freedom of the pipe from obstructions; and to enable fire hose to discharge water for flushing into the sewer. For such purposes lampholes are placed at points intermediate between manholes where the grade or line changes and where the distance between the manholes is so short that an intermediate manhole seems an unnecessary expense. In the experience of the author, lampholes are little used, and the economy is not proved. He would always use manholes as being altogether more satisfactory, as will readily be acknowledged should any obstruction occur in the line in question. The additional expense of manholes over that of lampholes is only a very small percentage of the total cost of the system. As temporary endings for laterals which will later be prolonged, he believes lampholes are useful, but elsewhere he prefers manholes. Fig. 881 shows the construction of a lamphole, and how the weight of the vertical pipe, which might otherwise crush down the sewer pipe, is supported by concrete. To still further save expense, the upper end of the Fig. 89 pipe is sometimes covered with a stone and left buried a foot or so below the street surface, the place being referenced so as to be found readily. Otherwise a cast-iron cover must be provided, not touching the pipe, but free to settle independently, as shown in Fig. 89.1 1 From Auburn contract drawings. J. W. Ackerman, City Engineer. CHAPTER VIII. CATCH-BASINS. WHERE the system of sewers is "combined," i.e. designed to receive both domestic sewage and storm water, or where the system is for storm water alone, adequate means must be provided for the admission of such storm water into the sewer. This is commonly done by making openings in the gutter so that the water flowing there will be intercepted and led off in a pipe to the sewer. There are a number of variations, however, in the method by which this is done, as the following discussion and illustrations will show: The location of the inlet is usually at or near the street corner in order that the rush of storm water across a street may be avoided. For example, in Fig. 90, the grades of the street being represented by the arrows, water coming down the gutters on A Street would flow across B Street unless intercepted at the points C and D. If so intercepted no inconvenience is experienced by pedestrians on the crossing below, and no channel is required across B Street, a great advantage to drivers. Similarly, to avoid a rush of water across A Street from left to right inlets should be provided at E and F. One inlet at G might take the place of E and C, but the water would then have to be led across or under both cross-walks. This arrangement will be modified by the topographical conditions. On the summit of a hill no inlets would be needed. In the center of a depression 8 inlets would be needed, and intermediate numbers would correspond to intermediate conditions. The method of construction of these inlets depends upon the use or non-use of catch-basins. Since gutter water presumably carries large quantities of sand, gravel, leaves, sticks, manure, and other street debris, engineers in the past have constructed, in connection with the street inlets, pits or basins through which the gutter water should pass, and by the reduced velocity deposit such debris. But in view of the expense of such basins, it is desirable to eliminate their construction if possible. In many cities such a basin is built in A STREET STREET חוה Fig. 90 connection with each inlet, but it is manifestly possible to bring the two inlets at each corner into one basin, as in Fig. 91, and this economical plan would reach its logical limit by having one basin into which all the inlets should discharge, as in Fig. 90 or Fig. 92. Many engineers to-day believe that with sewers of good grades, discharging freely into deep water, the basins are unnecessary, and that any material passing through the inlet grating will readily be carried to the outlet. Where the velocity of the sewage is less than 2 feet per second, or where a lightgrade sewer succeeds a steep-grade gutter, so that the velocity is much diminished, or if in the same sewer itself a steep grade precedes a light one, a catch-basin is desirable to prevent deposits of silt in the sewer. When the inlets are from dirt or macadam streets, catch-basins should be provided. The size of the basin or pit is determined by the condition of the surface drained and by the frequency of cleaning both streets and basins. If the pavement on the street drained is brick or asphalt, the pit need not contain more than two or three cubic feet. If the street surface is macadam, it should be of twice that size. It is better to have basins cleaned frequently to avoid the accumulation of decaying organic matter, and, therefore, too large a basin is objectionable. When the basin is too small, on the other hand, it may fill up in the first rain, and, if not at once cleaned out, become perfectly useless. The author believes that basins should be avoided when possible, but that the conditions of grades and street surface may sometimes require them, and that old sewers, or sewers badly designed or laid, may make the construction of basins imperative to prevent filling up the sewer. Gratings to hold back floating matter can generally be bought of any local foundry, although the forms of casting will differ materially. Fig. 93 Fig. 93 shows a rectangular flat-top grating manufactured by a foundry in Dayton, Ohio; 94 shows a circular flat-top grating made by a foundry in South Bend, Ind.; and Fig. 95 a circular elevated top; in fact, these gratings are formed of every conceivable design |