Changing Nature’s Course

Tracy Morris

David Trombley

A War of Two Rivers

• Atchafalaya• Controls Louisiana’s

crawfish crop• Basin encompasses

over 370,000 acres of fresh marsh, bottomland hardwoods, cypress swamps, and open water (the largest wetland in the US)

• Mississippi• Drains 41% of the

contiguous US• Basin covers more

than 1,245,000 square miles

• Lower alluvial valley is a flat plain of 35,000 square miles

Map of Southern Louisiana Today

The Old River connects the Mississippi to the Atchafalaya.

The Atchafalaya runs a much shorter course to the Gulf of Mexico.

Commerce has depended on the Mississippi for 200 years.

How It All Began

• 1831- Old River created by steamboat captain to shorten his upriver trip

• Old River flow slowed as the upper portion filled with silt

• 1839 – State of Louisiana began to clear 30 mile logjam on Atchafalaya River

• Flow in Atchafalaya began increasing

If It Did Switch Course?

• New Orleans ranks number 1 in the United States in tonnage of goods shipped through its port

• The Mississippi River is associated with 40% of the tidal wetlands in the contiguous United States

• Riverside development along Atchafalaya would be inundated

Change Isn’t Always Good

• Businessmen quickly realized that they needed the Mississippi to remain a navigable river

• In 1927, a 100-year flood gave us a second reason to control the flow

The Flood of 1927

• Inundated 16.5 million acres in 7 States

• Destroyed: $102 million of crops

160,000 homes

40,000 buildings

• Killed 500 people

• Prompted Congress to pass “Flood Control Act of 1928”

Flood Control

• Corps accepted over 300 proposals

• Winning proposal had three facets:

• Incorporate floodways to divert peak flow

• Create backwater areas

• Design all works on basis of a great hypothetical flood

Slowly but Surely

• 1953- Mississippi River Commission recommended an Old River Control Structure

• 1954- Corps engineers designed a lock, a dam over the Lower Old River and two control structures: Overbank and Low Sill

• 1955- Construction began

Old River Control Structure

• Completed in 1962– Kept Acadiana from flooding– Keep enough water in Mississippi so big

ships can navigate to New Orleans

• Composed of dams, weirs, and levees• Designed to allow 30% of the

Mississippi to flow down Atchafalaya under normal times

Old River Control Structure

• Without control structure, the straighter shorter Atchafalaya would capture the mainstream of the Mississippi

Old River Control StructureLow Sill Structure

• Made of eleven gates– 44 ft wide

• Total length 566 ft• Maximum water height allowed in forebay

– 69.8 ft above sea level

• Designed to withstand a 37 foot difference in water levels between Mississippi an Atchafalaya– Can only maintain 22 ft

Old River Control StructureOverbank Structure

• Contains 73 bays– Each 44 ft wide – Total length 3,356 ft

Old River Control StructureAuxiliary Control Structure

• Completed in 1986

• Total of 6 gates – 62 ft wide– Total length 442 ft

http://www.msstate.edu/dept/geosciences/CT/TIG/WEBSITES/LOCAL/Spring2002/Cathrine_Duex/oldriv14.html

http://www.msstate.edu/dept/geosciences/CT/TIG/WEBSITES/LOCAL/Spring2002/Cathrine_Duex/oldriv1a.html

Old River Control StructureNavigation Lock

• Allows travel between Mississippi, Red and Atchafalaya rivers

• 75 ft wide

• 1,185 ft long

• Floor 11 ft below sea level

Other Control Structures

• Bonnet Carre Spillway – Built 1937 – Diverted water to Lake Pontchartrain

References

• 1927 Flood Pictures: http://www.riddlelawoffice.org/1927Flood.shtml

• http://www.mvd.usace.army.mil/offices/pa/brochures/mrtbrochure.pdf

• http://www.biology.lsu.edu/webfac/cramcharan/Louisiana.html

• http://www.theadvertiser.com/news/html/BF4039D3-B1D9-497C-AB63-F8CB394C2B9F.shtml

• http://www.tulane.edu/~bfleury/envirobio/enviroweb/FloodControl.htm

• Pictures of control structure: http://www.msstate.edu/dept/geosciences/CT/TIG/WEBSITES/LOCAL/Spring2002/Cathrine_Duex/OldRiverControl.html

• http://www.dnr.state.la.us/sec/execdiv/pubinfo/newsr/archive/ crawfish.ssi

• http://www.lacoast.gov/cwppra/projects/atchafalaya/

Questions?Questions?