Virginia Administrative Code
Title 9 - ENVIRONMENT
Agency 25 - STATE WATER CONTROL BOARD
Chapter 790 - SEWAGE COLLECTION AND TREATMENT REGULATIONS
Part III - Manual of Practice for Sewerage Systems and Treatment Works
Article 1 - Collection and Conveyance Sewers
Section 9VAC25-790-320 - Design details

1. Sewer size shall not be less than eight inches in diameter, except under the following conditions:

2. Engineering calculations and justifications indicating that reduced line sizes are adequate shall be included with the submission.

1. Sewers shall be installed at a sufficient depth to prevent ice formation due to cooling of the wastewater flows, resulting in blockage of the flow channel. Sewers carrying nonsettled sewage and sewers carrying settled sewage shall be designed and constructed to give mean velocities, when flowing full, of not less than two feet per second and 1.3 feet per second, respectively, based on Manning's formula using a pipe material roughness coefficient ("n") value of 0.014. Use of other "n" values and slopes less than those specified herein shall be justified on the basis of pipe material specifications, research, or field data, presented with the submission for approval.

2. The following list represents the minimum slopes, which should be provided for gravity sewers; however, slopes greater than those listed are desirable:

3. Decreased slopes may be provided where the depth of flow will be 0.3 of the diameter or greater for design average flow. Whenever such decreased slopes are selected, design consultants must furnish, with their report, computations of the depth of flow in such pipes at minimum, average, and peak daily or hourly rates of flow. Otherwise, it must be recognized that decreased slopes may require available resources for additional sewer maintenance.

4. Sewers shall be installed with uniform slope between manholes.

5. Sewers constructed on 20% slope or greater shall be anchored securely with concrete anchors or equal. Suggested minimum anchorage is as follows:

6. Gravity sewers shall normally be installed with a straight alignment between manholes. Curved sewers should be installed only on curved streets, where the curve of the street and the curve of the sewer are concentric. The use of curved alignment for sewers may be considered, with the following restrictions:

7. Gravity sewer size shall normally remain constant between manholes. Where a smaller sewer joins a larger one, the relative elevations of the inverts of the sewers shall be arranged to maintain approximately the same energy gradient. An approximate method for securing these results, which may be used, is to align the 80% capacity flow level, or to align the internal pipe crown or top invert, of both sewers, at the same elevation.

8. Where velocities greater than 15 feet per second are expected, special provisions shall be made to protect against internal erosion by high velocity. The pipe shall conform to applicable ASTM, AWWA, ANSI, or other appropriate standards or specifications, which provide protection against internal erosion.

9. Any generally accepted material for sewers will be given consideration, but the material selected shall be adapted to local conditions such as character of industrial waste, possibility of septicity, soil characteristics, exceptionally heavy internal-external loadings, abrasions, and similar problems. The pipe material shall conform to applicable ASTM, AWWA, ANSI, or other appropriate standards and the pipe is to be marked with an approved identification such as the specifications standard.

10. All sewers shall be designed to prevent damage from superimposed loads. Proper allowance shall be made for loads on the sewer as a result of the width and depth of the trench.