Floods

Floods GE 41504150- Natural Hazards

Stream Equilibrium Discharge- the rate of water flow Dischargeexpressed as volume/time Available sedimentsediment- amount of sediment waiting to be moved Gradient-- slope of stream bottoms Gradient Channel patternpattern- sinuosity of the stream path

Case 1 Too much discharge (water) Erodes the stream bottom Increases sinuosity of channel Lengthens the flow path Slows water Natural Disasters 6th Edition Patrick L. Abbott

Meandering Water does not flow at even depths, but moves from sideside-to to--side, eroding the river bank on one side then the other Sediment is deposited on the inside bend of each meander Create oxbow lakes Human intervention to reduce meandering for boat travel

http://www.nps.gov/archive/miss/features/misshist/intro.html

Case 2 Too Much Load (sediment) Excess sediment builds up on the bottom of a stream Increases gradient Water will flow faster Can carry more load (sediment)

In this case the channels tries to become straight Braided streams are created when to much sediment load is present

http://www.geo.uu.nl/fg/palaeogeography/results/fluvialstyle

Flood Plains and Flood Frequency Floor of streams during floods Flood frequency is analyzed by plotting the flood size vs. recurrence interval The longer the historical flooding record, the more accurate the return times Need to be constructed on a stream by stream basis FEMA uses the 100 year flood for regulation

Flood Probability Statistically speaking the bigger the flood the longer the return period The 100% year flood has a 1% chance of occurring any year Cumulative probability: The longer the wait for a 100 year flood, the more likely it will occur But, there is still a 1% chance of that size flood occurring during any given year

Natural Disasters 6th Edition Patrick L. Abbott

Recurrence Interval Recurrence Interval = (N+1)/M N = number of years of flood records M = numerical rank of each year’s maximum flood discharge Natural Disasters 6th Edition Patrick L. Abbott

Flash Floods #1 weather related killer in the U.S. Most flood related deaths in the U.S. are caused by flash floods, 50% of those occur in automobiles Caused by excessive rainfall, dam breaks, or a release of water held back by ice Most are caused by slow moving thunderstorms http://www.dola.state.co.us/de m/public_information/flood.htm

Flash Floods

Big Thompson Canyon Colorado July 31, 1976 - 100th Birthday of Colorado 3,000 tourists in the canyon At 6 pm strong low level winds from the east pushed moist air upslope into the mountains Thunderstorm remained stationary and rain fall over 4 hours total the rainfall for the entire year

http://www.assessment.ucar.edu/flood/flood_summaries/07_31_1976.html

Big Thompson Canyon Colorado Rainfall created a flash flood within an initial wall of 20 feet high Flood moved 15 mph through the canyon, which left very little warning Over 400 automobiles on highway, many abandoned and went up hill and lived, or stayed in their cars and died

http://www.thedenverchannel.com/news/9596857/detail.html

http://www.survive-disasters.com/Flash%20Flood.htm

Big Thompson Canyon Colorado 6 pm river flowed at 137 ft3/sec, at 9 pm it was 31,200 ft3/sec Flow volume was 3.8 times greater than the estimated 100 year flood 139 killed, 6 were never found 418 houses and 52 businesses destroyed 400 cars damaged Total losses were $36 million

http://www.survive-disasters.com/Flash%20Flood.htm

http://www.casfm.org/images/big_thompson_flood_01.jpg

Regional Flooding High water inundate a region for weeks Fewer deaths than flash floods, but extensive damages In the U.S. about 2.5% of land is within a flood plain, and 6.5% of the population lives there

Natural Disasters 6th Edition Patrick L. Abbott

Mississippi River Third largest river basin in the world, drains 31 states and two Canadian provinces Of the 28 biggest rivers in the U.S., 11 are part of the Mississippi River Average water flow in the lower reaches is 18,250 m3/sec. This can be increased fourfold during floods

Mississippi River River has changed its course seven times in the past 4,600 years As sediments move into the delta, the river ‘s channel bottom grows higher. May build above the flood plain

Mississippi River When major floods occur and water overtopping banks and levees flow to lower elevations outside the channel, the river may adopt a new lower elevation course and abandon it’s old channel. This process is called avulsion. The Mississippi River wants to change course, but humans have prevented this from happening

Coast 2050 Report jointly developed Federal/State/Local plan to address Louisiana’s massive coastal land loss problems and provide for a sustainable coastal ecosystem by the year 2050 Within the last 50 years land loss rates have exceeded 40 square miles per year The loss represents 80% of the coastal wetland loss in the entire U.S. If nothing is done by 2050 the Louisiana coast will lose more than 630,000 acres of coastal marshes, swamps and islands

Wetland Loss Commercial fishing Ecotourism Habitats for threatened and endangered species

Navigation corridors and port facilities Water quality improvement Buffering for hurricane storm surge

Coast 2050 Goal To sustain a coastal ecosystem that supports and protects the environment, economy and culture of southern Louisiana, and that contributes greatly to the economy and wellwell-being of the nation

Strategic Objectives Sustain a coastal ecosystem with the essential functions and values of the natural ecosystem To restore the ecosystem to the highest practicable acreage of productive and diverse wetlands To accomplish this restoration through an integrated program that has the multiple use benefits, not only for the wetlands, but for all the communities and resources of the coast http://www.lacoast.gov/

CWPPRA Coastal Wetlands Planning, Protection and Restoration Act was passed in 1990

Funds wetland enhancement projects nationwide, designating $60 million annually to Louisiana