How a sewer model is built, measured, calibrated and used, from the elements in an EPA SWMM file to capacity and master plan analysis. Everything is on this page, free, with no sign-up.
A collection system model is a mathematical copy of your sewers, manholes and pump stations. It shows how full every pipe runs and where water rises out of a manhole. The question it answers most often is what happens when it rains, because rain that leaks into sanitary sewers causes most overflows.
Find the pipes that surcharge and the manholes that spill in a design storm.
Add future flow and see which trunk sewers and pump stations run out of room.
Rank basins by how much they respond to rain, and target rehabilitation where it pays off most.
Show that planned projects bring overflows down to the required level of control.
| System | Carries | Main modeling concern |
|---|---|---|
| Separate sanitary | Wastewater only, by design. In practice, also groundwater and rainwater that leak in. | Sanitary sewer overflows (SSOs) caused by infiltration and inflow. |
| Combined | Wastewater and stormwater in one pipe, as designed. | Combined sewer overflows (CSOs): how often and how much. |
| Storm | Stormwater only. | Flooding and drainage capacity. Covered in the free Stormwater Modeling Fundamentals course. |
This course focuses on separate sanitary systems. The same SWMM elements and process apply to combined systems.
Overflows, growth or rehab targeting set the detail needed.
GIS, inverts, pump stations, billing, plant flows.
Network, connectivity, sewersheds and dry weather loads.
Temporary flow meters and rain gauges through a wet season.
Dry weather first, then wet weather.
Check against storms not used to calibrate.
Design storms, capacity, deficiencies, alternatives.
Phase the fixes into a capital plan.
SWMM splits a model into hydrology, which turns rain into runoff, and hydraulics, which routes flow through the pipes. A sanitary model leans on the hydraulics plus a set of inflows loaded at manholes. Everything is saved in a plain-text .inp file.
| Element | Represents | Key inputs | Common mistake |
|---|---|---|---|
| Junction | A manhole or a pipe connection. | Invert elevation, maximum depth (rim minus invert), surcharge depth, ponded area. | A maximum depth that does not reach the rim, so the model reports flooding while the pipe is only surcharged. |
| Outfall | The downstream end: the treatment plant, a receiving water, or a cut point to another model. | Invert and a boundary type: free, normal, fixed stage, tidal or time series. | A free outfall where the plant headworks actually back water up the trunk sewer. |
| Storage unit | Any volume larger than a manhole: a wet well, an equalization basin, an in-line tank. | Invert, maximum depth, storage curve (area against depth). | Modeling a wet well as a junction, which loses the storage the pump cycles depend on. |
| Flow divider | A structure that splits flow between two pipes. | Diverted link and a cutoff, overflow, tabular or weir rule. | Expecting divider rules to apply in dynamic wave routing. There they act as plain junctions, so model the weir or orifice explicitly. |
| Element | Represents | Key inputs | Common mistake |
|---|---|---|---|
| Conduit | A gravity sewer or a force main. | Shape and size, length, Manning's n, inlet and outlet offsets, entry and exit losses. | Offsets entered as elevations when the model expects depths, or the reverse. |
| Pump | A pump in a lift station. | Pump curve, startup and shutoff depths in the wet well. Types 1 to 5 suit different stations (below). | An ideal pump used for design work, which passes any inflow with no capacity limit. |
| Orifice | An opening in a wall or the bottom of a structure, a gate. | Shape, size, discharge coefficient, optional flap gate. | Forgetting the flap gate on an outlet that cannot flow backward. |
| Weir | An overflow crest: CSO and SSO relief structures, diversions. | Type (transverse, side flow, V-notch, trapezoidal), crest height, length, coefficient. | Crest height as an elevation rather than a height above the node invert. |
| Outlet | A device with a known rating: flow against head or depth. | Rating curve or coefficient and exponent. | Using it where a pump or orifice describes the device better. |
| Type | Flow depends on | Typical use |
|---|---|---|
| Type 1 | Volume in an off-line wet well | Older stations with the wet well modeled as storage |
| Type 2 | Inlet depth, in steps | In-line pumps with stepped capacity |
| Type 3 | Head difference across the pump (head–flow curve) | Most lift stations; uses the manufacturer's curve |
| Type 4 | Inlet depth, continuously | Variable-speed pumps that hold a wet well level |
| Type 5 | Head difference, with speed scaling (affinity laws) | Variable-speed pumps with a known curve |
IF NODE WW1 DEPTH > 6 THEN PUMP P1 STATUS = ON.Gravity sewers flow part full, like small rivers, until they fill. Once full, they act like pressure pipes and the water level climbs up the manholes. A sewer model has to handle both states, and the switch between them.
The full-pipe capacity from Manning's equation is the starting point for every capacity check. Compare it with the peak flow the pipe must carry.
| Term | Meaning |
|---|---|
| d/D | Depth of flow divided by pipe diameter. 0.5 is half full, 1.0 is full. |
| q/Q | Flow divided by full-pipe capacity. It can exceed 1.0 when the pipe is pressurized. |
| Surcharge | The pipe is full and the hydraulic grade line (HGL) rises above its crown into the manholes. |
| Flooding / SSO | The HGL reaches the rim and water leaves the system. |
| Backwater | A downstream restriction (a smaller pipe, a pump station, a high river level) raises water levels upstream. |
| Self-cleansing velocity | About 2 ft/s, enough to keep solids moving. Lower velocities let grease and grit settle. |
| Method | What it handles | Use it when |
|---|---|---|
| Steady flow | Moves the inflow straight through each pipe with no storage or delay. | Quick screening only. |
| Kinematic wave | Delay and attenuation in part-full pipes. No backwater and no pressurized flow. | Simple tree-shaped systems that never surcharge. |
| Dynamic wave | The full equations: backwater, surcharge, pressurized flow, reverse flow and looped pipes. | Almost every sanitary capacity study. Use a short routing step and check the continuity error. |
Check continuity. SWMM reports the flow routing continuity error at the end of every run. Keep it under about 1 to 2%. A larger error usually points to too long a time step, very short pipes or a sudden change in size, not to real physics.
The flow at any meter is the sum of three parts, and each one is modeled differently. Separating them is the core skill of sewer modeling: it tells you how much capacity goes to customers and how much is lost to leaks and rain.
| Component | What it is | How it is estimated |
|---|---|---|
| Base sanitary flow (BSF) | Wastewater from homes and businesses, with a morning and evening peak. | Winter water billing times a return factor, or population times per-capita flow. Shaped by a diurnal pattern. |
| Groundwater infiltration (GWI) | Groundwater seeping through cracked pipes and joints. Steady over a day, higher in wet seasons. | Most of the minimum night flow, when almost no one uses water. A common shortcut takes 80–90% of it; the Stevens-Schutzbach method refines this. |
| Rainfall-derived infiltration and inflow (RDII) | Rain that enters through manhole lids, roof drains, cleanouts and saturated soil around leaking pipes. | Measured flow minus dry weather flow during and after storms, fitted with unit hydrographs. |
SWMM models RDII with three triangular unit hydrographs added together, each describing one path rain takes into the sewer:
Peaks within an hour or two. Roof drains, area drains and leaky manhole lids.
Peaks within several hours. Water moving through the trench backfill.
Lasts a day or more. Groundwater rising around the pipes.
Each triangle has three parameters: R, the fraction of rain it carries; T, its time to peak; and K, the ratio of recession time to time to peak. Optional initial abstraction terms make small storms produce less RDII than large ones, as they do in the field.
A sewer model lives or dies on its invert elevations. One wrong invert creates an adverse slope or a false bottleneck that no calibration can explain. Spend the time on the network before loading any flow.
| Data | Typical source | Used for |
|---|---|---|
| Sewer network | GIS (gravity mains, force mains, manholes), as-built drawings | Geometry, diameter, material, connectivity |
| Inverts and rims | Survey, as-builts, manhole inspections, LiDAR for rims | Slopes, capacity and flooding depth |
| Pump stations | Drawings, pump curves, wet well dimensions, SCADA set points | Pumps, storage units and controls |
| Customer loads | Water billing (winter months), land use, population, large users | Base sanitary flow |
| Condition | CCTV inspection, manhole inspection, smoke testing | Explaining I/I and checking suspect pipes |
| Flow and rain | Flow meters, rain gauges, plant influent records | Calibration (Sections 6 and 7) |
Master plans usually model trunk sewers, often 10 or 12 inches and larger, plus any smaller pipe with a known problem. Loads from the unmodeled pipes are collected at the nearest modeled manhole.
Confirm every pipe points downstream, every manhole connects, and every flow path reaches an outfall. Check the direction of flow at diversions and at pump station force mains.
Look for adverse slopes (an outlet higher than its inlet), drops that do not exist in the field, a pipe invert below the manhole invert, and pipes that get smaller downstream. Confirm suspect values with as-builts or a field visit.
Model each wet well as a storage unit with its real area, each pump with its curve and its on and off levels, and the force main as a conduit from the station to its discharge manhole.
Draw the area that drains to each loading manhole. Sewershed areas carry both dry weather loads and the RDII unit hydrographs.
Assign winter water use from billing to each sewershed, times a return factor, typically 80–95%. Where billing is not available, use population times per-capita flow, commonly 60–100 gallons per person per day. Add large users by hand.
Apply weekday and weekend diurnal patterns from meter data, then add GWI as a constant inflow by sewershed, once metering has measured it.
A sewer model cannot be calibrated without measured flow, and RDII cannot be measured without rain. A flow monitoring program places temporary meters through a wet season so that each meter basin sees several real storms.
Choose meter sites so each basin is a manageable size and similar in age, material and land use. Basins of a few thousand to tens of thousands of feet of pipe are common.
Area-velocity meters need straight pipe with steady flow. Avoid manholes just downstream of drops, bends, junctions or pump discharges, and avoid backwater from pump stations.
Typically 8 to 12 weeks, timed to catch at least three to five significant storms of different sizes.
Every meter basin should have a rain gauge nearby. Radar-rainfall data can fill the gaps between gauges.
Pick days with no rain in the previous few days. Average them into a weekday and a weekend diurnal curve for each meter.
Estimate GWI from the minimum night flow. What remains is base sanitary flow.
Subtract the dry weather curve for that day of the week from the measured flow. What is left is RDII.
Compare R-values and peak RDII per unit area or per inch-diameter-mile of pipe. The worst basins are where rehabilitation and investigation should start.
Calibrate dry weather first, then wet weather. If dry weather flow is wrong, every RDII parameter fitted on top of it is wrong too.
Adjust base flows and GWI by meter basin until each meter's daily volume matches.
Adjust diurnal patterns until the timing and size of the morning and evening peaks match.
Adjust the R values of the unit hydrographs until the RDII volume of each storm matches.
Split R between the short, medium and long-term triangles and tune T and K until the peak and the recession match.
Compare modeled and measured depths, especially where pipes surcharge. Mismatches here often point to a downstream restriction, a wrong invert or a pump station setting, not to the flows.
Run storms not used in calibration. Parameters that only fit the storms they were tuned on will not predict a design storm.
A widely used benchmark is the CIWEM Urban Drainage Group code of practice (formerly WaPUG). Many US studies adopt similar targets:
| Measure | Dry weather | Wet weather |
|---|---|---|
| Peak flow | ±10% | +25% to −15% |
| Flow volume | ±10% | +20% to −10% |
| Depth, not surcharged | ±0.1 m (≈4 in) | ±0.1 m (≈4 in) |
| Depth, surcharged | — | +0.5 m to −0.1 m (≈+20 in to −4 in) |
| Timing | Peaks and troughs line up | Peaks and recession line up |
The wet weather targets are deliberately looser, and lean toward overprediction: a model that slightly overstates peak flows is safer for design than one that understates them. Aim to meet them in at least two of three calibration storms.
With a calibrated model, apply a design condition and see which pipes, manholes and pump stations cannot carry it. The design condition and the pass and fail criteria are policy choices; agree them before you run anything.
Checks basic pipe capacity and the room left for growth. A pipe that is tight in dry weather has no margin for rain.
A synthetic storm such as a 5- or 10-year, 24-hour event, with depths from NOAA Atlas 14 and an SCS distribution. Run on wet antecedent conditions.
Years of real rainfall run through the model. It answers how often overflows happen, which a single design storm cannot.
| Criterion | Typical value | Checked at |
|---|---|---|
| Overflow | No SSO | Design storm |
| Freeboard | HGL at least 2 to 3 ft below the rim, or no surcharge at all | Design storm |
| Depth ratio, d/D | ≤ 0.5 for smaller pipes, ≤ 0.75 for larger trunks | Peak dry weather flow |
| Minimum velocity | ≥ 2 ft/s | Full-pipe or average dry weather flow |
| Pump station | Firm capacity (largest pump out of service) ≥ peak wet weather flow | Design storm |
| Force main velocity | About 2 to 8 ft/s | Pumping |
These are common values, not a standard. Use your state's design rules, the regulator's requirements and your utility's own policy.
A collection system master plan shows what it takes to carry today's and tomorrow's flow at the required level of service, as a sequenced list of costed projects. Every project in it should trace back to a model run.
Typically existing conditions, a near-term horizon of 5 to 10 years, and buildout.
Convert land use and population projections into future base sanitary flow, using unit flow factors by land use or per-capita flow. Assign it to the sewersheds where growth will happen. New sewers usually get lower RDII than old ones.
Run each horizon at peak dry weather and at the design storm. Existing conditions with the design storm shows today's deficiencies; buildout shows the full need.
List every pipe, manhole and pump station that breaks a criterion. Group neighboring deficiencies that share one cause into a single problem area.
For each problem area, test the options below in the model. A relief sewer upstream can push the problem to the next pipe down, so always re-run the whole system.
Apply unit costs per foot by diameter and depth, plus pump station and storage costs, contingency and soft costs. Compare life-cycle cost, not only capital cost: storage and pumping carry operating costs that pipes do not.
Existing overflows come first, then capacity needed for growth, timed to when development is expected. Coordinate with the rehabilitation program and with road works.
| Option | How it helps | Watch for |
|---|---|---|
| Upsize (replace) | More capacity in the same alignment | Bypass pumping during construction; more flow passed downstream |
| Relief sewer (parallel) | Adds capacity while keeping the old pipe in service | Room in the street; flow split between the two pipes |
| Storage | Holds the wet weather peak and releases it later | Odor, cleaning, operating cost; needs a site |
| I/I reduction | Removes RDII at its source by lining pipes, sealing manholes and disconnecting roof drains | Real removal is often less than planned, as water finds the next weakest point. Plan conservatively and confirm by metering. |
| Pump station upgrade | Larger pumps, more pumps, or a larger wet well | The force main and the gravity sewer downstream must take the extra flow |
| Diversion | Moves flow to a pipe or basin with spare capacity | Uses up capacity that growth may need later |
The best plans usually combine options: I/I reduction where basins respond strongly to rain, storage where peaks are short and sharp, and pipe where growth needs permanent capacity.
Most utilities do every step on this page by hand: weeks of invert cleanup, storm-by-storm RDII fitting and one design storm run at a time. The Wastewater practitioner track teaches you to automate the same workflow on your own system.
Build the SWMM model from GIS by script, run design storms and continuous simulations in the cloud, calibrate RDII automatically against your own flow meters, and drive it all through an AI agent. Two days, live online, on your own sewershed.
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