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206 Chapter 6 Water Distribution Systems: Components, Design, and Operation
3. Estimate the distribution system capacity at each section across the piping. To do this
(a) tabulate the number of pipes of each size cut; count only pipes that deliver water
in the general direction of flow; and (b) determine the average available hydraulic
gradient or frictional resistance, which depends on the pressure to be maintained in
the system and the allowable pipe velocity. Ordinarily, hydraulic gradients lie between
1% (per thousand) and 3%, and velocities range from 2 to 5 ft/s (0.60 to 1.5 m/s).
4. For the available, or desirable, hydraulic gradient, determine the capacities of ex-
isting pipes and sum them for total capacity.
5. Calculate the deficiency or difference between required and existing capacity.
6. For the available, or desirable, hydraulic gradient, select the sizes and routes of
pipes that will offset the deficiency. General familiarity with the community and
studies of the network plan will aid judgment. Some existing small pipes may have
to be removed to make way for larger mains.
7. Determine the size of the equivalent pipe for the reinforced system and calculate
the velocity of flow. Excessive velocities may make for dangerous water hammer.
They should be avoided, if necessary, by lowering the hydraulic gradients actually
called into play.
8. Check important pressure requirements against the plan of the reinforced network.
The method of sections is particularly useful (a) in preliminary studies of large and
complicated distribution systems, (b) as a check on other methods of analysis, and (c) as a
basis for further investigations and more exact calculations.

EXAMPLE 6.2 ANALYSIS OF A WATER NETWORK USING THE SECTIONS METHOD
Analyze the network of Fig. 6.10 by sectioning. The hydraulic gradient available within the network
proper is estimated to lie close to 2%. The value of C in the Hazen-Williams formula is assumed to
be 100, and the domestic (coincident) draft, in this case, only 150 gpcd (568 Lpcd). The fire demand
is taken from Table 4.13. Assume the population for each section as follows: section a-a. 16,000;
section b-b. 16,000; section c-c, 14,700; section d-d, 8,000, and section e-e, 3,000. Also assume that the
type of building construction in the high-value district is combustible and unprotected and that the max-
2
2
imum total surface area per building is 20,000 ft (1,858 m ). The area downstream of the high-value
2
2
district is residential with one- and two-family dwellings having a maximum area of 6,000 ft (557.4 m ).
Solution:
Calculations are shown only for the first three sections.
1. Section a-a population 16,000:
6
(a) Demand: domestic 16,000 150/10 2.4 MGD (9.1 MLD); fire (from Table 4.13)
3,750 gpm 5.4 MGD (20.4 MLD); total 7.8 MGD (29.5 MLD).
(b) Existing pipes: one 24 in. (600 mm); capacity 6.0 MGD (22.7 MLD).
(c) Deficiency: 7.8 6.0 1.8 MGD or (29.5 22.7 6.8 MLD).
(d) If no pipes are added, the 24-in. (600-mm) pipe must carry 7.8 MGD (29.5 MLD).
This it will do with a loss of head of 3.2% at a velocity of 3.8 ft/s (1.16 m/s).
2. Section b-b population and flow as in section a-a:
(a) Total demand 7.8 MGD (29.5 MLD).
(b) Existing pipes: two 20-in. (500 mm) at 3.7 MGD 7.4 MGD (28.0 MLD).
(c) Deficiency 7.8 7.4 0.4 MGD or (29.5 28.0 1.5 MLD).
(d) If no pipes are added, existing pipes will carry 7.8 MGD (29.5 MLD) with a loss of
head of 2.2%, at a velocity of 2.8 ft/s (0.85 m/s).