Spring Hill, Tennessee, approved 14,000 residential lots between 2018 and 2024. In January 2025, the city’s Board of Mayor and Aldermen unanimously approved a sewer moratorium framework after engineers discovered the wastewater system couldn’t keep pace. Not “would eventually struggle.” Couldn’t. Present tense. Homes already occupied. Pipes already undersized.
Sandy, Oregon, did the same thing. So did parts of Queens. In June 2023, Arizona’s Department of Water Resources froze new subdivision permits across the Phoenix Active Management Area after a groundwater model showed the aquifer couldn’t sustain projected development for the next 100 years. Buckeye and Queen Creek, two of the fastest-growing suburbs in the country, suddenly needed to secure alternative water sources before another lot could be platted.
Each of these crises had something in common. Nobody ran the model until it was too late.
A Promise, Not an Engineering Analysis
When a developer submits plans for a 200-home subdivision, the local water and sewer utility issues a document called a will-serve letter. It confirms the utility intends to provide service to the development. Banks require it before financing construction. Counties require it before issuing building permits.
What a will-serve letter almost never includes: a hydraulic model of the downstream system showing that the existing trunk sewer, lift stations, and treatment plant can handle 200 additional homes at peak flow. It is a statement of intent. A commitment to try. The engineering question of whether the system can physically move the sewage goes unanswered, because the modeling is expensive, slow, and nobody’s job to require.
For small infill projects, this gap barely matters. Five homes on a cul-de-sac won’t overwhelm an 18-inch trunk sewer. But when cities issue 14,000 lots over six years, each with its own will-serve letter, the accumulated load can exceed system capacity years before anyone checks. By the time engineers run the numbers, the homes are sold, the families are in, and the fix is a $50 million trunk sewer replacement paid for by all ratepayers.
What the Report Cards Say
ASCE’s 2025 Report Card for America’s Infrastructure gave drinking water a C-minus. Wastewater earned a D-plus. Stormwater received a D. None of these grades improved from 2021. Wastewater has never earned better than a D-plus in the report card’s 27-year history.
The EPA estimates 23,000 to 75,000 sanitary sewer overflows per year, not counting backups into buildings. That range is wide because reporting is inconsistent. Many municipalities don’t track overflows at all unless raw sewage reaches a waterway and triggers Clean Water Act reporting requirements.
Overlay that with building permit data. The metros issuing the most single-family permits in 2025, according to Census Bureau data analyzed by RealPage, are Houston, Dallas, Austin, Phoenix, Atlanta, Charlotte, Raleigh-Durham, Orlando, and Nashville. Every one is in the Sun Belt. Several have experienced moratoriums or capacity warnings in the past two years.
AI Can Model This in Under an Hour
Autodesk’s Innovyze division sells InfoWorks ICM, a hydraulic modeling platform that simulates sewage and stormwater flow through pipe networks under varying conditions. An engineer can add a proposed 200-home subdivision to the model, run 10-year and 25-year storm simulations, and see exactly where the system surcharges, backs up, or overflows. Total modeling time for a single development scenario: under an hour, assuming the base model exists.
Bentley Systems sells OpenFlows for the same purpose. Smaller platforms like Innovyze’s InfoWater Pro handle potable water distribution. These tools have existed for over a decade. What’s new is AI-assisted calibration that matches model predictions to actual flow sensor data, reducing setup time from weeks to days and improving accuracy by flagging pipe segments where the model diverges from reality.
A full InfoWorks ICM license runs $10,000 to $30,000 per year. For a city issuing 1,000 residential building permits annually, that works out to $10 to $30 per permit, buried in the noise of a $500 to $2,000 permit fee. Houston issued over 30,000 single-family permits in 2024. At $30,000 per year for the software, that is less than a dollar per permit.
Why Cities Don’t Do It
The tools exist. The cost is trivial. So why don’t most municipalities require hydraulic modeling before approving residential subdivisions?
Three reasons, and only one is technical.
First, many cities don’t have a calibrated base model of their sewer system. Building one requires accurate pipe inventory data (diameters, materials, slopes, invert elevations), flow monitoring, and engineering time. Older cities with 50-year-old sewer systems may have paper records that were never digitized. Without a base model, you can’t run scenarios against it.
Second, the institutional incentive runs the wrong direction. Building permits generate revenue. Impact fees fund capital improvement budgets. A moratorium generates lawsuits from developers and freezes tax base growth. Elected officials and city managers have every reason to keep issuing permits and no mechanism to internalize the cost of future capacity failures.
Third, there is no federal or state mandate requiring pre-development hydraulic analysis for residential subdivisions in most jurisdictions. Commercial developments over a certain impervious coverage threshold often trigger stormwater modeling requirements. Residential subdivisions get a will-serve letter and a handshake.
The Capacity Math Nobody Runs
A single-family home generates roughly 200 to 300 gallons per day of wastewater, depending on occupancy and fixtures. For a 200-home subdivision, that is 40,000 to 60,000 gallons per day of new sewage entering the collection system. Peak flow factors (the ratio of peak hourly flow to average daily flow) run 2.5x to 4x for residential sewage, pushing peak demand to 100,000 to 240,000 gallons per day.
An 8-inch gravity sewer at 0.5% slope carries approximately 230,000 gallons per day at full capacity. If the downstream trunk sewer receiving this new flow is already running at 70% capacity during peak events, a 200-home subdivision tips it over. Not eventually. On its first rainy Tuesday in November, when infiltration through cracked joints adds another 30% to the flow.
This is arithmetic a hydraulic model handles in seconds. Without the model, the failure mode is a sewer backup into someone’s basement, an SSO into a creek, or a wet-weather bypass at the treatment plant. All of which trigger EPA enforcement, rate increases, and emergency capital spending that costs 3x to 5x what proactive planning would have cost.
Strongest Counterargument
Municipal engineers and utility directors push back on this framing with a reasonable point: most cities maintain capital improvement plans that project future water and sewer demand based on population growth forecasts and comprehensive land-use plans. Will-serve letters are issued in the context of those plans. The utility isn’t blind to growth. It just models it differently, using population projections and per-capita demand factors rather than pipe-by-pipe hydraulic simulation.
That argument holds for cities with well-funded utilities, recent comprehensive plans, and conservative growth assumptions. It breaks down in fast-growth Sun Belt metros where actual development outpaces plan projections by 2x to 5x, where capital improvement plans are updated every five to ten years but permit approvals happen daily, and where the political cost of slowing growth exceeds the engineering cost of upgrading infrastructure. Spring Hill, Tennessee, had a comprehensive plan. It issued 14,000 lots anyway.
What You Can Do
If you’re buying in a new subdivision: Ask the builder for the utility’s will-serve letter. Then ask the utility whether they’ve completed hydraulic modeling for the sewer shed your lot feeds into. If the answer is no, check whether the city publishes a capital improvement plan online. Look for sewer or water main upgrades scheduled in your area. If none are planned and the subdivision adds 100+ homes, you are relying on a promise that hasn’t been stress-tested.
If you’re a builder in a fast-growth market: Commission a hydraulic analysis before you buy the land. A consulting engineer can model the downstream sewer capacity for $5,000 to $15,000. That is insurance against a moratorium freezing your $20 million project mid-construction. Builders in Spring Hill learned this the expensive way.
If your city issues more than 500 residential permits per year: The question isn’t whether to invest in hydraulic modeling software. It is how long until the first moratorium. InfoWorks ICM, Bentley OpenFlows, or EPA’s free SWMM (Storm Water Management Model) can handle capacity analysis for smaller systems at no license cost.
Limitations
This analysis connects aggregate infrastructure data (ASCE grades, EPA needs surveys, Census permit counts) to local capacity failures. Individual utility systems vary enormously. Some fast-growth cities have invested heavily in infrastructure and model diligently. Others with fewer permits may have worse systems. The connection between high permit volume and capacity stress is a pattern, not a universal rule. EPA’s SSO estimate of 23,000 to 75,000 events per year reflects severe reporting gaps, and the actual number may be higher. The per-permit cost calculation for modeling software assumes a calibrated base model already exists, which represents a significant upfront investment that this figure does not capture. Wastewater generation estimates (200-300 gpd per home) are national averages from EPA design standards. Actual flows depend on climate, fixture efficiency, and occupancy patterns.