VARIATION OF CHANNEL GEOMETRY & THE CHOKE CONCEPT

CHANNEL BOTTOM LEVEL CHANGE Bottom Rise (Upward Step) ‘ + z ’. *** CHOKE MAY OCCUR*** Sub-critical Flow Regime ii) Super critical Flow Regime

2- Bottom Drop (Downward Step) ‘ - z ’. i) Sub-critical Flow Regime ii) Super critical Flow Regime

B. CHANNEL BOTTOM WIDTH CHANGE   B. CHANNEL BOTTOM WIDTH CHANGE 1- Sudden Contraction ‘ B2 < B1 ’ *** CHOKE MAY OCCUR*** i) Sub-critical Flow Regime ii) Super critical Flow Regime 2- Sudden Enlargement ‘ B2 > B1 ’

CHANNEL BOTTOM LEVEL CHANGE 1- Bottom Rise (Upward Step) ‘ + z ’. *** CHOKE MAY OCCUR*** Sub-critical Flow Regime When the flow regime is Sub-critical and there is an upward obstacle along the channel width, the flow will try to jump over it by trying to approach to critical flow depth. To determine the flow depth over the step, the total energy just before the step value has to be compared with the minimum energy Emin over the step. For this calculation, the same discharge should be considered. Energy of ycr occuring over the step and hence the minimum energy Emin (i.e. Emin= 3/2ycr for rectangular channel cross-section) should be compared with the existing energy and the maximum step height +zmax. If the given step height +z < +zmax then; the flow depth over the step drops.

If the given step height +z < +zmax then; the flow depth over the step drops.

If the given step height +z = +zmax then; the flow over the step is passing on critical depth ycr.

If the given step height +z > +zmax then; the flow depth can not pass over the step hence, choke occurs just before the step so that the water level height increases and the energy level increases as well such that, the flow passes over the step at critical depth ycr. (i.e. at Emin). But immediately after the step Hydraulic Jump occurs so that, the gained energy due to choke will be absorbed, based on the uniform flow depth of the second reach. (if the slope is Mild slope after the step). Note that depending on the channel parameters FREE or SUBMERGED jumps may occur. E1 – EMIN = ΔZMAX IF ΔZ > ΔZMAX ∴ CHOKING OCCURS y1 yCHOKE (> y1)

EXAMPLE 11.1: A rectangular channel cross-section of bottom width b = 9.0 m has a uniform depth of water y1 = 2.0 m when the flow rate (discharge) Q = 27.0 m3/s. If along the reach, there is an upward step (rise) of: z = 30 cm, (No Choke) z = 200 cm; (Choke; only Free HJ) z = 310 cm; (Choke; Free HJ with M3 curve) z = 83 cm; (Choke; Drowned HJ)   determine the possibility of choke occurrence; discuss the possibility of Hydraulic Jump occurrence; depth just before the step, over the step and just after the step; draw the longitudinal variation the flow profile; draw the specific energy versus flow depth curve and show all the relevant data due to the effect of the step.

2.115 m

2.000

310 cm iii) 310 cm

- 0.307 m 0.329 m

iv)

EXAMPLE 11.2: A rectangular channel cross-section of bottom width b = 3.05 m has a uniform depth of water y1 = 1.53 m when the flow rate (discharge) Q = 9.91 m3/s with an equivalent Mannings roughness coefficient n=0.02. If along the reach, there is an upward step (rise) of z = 0.45 m at the middle portion of the reach;   determine the Energies E1 and E min; discuss the possibility of choke occurrence; discuss the possibility of Hydraulic Jump occurrence; depth just before the step, over the step and just after the step; draw the longitudinal variation the flow profile; draw the specific energy versus flow depth curve and show all the relevant data due to the effect of the step estimate the length of the H.J. Estimate the lengths of the occuring curves.

EXAMPLE 11.3: (old exam question)

2.28

271.5 m

0.001 271.5 m 271.5 m 0.001