Gravity Sewers & Collection Hydraulics

Gravity sewers move wastewater primarily because of elevation difference and pipe slope. Unlike force mains, they normally operate partially full, with wastewater flowing along the bottom of the pipe and air occupying the space above it.

Collection-system operators need to understand how slope, pipe size, flow depth, velocity, roughness, restrictions, and downstream conditions affect sewer performance. Hydraulic problems often show up as slow flow, solids deposition, surcharging, backups, manhole overflows, or unusually high upstream levels.

How Gravity Sewers Move Wastewater

Gravity flow occurs because wastewater moves from a higher hydraulic elevation to a lower one.

Important factors include:

• pipe slope;
• pipe diameter;
• flow depth;
• pipe roughness;
• obstructions;
• downstream water level.

Gravity Sewer Versus Force Main

A gravity sewer normally depends on slope and flows partially full.

A force main is pressurized by pumps and normally flows full.

Open-Channel Flow

When a gravity sewer is only partially full, it behaves hydraulically like an open channel.

The wastewater surface is exposed to air inside the pipe.

Flow Depth

Flow depth varies with:

• wastewater flow rate;
• pipe diameter;
• slope;
• roughness;
• downstream conditions.

Low Flow Depth

During low-flow periods, only a small portion of the pipe may contain wastewater.

Possible concerns include:

• low velocity;
• solids deposition;
• long wastewater residence time.

High Flow Depth

As wastewater flow increases, depth also increases.

If flow becomes high enough, the pipe may approach full-flow conditions.

Surcharge

Surcharge occurs when the wastewater level rises above the normal open-channel condition and the sewer begins operating under pressure.

Surcharging can result from:

• peak flow;
• wet-weather infiltration and inflow;
• downstream blockage;
• restricted pipe capacity;
• pump-station limitations.

Pipe Slope

Slope is the vertical change in elevation divided by horizontal distance.

A common expression is:

Slope = Vertical Drop ÷ Horizontal Length

Slope as Percent

Slope is often expressed as a percentage:

Slope, % = Vertical Drop ÷ Horizontal Length × 100

Slope Example

A sewer drops 4 feet over a horizontal distance of 1,000 feet.

Slope = 4 ÷ 1,000

Slope = 0.004 ft/ft

As a percentage:

Slope = 0.004 × 100

Slope = 0.4%

Why Slope Matters

Greater slope generally provides more hydraulic energy for flow.

Insufficient slope can contribute to:

• low velocity;
• solids settling;
• grease accumulation;
• frequent cleaning needs.

Too Much Slope Can Also Create Problems

Very steep slopes can produce:

• high velocities;
• abrasion;
• hydraulic turbulence;
• drop-related maintenance issues.

Velocity

Velocity is the speed at which wastewater moves through the sewer.

Velocity affects:

• solids transport;
• deposit formation;
• travel time;
• abrasion.

Low Velocity

Low velocity can allow solids to settle and accumulate.

Common deposited materials include:

• grit;
• sand;
• organic solids;
• grease-associated debris.

High Velocity

Higher velocity can improve solids transport but may increase:

• pipe wear;
• abrasion;
• hydraulic energy at structures.

Continuity Equation

A basic hydraulic relationship is:

Q = A × V

Where:

• Q = flow rate;
• A = flow area;
• V = average velocity.

Velocity Example

A sewer carries 8 ft³/s through a flowing area of 4 ft².

V = Q ÷ A

V = 8 ÷ 4

V = 2 ft/s

Flow Example

A flowing area is 3 ft² and average velocity is 2.5 ft/s.

Q = 3 × 2.5

Q = 7.5 ft³/s

Area Example

A measured flow is 6 ft³/s and average velocity is 3 ft/s.

A = Q ÷ V

A = 6 ÷ 3

A = 2 ft²

Full Circular Pipe Area

For a full circular pipe:

Area = π × Diameter² ÷ 4

Pipe-Area Example

A full pipe has a diameter of 2 feet.

Area = 3.1416 × 2² ÷ 4

Area = 3.1416 ft²

Partially Full Pipes

When a circular sewer is partially full, the flowing cross-sectional area is less than the full pipe area.

The relationship between depth, area, hydraulic radius, and velocity becomes more complex than the simple full-pipe calculation.

Diameter and Capacity

Larger pipe diameter generally provides greater hydraulic capacity.

Increasing diameter changes:

• cross-sectional area;
• hydraulic radius;
• potential flow capacity.

Pipe Capacity Is Not Determined by Diameter Alone

Capacity also depends on:

• slope;
• roughness;
• flow depth;
• downstream hydraulic conditions;
• obstructions.

Pipe Roughness

Rough pipe surfaces create more friction than smooth surfaces.

Roughness can increase because of:

• corrosion;
• mineral deposits;
• roots;
• grease;
• debris;
• deteriorated pipe surfaces.

Friction Loss

As wastewater moves through a sewer, energy is lost because of friction against the pipe surface.

Greater friction can reduce the flow that a sewer can convey at a given hydraulic condition.

Manning Relationship

Gravity sewer design and hydraulic analysis commonly use Manning-type relationships involving:

• pipe roughness;
• hydraulic radius;
• slope.

Operators may not always calculate complete sewer capacity in daily work, but they should understand that capacity decreases when roughness or restrictions increase.

Self-Cleansing Concept

A sewer should carry enough flow and velocity often enough to limit excessive solids deposition.

This is sometimes described as maintaining self-cleansing conditions.

Self-Cleansing Does Not Mean No Maintenance

Even well-designed sewers may still require cleaning because of:

• grease;
• roots;
• wipes;
• grit;
• structural defects.

Minimum Flow Periods

Late-night and other low-use periods can produce the lowest sewer velocities.

Problem areas may accumulate deposits during repeated low-flow periods.

Peak Flow

Peak flow is the highest flow occurring during a selected time period.

Peak conditions can be caused by:

• daily customer-use patterns;
• rainfall;
• snowmelt;
• industrial discharge cycles.

Average Flow Versus Peak Flow

A sewer that handles average flow easily may surcharge during peak conditions.

Capacity evaluations should therefore consider peak flow rather than average flow alone.

Wet-Weather Flow

Rain can rapidly increase collection-system flow through inflow and infiltration.

Possible hydraulic effects include:

• higher flow depth;
• higher velocity in some locations;
• surcharge;
• pump-station overload;
• sanitary sewer overflow risk.

Infiltration

Groundwater entering through cracked pipe, leaking joints, or damaged manholes may create a more sustained increase in flow.

Inflow

Direct stormwater entry can produce a rapid flow increase during rainfall.

Hydraulic Grade

The hydraulic grade represents the elevation to which wastewater would rise under the hydraulic conditions in the system.

In normal gravity flow, the wastewater surface provides an indication of hydraulic grade.

Hydraulic Grade During Surcharge

When a sewer is surcharged, the hydraulic grade may rise above the crown of the pipe.

If it rises high enough, wastewater can:

• back up into service laterals;
• rise in manholes;
• overflow from low structures.

Upstream and Downstream Levels

Comparing manhole levels can help locate hydraulic restrictions.

Restriction Pattern

A common pattern is:

• high wastewater level upstream;
• much lower level downstream.

This can indicate a blockage or severe restriction between the two points.

Backwater

Backwater occurs when downstream hydraulic conditions raise the water level upstream.

Possible causes include:

• downstream surcharge;
• high receiving level;
• blocked sewer;
• pump-station limitation.

Downstream Conditions Matter

A sewer may have adequate slope and diameter but still perform poorly if downstream water levels are abnormally high.

Blockages Reduce Effective Area

A blockage reduces the area available for flow.

Examples include:

• grease;
• roots;
• wipes;
• debris;
• collapsed pipe.

Partial Blockage

A partial obstruction can allow normal dry-weather flow while causing surcharge during peak flow.

Why Partial Blockages Can Be Hard to Detect

During low flow, the sewer may appear to function normally.

During rain or peak use, the restriction becomes hydraulically important.

Deposits Reduce Capacity

Accumulated grit or solids reduce the effective cross-sectional area of the sewer.

This can cause:

• higher flow depth;
• lower available capacity;
• greater surcharge risk.

Root Intrusion

Roots can both:

• reduce flow area;
• trap rags and other debris.

Grease Deposits

Grease can progressively reduce pipe diameter and create rough surfaces that trap solids.

Pipe Deformation

Deformed or collapsed pipe can create severe hydraulic restrictions.

Offset Joints

Misaligned joints can:

• reduce flow area;
• trap solids;
• increase turbulence.

Bellies and Sags

A sewer sag is a low section where wastewater can remain deeper than intended.

Sags can contribute to:

• solids deposition;
• septic conditions;
• reduced hydraulic performance.

Changes in Pipe Diameter

Transitions should allow wastewater to move without creating unnecessary hydraulic restrictions.

Changes in Direction

Sharp or poorly configured changes in direction can increase energy loss and create maintenance problems.

Manhole Channels

Smooth manhole channels help preserve hydraulic continuity through junctions and direction changes.

Junctions

At sewer junctions, multiple flows combine.

Operators should remember that downstream flow can be much greater than flow in any single tributary sewer.

Flow Balance Concept

At a junction, the total downstream flow approximately equals the sum of incoming flows, assuming no significant storage or loss.

Junction Example

Two upstream sewers carry:

• 1.2 MGD;
• 0.8 MGD.

The approximate combined downstream flow is:

1.2 + 0.8 = 2.0 MGD

Travel Time

Travel time depends on:

• distance;
• velocity.

A basic relationship is:

Travel Time = Distance ÷ Velocity

Travel-Time Example

Wastewater travels through 6,000 feet of sewer at an average velocity of 2 ft/s.

Time = 6,000 ÷ 2

Time = 3,000 seconds

Convert to minutes:

3,000 ÷ 60 = 50 minutes

Why Travel Time Matters

Long travel time can contribute to:

• septic wastewater;
• odor;
• hydrogen sulfide formation.

Dry-Weather Hydraulic Assessment

Useful observations include:

• flow depth;
• velocity;
• deposit buildup;
• odor;
• upstream and downstream levels.

Wet-Weather Hydraulic Assessment

During storms, operators may compare:

• rainfall;
• manhole levels;
• pump-station run times;
• treatment-plant influent flow;
• known surcharge locations.

Flow Monitoring

Temporary or permanent flow meters can help identify:

• average flow;
• peak flow;
• daily patterns;
• wet-weather response.

Depth Monitoring

Depth sensors can show when a sewer begins approaching surcharge.

Velocity Monitoring

Velocity measurements can help evaluate:

• solids transport;
• hydraulic capacity;
• metered flow.

Flow Meter Data Quality

Unexpected hydraulic data should be checked for:

• sensor fouling;
• incorrect calibration;
• debris;
• bad level measurements;
• data-communication errors.

Example: Upstream Level High, Downstream Level Normal

This pattern suggests a restriction between the monitoring points.

Investigate:

• blockage;
• roots;
• collapsed pipe;
• heavy deposits.

Example: Upstream and Downstream Levels Both High

This may indicate a larger downstream hydraulic problem rather than a local blockage.

Example: Sewer Surcharges Only During Rain

Possible causes include:

• infiltration and inflow;
• limited downstream capacity;
• partial blockage that becomes critical at high flow.

Example: Sewer Requires Frequent Cleaning

Review:

• slope;
• normal velocity;
• grease sources;
• root intrusion;
• structural defects.

Example: Flow Depth Slowly Increases Over Months

At similar wastewater flow, increasing depth can indicate:

• deposit buildup;
• progressive blockage;
• downstream restriction.

Example: Manhole Levels Rise Rapidly During Peak Use

The sewer may be approaching its available hydraulic capacity.

Example: High Flow Reaches Plant Earlier After Rain

Rapid response can indicate direct inflow sources.

Example: Flow Remains Elevated Long After Rain Ends

This pattern can indicate groundwater infiltration.

Example: Velocity Is Low in a Problem Sewer

Review:

• slope;
• flow rate;
• pipe size;
• deposits;
• structural condition.

Example: Velocity Appears High but Flow Is Low

Verify:

• flow area;
• depth measurement;
• velocity sensor;
• meter calibration.

Hydraulic Problems Can Create Maintenance Problems

Low velocity can lead to deposits.

Deposits reduce area and increase surcharge risk.

This can create a cycle of worsening hydraulic performance.

Maintenance Can Restore Hydraulic Capacity

Cleaning can remove:

• grit;
• grease;
• debris;
• some root intrusion.

Cleaning does not correct every structural problem.

Structural Repair May Be Necessary

Persistent hydraulic problems can require:

• spot repair;
• lining;
• replacement;
• other rehabilitation.

Hydraulic Capacity and Asset Condition

A sewer originally designed with adequate capacity may lose effective capacity because of deterioration or buildup.

Use Historical Data

Useful comparisons include:

• flow versus rainfall;
• depth versus flow;
• cleaning frequency;
• overflow history;
• customer backup history.

Common Gravity-Sewer Hydraulic Mistakes

• Assuming gravity sewers always flow full.
• Confusing gravity sewer flow with pressurized force-main flow.
• Looking at pipe diameter without considering slope and roughness.
• Using average flow when peak flow controls the problem.
• Ignoring downstream hydraulic conditions.
• Ignoring partial blockages that become critical only during high flow.
• Assuming low velocity is only a cleaning problem rather than a possible hydraulic or structural issue.
• Ignoring wet-weather flow patterns.
• Using questionable flow-meter data without verification.
• Failing to compare upstream and downstream levels when locating restrictions.

A Practical Gravity-Sewer Review

1. Review sewer size and slope.
2. Review current and normal flow.
3. Review flow depth.
4. Review upstream and downstream levels.
5. Review known deposits and cleaning history.
6. Review structural inspection results.
7. Review wet-weather behavior.
8. Determine whether the problem is hydraulic, structural, maintenance-related, or a combination.

A Practical Surcharge Review

1. Identify the highest upstream water levels.
2. Determine whether the condition occurs in dry or wet weather.
3. Check downstream levels.
4. Check for blockages or restrictions.
5. Review pump-station operation where applicable.
6. Review rainfall and I&I response.
7. Compare the event with previous surcharge history.
8. Correct the identified restriction or capacity problem.

A Practical Low-Velocity Review

1. Verify the flow and velocity data.
2. Review pipe slope.
3. Review pipe diameter and normal flow depth.
4. Inspect for deposits.
5. Review cleaning history.
6. Check for structural sags or deformation.
7. Determine whether preventive cleaning or structural correction is needed.

A Practical Hydraulic-Trend Review

1. Compare current flow with historical flow.
2. Compare depth at similar flow conditions.
3. Compare dry-weather and wet-weather response.
4. Review overflow and backup history.
5. Review maintenance frequency.
6. Identify locations where hydraulic performance is deteriorating over time.

What to Remember for the Exam

• Gravity sewers normally move wastewater by elevation difference and pipe slope.
• Gravity sewers usually operate partially full and behave as open channels.
• Surcharge occurs when wastewater rises above normal open-channel conditions and the sewer begins operating under pressure.
• Slope equals vertical drop divided by horizontal length.
• Low slope and low velocity can contribute to solids deposition.
• The continuity relationship is Q = A × V.
• For a full circular pipe, area equals π × diameter squared ÷ 4.
• Pipe capacity depends on diameter, slope, roughness, depth, downstream conditions, and restrictions.
• Roughness, roots, grease, deposits, and structural defects can reduce hydraulic capacity.
• Average flow can be acceptable while peak flow causes surcharge.
• Wet-weather infiltration and inflow can greatly increase sewer flow.
• The hydraulic grade shows the elevation to which wastewater rises under system conditions.
• High upstream level with lower downstream level can indicate a restriction between those points.
• Partial blockages may cause problems only during high-flow conditions.
• Deposits reduce effective pipe area and available capacity.
• Travel time can be calculated as distance divided by velocity.
• Long travel time can contribute to septicity and odor.
• Rapid wet-weather flow response can suggest inflow, while prolonged elevated flow can suggest infiltration.
• Cleaning can restore capacity lost to deposits, but structural defects may require repair or rehabilitation.
• Good collection hydraulics troubleshooting combines flow, depth, velocity, slope, pipe condition, upstream and downstream levels, rainfall response, and maintenance history.