Ho= [HL^2+2Q^2/gb^2HL]^1/2 HL = submerged depth in ft.
By the use
of the submerged discharge formula, the headloss would be minimized and the additional
head required in the project scheme would be eliminated.
The above
formulas are based on the momentum theorem and the use of simplifying
assumptions. These assumptions are: the kinetic energy of the water falling
into the trough does not contribute to longitudinal velocity, level inverts,
flow is substantially horizontal in direction, and the water surface curve is
approximated by a parabola.
To design
the trough, a sufficient freefall should be available at the upstream part of
the trough so flooding of the weirs does not occur. Total headloss from basin
water surface to the water surface in the outlet channel is therefore computed
by adding the difference between Ho and HL + freefall into the basin trough +
head on weir. On short runs of troughs, friction could be neglected, but
friction should be incorporated when the headloss is extremely critical and the
length of run is extensive.
When a
specific headloss (H0 – HL) is required for the basin collection trough, the
trial and error method must be used.
This headloss requirement is a result of the outfall condition mentioned
above. In many cases this headloss requirement will be the governing factor for
the basin trough design. And remember, by increasing the basin trough cross section,
the loss of head becomes smaller.
