. the prediction of surface profiles from the analysis of dh S0 - S 1- Fr

16 Classification of Profiles According to dy/dl dl 1) dy/>0; the depth of flow is increasing with the distance. (A rising Curve) ) dy/<0; the depth of flow is decreasing with the distance. (A falling Curve) 3) dy/0. The flow is uniform SfSo 4) dy/ -. The water surface forms a right angle with the channel bed. 5) dy/ /. The depth of flow approaches a zero. 6) dy/ S o The water surface profile forms a horizontal line. This is special case of the rising water profile

17 WATER SURFACE PROFILES CLASSIFICATION Classification of profiles according to dy / dl or (dh/). dh S0 - S 1- Fr

18 GRAPHICAL REPRESENTATION OF THE GVF Zone 1: y > y o > y c Zone : y o > y > y c Zone 3: y o > y c > y 3 c 3 o o y y 1 y y 1 S dh

19 Outlining Water Surface Profiles Please read your text book for the rest. Page 451

20 Example Draw water surface profile for two reaches of the open channel given in Figure below. A gate is located between the two reaches and the second reach ends with a sudden fall. (a) The open channel and gate location. (b) Critical and normal depths. (c) Water surface profile.

21 Example Draw water surface profile for two reaches of the open channel given in Figure below. A gate is located between the two reaches and the second reach ends with a sudden fall. (a) The open channel and gate location. (b) Water surface profile.

22 Jump Location and Water Surface Profiles If hydraulic jump is formed, two different locations are expected for the jump according to the normal depths y o1 and y o. y o1 is known Calculate conjugate depth of the jump y If y <y o Case I If y >y o Case II

23 Example A wide rectangular channel carries a specific discharge of 4.0 m /s. The channel consists of three long reaches with bed slope of 0.008, and Sc respectively. A gate located at the end of the last reach. Draw water surface profile. Manning s n First calculate yc, yo1, yo, and realize that yc yo3. To know whether the jump will occur in the first or second reach, calculate y (subcritical depth) of the jump. If y < yo then the jump will take place in the first reach. Please see Example in your text book.

24 Example

25 Example

26 CONTROL SECTIONS Bold squares show the control sections. Control section is a section where a unique relationships between the discharge and the depth of flow. Gates, weir, and sudden falls and critical depth of are some example of control sections. Subcritical flows have theirs CS at downstream Supercritical flows have theirs CS at upstream

27 CVE 341 Water Resources Computation of Water Surface Profiles

28 METHODS OF SOLUTIONS OF THE GRADUALLY VARIED FLOW 1. Direct Integration. Graphical Integration 3. Numerical Integration i- The direct step method (distance from depth for regular channels) ii- The standard step method, regular channels (distance from depth for regular channels) iii- The standard step method, natural channels (distance from depth for regular channels)

29 GRADUALLY VARIED FLOW Important Formulas H z y b + + V g E y + V g H z + b E d H dz + de de S S o f d y S 1- - S 0 f Fr

30 GRADUALLY VARIED FLOW COMPUTATIONS de _ S o S f d y S -S 1- Fr 0 f E: specific energy Analytical solutions to the equations above not available for the most typically encountered open channel flow situations. A finite difference approach is applied to the GVF problems. Channel is divided into short reaches and computations are carried out from one end of the reach to the other.

31 DIRECT STEP METHOD E D E x U S o _ S f S f : average friction slope in the reach A nonuniform water surface profile _ S f 1 (S fu + S fd ) Manning Formula is sufficient to accurately evaluate the slope of total energy line, S f n V u n VD S fu S 4 / 3 fd 4 / 3 R u R D

32 DIRECT STEP METHOD X E S D o E _ S f U ( ) ( y + V / g y + V / g) D D S o _ S f U U Subcritical Flow The condition at the downstream is known y D, V D and S fd are known Supercritical Flow The condition at the upstream is known y u, V u and S fu are known Chose an appropriate value for y u Calculate the corresponding V u, S fu and S f Then Calculate X Chose an appropriate value for y D Calculate the corresponding S fd, V D and S f Then Calculate X

33 Example A trapezoidal concrete-lined channel has a constant bed slope of , a bed width of 3 m and side slopes 1:1. A control gate increased the depth immediately upstream to 4.0m when the discharge is 19 m 3 /s. Compute WSP to a depth 5% greater than the uniform flow depth (n0.017). Two possibilities exist: OR

34 Solution The first task is to calculate the critical and normal depths. Using Manning formula, the depth of uniform flow: 1 n / 3 1/ Q yo 1.75 m AR Using the critical flow condition, the critical depth: S Fr Q ga T 3 y c 1.36 m It can be realized that the profile should be M1 since yo > yc That is to say, the possibility is valid in our problem.

35 Solution X E S D o E _ S f U ( ) ( y + V / g y + V / g) D D S o _ S f U U y A R E E S f S f So-S f x x yo + (0.05 yo)

36 THE STANDART STEP METHOD Applicable to non-prismatic channels and therefore to natural river Objectives To calculate the surface elevations at the station with predetermined the station positions A trial and error method is employed

37 THE STANDART STEP METHOD E S o S f This can be rewritten in finite difference form E _ s X(So Sf ) mean where mean refers to the average values for the interval X. This form of the equation may be used to determine the depth given distance intervals. The solution method is an iterative procedure as follows;

38 THE STANDART STEP METHOD g V α y X S h g V α y o f X + o 1 1 S y Z y Z g V α Z h g V α Z f g V α Z H ; g V α Z H f 1 H h H +

39 THE STANDART STEP METHOD H1 is known and X predetermined. 1) Assume a value for depth (Z ); simple add a small amount to Z 1 ) Calculate y from y Z - So X 3) Calculate the corresponding specific energy (E ) 4) Calculate the corresponding friction slope S 5) Calculate Η 6) Calculate H 1 H + S f X 7) Compare H and H 1 if the differences is not within the prescribed limit (e.g., 0.001m) re-estimate Z and repeat the procedure until the agreement is reached.

40 THE STANDART STEP METHOD X (m) Z (m) y (m) A (m) V (m/s) av/(g) H (1) R (m) Sf x hf H () E E-05 6E E-05 7E E-05 8E E S f