All sewers shall be designed and constructed with hydraulic slopes sufficient to give mean velocities, when flowing full, of not less than two (2) feet per second, based on Kutter's or Manning's formula, using an "n" value of 0.013. Use of other practical "n" values will be approved for the longer pipe sections if deemed justifiable on the basis of research or field data presented. The following are the minimum slopes which should be provided, especially where the depth of flow may be small, and are desirable minima in all parts of the system:
Minimum Slopes for Sewer Sizes | |
Sewer Size (inches) | Minimum Slope in Feet per 100 feet |
Minimum Slopes for Sewer Sizes | |
Sewer Size (inches) | Minimum Slope in Feet per 100 feet |
8 | 0.40 |
10 | 0.20 |
12 | 0.22 |
14 | 0.17 |
15 | 0.15 |
16 | 0.14 |
18 | 0.12 |
21 | 0.10 |
24 | 0.08 |
Under special conditions, if full and justifiable reasons are given, slopes slightly less than those required for the two (2) foot per second velocity when full may be permitted. Such decreased slopes will only be considered where the depth of flow will be three- tenths (0.3) of the diameter or greater for design average flow. Whenever such decreased slopes are selected, the engineer must furnish with his report his computations of the depths of low in such pipes at minimum, average, and peak rates of flow. It is recognized that such flatter grades may cause additional sewer maintenance expense and odor nuisance. The selection of the size of pipe shall be determined on the basis of the most desirable flow characteristics obtainable.
(Code 1991, § 12-488)