Floods

Return Periods

A 100 year flood is a flood that has a return period of 100 years A probability of 0.01 of occurring in any given year 1% chance of occurring any given year

What is the best description of a 100 year flood?

Any year, there is a 1% chance of a 100-year flood.

Floods (pt 2)

Are a natural, but humans have exacerbated some flooding Generally restricted to low-lying areas In the U.S about 50% of the flood rated deaths occur in vehicles

Severity if controlled by watershed characteristics

Drainage basin size (area) Topography Surface types/impervious surfaces/land use Vegetation/soil type

Historic Floods - Ellicott City (2016)

Flash Flood; total rainfall was 6.50 inches. In two hours they got more than half a foot of rain The rain was on the west side of the city (bullseye) Where the rain fell there was no soil there to absorb the rain

Floods

Have always been a problems for civilizations Rivers provide good sources of fresh water, transportation routes, and their floodplains have nutrient rich soil

Flash Floods

Heavy or excessive rainfall in a short periods of time, generally less than six hours Can happen in a s little as a few minutes Can also occur when a levee or dam fails Areas damaged by wildfires are particularly susceptible Rain cannot be absorbed by vegetation

Coastal Flood

Inundation of land areas along the coast, caused by higher than average high tide AND worsened by heavy rainfall and onshore winds. Common in places life Charleston, SC and Savannah GA. Low elevation and human impact

River Floods

Occurs when water levels rise over the top of river banks due to excessive rain over a short temporal period Rainfall normally caused by tropical cyclones making landfall, persistent thunderstorms over the same are for an extended period , combines rainfall/snow melt, or an ice/ debris jam

Causal Mechanisms

Rainfall that is falling at a fast rate Heavy rain and / or rapid snow melt Severity if controlled by watershed characteristics

Storm Surge

Related to the winds

Johnstown, PA (1889)

Rivers were overflowing Canamon lake grew a 30- 33% larger in size 2,229 people dies in this flood Started the first American relief organization; first time red cross was seen

What contributed to the Ellicott City flash flood?

Training thunderstorms tracking over Ellicott City A lot of impervious surfaces that led to increased runoff into the stream/river