Section 2 – Design Standards for Gravity Sanitary Sewers
2.1 General Requirements.
2.1.01 Sanitary sewers are to be provided solely for the collection and transport of sanitary waste. Under no circumstances shall any roof drains, foundation drains, or surface or subsurface drains be either directly or indirectly connection to sanitary sewers. The following design parameters include an adequate allowance for normal infiltration but will not accommodate the above forbidden connections.
2.2. Technical Design.
2.2.01 System Layout/.
- The overall layout and general design shall conform to the parameters set forth in the approved Engineering Report.
- Since all sanitary sewers must be accessible for operation and maintenance:(1) Locate all sanitary sewer in legally established road rights-of-way.
(2) Where it is impossible to avoid placing public sewers on private property, the sewer shall be installed in legally established permanent easements for such purpose, either existing or proposed in accordance with these Standards.
(3) Sewers shall be located outside of jurisdictional wetland areas whenever possible.
(4) Stormwater BMPs shall not encroach on the sanitary sewer.
- Construction shall be along the center line of rights-of-way or easements except when this location has been previously used by another utility, or when the width of a road right-of-way justify the use of two sewer lines. Exception to this specified location will be allowed only when it can be established that it is not practical to adhere to the standard location.
- All sewers shall be on continuous grade between manholes.
- Sewers should intersect in manholes at deflection angles not greater than 90 degrees. In the event that this is impractical the designer must satisfy the Department that adequate losses have been provided in the hydraulic analysis.
- Where sewer depth is 10 feet or less, sewer mains and manholes shall be located a minimum of 10 feet horizontally from any part of a building, structure, or its foundation. Where the depth of sewer is greater than 10 feet, the sewer mains and manholes shall be located a minimum of 15 feet from any part of a building, structure, or its foundation.
2.2.02 System Design.
The overall design shall be in accordance with the provisions of the approved Engineering Report in accordance with Paragraph 1.1.02 of these Standards.
(1) The design of carrying capacities of lateral, trunk, and interceptor sewers shall be based upon the total sewer shed area served by the line or lines in question. The design flow shall be based on acreage density, using the Henrico County Land Use Map or approved zoning, whichever allows higher densities.
(2) Equivalent flows from motels, schools, hospitals, etc. shall be based upon that of the Virginia Department of Environmental Quality Sewerage Regulations.
(3) In the absence of information on densities or equivalent flows, the designer shall supply sufficient information, substantiated by sound engineering judgment to verify the design. This information shall be subject to approval by the Department.
2.2.03 Capacity Design.
- Laterals shall be designed to carry ultimate tributary population with a 50 year projection as an upper limit. Proper allowance for peak flow, as shown on the Peak Flow Chart, Form F-2, shall be included.
- Trunks and interceptors shall be designed on the same basis as laterals, except in cases where capacities of system or parts thereof can be readily increased by future relief, allowing for shorter capacity design life of initial or subsequent lines.
- Computations of all lines shall be shown on a form similar to the sewer design form, Form F-4, including anticipated future relief lines that may be required.Computations shall be accompanied by an Overall (System Layout) Plan (Paragraph 1.1.02 B.), conforming basically to requirements of index map (Paragraph 1.2.02 C.) Map(s) shall show entire drainage area involved, location(s) of line(s) in system, and points of entry of flows, including any flows being received from other areas. Drainage area map shall be keyed to computation sheet (hydraulic analysis, Form F-4). Computations and maps shall be submitted to the Department for approval.
2.2.04 Hydraulic Design – Sewers.
- Minimum grades shall not be less than those required to produce a velocity of approximately two and one quarter (2.25) feet per second when the sewer size selected is flowing full or half full. Pipe sizes shall not be arbitrarily increased in order to take advantage of a flatter grade.
- The minimum size pipe to be used in systems shall be eight (8) inches.
- Allowable minimum grades shall be as follows:
Sewer Size (Inches) Minimum Slope in Feet/100 Feet 8 0.40 10 0.32 12 0.24 14 0.20 15 0.16 16 0.16 18 0.12 21 0.10 24 0.08 27 0.07 30 0.06 33 0.06 36 0.05
- Computations for velocity of flows shall be based upon the following values of “n” as used in the Kutter or Manning formula for velocity of flow.
(1) Sizes 8 inch through 21 inch: “n” equals 0.013
(2) Sizes 24 inch and above: “n” equals 0.012
- In cases where the calculated depth of flow is less than pipe flowing full, the velocity at actual depth of flow shall be computed.
- For sewage flow depth less than 1/4 full, an allowance shall be made for increased value of “n” and in no case shall velocities of less than 1.3 feet per second be permitted. The improved velocities shall be accomplished by steeper grades and not by changing pipe diameter.
- Generally the sizes of pipe shall be continually increasing with increase of tributary areas. However, when steep grades are available and length is such that a significant cost savings will result without jeopardizing the system, the size of pipe may be reduced no more than two (2) nominal diameters, but not below twelve (12) inches. Proper hydraulic allowances must be made for resulting head losses.
- Miscellaneous head losses at manholes and curves shall be computed as follows. Junctions of more than two (2) pipes will require special consideration.
(1) Manholes where radius of turn is less than 2 pipe diameters:
H = . 50 (angle /90