48 DESIGN 



outlet sewer through a pipe too small to carry the storm- 

 water flow. 



Sometimes where there are springs or another adequate 

 source of supply for the lake or pond, the entire volume of 

 storm-water is passed off into the outlet sewer direct. 



Calculation of Size. The usual methods of calculation 

 are employed. The coefficient of imperviousness, however, 

 will be low, due to the character of the surfaces, which are 

 mostly grassy. 



An outlet sewer from a lake need not be of sufficient 

 capacity to carry the entire flow of the storm if this flow is 

 admitted to the lake. The body of water may be used as 

 a reservoir whose elevation will rise slightly at time of a 

 storm and the surplus water will run off through the small 

 outlet sewer after the storm is over. 



2. Sanitary Sewers. These are used to carry the flow 

 from toilet rooms which are located in shelters and pavilions. 

 Such sewers are small. They should never discharge into 

 park lakes and water courses. Sanitary sewers are gen- 

 erally laid much deeper than storm-water sewers. It is 

 very important that the grade be flat in order that the 

 depth of flow may be as great as possible. The minimum 

 velocity should be 1.5 feet per second. I-n order to lay 

 the sewers on flat grades it may be necessary to build a 

 few drop manholes at places where the sewers join. The 

 size of sanitary sewers is 6 inches except for the mains. 

 Owing to the impossibility in many cases of obtaining a 

 proper depth of flow, say one-quarter of the diameter, 

 flush tanks will have to be used to keep the sewer free from 

 incrustation. 



3. Combined Sewers. These are but seldom used in 

 park engineering. Occasionally however, where a sanitary 

 sewer is available, storm-water sewers will be connected 



