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514 Chapter 14 Design of Sewer Systems
as receiving waters had degraded them into open sewers. By contrast, a separate system of
sewerage can exploit natural water courses hydraulically by discharging stormwater into them
through short runs of storm drains while preserving their aesthetic and recreational assets.
However, they may have to be channelized if they are to perform well. In the United States,
large sums of money have been and are being expended in sewer separation and related con-
struction in order to protect water courses from combined overflows. The construction of com-
bined sewers was forbidden by most health departments at the turn of the last century.

14.6 DESIGN INFORMATION
Much detailed information is required for the design of sewers and drains. Special surveys
are generally made to produce needed maps and tables as follows:
1. Detailed plans and profiles of streets to be sewered
2. Plans and contour lines of properties to be drained
3. Sill or cellar elevations of buildings to be connected
4. Location and elevation of existing or projected building drains
5. Location of existing or planned surface and subsurface utilities
6. Kind and location of soils and rock through which sewers and drains must be laid
7. Depth of groundwater table
8. Location of drainage-area divides
9. Nature of street paving
10. Projected changes in street grades
11. Location and availability of sites for pumping stations, treatment works, and outfalls
12. Nature of receiving bodies of water and other disposal facilities.
Much of the topographic information needed is assembled for illustrative purposes in
Figs. 14.13 and 14.14 for a single sanitary sewer in a street also containing a storm drain.
Aerial maps are useful.
Variations in flow to be handled by sanitary sewers are determined by (a) anticipated
population growth and water use during a chosen design period and (b) fluctuations in flow
springing from normal water use. Choice of the design period itself will depend on antici-
pated population increases and interest rates. By contrast, the design period for storm
drains and combined sewers is important principally in connection with expected effects of
drainage-area development on runoff coefficients and magnitudes of flood damage.
Required storm-drain capacity is primarily a matter of probable runoff patterns. Because
storms occur at random, adopted values may be reached or exceeded as soon as storm sew-
ers and drains have been laid.

14.6.1 Sanitary Sewers
Although anticipated wastewater volumes and their hourly, daily, and seasonal variations
determine design capacity, the system must function properly from the start. Comparative
flows are shown in the following schedule:
• Beginning of design period. (a) Extreme minimum flow 1/2 minimum daily flow.
Critical for velocities of flow and cleanliness of sewers. (b) Minimum daily flow
2/3 of average daily flow. Critical for subdivision of units in treatment works.
• Beginning and end of design period. (c) Average daily flow at beginning of design pe-
riod average daily flow at end of period. Critical for velocities of flow in force mains.