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The Hundred Year Flood Plain

From The BASIN Site: "The terms "10 year", "50 year", "100 year" and "500 year" floods are used to describe the estimated probability of a flood event happening in any given year. Their primary use is for determining flood insurance rates in flood hazard areas."

"Using historic weather and hydrograph data , experts derive the estimated rate of flow or discharge of a river or creek. A 10 year flood has a 10 percent probability of occurring in any given year, a 50 year event a 2% probability, a 100 year event a 1% probability, and a 500 year event a .2% probability. While unlikely, it is possible to have two 100 or even 500 year floods within years or months of each other."

The Mall of Memphis and The 100 Year Flood Plain

The grounds on which the Mall was built are bordered on one side by Nonconnah Creek, a mid-sized seasonal stream running largely parallel to I-240 South in Memphis. Pre-mall, the grounds were well below the Hundred Year Flood Plain - a problem that would have to be solved in order to insure the mall. The scale of the problem was noteworthy as the site was over 100 acres in total size and needed to be raised some 30 feet or more. That A LOT of fill material. To have to buy that much material would have skyrocketed the Malls cost, perhaps beyond economic feasibility.

A Built In Solution

Based on a conversation with Stanley H. Trezevant Jr. in late 2006, the bordering Nonconnah Creek was always part of the plan. Every rainy season, the creek drains massive amounts of runoff and the Creek becomes a raging river, depositing tons on soil and rock along its banks. It was unfortunately common for people to drown in the rain swelled creek each year when they underestimated the speed, amount and power of the water flowing by. Mr. Trezevant pointed out that in the 1980's EPA regulations tightly controlled what could be put IN the tributary but they had little to say about what could be taken OUT. In particular a number of developers along the Creek , including Trezevant took "borrow or fill material from the creek for their bordering projects. "It was a free, naturally replenished source of fill" said Trezevant in 2006. Nature takes time though and in order to get free fill, Mall development was delayed three years while the stream bed was graded for material. When everything had been scooped out that could be, the development had to wait close to a year for the next rainy season, when the Creek would again overflow its banks and deposit tons of dirt, silt and rocks along the mall property, which would be dutifully claimed by Mall developers. An unexpected hangup in this process was the discovery of Mammoth fossils in the excavated area. Trezevant allowed the University of Memphis to dig the site and remove all the bone fragments found.

This process was repeated until the complete Mall site was above the 100 Year Flood Plain. "Most of the area where Chuck Hutton Chevrolet was located (at Mt. Moriah and I240) was filled the same way", Trezevant said.

After fours years or so of this procedure, the site was ready for the next phase, Mall Construction.


BASIN-- the Boulder Area Sustainability Information Network