The National Weather Service flood definition states that "flooding is an overflowing of water onto land that is normally dry." Flooding happens when rain falls faster than the ground can absorb or natural channels can carry water away.

Flash Floods

The leading cause of weather-related deaths in the United States, flash floods happen within six hours of the rain event. Most flash floods occur due to heavy rains from slow-moving thunderstorms, repeated thunderstorms or the heavy rains of hurricanes or tropical storms.

River Floods

Seasonal rains, snow melt or stalled storms may lead to river floods. River floods occur as part of the natural seasonal cycle and may last a week or more.

Coastal Floods

Floods along coastlines happen when storms or winds push the ocean farther than the normal tide lines. Extreme low-pressure systems and intense on-shore winds, especially from hurricanes and tropical storms, cause coastal flooding. Seismic sea waves, better known as tsunamis or tidal waves, triggered by underwater earthquakes or volcanic eruptions also cause coastal flooding.

Urban Floods

As urban areas grow, so does the threat of flooding. Pavement and buildings prevent infiltration and increase runoff. Streets can become running rivers and low areas like underpasses and basements may fill with water.

Ice Dams and Log Jams

Sometimes ice or other natural materials like trees and shrubs temporarily block runoff. When these materials hold back runoff, the water builds pressure, behaving like a flash flood if the temporary dam breaks suddenly.

Other Flood Events

Severe flooding may occur when levees or dams break or when water must be released from impoundments to relieve pressure. Melting of snow by magma movement may also cause sudden flood events, as with the 1980 eruption of Mt. St. Helens.

Two key factors in flooding are rainfall amount and rainfall intensity, with influences from topography, soil conditions and ground cover. Each of these factors suggests possible projects. In general, a project on flood causes or outcomes will use models.

Topography impacts the rate of flow of water. Compare the speed of flow based on slope angle. Build or create a chute for the water. Calculate the speed of water flow using speed equals distance divided by time. Reset the chute to a steeper angle and calculate the speed again. Compare the speeds. Potential question: Does doubling the slope angle also double the speed of the water?

Consider how the width of a stream channel affects the speed of water. Use two different widths of chute. Measure the speeds and compare.

Evaluate how the depth of water changes as the channel narrows. Flash floods can result in a wall of water 30 feet high in a narrow canyon. Create or build a narrow chute and a wide chute. The amount of water flowing must be the same for both chutes. Measure the height of the water line in each case. Alternately, create a chute that gradually changes from wide and shallow to narrow and deep. Mark the water line. Extend the project by comparing the water speed in the wide section to the speed in the narrow section.

Nearly half of flash flood related deaths happen in automobiles. Calculate the force required to move an average automobile. How deep does water have to be to move a car?

Some floods result from breaking ice dams, levees or dams. In fact, one of the worst dam breaks in US history was the May 31, 1889 Johnstown flood. Research and design a dam. Build a model of the dam across a chute. Determine the force required to break the dam. Evaluate and redesign to improve. Alternatively, design a bridge to minimize debris jams. Evaluate the pressure on a bridge due to debris or ice jams.

What do sedimentary deposits from a flood look like? Build or create two chutes. Place one chute at a shallow angle and the other at a steeper angle (in general, the steepest natural slopes range between 45 and 60 degrees). Fill both chutes with equal amounts of silt, sand and rocks arranged as much alike as possible. Place a clear plastic box at the bottom of each chute. Let water flow down through each chute to carry the silt, sand and rocks into the plastic boxes. Compare the final arrangements of sediments. Alternatively, let the water and sediments flow out. Measure and compare how far the water carries the sediments.

Soil type affects the rate of rainfall infiltration. Using shallow plastic boxes, fill one with silt, one with sand and one with pebbles, filling to the top of the plastic box. You can extend the idea by using two different boxes for each soil type, leaving sediments loose in one box and packing the sediments tightly in the other. Place each plastic box in a larger box to catch run-off. Use a sprinkler system to "rain" on the boxes. Measure and compare the amount of water that sinks into the sediments and measure the amount of water that runs off. You might also want to reset the boxes and increase the rate of rainfall.

Plant cover affects rainfall runoff. Use two clear plastic containers. Fill both with soil. Plant grass seed in one container. Once grass has become established, use a sprinkler to rain on both containers. Capture and measure the amount of runoff. Alternately, use toothpicks to simulate plants in one container. Measure the amounts of water that infiltrates and that runs off.

Educate the public with a flood emergency project. Research the flood zones in the area. Create a public awareness campaign. Share emergency preparedness checklists. Write an article for the local paper or newscast. Work with local authorities to create and post emergency escape route signs from flood zones.

Online citizen science projects allow individuals to gather and add data to scientific studies. SciStarter and Citizen Science Alliance (see Resources) are two online sites seeking public input.