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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
Snow Ratio: The percentage of water within a sample of snow is called 'snow ratio'. An old rule of thumb was that for every to inches of snow, there would be 1 inch of water (10:1). However, snow ratios can vary dramatically around the country and from event to event.
Variables that affect snow ratio: 1) Depth of the 'warm' layer from the surface into the snow-producing cloud. Amount of ice in the snow-producing cloud. If its windy, snowflakes can fracture, losing their 'lacy' structure. Deep cold leads to higher snow ratios.
Snow Squall: An intense, but limited duration period of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possibly lightning. Snowfall rates may be significant. Hazard: Heavy snow and blowing snow, wind gusts up to 40 mph. Source: Radar indicated. Impact: Dangerous life-threatening travel. What you can do: Try to safely exit the highway, drive slowly, increase your following distance.

 

 

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