Have you ever noticed how quickly water runs across a parking lot during a storm?
Or why a grassy field often stays relatively dry while a nearby street develops puddles almost immediately?
The answer comes down to something called impervious surfaces.
Impervious surfaces are hard materials that prevent water from soaking into the ground. They are a normal part of every community, but they also change how stormwater behaves.
Understanding the difference between natural land and paved surfaces helps explain why flooding can occur more frequently in developed areas.
What Is an Impervious Surface?
An impervious surface is any surface that water cannot easily penetrate.
Common examples include:
- Streets and roads
- Parking lots
- Sidewalks
- Driveways
- Rooftops
When rain falls on these surfaces, it cannot soak into the soil. Instead, it becomes runoff.
That runoff must travel somewhere—usually toward storm drains, ditches, ponds, and waterways.
How Natural Land Handles Rain
Before communities were developed, much of the landscape was covered by:
- Prairie grasses
- Trees
- Wetlands
- Natural depressions
- Permeable soils
When rain fell, much of it:
- Soaked into the ground
- Recharged groundwater
- Was absorbed by vegetation
- Evaporated back into the atmosphere
Only a portion became surface runoff.
This natural process helped reduce flooding and improve water quality.
What Changes When Land Is Developed?
Development changes how water moves across the landscape.
As more rooftops, roads, and parking lots are built:
- Less water infiltrates into the ground
- More water becomes runoff
- Runoff moves faster
- Larger volumes reach the stormwater system at the same time
The result is often higher peak flows during storms.
Think of the difference between pouring water onto a sponge and pouring water onto a sheet of glass.
One absorbs water.
The other sheds it immediately.
Why Speed Matters
The amount of runoff is important, but so is the speed.
When rain falls on pavement:
- Water reaches storm drains faster
- Multiple areas contribute runoff simultaneously
- Water levels rise more quickly in pipes and ditches
This can increase the likelihood of:
- Street flooding
- Ponding in low areas
- Erosion along channels and waterways
- Stress on stormwater infrastructure
Even relatively small storms can create noticeable runoff when large amounts of impervious surface are present.
Flooding Isn’t Always Caused by a Lack of Drains
A common assumption is that flooding happens because there aren’t enough storm drains.
In reality, runoff volume is often a major factor.
Imagine a neighborhood where:
- Most yards are paved
- Large driveways connect directly to streets
- Rain falls quickly
Even a well-designed stormwater system can be challenged if runoff enters faster than it can be moved downstream.
This is why engineers look at both infrastructure capacity and land use when planning for stormwater management.
How Communities Reduce Runoff
Modern stormwater management often combines traditional infrastructure with natural solutions.
Examples include:
Rain Gardens
Shallow landscaped areas that capture and soak in runoff.
Native Vegetation
Deep-rooted plants help improve infiltration and reduce erosion.
Detention and Retention Ponds
These systems temporarily store water and release it more slowly.
Vegetated Buffers
Plants along waterways help slow runoff and filter pollutants.
Together, these practices reduce the amount of water entering storm drains all at once.
What Residents Can Do
While no one can eliminate runoff entirely, homeowners can help reduce it.
Consider:
- Directing downspouts toward lawns or landscaped areas
- Planting native vegetation
- Installing rain gardens where appropriate
- Reducing unnecessary paved surfaces
- Keeping soil healthy and vegetated
Even small changes can help water soak into the ground instead of rushing directly into streets and drains.
Why It Matters
Impervious surfaces are a necessary part of modern communities, but they change the natural water cycle.
As more rainwater runs off instead of soaking in, stormwater systems must work harder to manage larger volumes of water moving at greater speeds.
Understanding how streets, parking lots, rooftops, and other hard surfaces affect runoff helps explain why flooding occurs—and why both gray and green infrastructure are important parts of the solution.
The goal isn’t to eliminate development.
It’s to manage water in ways that protect homes, businesses, waterways, and the community as a whole.
Our Water, Our Responsibility.