Rivers and Flooding 4 Flashcards
What is a flood?
a flood is an overflow of water that submerges land which is usually dry
FEMA’s definition of a flood(part 1)
A general and temporary condition of partial or complete inundation of two or more acres of normally dry land area or two or more properties (a least one of which is your property) from:
a. Overflow of inland or tidal waters
b. Unusual and rapid accumulation of runoff
c. Mudflow
FEMA’s definition of a flood(part 2)
collapse or subsidence of land along the shore of a lake or similar body of water as a result of erosion or undermining caused by waves or currents of water exceeding anticipated cyclical levels that result in a flood as defined in 1.a. above
What causes floods?
- Heavy rain
- Flash floods
- Fires
- Dams
- Levees
- New development
- Change in land management
- Snow melt
- Spring thaw
- Ice jams
- Hurricanes and tropical storms
- Storm Surges
- Climatic cycles(La Nina…)
Formula for Discharge
Q= width X depth X velocity
Periodicity of Flooding
Bank full Stage 1-2 years: filled channel
Small Flood 2-3 years: local splays
10 year flood 10 years: 1.2-2x m.a.fl.
50 year flood 50 years 2x 10 year flood
Stages of a River
Low Flow: 95% of time
Mean annual flow: 30% of time
Bank full flow: 2 times in 3 years on average
Moderate flood: every 10 years on average
Flood Control
- artificial levees
- flood control dams
- retention ponds
- channelization/river restoration
- non structural approach
What is a “regulatory flood way”?
means the channel of a river or other watercourse and the
adjacent land areas that must be reserved in order to discharge the base flood without
cumulatively increasing the water surface elevation more than a designated height. Communities must regulate development in these floodways to ensure that there are no
increases in upstream flood elevations.
channel restoration
the process of returning a stream and adjacent areas to a more natural state
- cleaning urban waste from the channel;allowing the stream to flow freely
- protecting the existing channel banks by not removing existing trees
- planting additional native tress and vegetation
