🚀 go-pugleaf

Gary E. Freeman wrote:
> 
> 
> First I should note that the 10% figure was based on the whole basin.
> The numbers for some of the basins were a little higher but we felt
> like a realistic figure was probably more like 5% with 10% being the
> upper limit.
> 
> Now about levees:
> 
> Our study based on the portion of the Mississippi River from Hannibal,
> Missouri to Cairo Illinois (300+ River Miles) showed that for the 1993
> flood the levees had little effect.  Part of the reason was that most
> of the levees failed.   We looked at a scenario where none of the
> levees failed and found that the maximum increase in water surface,
> for this reach was 2 feet out of approximately a 20-30 foot depth above
> flood stage.  For an option where the levees were removed the lowering
> of stage varied depending on what was assumed for the
> roughness value of the floodplain that replaced the current agricultural
> lands.  In the midwest the floods correspond to the farming season,
> unlike California where the floods come during the winter, so land
> use would change in the floodplain to something other than current uses.
> 
> The reduction in flood depths for a no levee scenario varied from 1.1
> feet to 7-10 feet in an area where the floodplain was several miles
> wide.  Even in the area with a 7-10 foot reduction in stage the flood
> waters were 10+ feet deep and only fringe areas would be unflooded.
> Nearly all, if not all, areas that were damaged would still be damaged
> with the levees removed.
> 
> The follow on study by the Corps looked at the whole Upper Mississippi
> River Basin (much larger than any basin in California or the West)
> and found that the no levee failure option would have increased flood
> heights by up to 6 feet (6 on top of about 20 feet as occured) - a
> significant increase.
> 

I don't remember, if I ever heard, what magnitude those floods were considered to be.  
If those flows represented a 50 year event how much less damage would be caused in the 
floodplain by a ten year?  

What I am trying to suggest is that homes and structures need full time permanent 
protection from flooding, above the 100 yr line.  But perhaps ag fields, and related 
structures, could be left in a gamblers position of being suceptible to the 25 year 
event?  I know the area affected would change depending on the topo of the floodplain. 
 Was/is this looked into with regard to the rebuilding of levees?  What % of the flood 
plain is protected from the 25 year, 4%, event?