1993 Missouri River flood
1993 Missouri River flood
Two photographs of the July 1993 Missouri River flood, taken north of Jefferson City.
Flooding occurs when water arrives at a rate faster than it can drain downhill, evaporate, or absorb into the ground. Floods are commonly caused by severe storms (hurricanes, tropical storms, monsoons, thunderstorms), melting snowpack, and sudden releases of natural or man-made water barriers (dams, levees, glaciers). Deserts are prone to flash flooding because of violent heavy thundershowers and flat impermeable soils. Urban areas are also at increased risk of flooding because of the high percentage of non-absorbent surfaces (concrete, brick, etc.). Click on this link for a simulation of a hypothetical flash flood in LA, both urban and desert, “Flash Flood 20 Feb. 2012”.

In addition to the danger of drowning, floods also create severe secondary problems. Typically, transportation to flooded areas is compromised, limiting the efforts of rescue crews. Additionally, flooding often contaminates water sources with sewage, which can create clean water shortages and outbreaks of disease for months after the waters subside. In the long run, lost crops and buildings may severely affect the local economy.

In the U.S., the areas most at risk for flooding are identified and mapped by the Federal Emergency Management Agency (FEMA). Properties within a Special Flood Hazard Area have a 1% or greater annual chance of flooding. In other words, they are expected to flood at least every hundred years. The 1-percent-annual-chance floodplain is referred to by some agencies and publications as the base floodplain or 100-year floodplain. To find out if you live in a Special Flood Hazard Area, use our Hazard Viewer.

Flooding and Earthquakes

Lower San Fernando Dam
The Van Norman Dam after the 1971 San Fernando Earthquake.
Earthquakes can indirectly cause flooding in a number of ways: triggering tsunamis, destabilizing dams, breaking and/or lowering protective levees. The 2011 Japan Earthquake caused devastating flooding through a combination of these means. The earthquake disrupted the ocean floor, creating a tsunami. Furthermore, whole sections of Japan’s coastline dropped in elevation because of the tremor, effectively shortening sea walls and protective levies by up to three feet[1]. Compromised and lowered barriers exacerbated the tsunami’s damaging effect.

There are also historical examples from the United States. The 1971 San Fernando Earthquake, for instance, significantly damaged the Van Norman Dam, which precariously held back 15 million tons of water from the densely populated San Fernando Valley below. Worried authorities evacuated tens of thousands of valley residents and then proceeded to lower the water level in the reservoir over the course of three days, effectively reducing the danger and the pressure on the fragile dam remains[2]. As a result of this narrowly averted disaster, California law requires all dam owners to file dam inundation maps with the California Emergency Management Agency. Other states, too, provide similar resources to assess the risk of flooding from a ruptured dam. These maps are the basis for our dam inundation mapping in the OpenHazards Hazard Viewer. While modern dams in seismically active areas are generally built to withstand significant earthquakes, all residents in dam inundation zones should be aware of their potential risk and evacuation routes.

[1] “Japan's Killer Quake: An eyewitness account and investigation of the epic earthquake, tsunami, and nuclear crisis.” Aired February 29, 2012 on PBS. http://www.pbs.org/wgbh/nova/earth/japan-killer-quake.html (Retrieved 8/15/2012).

[2] “The Los Angeles Dam Story.” USGS. http://earthquake.usgs.gov/learn/publications/la-damstory/ (Retrieved 9/3/2012).

Risk Alert