100-Year Rainfall vs 100-Year Flood Explained
August 12th, 2018
Piece by Scott Saenger, PE, CFM
Jones & Carter
Contrary to how it may sound, a 100-year rainfall event is not something that happens every 100 years. Rather, it means that there is a one percent chance that a certain amount of rain will fall in a particular location over a set amount of time in a given year.
A 100-year rainfall event depends on the duration (length of time) of the rainfall, the amount of rain that falls during that time, and where it rained geographically. Once these components are put together, the frequency (100-year, 50-year, 5-year, etc.) of the event can be determined.
If four inches of rain fell on both Houston and El Paso over a 24-hour period, Houston wouldn’t blink but El Paso would have just received a 100-year rainfall. Let’s say approximately 12 inches of rain fell in the same locations over 12 hours, Houston would have had a 100-year rainfall event while El Paso would have seen more than their average amount of annual rainfall, which would likely cause major issues.
An important distinction is that even though both locations had 100-year rainfall events in these scenarios, they didn’t necessarily have 100-year floods.
Although not every 100-year rainfall event leads to a 100-year flood, there are circumstances that may allow for a minor storm to lead to severe flooding. Flooding happens when water flows into a region faster than it can be stored in a lake or reservoir, absorbed into the ground, or removed by runoff into a drainage system.
On July 4th, parts of Harris County received a 100-year rainfall with more than six inches of rainfall over a short time. The rainfall led to localized street flooding, water pooling in low lying areas, and parts of Interstate 10 becoming impassible for a few hours but all-in-all most of the area was fairly unscathed. While water did collect on some roadways, it still did not constitute a 100-year flood.
To learn more about rainfall depth-duration frequency in Texas, view this United States Geological Survey (USGS) report. The USGS report is the most recent rainfall analysis for Texas. However, the Atlas 14 study, which is an ongoing rainfall analysis, is scheduled for completion this fall.
100-year rain – a rainfall event which has a 1% probability of occurring in any given year.
100-year flood – a flood event that has a 1% probability of occurring in any given year.
Atlas 14 – an ongoing study used to analyze historical rainfall data to update statistical hypothetical rainfall events in Texas.
Channel – the bed where a natural stream of water runs.
Floodplain – an area of low-lying ground adjacent to a river, formed mainly of river sediments and subject to flooding.
Watershed – a land area that channels rainfall and snowmelt to creeks, streams, and rivers, and eventually to outflow points such as reservoirs, bays, and the ocean.