CEMP-E
EI 11C201
1 March 1997
Q = 0.265 mgd dc/D = 0.26 < 0.30
B-3. Design 1000 feet of lateral sewer for a small tributary area on the above installation with the
following flows.
Offices
2 buildings with 100 employees working 8-hour shifts (30 gpcd is a typical allowance)
Theater
300 seats--open 10 hours per day (3 gpd/seat typical allowance)
Shop
30 employees working 8-hour shifts (30 gpcd is a typical allowance)
No industrial wastes
Domestic Flows
Offices, Theater and Shop
100 x 30
= 3000 gpd over 8 hours
300 x 3
= 900 gpd over 10 hours
30 x 30
= 900 gpd over 8 hours
total average daily flow
= 4800 gpd
Use an 8-hour basis since tributary area is small and all occupants are short term.
Average hourly flowrate
= 4800/8 = 600 gph
Extreme peak flowrate
= 22 5/(600)0.167 = 7.73
R
7.73 x 600
= 4637 gph
Peak diurnal flowrate
= 1/2 x 4637 = 2319 gph
Infiltration Allowance
Assume an 8-inch sewer (minimum size).
1000 x 8 x (1000/5280)
= 1515 gpd = 63 gph
Design Flows
Extreme peak
= 4637 + 63 = 4700 gph
Diurnal peak
= 2319 + 63 = 2382 gph
Average hourly
= 600 + 63 = 663 gph
Typical Sewer Design
Try an 8-inch sewer on a 3.0 percent slope (n = 0.013).
Flow depths and velocities
Q = 4700 gph
d/D = 0.20
V = 3.5 fps
Q = 2382 gph
d/D = 0.13
V = 2.7 fps > 2.5 fps
Q = 663 gph
d/D = 0.05
V = 2.0 fps = minimum
Critical depths
Q = 4700 gph
dc/D = 0.29 > 0.20*
Q = 2382 gph
dc/D = 0.20 > 0.13*
* Note that supercritical flow will result here. The critical slope equals 0.65 percent.
However, a slope of 3.0 percent is required to produce the minimum velocity of 2.0 fps.
Considering that the sewer is of minimum size, and that a flatter slope is not feasible if adequate
velocity is to be provided, supercritical flow would be justified in this case.
B-3
Previous Page
