In one of the epicenters of damage from Houston's 2016 Tax Day flood, planners have drafted a list of specific improvements they hope to have funded by federal money slated for allocation this summer.

The North Houston District, formerly the Greenspoint District, commissioned among the most advanced hydrological study yet conducted in Houston after floods last spring forced area residents to evacuate apartment complexes floating in canoes, air mattresses and even a refrigerator.

The district is now citing that $75,000 study as it vies for its share of federal flood money that will be allocated throughout Harris County in June or July.

"Through this study we think we have some good projects to spend that money on," said Robert Fiederlein, vice president at the North Houston District. "We have a pretty good idea of what needs to be done. Now it's the political battle to secure the funding."

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Solutions include millions of dollars to buy out apartment complexes, renovate Greens Road and turn former residential areas into flood control basins.

It took an exhaustive analysis from a high-powered computer to attain that insight. Just like the advent of LIDAR revolutionized drainage understanding almost two decades ago, new computing power allows engineers to model how water flows over the complex surface of the land, instead of simply modeling how it enters and moves through the system of drainage pipes.

"Advances in technology have allowed us to now do this type of analysis," said Derek St. John, an engineer with Lockwood Andrews and Newnam and the principal author of the study. "We can evaluate the complete drainage system and understand where they're experiencing flooding."

The difference is between one- and two-dimensional analyses. One-dimension shows how water would enter the multitude of inlets to the drainage system, then travel in one direction down pipes and into bayous. Two-dimensional analysis includes the ground, modeled to account for coverings like concrete, buildings, forests or grass that would change the way water moves. All of that overland flow previously had to be estimated.

Pipes too small

What the study reveals for the Greenspoint area: really enormous problems. The pipes are too small, the lots and roadways are not adequately designed to direct water out of developed areas and large apartment complexes are built at the bottom of a large basin that naturally collects water.

Those problems owe themselves to a more primitive past - until the mid-1980s, countywide drainage criteria called for smaller pipes than are required today and didn't mandate use of roadways to convey floodwaters to the nearest waterway.

The Tax Day torrent hit Greenspoint in two charges, the first of which struck shortly after midnight with 4.9 inches of rain in one hour, filling the drainage pipes and ditches to their brims but retreating before dealing catastrophic damage. The rain returned around 4:30 a.m. with 5.2 inches in about 1.5 hours falling on a drainage system already at capacity.

The more intense downpours that fell upstream in the northwest county were raging down Greens Bayou, picking up speed until they moved too quickly to make a southward turn in the bayou's path, and instead blasted westward over the bank at about 1,900 cubic feet per second.

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It filled Greens Road, which was already surrounded by substantial pooling, fueling a westward current hundreds of yards wide that flowed at 2,800 cubic feet per second over a floodplain covered in hundreds of buildings. The water made for the low point near Greens and Hardy Street, the site of Arbor Courts apartments, where residents paddled their way to safety atop whatever would float.

Stiff limitations

Potential solutions face some stiff limitations.

For one, the district can't just put more water into Greens Bayou, as that would force it out of bank downstream. The U.S. Army Corps of Engineers, with help from the Harris County Flood Control District, plans to widen 11 miles of Greens Bayou over the next decade or more, but even that wouldn't create enough space to receive more water from Greenspoint. The district is also currently overseeing construction of two detention basins totaling 460 acres upstream from Greenspoint, scheduled for completion in 2018. The study included a model of the Tax Day flood with those basins completed to assess their impacts, finding they would have decreased the water level at the Arbor Courts apartments from 4.79 feet to 4.33 feet. So the area requires a local solution.

The basis of that solution, Fiederlein hopes, will be city buyouts of Arbor Court and the neighboring Biscayne apartments, which should be demolished and turned into floodwater detention.

"Buying out an apartment complex is kind of a big deal," Fiederlein said. "It's not going to be cheap."

He estimated between $7 million and $9 million to buy the apartments and between $3 million and $4 million to turn them into basins that double as parks.

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In addition, the district recommends upsizing the drainage pipes beneath Greens Road, Imperial Valley Drive and Hardy Street - a tedious process which requires ripping up the streets and replacing small pipes with larger ones that run to the newly created detention basin. Fiederlein estimated those improvements could cost $10 million.

He hoped for funding from Harris County's $61 million share of a $150 million federal flood recovery grant administered through the Texas General Land Office in March of this year. The county's community services division has until Aug. 1 to announce allocation of that money, a spokesperson said. Allocation will depend on factors including community input, levels of unmet needs, amount of flood damage sustained, numbers of low income residents and more.

"We could probably use all of Harris County's $61 million," Fiederlein said. "But of course we know that won't happen."