(Determines how much water flows through the grate)

Capacity of Neenah Grates for Ponding Conditions

For flow through a grate under ponding conditions, orifice type flow occurs. If the net open area is used as the orifice area, the dischange can be computed using the orifice equation Q = CA2gh where C has a value of .6, A = area in square feet, g = the constant value 32.2, and h = depth in feet of water over the grate.

The orifice equation is most accurate when large depths of ponding are encountered. At shallower depths, the equation loses its accuracy. The determining factor is the grate's free open area. The orifice equation remains accurate for grates of smaller area when depths are in the .4 to .3 foot range. Grates of larger free open area are not able to sustain ponding conditions at shallow depths.

It should be left to the designer to make any necessary reductions in the discharge due to debris restriction.

For an example of the relationship between the weir and orifice equations and the expected flow through a grate under varying conditions see the section on weir flow.

Note on Beehive Grates

To determine orifice flow capacities of beehive grates, the ponding depth is zero at the top of the grate. This is conservative, but it is usually better to underestimate than overestimate the grate capacity.

For additional information regarding Neenah Inlet Grate Capacities, please contact Joe Falle P.E., at (920)729.3653 or email at joseph.falle@neenahenterprises.com.