B.4 - CHART 4 - Rectangular Inlet With Through Pipeline and Grate Flow

Pressure change coefficients are presented in this chart for use in determining the common elevation of the upstream in-line pipe pressure line and the water surface in the inlet. The in-line pipes connect at the inlet sides (long dimension) and must meet the alignment requirement stated for Figure B.3 Chart 3. As much as half the total flow may enter through a top grate. The main graph of Figure B.4 Chart 4 includes effects of various proportion of grate flow for a relative water depth d/D _O of 2.5. Increments of K _U for other relative depths are shown in the supplemental graphs; positive increments for d/D _O less than 2.5 and negative for greater depths.

To use the chart:

1.

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

2.

Calculate velocity head in the outfall - Gen. Instr. 2.

3.

Calculate the ratios D _U /D _O and Q _U /Q _O - Gen. Instr. 3. (The grate flow ratio Q _G /Q _O = 1 - Q _U /Q _O ).

4.

Estimate a value for the water depth d.

a.

Follow Gen. Instr. 4.

b.

Estimate K = 3 Q _G /Q _O .

5.

Calculate the corresponding relative water depth d/D _O .

6.

If the estimated d/D _O is approximately 2.5, enter the lower graph on Figure B.4 Chart 4 at the pipe size ratio D _U /D _O and read K _U at the curve or interpolated curve for Q _U /Q _O ; Then proceed as in step (9).

7.

If the estimated d/D _O is other than 2.5, follow step (6), then enter the upper graph on Figure B.4 Chart 4 at the given D _U /D _O and determine the increment of K _U required to account for the effects of the estimated relative water depth d/D _O .

8.

Add K _U from step (6) and the increment from step (7) to determine the total value of K _U . Note that negative values of K _U may occur.

9.

For a rounded outfall pipe entrance or one consisting of a pipe socket, reduce K _U , according to Gen. Instr. 6.

10.

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

11.

Add h _U , to the elevation of the outfall pressure line at the inlet center to obtain the elevation of the upstream in-line pipe pressure line at the same location. The water surface elevation will correspond.

12.

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

13.

Repeat the above procedure with the improved value of d from (12), if necessary. Such repetition may not be necessary if the original estimated d/D _O of step (5) was reasonably accurate.

14.

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

Figure B.4 Chart 4 - Rectangular Inlet With Through Pipeline And Grate Flow
(Source University of Missouri E.S.B. #41)