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Plan

1. Combined through and terminal manhole

3. Through and drop manhole

1. Combined through and terminal manhole

2. Through manhole with no change in grade

3. Through and drop manhole

4. Through manhole with change in grade

2. Through manhole with no change in grade

5. Terminal manhole

Water supply

Syphon

6. Automatic flush tank about to discharge

Water supply

Syphon

4. Through manhole with change in grade

5. Terminal manhole

6. Automatic flush tank about to discharge

FIG. 7.11.1 Plan, profile, and construction details of sanitary sewers.

and the sewer cleaning equipment. Manholes, however, should not be located in low places where surface water can enter. If such locations are unavoidable, special water-tight manhole covers should be provided.

Part a in Figure 7.11.2 shows a typical manhole; part a in Figure 7.11.3 shows the details of a terminal cleanout.

Drop Manholes

A drop manhole reduces the turbulence in the manhole when the elevation difference between incoming and outflow sewers is greater than 0.5 m (1.5 ft). Turbulence due to a sudden drop of wastewater can cause splashing, release of odorous gases, and damage to the manhole. Part b in Figure 7.11.2 shows the details of a drop manhole.

Flushing Manholes

In their upper reaches, most sewers receive so little flow that they are not self-cleaning and must be flushed from time to time. This flushing is done by the following means:

Damming the flow at a lower manhole and releasing the stored water after the sewer has almost filled. Suddenly pouring a large amount of water into an upstream manhole.

3/8" Plaster Coat

3/8" Plaster Coat

Manhole Frame and Cover
FIG. 7.11.2 Typical designs of manholes. 1 ft = 0.3048 m. (Reprinted, with permission, from National Clay Pipe Institute, 1978, Clay pipe engineering manual, Washington, D.C.: National Clay Pipe Institute.)

Providing a flushing manhole at the uppermost end of the line. A flushing manhole is filled with water through a fire hose attached to a nearby hydrant before a flap valve, shear gate, or similar quick-opening device leading to the sewer is opened. Installing an automatic flush tank that fills slowly and discharges suddenly. Apart from the cost and maintenance difficulties, the danger of backflow from the sewer into the water supply is a negative feature of automatic flush tanks.

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