EI 11C201
CEMP-E
1 March 1997
B-4. Design a short lateral sewer to serve a battalion headquarters area with 5 companies, each
consisting of 100 men. Facilities include a 500 man barracks complex, administrative of fices,
motor pool and mess hall. During the normal 8-hour workday a total of 300 people work in the
area, 100 residents (out of 500 total) plus 200 nonresiding officers, NCO's and civilian
employees. No industrial wastes are generated. A per capita allowance of 50 gpd from Table
3-1 will be used for the barracks buildings, and 30 gpcd is a typical allowance for office and shop
workers.
Domestic Flows
300 x 30
= 900 gpd on an 8-hour basis
500 x 50
= 25,000 gpd on a 16-hour basis
total average daily flow
= 34,000 gpd
Compute average hourly flowrates
For 8 hours - 9000/8
= 1125 gph
For 16 hours - 25,000/16
= 1563 gph
Use the 1563 gph as the average hourly flowrate
Extreme peak flowrate
R = 22.5/(1563)0.167
= 6.59
6.59 x 1563
= 10,300 gph
Peak diurnal flowrate
= 1/2 x 10,300 = 5150 gph
Design Flows
Extreme peak
= 10,300 gph
Peak diurnal
= 5150 gph
Average hourly
= 1563 gph
Typical Sewer Design
Try an 8-inch sewer on a 1.4 percent slope (n = 0.013)
Flow depths and velocities
Q = 10,300 gph d/D = 0.35
V = 3.3 fps
Q = 5150 gph
d/D = 0.25
V = 2.8 fps > 2.5 fps
Q = 1563 gph
d/D = 0.14
V = 2.0 fps = minimum
Critical depths
Q = 10,300 gph
dc/D = 0.43 > 0.35*
Q = 5150 gph
dc/D = 0.30 > 0.25*
Q = 1563 gph
dc/D = 0.17 > 0.14*
* Again flow would be supercritical, but since a slope of 1.4 percent is required to produce the
maximum velocity of 2.0 fps (Sc = 0.7 percent), and the sewer is of minimum size, supercritical
flow is justified.
B-4