National Weather Service United States Department of Commerce

Flood Infographics

 

Flood Science - Debris Flows
Debris Flows: Dangerous land and water flow caused by rainfall, terrain and loose-bare soil. Flash flooding and debris flows are common in or near burn scars. 
Debris flows carry everything: A debris flow is a moving mass of loose mud, sand, soil, rock, water and air that travels down a slope under the influence of gravity. To be considered a debris flow, the moving material must be loose and capable of 'flow', and at least 50% of the material must be sand-size particles or larger. In areas of very steep slopes they can reach speeds of over 100 mph. 
Burn scars are notorious for debris flows: Burned soil can be as water repellant as pavement. When vegetation is burned at high intensity, water repellent compounds are vaporized, and condense on the soil layers below, which prevents soil from absorbing water. As a result, much less rainfall is required to produce a flash flood. 
Rainfall and gravity take over: As water runs downhill through burned areas it can create major erosion and pick up large amounts of ash, sand, silt, trees and boulders. The force of the rushing water and debris can damage or destroy culverts, bridges, roadways, and buildings even miles away from the burned area.
The risk of debris flow could last years: Most burn areas will be prone to this activity for at least two years. Each wildfire burn area poses its own unique risk of flash flooding due to many factors including proximity to population centers, burn severity, steepness of terrain, and size of the burned area. weather.gov/flood
Flood Science - Snowmelt Process
Snowmelt Processes: During certain times of the year, water from snowmelt can be responsible for almost all of the streamflow in a river. It's important for hydrologists to understand these processes in order to accurately forecast river floods.
Snow Distribution: The path that weather systems take is the most important factor in determining snowpack, but terrain and vegetation also influence how snow accumulates on the ground.
Snowpack Characterisitcs: The temperature and the amount of water (snow water equivalent) in the snowpack and important to the melting process. Before rapid melting can occur, the snowpack as a whole needs to be warmed to 32 degress F.
Snow Energy Exchanges: Incoming solar radiation, emitted longwave radiation, turbulent transfer of heat, ground conduction, and heat transferred during rainfall are all important factors in heating or cooling the snowpack.
Weather Factors: Strong winds and high dew point temperatures aid in melting by limiting the effects of evaporative cooling and allow the layer directly above the snowpack to remain warm due to turbulent mixing. Rain falling on a snowpack can accelerate the melt process as well.
Where the Water Goes: Once rapid melting begins, the water will either infiltrate into the soil, run off into streams and other bodies of water, pool in place and potentially refreeze as ice, or a combination. Ice jam floding can occur if the river channel has excessive ice cover. weather.gov/flood
Flood Science - Streamflow Routing
Streamflow Routing: Describes the movement of water volume from one point to another along a river. Hydrologists use this to predict flood peaks.
Hydrologic Routing Techniques: Advanced formulas are used to determine the behavior of flow from point A to point B in a stream, creek or river.
Streamflow Characteristics: The geometry of the channel may vary at different points along a stream or river and will affect the amount of discharge for a given volume of water.
Watershed Characteristics: Additional inflows to a stream between point A and point B further complicate the predictability of the flow.
Rating Curve: A rating curve is a relationship between stage and discharge at a cross section of a river. The output from a hydrologic modle is a discharge or flow, which can then be converted stage - a measure of the water level at a given point on the river.
Floodplains: These are lowland areas adjacent to the river or stream that are prone to flooding due to increases in streamflow on the channel -- which may result from water that is routed downstream. weather.gov/flood
SCIENCE OF RIVER FLOODING -
Water is essential for life on Earth. But in large enough quantities, the very substance we drink and use to grow crops can destroy homes, businesses and cause fatalities.

River flooding occurs when river levels rise & overflow their banks or the edges of their main channel and inundate normally dry areas.

River flooding can be caused by heavy rainfall, dam failures, rapid snowmelt and ice jams.

Six Steps to Create a River Model
Hydrologic Cycle: Hydrologists try to understand and simulate the natural hydrologic cycle, which is the intricate combination of many processes such as evaporation, transpiration, precipitation, infiltration, interflow, groundwater storage, and runoff.

Precipitation: Precipitation is the primary input to basin hydrologic processes and serves as the primary driver of hydrologic models. Accurate representation of precipitation input is an important intial step. Small river channel systems are very sensitive to rainfall.

Runoff: The next step is to compute the amount of precipitation that appears in surface water within a relatively short time from the onset of a storm event. This is runoff. Runoff consists of 3 components: 
overland flow, rain falling directly on surface water bodies, and interflow.

Unit Hydrograph: After computing basin runoff, the next step is to calculate a forecast hydrograph in units
of discharge. A hydrograph is a plot of the change of stage or discharge with respect to time. Discharge is the volume of water flowing past a location per unit time and is usually expressed in cubic feet per second (cfs).

Streamflow Data: Scientists use streamflow measurements to capture the vital relationship between discharge (volume flow rate) and stage (height) for a given location. This can only be done by taking streamflow measurements at different river levels and noting the corresponding stages. This relation is called a rating curve.

Routing: Hydrologists analyze and interpret how the water moves once it’s in the river and how a flood wave is modified due to the effects of storage and friction as it moves downstream. So, what happens upstream affects the entire downstream community.
Don't underestimate the power of water! 6 inches of fast-moving water can knock over and carry away an adult. 12 inches of fast-moving water can carry away a small car. 18-24 inches of fast-moving water can carry away most large SUVs, vans and trucks.
Do you really know how deep the water is?  6 inches of fast-moving water can knock over and carry away an adult.  12 inches of fast-moving water can carry away a small car.  18-24 inches of fast-moving water can carry away most large SUVs, vans and trucks.
Flood Warning: A Flood Warning is issued when flooding is happening or will happen soon.  Move to higher ground.  Never drive through flooded roads.  Flood Watch: A Flood Watch is issued when flooding is possible.  Stay tuned to radio/TV, follow weather.gov and be ready to seek higher ground.  Learn more at weather.gov/flood
Flooding At Night - Sometimes difficult to see during the day. Impossible to see at night.