Page 555 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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14.5 Choice of Collecting System 513

Weir Overflow
Adjustable lip
To overflow

To intercepter
To intercepter
(a) Diverting weir-plan (b) Leaping weir-vertical section

Combined
sewer Combined
Intercepter sewer

To
Dry-weather
overflow Regulating
flow continues
Float valve
to intercepter
Telltale pipe
(c) Siphon spillway-vertical section (d) Mechanical diverter, or regulator–
vertical section; actual mechanisms
are more complicated.
Figure 14.12 Regulation of Stormwater Overflow.

Hydraulic separation of excess flows from dry-weather flows is accomplished by de-
vices such as the following:
1. Diverting weirs in the form of side spillways leading to overflows, with crest lev-
els and lengths so chosen as to spill excess flows that, figuratively speaking, over-
ride the dry-weather flows, which follow their accustomed path of the interceptor
(Fig. 14.12a)
2. Leaping weirs, essentially gaps in the floor of the channel over which excess flows
jump under their own momentum, while dry-weather flows tumble through the gap
into the interceptor (Fig. 14.12b)
3. Siphon spillways that carry flows in excess of interceptor capacity into the over-
flow channel (Fig. 14.12c)
4. Mechanical devices, in which diversion of stormwater flows is generally regulated by
float-operated control valves activated by flow levels in the interceptor (Fig. 14.12d).

14.5 CHOICE OF COLLECTING SYSTEM
As explained in a previous chapter, apart from questions of economy, the combined system
of sewerage is at best a compromise between two wholly different objectives: water car-
riage of wastes and removal of flooding runoff. In the life of growing communities, initial
economies are offset in the long run (a) by undesirable pollution of natural water courses
through stormwater spills and consequent nuisance or, at least, debased aesthetic and
recreational values of receiving bodies of water; (b) by the increased cost of treating and
pumping intercepted wastewater; and (c) by more obnoxious conditions when streets and
basements are flooded by combined sewage instead of stormwater.
In the past, small streams, around which parks and other recreational areas could have
grown, have been forced into combined sewerage systems because pressing them into service