Gravity sanitary sewers work because water flows downhill. The pipe must be laid at a slope steep enough to maintain a self-cleansing velocity of at least 2 feet per second (ft/s) at the design flow. If the slope is too flat, solids settle in the pipe, causing blockages and odors. If the slope is too steep, the liquid outpaces the solids, leaving them stranded. Getting the slope and pipe size right is one of the fundamental tasks of sanitary sewer design.

Minimum Slope Requirements

Minimum slopes are established to achieve the self-cleansing velocity of 2 ft/s when the pipe is flowing at design capacity (typically half-full for gravity sewers). The slopes are calculated using Manning's Equation and vary by pipe diameter. Most jurisdictions adopt the following minimums, based on standards from the Gravity Sanitary Sewer Design and Construction manual (ASCE MOP No. 60 / WEF MOP No. FD-5):

Maximum Slope and Velocity

Maximum velocity in a gravity sewer should not exceed 10 ft/s for standard PVC and ductile iron pipe. Velocities above this cause excessive pipe erosion, turbulence, and noise. For steep sites, the options include:

• Drop manholes — a vertical drop inside the manhole structure, allowing the sewer to descend rapidly without increasing pipe slope. Internal drops of up to 24 inches are common; external drops handle greater elevation changes.
• Energy dissipation structures — for large-diameter sewers on steep slopes where drop manholes are insufficient.
• Flatter pipe slope with more frequent manholes — sometimes the geometry allows the pipe to be laid flatter than the ground slope by increasing the depth at each manhole.

Pipe Sizing by Flow

Pipe sizing is based on the peak wastewater flow. The design flow is calculated as:

Peak Flow = Average Daily Flow x Peaking Factor + I&I Allowance

Average daily flow is based on the land use and number of connections. Common design values from EPA and Ten States Standards:

Peaking Factor

The peaking factor accounts for the fact that flow varies throughout the day. Peak flows are highest in the morning and evening and lowest at night. The peaking factor is a multiplier applied to the average daily flow to estimate the peak instantaneous flow. Common formulas include:

• Harmon Formula: PF = 1 + 14 / (4 + P^0.5), where P is the contributing population in thousands. For 5,000 people, PF = 3.5; for 50,000 people, PF = 2.4.
• Ten States Standards: PF = 18 / P^0.5 for populations greater than 1,000, with a minimum of 1.5 and maximum of 4.0.

Infiltration and Inflow (I&I)

I&I is groundwater and stormwater that enters the sanitary sewer through defective pipes, joints, and manholes. An I&I allowance is added to the peak flow for pipe sizing. Common design allowances range from 200 to 1,000 gallons per day per inch-diameter per mile (gpd/in-dia/mi) of pipe, depending on soil conditions and groundwater levels. A typical value is 500 gpd/in-dia/mi.

Manning's Equation for Pipe Capacity

The capacity of a gravity sewer is calculated using Manning's Equation:

Q = (1.486 / n) x A x R^(2/3) x S^(1/2)

Where Q is flow in cubic feet per second, n is Manning's roughness coefficient, A is the cross-sectional flow area, R is the hydraulic radius (A divided by wetted perimeter), and S is the pipe slope in ft/ft. For sewer design:

• PVC (SDR 26 or SDR 35): n = 0.010 to 0.013
• Ductile Iron (cement-lined): n = 0.013
• Vitrified Clay: n = 0.013 to 0.015
• Concrete: n = 0.013 to 0.015

Most districts require n = 0.013 for design regardless of pipe material, providing a conservative estimate.

Pipe Capacity at Minimum Slope

Design Depth and Cover

Sanitary sewers must have adequate cover (depth of soil above the pipe crown) to protect against crushing from traffic loads and to prevent freezing. Minimum cover requirements vary by jurisdiction:

• Under roadways: typically 3 to 5 feet minimum cover
• Outside roadways: typically 2.5 to 3 feet minimum cover
• In cold climates: the sewer must be below the frost line, which can be 4 to 8 feet in northern states

Maximum depth is limited by construction cost and the need for manholes. Sewers deeper than 20 feet require deep trenching equipment, heavy shoring, and more expensive manholes, significantly increasing construction costs.

Manhole Spacing and Placement

Manholes provide access for maintenance and are required at every change in pipe direction, slope, size, or alignment. Maximum spacing between manholes varies by jurisdiction but is typically 300 to 400 feet for pipes 15 inches and smaller, and up to 500 feet for larger pipes. Some districts allow up to 600 feet with clean-out provisions.

Drop manholes are used where the invert of the incoming pipe is more than 24 inches above the outgoing invert. The drop is constructed inside or outside the manhole to reduce turbulence and prevent sewer gases from being released into the manhole.

Common Design Mistakes

• Designing for full-pipe flow. Gravity sewers are designed for flow at half-full (d/D = 0.5) or no more than 75 percent full at peak flow. Full-pipe flow means the pipe is surcharging, which causes backups and overflows.
• Ignoring I&I allowance. A sewer designed for dry-weather flow alone will surcharge during wet weather when groundwater infiltrates through joints. Always include the I&I allowance.
• Using minimum slope everywhere. Minimum slope produces minimum velocity. On long flat runs, slightly steeper slopes provide a safety margin against blockages. Use minimum slope only where the topography forces it.
• Forgetting about future connections. Size trunk sewers for ultimate buildout capacity, not just the first phase. Adding capacity later requires parallel sewers, which is far more expensive than sizing correctly the first time.