At New Orleans the Mississippi River is a fascinating monster. New
Orleans is 94 miles from the Head of Passes where the rivers splits
into Southwest Pass, South Pass, and Pass à Loutre to enter the Gulf.
The navigation channel from the Gulf past New Orleans and all the way to Baton Rouge is maintained at a
minimum depth of 45 feet year round.
The river is
less than 1 km wide at New Orleans and about 200 feet deep off Algiers
Point. The water level ranges from 2 feet to 18
feet above sea level (NGVD) with the spring flood stage being the highest level. Flow
averages 600 thousand cubic feet of water per second (100k-1m normal
range). Typically velocity is around 4-5 mph at the surface
reaching almost 9 mph in flood stage. Project Flood at New Orleans with
all diversions open is 1.5
million cfs.
(Just for fun - 600,000 cfs is about 5 million gallons per second. This
flow would fill an Olympic swimming pool in 0.04 seconds.)
Since the bottom of the river is deeper than the surface level of the Gulf, and saltwater is heavier than fresh water,
saltwater from the Gulf can penetrate up river when the current is slow. Unchecked the
saltwater wedge could reach as far as Natchez so engineers build a ridge
across the river bottom near Myrtle Grove (64 miles from the Head of
Passes) to stop the saltwater when river flow drops below a minimum target level.
Here's the story on August 15, 2012. With record drought savaging the crops in the Midwest, New Orleans begins to feel the effects:
It takes 90 days for a drop of water to move 2320 miles from Lake Itasca to the Gulf at an average speed of 1.07 mph.
Just before Katrina blew though in August 2005 the river was at low stage, just 2 feet above sea level. The
storm caused the river level to rise to 16 feet overnight. Debris was blown over the top of the levee on
north facing sections of the Westbank directly across from the Chalmette Battlefield. 100+ mph winds from the North pushed the waters towards the Westbank. Ships moored in the river strained at their anchors. A barge was thrown onto the batture and nearly struck the levee. Wave action
threatened to top the levee. In
some places the debris line actually reached the top of the levee
indicating some splash over. A crevasse in that levee would have
flooded Algiers and most of West Jefferson more violently than anything
that happened in the lower Ninth Ward. Those levees are not armored,
but are long established and well sodded with grass.
Rita caused a smaller spike in river level about a month later topping out under 8 feet.