Question 10.5: The design flood with a 20-year return period is 15 m³ s^-1.......
The design flood with a 20-year return period is 15 \, m ^3 s ^{-1} . Design the culvert-type cross-drainage structure with a high embankment with the following data: culvert length=30 m; slope =1.5 %; available pipe barrel, corrugated pipes in multiples of 250 mm diameter; Manning’s n = 0.024. The barrel protrudes from the embankment with no end walls, with an entry loss coefficient of 0.9. The maximum permissible head water level is 4 m above the invert with the barrel flowing full.
For full pipe flow the energy equation gives
H+30\times(0.015=D+0.9V^{2}/2g+(V n)^{2}L/R^{43}+V^{2}/2g. (vi)
Equation (vi) gives the following results: (see the Table)
Therefore provide a 2.25 m diameter barrel for H ≤ 4.0 m.
Check for the flow conditions:
H/D = 3.70/2.25 = 1.65 > 1.2.
Hence the inlet is submerged. Using Manning’s equation with the maximum discharge, the required diameter for the flow to be just free is 2.32 m, which is greater than the diameter provided. Hence the barrel flows full (under pressure).
Note that an improved entrance would considerably reduce the head loss and allow a smaller-diameter barrel to discharge the flood flow. For example a flare-edged entry (loss coefficient = 0.25) would produce a head of 3.93 m (<4.0 m) with a barrel of 2.00 m diameter.
| D (m) | H (m) |
| 1.500 | 12.61 |
| 2.000 | 4.74 |
| 2.500 | 3.25 |
| 2.250 | 3.70 |