Late last week Hurricane Harvey, the most powerful hurricane to hit Texas in more than 50 years, struck the Houston area, causing an unprecedented flood. Localized rain amounts of up to 50 inches are possible by the end of this week. The result, as the Weather Channel observes, is this may end up being one of the worst flood disasters in U.S. history.

To fully understand the problem, though, requires knowing flood-related terminology that may be foreign to most Americans. Here are seven terms you need to know to make sense of the news about the disaster in Houston. 

1. Flooding

Flooding typically occurs, the National Weather Service says, when prolonged rain falls over several days, when intense rain falls over a short period of time, or when an ice or debris jam causes a river or stream to overflow onto the surrounding area. Flooding can also result from the failure of a water-control structure, such as a levee or dam.

The most common cause of flooding— like that in Houston—occurs when water due to rain and/or snowmelt that accumulates faster than soils can absorb it or rivers can carry it away. Approximately 75 percent of all U.S. presidential disaster declarations are associated with flooding.  

2. Runoff

When precipitation such as rain or snow comes in contact with the land, it is affected by gravity in one of two ways: (1) the water is pulled directly into the soil and collected as groundwater, or (2) the water flows downhill as runoff. Runoff (sometimes called stormwater runoff or surface water runoff) is water from rain or melting snow that “runs off” across the land instead of seeping into the ground. This runoff usually flows into the nearest stream, creek, river, lake, or ocean.

Because Houston has tight clay soils, making difficult for the land to absorb water, the area relies on runoff to relieve flooding.

3. Drainage  

“There are just two things you can do with water,” says Mike Talbott, an engineer for the Harris County Flood Control District. “You can move it or you can store it.”

Drainage is the process, either natural or artificial, for moving runoff from an area of land either to a storage area or to a watershed.

Houston maintains 2,900 miles of roadside ditches, 3,600 miles of storm sewer pipes, and 120,000 sewers, roadway grates, and other inlets for floods to enter the system. (The city’s newest parts of the drainage system are designed to carry about 13 inches of rain over 24 hours.) The roadways are also designed to serve as a drainage system, which is why the roads in Houston are especially flooded.

All of these drainage areas lead to Harris County's 2,500 miles of watersheds, which empty into Galveston Bay, an estuary connected to the Gulf of Mexico.

4. Watershed

A watershed is a land area or “drainage area” that ultimately drains runoff to a common body of water. In the Houston area, watersheds drain into one of nearly two-dozen major creeks and bayous.

5. Bayou

bayou is a slow-moving stream, either a small river or a large creek.

Houston is known as “The Bayou City” because it has 22 bayou systems and waterways in the greater Houston region. As Lisa Gray says, “if straightened out and laid end to end . . . they'd stretch from [Houston] to Boulder and back, with enough left over for a round trip to Austin.”

5. Floodplain

floodplain is flat or nearly flat land adjacent to a stream or river that experiences occasional flooding.

Many areas of Houston are built on floodplains. As Dylan Baddour explains, “By the time planners made maps in the mid-1980s to show that low-lying areas beside waterways were prone to flooding, those areas were full of buildings and inadequate drainage systems.”

7. 500-Year Flood

Terms such as “100-year flood” or “500-year flood” are confusing and potentially misleading. For example, a 500-year flood (like the one in Houston) does not mean a flood that will occur once every 500 years.

The number (i.e., 1, 10, 100, 500) refers to the annual exceedance probability (AEP) flood as used for the basis for the National Flood Insurance Program. So a 100-year flood would be a 1 percent AEP flood, a flood that has a 1 in 100 chance of being equaled or exceeded in any one year. The 500-year flood therefore corresponds to an AEP of 0.2 percent, which means a flood of that size or greater has a 0.2 percent chance (or 1 in 500 chance) of occurring in a given year.

Floods are independent events, so the probability of a flood in one year (e.g., 2015) has no correlation with the probability of a flood in another year (such as 2017). The percentages are true every year, regardless of severity or how many floods have occurred in previous years. In fact, as the Harris County Engineering Department notes, “If your home is in the 100-year floodplain, it has a 26 percent chance of getting flooded over a 30-year mortgage period, which is about five times higher than the risk for a severe fire!”