STORMWATER & FLOODING
Why a 1-Hour Thunderstorm Can Cause More Flooding in Columbia Than a Day of Rain
Travis Basnett, PE · Licensed in SC, NC & GA · August 7, 2026 · 7-minute read
Total rainfall matters, but in the Midlands, short-duration rainfall intensity can sometimes be what overwhelms a drainage system.
Turn on your bathroom faucet, and water can run for hours without overflowing the sink because the drain carries it away about as fast as it arrives. But pour a five-gallon bucket into that same sink in three seconds, and water spills onto the floor, even though you used far less water overall.
Storm drainage systems respond in much the same way.
When summer thunderstorms roll through Richland and Lexington counties, residents are often understandably puzzled. A storm might drop “only” two or three inches of rain, yet within 20 minutes traffic slows at flooded intersections, water approaches storefronts, and streets temporarily become shallow ponds. A few hours later, much of that water may be gone.
Naturally, people compare these quick cloudbursts with larger 24-hour storm events and wonder why a brief afternoon thunderstorm caused so much disruption.
For short-duration urban flooding, total rainfall is only part of the story. The rate at which that rainfall occurs can be just as important, and sometimes more important, than the total storm depth.
Total Rainfall vs. Rainfall Intensity
A slow-moving weather system that delivers four inches of rain over 18 hours creates a substantial volume of runoff. But because that rainfall occurs over a longer period, storm pipes, roadside ditches, detention ponds, and receiving streams have more time to collect, convey, store, and release the water.
A summer thunderstorm that produces three inches over a much shorter period creates a different hydraulic problem.
During its most intense portion, water may reach street inlets, low points, pipes, and channels faster than those systems can accept and convey it.
That distinction is important:
A shorter storm does not necessarily produce more total runoff. It can produce a much higher rate of runoff for a short period.
Consider a simplified five-acre commercial site similar to a small shopping center and parking lot.
Illustrative Site Assumptions
- Drainage area: 5.0 acres
- Predominantly impervious commercial development
- Rational Method runoff coefficient: C = 0.90
- Approximate NRCS Curve Number: CN = 95
- Time of concentration: 10 minutes
For the peak-flow comparison below, the Rational Method is used:
Q = C × I × A
where rainfall intensity corresponds to the approximate 10-minute time of concentration. The calculations are:
1-hour thunderstorm: Q = (0.90)(7.2 in./hr)(5 acres) = 32.4 cfs
24-hour frontal system: Q = (0.90)(2.5 in./hr)(5 acres) = 11.3 cfs
Figure 1: Illustrative Storm Comparison
Illustrative comparison for a 5-acre commercial site. The 1-hour thunderstorm includes a 1.2-inch burst in 10 minutes, equivalent to 7.2 in./hr; the 24-hour system uses a peak 10-minute intensity of 2.5 in./hr. Peak flows are calculated using the Rational Method. CN 95 describes the highly impervious site but is not used in the peak-flow calculation; the hydrograph shapes are schematic.
Despite having less total rainfall—2.0 inches versus 2.8 inches—the short-duration thunderstorm produces nearly three times the calculated peak runoff rate in this example.
The example is intentionally simplified and is not intended to represent a particular property or design storm. Actual peak flow depends on rainfall distribution, site characteristics, drainage geometry, and other factors. But it illustrates the fundamental issue: how quickly water arrives can matter as much as how much water ultimately falls.
That brief surge can be enough to inundate a road, stall a vehicle, enter a building, displace a manhole cover, or erode a channel. The fact that the water recedes 45 minutes later does not undo the damage that occurred during the peak.
The Journey of a Raindrop: Where the System Can Become Limited
When rain falls on a forested or vegetated area, more of it can infiltrate into the soil, while vegetation and rough ground surfaces slow the movement of runoff.
When the same rain falls on asphalt, concrete, rooftops, and other impervious surfaces, a much larger percentage becomes runoff, and that water reaches the drainage system much more quickly.
Engineers call the time it takes runoff from the hydraulically most distant part of a drainage area to reach the point being analyzed the time of concentration, commonly written as Tc.
A small, heavily paved drainage area may respond within minutes. A larger or more natural watershed may respond over a considerably longer period.
This matters because rainfall intensity varies dramatically with duration. If a drainage area responds in ten minutes, the most intense ten-minute portion of the storm may be particularly important in determining its peak flow.
Once runoff begins moving, it typically passes through a sequence of systems:
Surface travel: Water moves across rooftops, parking lots, yards, and streets.
Collection: Curb inlets, catch basins, and roadside ditches intercept the runoff.
Conveyance: Pipes, culverts, swales, and channels carry it downstream.
Storage: Detention facilities temporarily store runoff and release it at a controlled rate.
Outfall: Water eventually reaches larger streams, rivers, lakes, or other receiving systems.
Any one of these steps can become the limiting point during a particular storm.
For example, an underground storm pipe may have available capacity, but the street-level curb inlets may not capture water as quickly as it arrives. Water therefore accumulates in the street even though the downstream pipe is not completely full.
Conversely, the inlets and pipes may function properly, but if the receiving stream is already high, the downstream water surface can restrict the system’s ability to discharge.
That is why flooding rarely has one universal explanation.
What a “10-Year Storm” Really Means
Terms such as a “10-year storm” or “25-year storm” are statistical descriptions, not schedules.
A 10-year rainfall event has approximately a 10% probability of being equaled or exceeded in any given year.
A 25-year event has approximately a 4% annual probability.
It is entirely possible to experience two 25-year events within a relatively short period. The term does not mean that 25 years must pass between storms.
More importantly, there is no single universal “25-year storm.”
Rainfall frequency depends on duration.
For example, rainfall can be evaluated over:
- 15 minutes
- 30 minutes
- 1 hour
- 3 hours
- 6 hours
- 24 hours
A storm can appear relatively ordinary when judged by its entire 24-hour rainfall total while being statistically unusual when judged by its most intense 15 minutes or hour.
Figure 2: NOAA Atlas 14 Precipitation Frequency — Columbia Area
NOAA Atlas 14 precipitation-frequency estimates for Columbia, South Carolina. Rainfall depths are shown in inches; confidence intervals are omitted for clarity. Approximate values include 0.92, 1.24, and 1.68 inches for the 15-minute 2-, 10-, and 100-year events; 1.75, 2.47, and 3.52 inches for the corresponding 1-hour events; and 3.56, 5.15, and 7.68 inches for the corresponding 24-hour events. Exact values vary somewhat by location.
This is one reason a daily rainfall total by itself can be misleading when trying to understand localized urban flooding.
Real Weather Does Not Follow a Single Design Pattern
Civil engineers need standardized rainfall patterns so that drainage systems can be designed and compared consistently.
One commonly used method in South Carolina has historically been the NRCS Type II 24-hour rainfall distribution. It takes a specified 24-hour rainfall depth and distributes that rainfall over time according to a standardized pattern.
Type II is not a mild storm. In fact, it intentionally concentrates a significant portion of the total rainfall near the middle of the event. Approximately 45% of its 24-hour rainfall occurs within the central hour.
The limitation is not that the Type II storm lacks intensity.
The limitation is that it represents one predetermined rainfall pattern.
Real thunderstorms may distribute rainfall differently. Their most intense 15 minutes or hour may occur earlier or later, may be more or less concentrated, and may affect only part of a larger watershed.
NOAA Atlas 14 data also show that the relationship between short-duration and 24-hour rainfall varies with location and recurrence interval. NRCS itself has recognized that legacy regional distributions such as Type II can either overestimate or underestimate peak discharge depending on how well the assumed temporal pattern represents local precipitation characteristics.
That does not mean the Type II distribution is incorrect or that projects designed using it are deficient.
It means that every model has a purpose and limitations.
A standardized 24-hour storm remains particularly useful for evaluating runoff volume, detention storage, and overall system performance. Short-duration rainfall intensity becomes increasingly important when evaluating the peak flow reaching an inlet, pipe, culvert, or other relatively fast-responding drainage feature.
Nature does not deliver a “design storm.” It delivers a real storm with its own timing, intensity, movement, and geographic footprint.
What Does Flooding Tell Us About the Drainage System?
Flooding deserves investigation, but by itself it does not identify the limiting condition.
Flooding can indicate a correctable condition such as:
- Clogged inlet grates
- Sediment accumulation
- A damaged or collapsed pipe
- An obstructed ditch
- A poorly maintained detention pond outlet
- Grading that blocks an intended overflow route
- A known capacity restriction
Those conditions should be identified and corrected.
In other situations, a drainage system may be functioning substantially as intended but encounter rainfall intensity, downstream conditions, or a combination of circumstances beyond the level of service it was designed to manage.
There is also a third possibility: the visible flooding location may not be where the actual limitation exists.
Increasing the size of one pipe may accomplish very little if the downstream pipe, culvert, channel, or stream is already at capacity. In some cases, increasing upstream conveyance can simply move water downstream faster.
That is why increasing pipe size is not automatically the solution.
A drainage evaluation should instead ask several questions:
Is maintenance contributing?
Clogged inlet grates, accumulated sediment, vegetation, or damaged infrastructure can reduce otherwise available capacity.
Is collection the limitation?
Water may reach a street faster than curb inlets can capture it, particularly where flow is shallow, fast-moving, or approaching an inlet from an unfavorable direction.
Is conveyance the limitation?
The pipe or channel itself may simply be carrying as much water as it can.
Is the downstream system controlling?
A local drainage system cannot discharge freely if the receiving system is already full or elevated.
Where does the water go when the underground system is exceeded?
Overland drainage paths are an important part of stormwater planning. When pipes eventually reach capacity, excess runoff needs somewhere to go that minimizes risk to buildings, roads, and critical infrastructure.
Understanding the Problem Comes Before Choosing the Solution
Reducing flood risk can involve many different approaches:
- Maintaining existing infrastructure
- Improving inlet capacity
- Replacing damaged pipes
- Increasing conveyance
- Adding or modifying detention storage
- Improving overland flow routes
- Addressing downstream bottlenecks
- Updating hydrologic assumptions where appropriate
But no individual solution is appropriate everywhere.
The better questions are:
Where did the water come from?
How quickly did it arrive?
Why did it collect at this particular location?
What was the limiting part of the drainage system?
And what improvement would reduce the greatest amount of risk without simply transferring the problem somewhere else?
Short-duration storms across the Midlands regularly demonstrate how quickly a concentrated rainfall burst can affect a developed watershed.
Understanding the difference between rainfall depth, rainfall intensity, runoff volume, and peak flow is an important first step toward understanding why.
From there, each flooding location has to be evaluated on its own facts.
About the Author
Travis Basnett, PE is a civil engineer with 16 years of experience in civil site development, stormwater design, drainage evaluation, and flooding investigations. He provides expert witness support and independent technical review for civil engineering disputes and is licensed in South Carolina, North Carolina, and Georgia.
Sources
- Precipitation-frequency estimates: NOAA Atlas 14 Precipitation Frequency Data Server, NOAA National Weather Service, Hydrometeorological Design Studies Center.
- Rainfall temporal distributions: NRCS National Engineering Handbook, Part 630, Chapter 4 — Storm Rainfall Depth and Distribution, USDA Natural Resources Conservation Service.
Related reading: Why stormwater facilities fail: design, construction, maintenance, or changed conditions
Dealing with a flooding issue? I provide expert witness support and limited independent technical review for drainage and stormwater disputes across South Carolina.
Disclaimer: This article is intended for general informational purposes and does not constitute engineering advice regarding any specific property, project, or drainage system.