DISCUSSION
Exercise:
12. The maximum flow rate between the manholes will occur between the house
connection and the downstream manhole. This would consist of the 125 gpm from the
upper manhole, the 40 gpm from the house connection, and
gpm
due
to
infiltration. Hence, the maximum flow would be 125 + 40 + 4 = 169 gpm.
15. Enter figure 14 at 400 gpm on the right vertical scale, and move horizontally
to the left until the 1.5 percent slope line is
intersected.
This point of
intersection is above the 6-inch diameter pipe size, so 6-inch pipe will not handle
the required flow at this slope. 8-inch diameter pipe will be required.
16. The velocity at full capacity is found by entering figure 14 at the design
slope of 1 percent along the bottom horizontal axis, and moving up vertically until
the design pipe size of 12 inches is intersected.
At this point the velocity is
read as 4.4 fps.
17. The discharge ratio is the ratio of the actual discharge (Qa) to the
discharge at full flow (Qc). The actual flow is given as 500 gpm. Entering figure
14, attached memorandum, at a slope of 2.0 percent and proceeding vertically upward
until the curve for 8-inch pipe is intersected, a full flow of 700 gpm is read.
Therefore, the discharge ratio is
.
18. Enter figure 15, attached memorandum, at a value of 0.34 (the discharge
ratio) along the bottom horizontal scale.
Proceed vertically to the discharge
curve, horizontally from this point to the velocity curve, then proceed vertically
downward from this point and read the velocity ratio (0.9) along the bottom
horizontal axis.
The actual velocity is then the velocity ratio times the full
flow velocity or 0.9 x 5 = 4.5 fps.
19. The percent slope is
percent.
Referring to figure 14, a slope of
2.0 percent and a flow of 180 gpm intersect between the 4" and 6" rectangles.
Hence, 6-inch pipe is required.
20. The house connection is 300 feet downstream from the manhole.
Since the
slope is 2 percent, the connection is .02 x300 feet or 6 feet lower than the
manhole outlet invert.
Hence, the elevation of the connection is 390 - 6 = 384
feet.
4-2
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