Sanitary pipe system design guidelines

The information and guidelines contained herein are to be used whenever applicable. All sanitary sewers shall be designed in accordance to City Standards and to accepted engineering principles. Always consult design guidelines with the City Engineer to ensure local regulations applied to the specific project and location.

Design submittals shall show all lines necessary for the development or improvement, including pipe size, material, appurtenant, manholes & laterals locations, all sewer profiles (slopes, invert elevations, and finished surfaced elevations), cleanouts, backflow devices, lots to be serviced, property lines, easements, etc.

Pipe Materials:

*Vitrified Clay Pipe (VCP): shall conform to the materials section of the current Clay Pipe Engineering Manual published by the National Clay Pipe Institute www.ncpi.org and shall conform to ASTM requirements, Designation C700, Class II. VCP shall have a dense wall and shall not leak through the barrel more than the allowable when tested in accordance with “Hydrostatic Pressure Test” as described in “Clay Pipe Engineering Manual”.

*Polyvinil Chloride (PVC) Pipe: shall conform to ASTM D3034 (SDR 35) for pipe diameter four (4) inches through fifteen (15) inches and ASTM F679 for pipe diameters from eighteen (18) inches through twenty-four (24) inches. PVC sewer pipe shall only be allowed for residential sewage flows. PVC standard laying length shall be 20 feet minimum, bell and spigot joints with elastomeric gaskets (watertight). www.plasticpipe.org ; www.uni-bell.org

*Reinforced Concrete Pipe (RCP): shall be Class II and conform to ASTM C76 for sewer pipe and lined with PVC T-lock sheets applied at the top 300 degrees of the pipe. RCP standard laying segments are 6 to 8 feet, bell and spigot with rubber gasket joints. www.concrete-pipe.org

*Ductile Iron Pipe (DIP): for sewers shall conform to ANSI A21.50 (AWWA C150) and shall be pressure class 150 minimum unless otherwise shown on plans. Asphaltic or polyethylene outside coating/film and interior ceramic epoxy or cement liner (40 mils) is required. DIP shall only be allowed from manhole to manhole when outside of roadways or sewer laterals, unless otherwise specified.
DIP and Cast Iron restraints calculations and other engineering information can be found on the DIP Research Association (DIPRA) www.dipra.org

Horizontal Alignment: shall be preferably located at the center of street and never under the storm water system. Otherwise, sewer lines shall be located within the paved area of the road with not less than one (1) foot between the outside surface of the pipe and the nearest lip of the gutter or edge of improved road shoulder.
*In general, design public sewer mains in straight street sections to run parallel to the street centerline.
*In curved streets design them on one side of the center line to allow installation of other facilities such as water, storm drains, etc.
*Criteria for the separation water and sanitary sewer most conform to the State of California Department of Health Services (DOHS). The minimum separation required is ten (10) feet clearance between walls otherwise DOHS guidelines must be followed.
*Separation from other wet utilities (Storm drain, sewer, recycle water) will be a minimum of five feet (5) clear between pipes except at crossings.
*Separation from other dry utilities (Gas, electrical cable, etc) will be a minimum of four feet (4) clear between the pipes except at crossings.
*Separation from structures, building over-hangs, gutters, property lines, or edges of easement must be a minimum of five (5) feet clearance and three (3) feet from all monuments, and/or lips of gutters. The alignment will be designed so that any 48 inch manhole shall be centered a minimum of three (3) feet from the lip of gutter and any sixty (60) inch manhole shall be centered a minimum of four (4) feet from lip of gutter.
*Horizontal curves in gravity sewer mains are not allowed.

Vertical Alignment: shall conform to the State of California Department of Health Services (DOHS) “Criteria for separation of Water and Sanitary Sewer”.
*Generally provide a minimum of a foot (1) vertical separation from wet utilities and six (6) inches from dry utilities. When the minimum cannot be maintained other measures, such as concrete encasement or ductile iron pipe, may be submitted for approval of the Director of Utilities.
*Vertical curves in gravity sewer mains are not allowed.


Main Sizing Criteria: will be sized to serve the entire tributary area at build-out densities. Large developments may be required to provide trunk or collection system calculation or have a wastewater model run performed.
*Design Flow: is the average domestic flow, around 125 gallons per person per day, multiplied by the Peak Load Factor (may vary from 1.8 to 3.5). In addition, public sewers shall be designed to carry infiltrated water at the rate of 7% of the design flow. *Collection System Capacity Requires (Q) = Peak Factor (PF) x Average Daily Flow (QADF) + Rainfall Inflow and Infiltration (I/I) + Groundwater Infiltration (GWI)
* The minimum pipe size for main sanitary sewers is eight (8) inches inside diameter.

Hydraulic Calculations: Pipe size, flow rates, velocities, and depth for sanitary gravity flows are determined by solving the various parameters of Manning’s Equation.
Q= vA =(1.49/n)*A*{R^(2/3)}*{S^(1/2)}
Q = flow (ft3/sec)
n = Manning roughness coefficient (RCP-DIP-ABS-VCP n = 0.013; PVC n = 0.010)
R= Hydraulic radius (ft) = A/P
A= Area (ft2)
P= Wetted perimeter (ft)
S= Slope = {(∆E difference in Elev.) / (Horizontal distance)}
.v = velocity (ft/s)
D = Pipe diameter (ft)
.d= depth of flow (ft)
*Design all gravity sewers to achieve a minimum velocity of 2 fps when the pipe is flowing full (Qfull) and do not exceed a maximum velocity of 10 fps.
*Maximum depth of flow (at peak flow conditions) shall be 2/3 D; d/D=0.67 for pipe size ten (10) inch or smaller and 3/4 D; d/D= 0.75 for pipe size twelve (12) inch or larger.
*The preferred minimum slope (S) for gravity sewer is 0.005 (0.5%). Flatter slope conditions may be approved (City Standards)
*The maximum slope is 0.15 (15%), or 15 feet per 100 ft. Consideration to relevant factors such as steep terrain, steeper sewers may be allowed with the use of restrained joints.

Hydraulic Grade Line: The hydraulic grade line shall be determined from the design flows, based upon 100 percent development of the tributary area. Hydraulic grade line calculations must be submitted for the design of all lines 12 inches in diameter or larger.

Cover (Bedding): Minimum cover for all gravity sewers shall be 48 inch from top of the pipe to finish grade (City Standards). Per public works construction standard specifications (“Green book”), for cover less than four (4) feet or more than fifteen (15) feet special bedding is required.

Manholes:
*A manhole (MH) is required at every horizontal or vertical change in alignment (i.e. mains intersection).
*Spacing between manholes is 300 feet maximum (City Standards).
*A manhole is required at the end of every main in excess of 200 feet in length however a rodding inlet or cleanout may be installed in lieu if the main size is ten (10) inch or less.
*Mains eighteen (18) inch or larger in diameter and/or eight (8) feet in depth or more require sixty (60) inch diameter M.H.
*When a change of alignment is greater than 30 degrees allow a minimum drop of 0.1 foot (1.2 in) to compensate for energy loss caused by the change of alignment.
*When pipe sizes change at structures, design the inlet crown at least as high as the outlet crown.
*The change of direction at the manhole between the downstream and any incoming sewer shall be a minimum of 90 degrees.
*Drop manholes are not encouraged. They are typically required when the difference in elevation between the incoming and outgoing sewer is greater than two (2) feet. Upstream slope changes should be used to avoid the need for a drop manhole. If one (1) drop manhole can not be avoided at a connection, use a sixty (60) inch M.H., if two (2) or more are required, use a seventy two (72) inch diameter M.H.

Future sewer extension: when a sewer main extension ends at a manhole and the sewer will be extended further in the future, include in the design a three (3) feet long stub out of the MH with a plug or cap for future connection.

Sewer laterals: must be provide for each lot. The minimum sewer lateral size is four (4) inch and will be located on the property frontage.
*The minimum slop of sewer laterals is two (2 %) percent or ¼ inch per foot for four (4) inch diameter laterals and one (1%) percent or 1/8” per foot for six (6) inch laterals.
*Locate sewer laterals outside of driveway areas where possible. In general, sewer laterals will be in the center third of lots when driveway locations are unknown and a minimum of 10 feet from trees whenever possible. For hillside development, place sewer laterals on the low side of property frontages when not in proposed driveway.
*Laterals serving plumbing fixtures below the nearest upstream sewer manhole rim require an approved backflow overflow device.

Grease interceptors: shall be required for any business having the potential of producing grease. General commercial/retail buildings shall require dedicated grease line for future use.

Abandonment of sewer mains: Sewer mains that are to be abandoned will be securely closed at all pipe ends with a cap or at manholes with a concrete plug. Further, mains twelve (12) inch and larger must be filled with a sand slurry (City Standards).

Easements: When sewer cannot be located within the street, it shall be located in an approved easement. Sewer easement shall be a minimum 15 ft in width if it only contains a sewer main or 20 feet wide if it contains another facility. For deep pipe the easement shall be ((2 x depth) – OD) to a maximum 25 feet.
*No structures may encroach on, above, or below the surface of the ground in any public easement. This includes footings, eaves, decks, etc.
*No trees may be planted in a public sewer easement without first obtaining approval of the Director of Utilities.

Access roads: Clear access must be provided and maintained to all structures on the sewer system. Access roads must be a minimum of twelve (12) feet wide and its location must be approved by the streets and sanitation division.

Creek crossings: Advance approval of the Environmental Utilities Director, City Engineer, and other appropriate agencies is necessary to initiate design.
*Sanitary sewers crossing creeks shall be class 52 D.I.P. encased in reinforced concrete or 3/8 inch thick steel carrier pipe (Casing). The top of the encasement shall have a minimum four (4) feet of cover at the lowest point of the crossing.

Water well clearance: Sewer lines shall maintain a minimum 100 foot separation from all public or private wells (properly abandoned wells are not included). If a clearance of less than 100 feet is approved the pipe material shall be approved by the environmental Utilities Director. In no case shall a clearance of less than 50 feet be allowed.

Performance and testing: all sanitary sewers including laterals will be visually inspected prior backfill placement. Once backfill is completed all sanitary sewers will be air-tested for leakage using current ASTM standards prior to acceptance.

Pretreatment: Special pre-treatment may be required per City code for discharge of individual or commercial wastewater to the sewer at the Developer’s expense. Inquire with the City Water Pollution Control if an industrial wastewater discharge permit is required.

Permitting: Permits are required for all sewer work, including but not limited to the following – Excavation; Obstruction; Traffic control; other as necessary.