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This Appendix presents methods and charts for determining pressure changes at inlets,
square and round junction boxes (manholes) on storm drain systems flowing full. The
following instructions and charts are from the University of Missouri Engineering
Bulletin No. 41 "Pressure Changes at Storm Drain Junctions"
The University of Missouri Engineering Bulletin No. 41 "Pressure Changes at Storm
Drain Junctions" was the results of flume model testing.
Figure B.2 Chart 2 through Figure B.7 Chart 7 each dealt with a rectangular inlet
box admitting grate flow and having a specific arrangement of pipelines. Figure B.8
Chart 8 through Figure B.10 Chart 10, supplemented by Figure B.3 Chart 3, apply to
square and round manholes with various pipeline arrangements but with no flow admitted
through the top of the junction structure. Since no one chart presents a complete
solution for manholes, the following explanation of methods for the determination
of pressure changes is divided into sections relating to the particular configuration
instead of into sections relating to each chart, as was done in the case of inlets.
Pressure change coefficients are presented in Figure B.3 Chart 3 for use in determining
the elevation of the pressure line of an in-line pipe upstream from a manhole with
through flow only. The pressure change coefficient is controlled primarily by the
relative diameters of the upstream and outfall pipes, and secondarily by the distance
across the open manhole if the outfall pipe entrance is square-edged. Rounding the
outfall entrance eliminates the effects of manhole size relative to the pipe and reduces
the coefficients to a limited degree. Manhole cross-section shape is not significant.
Thus, the values of Figure B.3 Chart 3 apply equally to round, square, or rectangular
manholes. Shaping of the bottom of a manhole to continue a portion of the lower sector
of the pipe cross-section through from pipe to pipe is ineffective in reducing losses.
The nomenclature used for all manhole types is given in Figure B.1 Chart 1.
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