Fig. 32.—Baltimore Standard Lamphole.
57. Lampholes.—A lamphole is an opening from the surface of the ground into a sewer, large enough to permit the lowering of a lantern into the sewer. Lampholes are used in the place of manholes to permit the inspection or the flushing of sewers, and to avoid the expense of a manhole. They are located from 300 to 400 feet from the nearest manhole in such a manner that a lamp lowered in the lamp hole can be seen from the two nearest manholes.
Lampholes should be constructed of 8– to 12–inch tile or cast-iron pipe. The lower section should be a cast-iron T on a firm foundation, but if constructed of tile it should be reinforced with concrete to take up the weight of the shaft. The details of the Baltimore standard lamphole are shown in Fig. 32. Lampholes are not commonly used on sewerage systems on account of their lack of real usefulness and the troubles encountered by breaking of the pipe below the shaft.
58. Street Inlets.—A street inlet is an opening in the gutter through which storm water gains access to the sewer. The types used in different cities vary widely. Details of an inlet in successful use are shown in Fig. 33. The figure shows also a common form of connection to the sewer. A water-seal trap is sometimes used to prevent the escape of odors from the sewer. Care must be taken in design that such traps do not freeze in winter nor dry out in summer, although it is not always possible to prevent these contingencies.
Fig. 33.—Details of an Untrapped Street Inlet, without Catch-Basin.
The important features to be observed in the design of a street inlet are: height and length of opening, character of grating, and location. The general location of inlets is discussed in Chapter V. The clear height of opening commonly used is from 5 to 6 inches, with a clear length of 24 to 30 inches or longer. Inlets of this size have given satisfaction on paved streets with moderate slopes, where the drainage area is not greater than 10,000 to 12,000 square feet of pavement. W. W. Horner states:[[35]]
The St. Louis type of inlet “old” style was a vertical opening in the curb 8 inches high and 4 feet in length with a horizontal bar making the net opening about 5 inches. It has a broad sill extending under the sidewalk. The “new” style inlet is 4½ feet long with a clear opening of 6 inches and no bar. The sill is done away with and the opening drops down directly from the curb. Tests were made of the capacity of this inlet on pavements on different slopes with sumps of depths varying from 0 to 6 inches in front of the inlet, extending out 3 feet from the gutter, and returning to the elevation of the gutter at a slope of 3 inches to the foot. The results of these tests are shown in Table 22. The capacity of the inlet is expressed as the amount of water entering just before some water begins to lap past. If a large amount of water is allowed to flow past much more water will enter the inlet thus furnishing a factor of safety for large storms. It was noted that by beginning the rise in the pavement about opposite the middle of the inlet the capacity of the inlet was increased from 20 to 50 per cent.
| TABLE 22 | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Capacities of St. Louis Street Inlets | ||||||||||||||||
| From tests by W. W. Horner. Cubic feet per second | ||||||||||||||||
| Slope in Per Ct. | 0.42 | 1.5 | 2.85 | 4.5 | ||||||||||||
| Depth of Sump, Inches | 0.0 | 2 | 4 | 6 | 0 | 2 | 4 | 6 | 0 | 2 | 4 | 6 | 0 | 2 | 4 | 6 |
| Capacity, old style | 1.27 | 0.03 | 0.25 | 0.78 | 1.49 | |||||||||||
| Capacity, new style | 0.1 | 0.5 | 1.5 | 2.5 | 0.08 | 0.4 | 1.1 | 2.1 | 0.03 | 0.28 | 0.87 | 1.62 | 0.02 | 0.15 | 0.45 | 1.0 |