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Is "Going by the Book" Enough for Flood Mitigation?

 

by John Howell
Storm & Flood Products Manager

Living near water requires that we plan for situations when we have too much of it. Throughout history we have dealt with flooding when weather events exceed “normal” parameters. Over time we have found different ways to mitigate the risk of flooding, up to a point. Levees, dams, drainage systems, building codes, urban planning; all have a place in reducing risk from flooding. Some of our mitigation strategies have been codified into law. However, rules and regulation, building codes and laws are not enough. Engineers and water professionals need to apply creativity and critical thinking when designing and implementing infrastructure to mitigate flood risk. A recent event in my hometown illustrates this need.

Huntsville, Alabama rests on the eastern edge of the Tennessee Valley. The traditional eastern boundary of the city is the western edge of the Appalachian Mountains. Here the mountains are separated by flat valleys, typically bisected by a stream, creek or river. Huntsville has experienced explosive growth since the 1950’s growing from a small city of 20,000 to a metro area of 500,000 people. The demand for housing dictated that the once peaceful valleys of the foothills are now suburbs.

One suburb in particular very recently experienced a situation that highlights the importance of comprehensive planning. Hampton Cove is a planned community of 1,200 homes just east of Huntsville in a valley between Mount Monte Sano and Keel Mountain. Before the construction of Hampton Cove the valley existed almost entirely within the 100 year floodplain of the Flint River, a tributary to the Tennessee River. Hampton Cove was carefully planned to mitigate flood risk. All of the land was graded so that lots were 10 feet above the 100 year floodplain. The community was designed around a man-made lake that is basically a large detention pond. The pond has an overflow spillway that drains to the Flint River.

In the 72 hours between December 24th and 27th the Huntsville area received over 8 inches of rainfall (Click Here for Gauge Data). The 3 inches received on December 24th was followed by 5 inches on the 26th.  A 5 inch rain event normally wouldn’t overtax the flood control structures already in place. A 5 inch event occurring so closely after a 3 inch event, with the ground already saturated and downstream capacity limited proved to be problematic.

The stormwater infrastructure designed for Hampton cove worked as planned. The lake did reach maximum storage capacity and the spillway received flow for about 3 hours on December 26th. None of the homes in the subdivision experienced flooding.

However, the subdivision is served by two roads; one road entering the valley from the north and the second entering from the south. Both roads have stretches that are barely above the 100 year flood line. On the night of December 26th the Flint River topped its banks, and water rose to 7 feet above flood stage. While the homes in the subdivision were high and dry, both roads entering the area were under several feet of water in some areas. The homeowners of Hampton Cove had no way to leave the area for the better part of a day. Residents returning home from Christmas festivities couldn’t travel the last mile home.

Residents of the area are asking questions as to why road elevations weren’t considered when the plans were being drawn up for the subdivision. The basic answer was that the law didn’t require it.

The roads that flooded have been in existence for over a hundred and fifty years, and they travel through two municipalities and one unincorporated area of Madison County. The building codes and regulations relative to new housing don’t consider the traffic infrastructure outside the scope of the housing development. Their concern was mitigation of flood waters directly at the site which did indeed work as intended.

This is not an isolated incident.  Traditional methods of flood management do not always take into account what is happening directly outside the boundaries of a specific project.  But the fact remains that upstream and downstream conditions affect and are affected by flooding situations, and those are not always completely handled by municipal or federal regulations.

The resolution that can prevent future flooding in situations like this is for governing bodies that are stakeholders to cooperate in developing an integrated flood management plan for the valley. Coordinating future road and housing development with an overarching flood management plan can help resolve issues that can occur when we simply “go by the book”.


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