1. Variances. These design criteria are not intended to cover extraordinary situations. Variances may be allowed or may be required in those instances where considered justified by the City Council.
2. Design factors. Sanitary sewer systems should be designed for the ultimate tributary population. Due consideration should be given to current regulations and reports, where applicable. Sewer capacities should be adequate to handle the anticipated maximum hourly quantity of sewerage and industrial waste, together with an adequate allowance for infiltration and other extraneous flow.
3. Maximum size. The diameter of sewers proposed shall not exceed the diameter of the existing or proposed outlet, whichever is applicable, unless otherwise approved by the Planning Commission.
4. Minimum size. No public sewer shall be less than eight inches in diameter, except that the use of six inch diameter sewer may be permitted in situations where it cannot be extended and no more than 400 feet will be installed in any one place.
5. Velocity of flow.
a. All sewers shall be designed and constructed with hydraulic slopes sufficient to give mean velocities, when flowing full, of not less than two feet per second (based on Kutter’s on 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 researcher 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 minimal 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 | 0.29 |
12 | 0.22 |
14 | 0.17 |
16 | 0.15 |
18 | 0.14 |
21 | 0.10 |
24 | 0.08 |
b. Under special conditions, if full and justifiable reasons are given, slopes slightly less than those required for the two-foot per second velocity, when full, may be permitted. Such decreased slopes will only be considered where the depth of flow will be 0.3 of the diameter or greater for design average flow. Whenever such decreased slopes are selected, the engineer must furnish with his or her report his or her computations of the depths of flow 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 characteristic obtainable.
6. Alignment. All sewers shall be laid with straight alignment between manholes, unless otherwise directed or approved by the Planning Commission or the City Council.
7. Manhole location. Manholes shall be installed:
a. At the end of each line;
b. At all changes in grade, size or alignment;
c. At all intersections; and
d. At distances not greater than 400 feet for sewers 15 inches and smaller, and 500 feet for sewers 18 inches in diameter and larger.
8. Manholes. The difference in elevation between an incoming sewer and the manhole invert shall not exceed 12 inches except where required to match crowns. The use of drop manholes will require approval by the City Council. The minimum inside diameter of the manholes shall conform to those specified by the City Council or the Planning Commission. Inside drop manholes will require special considerations; however, in no case shall the minimum clear distance be less than that indicated above. When a smaller sewer joins a larger one, the crown of the smaller sewer shall not be lower than that of the larger one. The minimum drop through manholes shall be 0.2 feet.
9. Sewerage locations. Sanitary sewers shall be located within street or alley rights-of-way, unless topography dictates otherwise. When located in easements on private property, legal public access shall be available to all manholes. A manhole shall be provided at each street or alley crossing. End lines shall be extended to provide access from street or alley rights-of-way where possible. Imposed loading shall be considered in all locations. Not less than six feet of cover shall be provided over the top of pipe in street and alley rights-of-way, or four feet in all other areas.
10. Water supply interconnections. There shall be no physical connection between a public or private potable water supply system and a sewer, which will permit the passage of any sewage or polluted water into the potable supply. Sewers shall be kept removed from water supply wells and other water supply sources and structures.
11. Relation of sewers to water mains. A minimum horizontal distance of ten feet shall be maintained between parallel water and sewer lines. At points where sewers cross water mains, the sewer shall be constructed of cast iron pipe or encased in concrete for a distance of ten feet in each direction from the crossing, measured perpendicular to the water line. This will not be required when the water main is at least two feet above the sewer.