B.2 - CHART 2 - Rectangular Inlet With Grate Flow Only

Pressure change coefficients are presented in this chart for use in determining the elevation of the water surface in a rectangular inlet with all inflow entering through a top grate. Separate curves are included for the outfall pipe connected at the box end (short dimension) and the box side (long dimension). The coefficient K _G depends on the pipe position and the depth of water in the inlet.

To use the chart:

1.

Note whether outlet is at end or side.

2.

Determine outfall pipe pressure line elevation - Gen. Instr. 1.

3.

Calculate outfall velocity head - Gen. Instr. 2.

4.

Estimate a value for water depth d.

a.

Outfall pressure line elevation minus inlet bottom elevation plus 
equals d.

b.

Estimate K _G as follows:

For pressure line to bottom, not over 2 pipe diameters

i.

7.0 for end outlet

ii.

5.0 for side outlet

For higher pressure lines

iii.

4.0 for end outlet

iv.

3.0 for side outlet

5.

Calculate the estimated relative water depth d/D _o .

6.

Enter Figure B.2 Chart 2 at this depth d/D _O and read K _G from the curve for the particular outfall pipe location.

7.

Calculate h _G as indicated on the diagram on the chart and by Gen. Instr. 7.

8.

Add h _G to the elevation of the outfall pressure line at the inlet center to obtain the water surface elevation in the inlet.

9.

From this water surface elevation subtract the elevation of the inlet bottom to obtain a more precise value for the water depth d.

10.

Repeat the above procedure with the improved value of d from step (9), if necessary. Such repetition may not be necessary if the estimated d/D _O of step (5) was reasonable accurate.

11.

Check to be sure the inlet water elevation is below the top of the inlet so that inflow may be admitted.

Note: The designer may consider the use of K _G = 1.5 and K _E = 0.5 for design instead of Chart 2. K _E is the entrance loss. The high values of K _G for curb inlet or grate is questionable.

Figure B.2 Chart 2 - Rectangular inlet with grate flow only (Source University of Missouri E.S.B. #41)