Combination Inlet in Sag

According to HEC-22, combination inlets are considered advisable for use in sags where hazardous ponding occurs.

Conservatively, the ponding depth computed at a combination inlet in sag will be the depth which corresponds to the smaller of orifice or weir governing intercept, at each opening. Often, the curb and grate inlet openings are of equal length. However, for improved efficiency a longer curb inlet opening may be desirable.

Equal Length Inlets

Equal length inlets refer to a grate inlet placed along the side of a curb-opening inlet of identical length. At lower flow depths, the grate inlet is operating as a weir and the interception capacity of the curb is negligible (unless the grate is clogged, in which case the curb is intercepting some flow). The flow Q_iw intercepted by the combination is then:

Q_iw = C_wPd^1.5

C_w	=	Weir coefficient (typically 1.66 SI, 3.0 U.S. customary)
P	=	Perimeter of grate, disregarding side along curb (m, ft)
d	=	Flow depth at curb (m, ft)

At higher flow depths, both the grate inlet and the curb-opening inlet are operating as orifices.

Note: The clear opening area of the grate depends on the opening ratio of the grate (HEC-22 defines an opening ratio for each grate type), as well as the clogging factor you specify.

The flow Q_io intercepted by the combination inlet operating as an orifice is:

Q_io = C_oA_g(2gd)^0.5 + C_ohL(2gd_o)^0.5

C_o	=	Orifice coefficient (C_o = 0.67)
A_g	=	Clear opening of grate (m² , ft² )
g	=	Gravitational acceleration (9.81 m/sec.² , 32.16 ft/sec.² )
h	=	Height of curb-opening inlet (m, ft)
d_o	=	Head, measured from the center of the orifice throat (m, ft)

Sweeper Inlet

A sweeper inlet refers to a grate inlet placed at the downstream end of a longer curb opening inlet. A sweeper inlet is more efficient than an equal length combination inlet in intercepting debris.

Note that since the HEC-22 manual is not very explicit about this type of inlet in sag, some assumptions were made in order to define the flows for this inlet.

The flow Q_i intercepted by a sweeper inlet is the sum of the flow Q_ie as calculated above for an equal length combination inlet of length L (where L is the length of the grate) and the flow Q_ic intercepted by the additional length L (upstream of the grate) of the curb opening.

Q_i = Q_ie + Q_ic