Slope Sewer pipes must be adequately sloped to reduce solids deposition and production of hydrogen sulfide and methane. Table 4-2 presents a list of minimum slopes for various pipe sizes. If a sewer slope of less than the recommended value must be provided, the responsible review agencies may require depth and velocity computations at minimum, average, and peak flow conditions. The size of the pipe may change if the slope of the pipe is increased or decreased to ensure a proper depth below grade. Velocity and flow depth may also be affected if the slope of the pipe changes. This parameter must receive careful consideration when designing a sewer.
Minimum Slopes For Various Pipe Lengths
Diameter (inches): 8 Pipe Length up to 5 ft: 0.47 Length 6 ft or more: 0.42
Diameter (inches): 10 Pipe Length up to 5 ft: 0.34 Length 6 ft or more: 0.31
Diameter (inches): 12 Pipe Length up to 5 ft: 0.26 Length 6 ft or more : 0.24
Diameter (inches): 14 Pipe Length up to 5 ft: 0.23 Length 6 ft or more: 0.22
Diameter (inches): 24 Pipe Length up to 5 ft: 0.08 Length 6 ft or more: 0.088
Diameter (inches): 30 Pipe Length up to 5 ft: 0.07 Length 6 ft or more: 0.07
Calculating Percent Grade
The invert elevation for a sewer line at adjacent manholes is 1400 feet apart. Elevation 1 is 456 feet and elevation 2 is 450 feet?
The percent grade is the rise in feet over the run in feet times 100. First subtract to find the rise.
Rise in feet = 456-450 ft = 6 ft
% grade = (6 feet) / (1400 feet) x 100 = 0.43 %
The grade for a sewer pipe is 0.5% for a length of 8,000 ft and then changes to 0.3% for an additional 4,000 feet, how many feet lower would the downstream end of the pipe be than the upstream end?
Given the percent grade and the run, solve for the rise. Multiply the grade by the distance for each section and then add them together.
0.005 x 8,000 = 40 ft AND 0.003 x 4000 ft = 12 ft
40 ft + 12 ft = 52 ft
Depth of Bury
Depth of bury affects many aspects of sewer design. Slope requirements may drive the pipe deep into the ground, increasing the amount of excavation required to install the pipe. Sewer depth averages 3 to 6.5 feet below ground surface. The proper depth of bury depends on the water table, the lowest point to be served (such as a ground floor or basement), the topography of the ground in the service area, and the depth of the frost line below grade.