Design the outlet chamber. a. At the depressed-sewer outlet, the junction of the three pipes with the

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Design the outlet chamber.
a. At the depressed-sewer outlet, the junction of the three pipes with the 750-mm gravity sewer should be designed to reduce the opportunity for eddies to carry sediment back into the pipes that frequently have no flow through them, but are full of standing water. This is especially important with the 500-mm pipe, which will not be in operation except at unusual rates of flow.

b. The design is completed by locating the two larger pipes, or the inverts of corresponding channels within the junction chamber, as high as possible to avoid pooling and reducing the velocity in the chamber. As a further precaution, the outlet of the 500-mm pipe (least frequently required) may be raised so that the invert of its channel has a sharp forward pitch toward the invert of the 750-mm sewer. The crown of the pipe must not be raised above that of the 750-mm sewer, or a portion of it will be above the hydraulic grade line.
In designing the depressed sewers, it was assumed that flexible joint pipe was used. This type of pipe, which allows joint deflections, permitted the three pipes to be de-pressed the required amount. If bends amounting to 10 or 20° are required to make the depression, an additional head loss allowance must be made for each bend. Since the actual inside diameters of ductile-iron pipe suitable for depressed sewers, such as the one in this example, are about 4 percent larger than nominal, a built-in extra capacity of about 10 percent is available even without adjusting the n value. However, this extra capacity could be lost because of a buildup of coatings on the pipe walls. A change of n from 0.013 to 0.015 would provide an additional 15 percent capacity. For ductile iron, the use of a larger n value is not recommended since the extra capacity provided is not needed. For pipe where the actual inside diameter is the same as nominal, an increased n may be justified, although sufficient extra capacity may result merely from using standard pipe sizes.