Primary Outlet



General Design Considerations



(a) Flow Regulation

Flow regulating devices are often called upon to perform what may appear to be conflicting tasks, such as limiting flow rates, be free of clogging, and be relatively maintenance free.



(b) Location of the Flow Regulator

Flow regulating devices for above-ground storage are typically housed in an outlet structure, called a discharge control pit (DCP), which is an important component of the storage facility. It not only controls the release rate, but also determines the maximum depth and volume within the storage. Flow regulating devices for below-ground storage should be located within, at or near, the bottom  of the storage facility. It some cases, where the topography does not permit emptying of storage facility by gravity, pumping will be required to regulate the flow rate.



(c) Protection from blockage

Primary outlet build in OSD is relatively small to reduce the site discharge and therefore it can be highly susceptible to blockage by debris. Hence, the best solution is to provide screens around the primary outlet.

Figure 19.11 Primary Outlet Flow Regulator



The simplest flow regulating device is an orifice. When the outlet is small in comparison to the depth of water, the discharge through the orifice can be calculated using the orifice equation. (Equation 19.3)




Q    = the orifice flow rate (m3/s)

= orifice discharge coefficient (use 0.60 for orifice diameter Do < 50 mm, 0.62 for Do > 50 mm)

= area of orifice (m2)

= effective head on the orifice measured from the centroid of the opening (m)
g   = acceleration due to gravity (9.81 m/s2)


The orifice equation assumes that there is no back pressure from the downstream drainage system. To ensure that free discharge is maintained, the outlet needs to be well ventilated and the outlet pipe needs to be large enough to prevent submergence. The outlet pipe from the storage has a just-full capacity of at least twice the PSD.

Figure 19.12 Typical Orifice Plate Details


The minimum orifice diameter shall be 25 mm to minimise the potential for blockage.


Flow Restricting Pipe


Advantages of using a flow restricting pipe as a storage outlet is:

  • It is difficult to modify the hydraulic capacity of the pipe, unlike an orifice which can be easily removed.
  • The required flow reduction may be archived using a larger diameter opening than an orifice, which considerably reduce the possibility of blockage outlet

‘If the pipe is assumed to be flowing full, the outlet capacity can be calculated using equation 19.6.






Q = pipe capacity (m3/s)

A = cross-sectional area of the pipe (m2)

g = acceleration due to gravity (9.81 m/s2)

= water depth at the upstream invert of the pipe (m)

= water depth at the downstream invert of the pipe (m)

S = pipe longitudinal slope (m/m)
L = pipe length (m)

= sum of loss factors for the pipe system

Editor :- Dilah – 13 Feb 14

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