Application of Critical Flow

1. Application of Critical Flow • Discharge and flow depth have a unique relationship when flow is critical ( • Critical Flow meters • Discharge is maximum when flow is critical for a given specific energy. • Bridge lengths may be reduced by producing critical flow at that section • Critical Flow may be produced in a channel by raising the channel bottom, reducing the channel width, or by combination of these measures.

2. Example • A bridge is planned on a 50 m wide rectangular channel carrying a flow of 200 m3/s at a flow depth of 4.0 m. For reducing the length of the bridge, what is the minimum channel width such that the upstream water level is not influenced for this discharge?

3. Computation of Critical Depth • The critical flow occurs at the point of minimum channel width. • , • Define section factor • (3-52) • Critical flow at a given value of in a channel is possible only for one value of discharge • Critical depth may be determined by using the design curves, solving Eq. 3-52 by a trial and error, or by using numerical methods

4. Example • Derive an equation showing critical discharge through a rectangular channel section in terms of the channel width and the total head?

5. Example • For a trapezoidal channel with base width b = 6.0 m and slide slope z = 2, calculate the critical depth of flow if Q = 17 m3/s 1 z 6 m