Reflections -- A False Premise Established FEMA’s Original Flood Elevations
By Allan Von Werder
Determining flood elevations is one of the trickiest of sciences due to the ever-changing variables that can bring floodwaters where they’re not welcome.
In St. Mary Parish, out-of-state contractors toiling away in the 1980s determined 100-year and 500-year flood elevations. Reading the original report, one suspects the out-of-state contractors were clueless.
For example, the 100-year flood elevations were determined under a flood scenario that included moderate flooding of the Atchafalaya River with a Category 5 hurricane at the river’s mouth.
While theoretically possible, it is certainly not a 100-year event to have a Category 5 hurricane in the spring, nor is it a 100-year event to have moderate flooding of the Atchafalaya in late August or early September. Those two events always occur months apart from each other.
So, FEMA’s original flood maps were not only built on a false premise, it is practically an impossible premise.
St. Mary did not suffer another variable that affected Cameron Parish in the original batch of FEMA maps. The California contractor that extrapolated Cameron’s levels used a contour map with contours at five-foot intervals. Even though the flood elevation threshold was around six feet, Cameron was told 10 feet was the threshold because that was the next contour level on the contractor’s map.
Welcome to the federal bureaucracy, where people who don’t care hire people who don’t know in order to force you to do or buy something you don’t need.
FEMA’s nonsense would be tolerable if it were just a “we think you should be warned about this” informational campaign. But this bad data is being used to force people to buy insurance on properties that have never flooded and probably never will.
The forced insurance buy is another curiosity. Nationwide statistics are very clear on flooding. They show that 90 percent of all floods occur in non-flood zones. That begs the question of why people that live in flood zones, the least likely areas to flood by historical data, have to buy insurance while people who live in areas statistically more likely to flood do not. It also proves that when establishing “flood zones,” neither FEMA nor anyone else is very proficient in their predictions.
A case in point — nearly all of the flooding in New Orleans after Katrina was in non-flood zones.
There are generally two types of flooding — riverine and still water. Riverine is when a river, creek or bayou rises and spills over its banks to surrounding areas. Still water is a rain event.
Floods occur when the ground can no longer absorb the excess water and levels begin to rise. Bare ground has what’s known as “field capacity.” Because one cubic inch of soil has so much surface area, it can hold enough water to wet a 25 square foot area. That translates to each foot of earth being able to absorb two or three inches of water.
Buildings, roads, sidewalks, etc. all reduce the field capacity of an area so every time you build something or change something, flood risks change too. Development doesn’t have to be close to an area to change its flooding risk.
If, for example, you develop an area a mile away it’s possible that the new area may have been where your excess water used to go — especially if it was originally lower in elevation. You could now flood due to a project you thought had no connection to where you are a mile away.
Large developments that take acres of field capacity out of an area are required to include retention ponds, open areas where water can pond as it’s waiting to be drained. The Walmart in Bayou Vista has two such areas by the garden center.
Storm surge induces riverine flooding by essentially making a river, canal or bayou flow backward. On a beach, the tide simply rises — a lot.
Storm surge is not much of a threat in the Tri-City area mainly because it’s a 20-mile trip to the gulf. In the Franklin area it’s only seven miles to the bay as the crow flies (on a clear day you can see the bay from the top of the courthouse). Hurricane Andrew had the perfect storm track to deliver a massive surge on the Atchafalaya. It was a non-event due to the distance.
Stopping a storm surge does not require the same kind of levee for containing a river. During Hurricane Katrina, the storm surge in Mississippi was stopped by the railroad tracks, indicating that any simple barrier will do. Storm surges are short events.
In the 1950’s, congress ordered railroads near the gulf to elevate their tracks to nine feet. That stopped Katrina’s surge in Mississippi and means St. Mary has a nine-foot storm surge levee protecting most of the populated areas. FEMA thinks the railroad impedance does not exist because it is not a “certified” levee blessed by the U.S. Army Corps of Engineers. However, history shows that elevated railroad tracks work just fine for containing storm surges. All of the waterways that flow under the tracks in St. Mary are or will soon be contained.
FEMA’s new flood maps discount the existence of railroad tracks, local levees, pumping stations and other flood prevention structures and then declare we’re at serious risk of flooding.
With the new containment structures on Franklin area waterways, St. Mary is no longer at risk from riverine flooding due to storm surge. Oddly enough, FEMA raised the elevation threshold in the Franklin area by a foot before these new structures were in place while lowering Morgan City’s elevation threshold by several feet.
While it is true that Louisiana suffers elevation loss from subsidence, all the studies note that subsidence stops in Gibson and is virtually nonexistent westward. Much of the populated corridor of St. Mary sits atop the Teche Ridge, a very solid piece of ground.
The record shows that FEMA was wrong in their initial flood premise, continued being wrong while the old flood maps prevailed, and is even more wrong today in their new assumptions.
Piling mistakes and false assumptions on top of each other will always produce an alarming result. But wrong is wrong and with so much bad data underneath FEMA’s assumptions, they have reached a point where it’s best to start over.
Historical data and local input on flood structures will paint a much more reasonable, accurate picture.