Types of Flood

The water cycle, also called the hydrologic cycle, is the natural process by which moisture from oceans and other water bodies moves into the atmosphere as evaporation and then falls back down to land as precipitation in the form of rain, sleet, hail, or snow. Due to our state’s varied landscape and location along the Gulf of Mexico, Texas’ version of the water cycle frequently results in flooding, with rainfall serving as the driver of most events. The primary types of flooding that impact the state are summarized below.

Riverine flooding

Abundant rainfall can result in more runoff entering a river channel than can be contained within its banks. When water levels exceed the capacity of a channel, the river overflows onto adjacent lands, called the floodplain. On steep, narrow floodplains, these excess overflows can create flood conditions suddenly (see flash flooding below). Where land is flat and floodplains are more expansive, greater volumes of runoff are required to cause flooding, the impacts of which may take hours or days to reach locations downstream (see slow-rise flooding below).

Flash flooding

A type of riverine flooding, flash flooding is characterized by a short time lag (less than six hours) between the rain event and rapidly rising water levels (NWS, 2018b). Flash flooding can occur anywhere rainfall intensity exceeds the infiltration capacity of the soil, causing rapid surface runoff. Areas with large amounts of impervious surfaces, exposed bedrock, or other solid surfaces that reduce infiltration and increase runoff, are especially susceptible to flash flooding. Near El Paso, runoff from steep slopes flows rapidly over dry, impenetrable soils transporting and depositing eroded materials across the landscape.

Slow-rise flooding

This second type of riverine flooding occurs when rain events near the top of the watershed, or far upstream, cause flooding that continues unabated downstream, impacting communities where no rain fell. For example, slowrise flooding occurs along the Guadalupe River. When intense rains in the Hill Country cause the river to swell in New Braunfels, the City of Victoria, located 230 river miles downstream, can expect floodwater to arrive roughly one to two days after it passes underneath Interstate 35.

Coastal flooding

Low pressure systems may gain strength as they travel across the warm waters of the Gulf of Mexico, sometimes developing into tropical storms or hurricanes. As these systems approach the Texas coast, stronger winds combined with changes in water surface elevation can produce a storm surge that drives ocean water inland across the flat coastal plain. High tide events also may cause frequent, localized flooding of low-lying coastal lands.

Stormwater flooding

This type of localized flooding occurs when rainfall overwhelms the capacity of engineered drainage systems to carry away rapidly accumulating volumes of water. It typically dissipates quickly, except in situations such as when pumping equipment fails due to loss of power, inflows exceed pumping or conveyance capacity, or debris blocks the passage of water. The solid surfaces of buildings and streets (also called impervious cover) prevent rainfall from soaking into the ground, resulting in runoff. Because this type of flooding is most common in urban environments, it is sometimes called urban flooding.

Structural failure flooding

Though uncommon in Texas, failure of man-made infrastructure, such as dams or levees, can occur when intense or extensive rainfall results in the uncontrolled release of floodwaters. Failures may arise if a rain event exceeds the design capacity of a structure, such as when Callaway and McGuire dams failed in Robertson County in May 2004 (TDEM, 2013).