206 SEWER DESIGN 



designed. At the manholes the confluent streams are made to 

 merge gradually by means of tongues formed in the bed of the 

 manhole by which means the streams are guided together as 

 smoothly as possible. If one sewer at a small depth enters a 

 main at great depth, it is better to allow the lateral a straight 

 drop with a bend at the bottom than to let the flow shoot down 

 a steep incline to have its solid matter deposited at the foot. 

 When two laterals enter the main from opposite sides it is 

 especially desirable that the streams be guided into the main 

 rather than that their opposing currents meet and form eddies 

 which will tend to the formation of deposits. 



It is sometimes possible to use the volume of water brought 

 down by a lateral as a source of flushing, a gate or storage 

 reservoir in the manhole being arranged, and in this case an 

 incline is better than a straight fall. 



PROBLEMS 



79. Consult collections of plans of sewer systems such as may be found 

 in the report of the New York State Department of Health, 1900-1904, 

 and select distribution systems to illustrate, as well as possible, the dif- 

 ferent systems described in this chapter. 



80. Make a numerical comparison of the results to be obtained by using 

 designs of either Figs. 54 or 55. Assume the length of each square of 

 the figure to be 500 feet, that the amount of contributing sewage is 

 10,000 gallons per day for each such unit distance. Assume a uniform 

 velocity of flow of 2 feet per second and compare the relative elevations 

 of the two outfalls. 



81. If a flush tank installed costs $60, and two sewers are leaving a 

 ridge street on the same cross street, would it be more economical to start 

 the two lines from one flush tank in the middle of the ridge street or to 

 build two flush tanks 180 feet apart on the cross street? Assume the pipe 

 to be 8 inches, costing 14 cents a foot, and the excavation to cost 60 cents 

 per cubic yard. 



