Critical flow condition develops when Froude no equals to 1.0. In open channel flow design, critical state of flow should be avoided as under such condition the water surface becomes unstable and wavy. It is, therefore, recommended that the open channel flow should be designed so that the Froude no should not exceed 0.8
Design velocities to be ensured in gravity storm conduits/channels
S. No. Criteria Value
1 Minimum velocity 0.6 m/s
2 Maximum Velocity 3 m/s
Note:
For hilly regions, maximum velocity to be permitted in storm water conduits should be 6.0 m/s for plastic pipes or other pipes lined with plastics.
Freeboard in open channel The freeboard is the vertical distance from the water surface of designed flow condition to the top of the channel. The importance of this factor depends on the consequence of overflow of the channel bank. Freeboard should be sufficient to prevent waves, super elevation changes, or fluctuations in water surface from overflowing the sides. Recommended value of minimum freeboard for different width is as given in Table
Minimum Free Board for open channels
Drain Size Free Board
(i) Beyond 300 mm bed width 10 cm
(ii) Beyond 300 mm & up to 900 mm bed width 15 cm
(iii) Beyond 900 mm & up to 1500 mm bed width 30 cm
Source: IRC SP 50 – 2013
For larger drains, the freeboard shall be higher up to 90 cm depending upon the discharge. For storm conduits, freeboard is not defined as they are supposed to run full.
Curves/bends in drains
Curves and bends are sometimes unavoidable in drain alignments. The complexity in the design arises due to increase in friction losses along the curve that causes serious local erosion due to spiral flow motion induced by the centrifugal force which is very pronounced and irregular in the bend.
Therefore, in order to reduce the super elevation of the water surface that occurs due to the difference in elevation of water surface between inside and outside wall of the bend at the same section and maintain the freeboard, a minimum radius of curvature of 3 times the width of the drain should be provided in the horizontal curve. Benching should be provided at the bend to minimize the sedimentation at the inner side of the bend.
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Urban Infrastructure and Network Study notes for M. plan Sem-III
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