TM 5-814-3/AFM 88-11, Volume III
(1) Proportional weir design formula.
= static head above weir crest;
Q = total discharge past weir (cfs);
= weir coefficient (0.61 for practical design);
= velocity through grit chamber (fps);
B = 3.1416 when arctan
Other symbols are as indicated in figure C-1a and all dimensions are expressed in feet.
(2) Parshall flume design formula. In the illustration (fig C-1b), the depth of the liquid in the grit
chamber above the grit space is the same as Ha, the depth in the Parshall flume ahead of the throat. The
velocity through a channel of the illustrated cross-section can be determined from the following formula:
Q = discharge of Parshall flume (cfs);
N = number of grit settling channels in service;
Ha = depth of liquid in the grit chamber and in the Parshall flume ahead of the throat, and other
symbols are as previously stated or as indicated in figure C-1b.
Table C-2 illustrates the variations in velocity through channels of different sizes used in conjunction with
Parshall flumes of different sizes discharging various quantities under free-flow conditions. The quantities
and values of Ha are taken from Table C-2. Table C-3 can serve as a guide in selecting the size of Parshall
flume and the number and approximate size of channels to suit a specified set of flow conditions.