Inlets in Sags

An inlet in a sag, or sump, has no longitudinal slope, i.e., the gutter slope equals zero. In addition, inlets in sags capture 100% of the flow and thus no bypass flow.

Inlets in sag locations operate as weirs under low head conditions and as orifices at greater depths. Orifice flow begins at depths dependent on the grate size or the curb opening height. At depths between those at which weir flow definitely prevails and those at which orifice flow prevails, flow is in a transition stage. At these depths, control is not well defined and may fluctuate between weir and orifice control.

The efficiency of inlets in passing debris is critical in sag locations because all runoff which enters the sag must be passed through the inlet. Total or partial clogging of inlets in these locations can result in hazardous ponded conditions.

Grate inlets alone are not recommended for use in sag locations because of the tendencies of grates to become clogged. Combination inlets, curb-opening and Drop Curb inlets are recommended for use in these locations.

Drop Curb inlets are always in a sag.

Curb Inlets in Sag

The capacity of a curb-opening inlet in a sag depends on water depth at the curb, the curb opening length, and the height of the curb opening. Curb inlets operate as weirs to depths equal to the curb opening height and as orifices at depths greater than 1.4 times the throat height. At depths in between, flow is in a transition stage.

Depressed Curb Opening

The equation used for the interception capacity of the inlet operating as a weir is:

Where:
Cw = 2.3 (1.25)
L = length of curb opening in ft (m)
W = gutter width in ft (m)
d = depth at the face of curb measured
from the cross slope, Sx, in ft (m)

Note: If L > 12 feet (3.65 m) then the equation for non-depressed inlets is used, per HEC-22.

Horizontal throat curb inlet with local depression

Without Depression

The equation used for the interception capacity of the inlet operating as a weir is:

Where:
Cw = 3.0 (1.60)
L = length of curb opening in ft (m)
d = depth at the face of curb measured from the water surface to cross slope, Sx, in ft (m)

The equation used for the interception capacity of the curb inlet (depressed and non-depressed) operating as an orifice is:

Where:
Co = 0.67
h = total height of curb opening in ft (m)
L = length of curb opening in ft (m)
g = 32.2 (9.8) gravity
do = depth measured from water surface to the center of the inlet opening in ft (m)

Note that the inlet depths reported are measured from the lip of the curb opening, i.e., Depth = do + (h / 2) where h is the total opening. Inclined Throat Depth = Do + (h / 2)Sin(45). In transition flow, both equations are evaluated. The smallest Q is selected.

Horizontal Throat vs. Inclined

The height of the orifice in the above orifice equation assumes a vertical orifice opening or horizontal throat. As illustrated below, inclined throats can reduce the effective depth on the orifice and the dimension Do.

A limited curb opening length could reduce the capacity of the curb-opening inlet by causing the inlet to go into orifice flow at depths less than the height of the opening.

Inclined Throat
Horizontal Throat

Grate Inlets in Sag

Grate inlets in sags operate as weirs to a certain depth dependent on their bar configuration and operate as orifices at greater depths. Studio Express uses both orifice and weir equations at a given depth. The equation that produces the lowest discharge is used. The standard orifice equation used is:


Where:
Co = 0.67
Ag = clear opening area in sqft (sqm)
g = 32.16 (9.8) gravity
d = average depth of water over the grate in ft (m)

The weir equation used is:

Where:
Cw = 3.0 (1.66)
P = perimeter of the grate in ft (m) disregarding side against curb
d = average depth of water over the grate in ft (m)

Combination Inlets

Combination inlets consisting of a grate and a curb opening are considered advisable for use in sags where hazardous ponding can occur. Equal length inlets refer to a grate inlet placed along side a curb opening inlet, both of which have the same length.

A sweeper inlet refers to a grate inlet placed at the downstream end of a curb opening inlet. The curb opening inlet is longer than the grate inlet and intercepts the flow before the flow reaches the grate. The sweeper inlet is more efficient than the equal length combination inlet and the curb opening has the ability to intercept any debris which may clog the grate inlet.

Sweeper Inlet

Combination InletUnfortunately, HEC-22 methods ignore the curb opening.

 

Updated on May 9, 2018

Related Articles