After a heavy rainstorm, it’s common to see water standing in streets, parking lots, or low areas around town. When that happens, many people assume something must be clogged or broken.
Sometimes that’s true.
But often, the stormwater system is working exactly as designed—it’s simply receiving more water than it can move all at once.
Understanding the difference can help explain why flooding sometimes occurs, even when storm drains, pipes, and ditches are functioning properly.
Stormwater Systems Have Limits
Stormwater infrastructure is designed to reduce flooding and move water safely through the community.
This includes:
- Storm drains
- Underground pipes
- Ditches and channels
- Culverts
- Detention and retention ponds
These systems are built to handle specific storm events. Like roads, bridges, and water systems, they are designed around expected conditions.
But not every storm is an average storm.
Rainfall Intensity Matters
When most people think about rainfall, they think about how much rain falls.
Engineers also pay attention to how quickly it falls.
Consider these examples:
- One inch of rain spread over an entire day
- One inch of rain falling in twenty minutes
The total rainfall is the same.
The impact on the stormwater system is very different.
When rain falls faster than water can enter drains, travel through pipes, or flow through ditches, water begins to collect on the surface.
This is called exceeding system capacity.
Think of It Like a Funnel
Imagine pouring water into a funnel.
If you pour slowly, the water flows through without a problem.
If you dump the entire container at once, water backs up and spills over the edge.
Stormwater systems behave similarly.
When rainfall arrives faster than the system can move it, temporary ponding and localized flooding can occur—even when nothing is blocked.
Why Streets Sometimes Become Temporary Storage Areas
Many streets are intentionally designed to carry shallow amounts of water during larger storms.
This may sound surprising, but it is often safer to temporarily store water in streets than to allow it to enter homes and businesses.
During intense rainfall:
- Storm drains collect as much water as possible.
- Pipes and ditches continue moving water downstream.
- Excess water temporarily remains on the surface.
- As the storm passes, the system catches up and water levels recede.
This is why some flooded streets clear within a few hours after the rain ends.
The Role of Detention and Retention Ponds
Stormwater ponds help reduce the strain on the system.
They work by temporarily storing runoff and releasing it more slowly.
Without these facilities:
- Water would move downstream faster
- Peak flooding would be higher
- More pressure would be placed on pipes and channels
Ponds do not eliminate flooding, but they help reduce the severity of flooding during larger storms.
Why New Development Can Affect Runoff
As communities grow, more land becomes covered by:
- Roofs
- Streets
- Sidewalks
- Parking lots
These hard surfaces prevent water from soaking into the ground.
Instead, runoff reaches the stormwater system more quickly and in greater volumes.
Modern development standards often include stormwater controls designed to offset these impacts, but intense storms can still challenge the system.
What About Clogged Drains?
Blocked storm drains can absolutely contribute to flooding.
Common causes include:
- Leaves
- Grass clippings
- Trash
- Sediment buildup
However, not every flooded street is caused by a blockage.
Many flooding complaints occur during storms that temporarily exceed the system’s ability to move water.
Understanding the difference helps explain why flooding can occur even when infrastructure is functioning properly.
What Residents Can Do
While residents cannot control how much rain falls, they can help the system work more effectively.
You can:
- Keep leaves and debris away from storm drains
- Avoid placing grass clippings in streets
- Report damaged or blocked inlets
- Use rain gardens and other infiltration practices when possible
- Direct downspouts toward vegetated areas rather than pavement
Small actions help reduce the amount of runoff entering the system during storms.
Why It Matters
Stormwater systems are designed to manage rainfall, not eliminate every instance of standing water.
During intense storms, there may simply be more water arriving than the system can move at one time.
That doesn’t necessarily mean something is broken.
It means the system is experiencing conditions beyond its normal capacity.
Understanding how stormwater systems respond during large rain events helps explain why temporary flooding occurs—and why slowing, storing, and managing runoff throughout the community remains so important.
Our Water, Our Responsibility.