Why does the Nashville area flood?

Sunday, May 9, 2010 at 11:45pm

Why are parts of Nashville fetid, stagnant bathtubs? Why did the Cumberland River swallow its well-established banks and sprawl across downtown, transforming Lower Broadway into a new riverfront and Wildhorse Saloon into a pool bar?

The short answer to those questions is rain. Not just once-in-a-lifetime rain. Not even once-in-a-New-Testament rain. More like the only rain of its kind in recorded history — like the last rain of similar volume could have predated the Bronze Age. Experts are saying the roughly 13 inches dumped on Nashville over 48 hours during last weekend occurs, on average, once every 5,000 to 8,000 years; like an entire season packed into two sodden days of unremitting precipitation.

The long answer to those questions is one whose details will be mulled by civil engineers and hydrologists for some time to come, but The City Paper spoke with a few of them about some of the broader factors that delivered a dousing unlike any Nashville has ever seen.

Rain, rain, go away …

Middle Tennessee wasn’t the only region getting dumped on.

It was pouring all the way up into Kentucky. Imagine all that rain from all those storm clouds stretching for hundreds of square miles, draining and falling into the creeks that join the smaller rivers like the Harpeth and the Red and, past downtown, the Stones, all of which eventually join the Cumberland. Then imagine all of that thundering its way downstream, getting cumulatively worse the farther it goes until, finally, it arrives in Music City.

But it could have been worse. Much worse.

Now envision the series of U.S. Army Corps of Engineers-operated dams situated along the Cumberland and its tributaries as a system of stoplights, with reservoirs acting as temporary rest spots for the seething floodwater. If the rain is localized, the nearest dam will contain the flood. The river downstream won’t swell as much as it would if it were free flowing. When a wide swath of the watershed absorbs record-smashing rains, however, all bets are off.

The Cumberland is first held back at the Wolf Creek Dam in southern Kentucky. Then the Cheatham Dam in Ashland City. The J. Percy Priest Dam holds back much of the Stones. The difference is yawning — a swamped Lower Broad, for example, instead of a completely drowned downtown.

If it keeps on rainin’ …

The difference between a biblical deluge in Old Hickory and parts of East Nashville, and a nasty, house-swallowing flood was measured in inches on Sunday.

Notably, the Old Hickory Dam came within 7 inches of reaching its maximum elevation. That doesn’t mean that the water was lapping only inches from the tippy-top of the dam. It means it was inches from overrunning the facilities the Army Corps of Engineers uses to manage the water level of Old Hickory Lake and the releases into the Cumberland.

Corps spokeswoman Allison Jarrett said they held the water back for so long because they were waiting for some of the flooding in Nashville to go down before they added to it. By Sunday night, the releases were curtailed — a true for-the-good-of-the-many trade-off, no doubt to the consternation of the few. Allowing the water to rise any higher could have compromised the dam, resulting in catastrophe.

Big River

Is there a poultice for the swelling of the Cumberland River? And if so, could it have reduced the flooding that inundated our streets and infiltrated our houses? Depends on whom you ask.

Vanderbilt Univeristy civil engineering professor Dr. Prodyot Basu said the Harpeth and the Cumberland are in need of a good dredging.

“The Harpeth River floods all the time. Why don’t they make arrangements to do the work on the river so that it does not overflow its banks and increase the dredging so the water flows out? Increase the channel area basically,” said Basu, who lives in beleaguered Bellevue. “None of those actions are taken. It comes and it floods everything and brings a lot of problems to people. Then people forget about it, and it comes back. It goes in a cycle.”

The logic goes: The deeper the Cumberland, the better it drains. And the better it drains, the more readily it accepts the stormwater carried to it by its tributaries. Instead, already way beyond capacity, the Cumberland couldn’t take anymore, and the tributaries that feed the river, and the creeks that feed the tributaries, backed up and spilled out into your living room.

Dr. Bruce Tschantz, emeritus professor of civil engineering at the University of Tennessee, doesn’t buy it. He thinks constructing a large reservoir along the Cumberland could contain enough of the river to prevent massive flooding — another rest area at the stoplight, so to speak.

Water, water everywhere …

Rain doesn’t just fall into rivers. It lands in streets, on sidewalks; it fills ditches and sluices through culverts and pools in topographical depressions both natural and manmade.

Metro Nashville has a drainage system designed to slough the water off the streets and out into pipes that carry it away. Metro is still assessing this infrastructure, but a Metro Water Services spokesperson told The City Paper that many pipes had been completely washed out — either because the culverts were too small to accommodate the runoff, or they were blocked by debris that choked the flow.

Most new minor Metro construction is equipped to handle the kind of flooding that occurs once every 10 years. Some of the larger pipes are built to deal with 100-year floods. Hit them with an 8,000-year flood and here we are.

“If the capability of the drainage system is less than what comes in, something’s gotta give,” Tschantz said. “It builds up in the streets and the backyards. Finally, the backyards spill over and it gets into places that have never been flooded before.”

The recently completed stormwater projects at Trousdale Drive, Holt Road and Stillwood Drive weren’t overrun like they would have been, had they been equipped with the older culverts.

Much of Nashville, though, isn’t so fortunate, and it found itself woefully unable to cope with the rising water. We’ll continue to see a waterworld, to some extent, until the Cumberland subsides. According to Tschantz, first the Cumberland must drop, then the tributaries, creeks and flooded areas nearest the river, and, finally, everything else.