Storm events that used to be labeled “50-year” or “100-year” now seem to be occurring annually. This change is affecting the operational stability of municipal sewer systems. Sanitary and combined sewer networks are experiencing flows far beyond what they were originally engineered to handle. The issue isn’t just volume but duration, frequency, and unpredictability.
With every storm surge, excess water finds its way into aging infrastructure through cracks, joints, and poorly sealed lateral connections. The results include hydraulic overloading, system surcharging, treatment inefficiencies, and rising risks to public health and the environment.
Hidden pressure beneath the surface
When rainfall saturates soil over extended periods, hydrostatic pressure builds against buried pipelines. In older systems or networks with poor sealing at manholes, service connections, or joints, this pressure forces water into the sewer system. Even when a pipe isn’t visibly cracked or broken, small leaks become large entry points under pressure.
The invisible load compromises system efficiency and shortens the life of even structurally sound pipelines. Constant exposure to high-moisture conditions can deteriorate joint seals, cause differential pipe movement, and create long-term infiltration issues.
Infiltration and inflow
Infiltration and inflow (I&I) is a major challenge for many municipalities. Infiltration from groundwater and inflow from cross-connected storm drains, roof leaders, or illegally tied sump pumps rapidly multiply inflows. In some systems, wet weather flows triple or quadruple dry-weather averages.
Beyond volume, the velocity and suddenness of the inflow can cause backflow, surcharge conditions, and overflows at manholes or low points in the system. Successfully preventing these problems often involves using trenchless technologies like ultraviolet light-cured, glass-reinforced pipe (UV GRP) liners, which can deliver long-lasting results without excavation.
Wastewater treatment plants under strain
Overloaded systems not only flood streets but also send diluted wastewater to treatment plants. This affects everything from microbial treatment cycles to chemical dosing strategies. Flow equalization becomes difficult or impossible when influent volume exceeds plant capacity.
By allowing stormwater to enter the sanitary system unchecked, treatment efficiency drops and discharge quality suffers. Overflow events risk permit violations, costly fines, and real environmental damage. When the treatment train is overwhelmed, water doesn’t get the contact time or process control it needs. Bacteria, nutrients, and solids may bypass full treatment and reach receiving water bodies.
The consequences of overflow events
Every bypassed gallon has consequences. When stormwater pushes untreated wastewater into rivers, lakes, or aquifers, the damage isn’t easily reversed. In many municipalities, overflows are routed to outfalls that lead to sensitive watershed areas.
Even when discharge complies with regulatory minimums, the increased volume of partially treated or untreated water alters the chemical and biological makeup of surface waters. This can trigger algal blooms, reduce oxygen levels, and degrade aquatic habitats.
When sanitary and storm networks fail together
Many cities still rely on combined or partially separated sewer systems, which makes the interactions between storm and sanitary networks even more complex. During extreme rain, pressure differentials can cause sanitary flow to exfiltrate through cracks or cause inflow to overwhelm sanitary pipes.
Manholes in flood-prone zones often become high-risk entry points. Without watertight covers or riser seals, they let in sheet flow from streets or standing water after rainfall. In the worst cases, these points become geysers, creating unsafe conditions for pedestrians and vehicles.
What municipalities should do
The most actionable path forward involves proactive trenchless rehabilitation strategies. UV GRP liners like Reline America’s Alphaliner deliver consistent wall thickness, minimal cure variability, and high tensile and flexural modulus — all of which contribute to a long-lasting, watertight seal that addresses I&I at the source.
Unlike traditional methods, UV GRP cures on demand. That means tighter construction windows, less site disruption, and more reliability under shifting weather conditions. Combined with manhole rehab and lateral sealing, this approach helps municipalities regain control of flow conditions before storms strike.
Prepare your sewer system for what’s coming
Extreme weather is no longer a theoretical risk. Municipalities must adapt their sewer systems to handle the increasing pace and intensity of these weather events. By leveraging UV GRP trenchless technologies and investing in proactive solutions, public works teams can reduce overflows, maintain water treatment performance, and protect their surrounding watersheds.
Learn more about how to strengthen your sewer system against extreme weather. Connect with the experts at RelineAmerica.com to help determine the best strategy for your system.
