§ 8.3.4. Outlet Structure Design

There are two basic types of outlet control structures: those incorporating orifice flow and those incorporating weir flow. Rectangular and V-notch weirs are the most common types.

Generally, if the crest thickness is more than 60% of the nappe thickness, the weir should be considered broad-crested. The coefficients for sharp-crested and broad-crested weirs vary. The respective weir and orifice flow equations are as follows:

A.

Rectangular Weir Flow Equation (See Figure 8-2 in Appendix D of this manual)

Q = CLH ^3/2 (Eq. 8-1)

Where

Q = Weir discharge, cubic feet per second

C = Weir Coefficient

L = horizontal length, feet

H = Head on weir, feet

B.

V - notch Weir Flow Equation (See Figure 8-2 in Appendix D of this manual)

Q = 4.28 C _e tan (Θ/2)H ^2.5 (Eq. 8-2)

Where

Q = Weir Discharge, cubic feet per second

C _e = Weir Coefficient, 25° < Θ < 100°

Weir Coefficient for V-notch weir Equation 8-2

Θ = Angle of the weir notch at the apex (degrees)

H = Head on Weir, feet

C.

Orifice flow equation (See Figure 8-2 in Appendix D of this manual)

Q = C _o A(2gH) ^0.5 (Eq. 8-3)

Where

Q = Orifice Flow, cubic feet per second

C _o = Orifice Coefficient (use 0.6)

A = Orifice Area, square feet

g = Gravitation constant, 32.2 feet/sec

H = Head on orifice measured from centerline, feet

Analytical methods and equations for other types of structures shall be approved by the Watershed Protection Department prior to use.

In all cases the effects of tailwater or other outlet control considerations should be included in the rating table calculations.

Source: Rule No. 161-19.01 , 3-14-19.