Explanation:
For a given flow rate, open channel flow based design requires larger conduit sizes than those dimensioned based on pressure flow. While it may be more expensive to build designed storm drainage systems
Based on open channel flow, this design procedure provides a margin of safety by providing headroom in the duct to accommodate an increase in flow above the design discharge. Beneath the majority
Under normal conditions, it is recommended that the size of storm drains be based on a gravity flow to full flow criteria or almost full. The design of the pressure flow may be justified in certain cases. As the hydraulic calculations are performed, frequent verification of the existence of the desired flow condition should be performed.
Storm drainage systems can often alternate between pressure and open channel flow conditions from one section to another (Federal Highway Administration, US Department of Transportation, 1996).
For gravity flow conditions, the Manning formula must be edited as described below.
ܳ ൌ
1,486
݊
ܣܴ ଶ / ଷ ܵ ଵ / ଶ
Where:
Q = Discharge, cubic feet per second
A = flow cross-sectional area, square feet
n = Manning roughness coefficient (see Table 5603-1)
ܴ ൌ
ܣ
ܲ
R = hydraulic radius, feet
S = slope in feet per foot
P = wet perimeter in feet
