Fifth in a series

PATTISON — Coffee brown water flows through ditches in rural Waller County, the remnant of storms that drenched the Katy Prairie during Houston's Tax Day flood.

Weeks after that epic rainfall, the prairie is awash in daisies and blue and purple horesemint flowers. Beavers take advantage of ponds brimming with water, and nearby dirt roads show little evidence of being recently inundated.

"This is how the land is supposed to act," said Mary Anne Piacentini, executive director of the Katy Prairie Conservancy, a nonprofit land trust. "It's supposed to absorb water and filter out pollutants. It's not supposed to send it roaring into the rivers and bayous and homes."

In the greater Houston area, though, the staggering increase of impervious surfaces — roads, sidewalks, parking lots, anything covered with asphalt and concrete — has exacerbated the effects of flooding as development in the region has exploded. When land is covered by these surfaces, it loses ability to act like a sponge and soak up water. Things are further complicated in flat-as-a-pancake Houston, where much of the soil is heavily compacted and acts like pavement anyway, sending sheets of storm water to the nearest low-lying area.

A recent analysis of federal satellite data by the Houston Advanced Research Center for the Houston Chronicle shows that 337,000 acres of 1.1 million acres in Harris County were covered by impervious surfaces in 2011, the most recent year of data. That dwarfs surrounding counties, but the analysis shows many are catching up as the onslaught of development continues pushing from the city farther into the suburbs.

Between 2001 and 2011, Fort Bend County, for example, had a 53 percent increase in impervious surfaces, more than twice the percentage increase in Harris County during the same period. Waller County, home to much of the Katy Prairie, saw a 17 percent increase.

That kind of development comes with a price, namely the loss of the region's natural landscape, including wetlands, prairies, coastal marshlands and forests, and thereby a greater risk of flooding. Even with federal regulations in place to preserve wetlands, the 14-county Houston region lost more than 54,000 acres of wetlands between 1996 and 2010, according to HARC's analysis.

"Pitiful," said John Jacob, a Texas A&M University professor and director of the Texas Coastal Watershed Program.

As large tracts of undeveloped land have been transformed by new roads, homes and businesses, city and county planners have relied almost exclusively on detention basins — often referred to as detention ponds — to solve the water runoff problem created by the region's vast asphalt and concrete surfaces.

Detention basins are man-made structures designed to capture storm-water runoff and temporarily store it. Harris County first began requiring them of developers in the early 1980s, and neighboring counties quickly followed. Today, each county in the region has hundreds of the ponds, both dry and wet.

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However, these detention requirements have fallen short in attempting to tackle the source of the flooding problem because they do not require developers to eliminate runoff from their projects.

Many Houston-area homeowners blame inadequate storm-water mitigation rules for their flooding woes. City and county officials disagree, but concede it's difficult to untangle the effects of new development, flood control projects and climate change when trying to determine the culprit for the region's worsening flood problem. The issue came to a head recently for a group of west Houston residents who sued the city a couple of months ago claiming it is allowing developers to circumvent storm-water safeguards.

"I can show you on any individual project how runoff has been properly mitigated," Montgomery County Engineer Mark Mooney said. "Having said that, when you see the increase in impervious surfaces that we have, it's clear the way water moves through our county has changed.

"It's all part of a massive puzzle everyone is trying to sort out."

'Lakes' of Fort Bend

On an unusually warm morning in June, Nita Khan strolled around one of the many wet detention ponds in Riverstone, one of the seven mega master-planned communities in Fort Bend County.

Khan moved there two years ago, drawn from Sugar Land by word of mouth from relatives and friends.

"I have a large family, so the larger houses were very enticing," she said. "And there's just so much to do here."

Move the slider left-to-right to illustrate how this section of northwest Harris County has transformed within the last 15 years. In the greater Houston area as development in the region has exploded, the effects of flooding have exacerbated. When land is covered by concrete surfaces it loses ability to act like a sponge and absorb water. (John D. Harden/Houston Chronicle)

For Khan, that includes walking around neighborhood "lakes," which actually are detention ponds covering about 200 acres.

Nowhere in the booming Houston area has seen the kind of population growth — and the ensuing increase in those impervious surfaces — than Fort Bend County, one of the nation's fastest-growing counties. In 2001, there were about 30,000 acres of such surfaces; a decade later, that number increased to 47,000 acres.

Today, there are also hundreds of detention ponds in Fort Bend, widely considered to have the most stringent set of rules to mitigate storm-water runoff in the Houston region. Those ponds, designed to resemble amenities, are credited with helping Fort Bend avoid the worst of the flooding that has plagued other areas of the Houston metro region.

"Harris County had to play catch-up with their development," said Mark Vogler, director of the Fort Bend County Drainage District. "We, and many other counties, were in a position to learn from them, and that's benefited us."

In the early '80s, Fort Bend hired Larry Dunbar, a consultant, to help draft the first set of detention pond requirements. Until then, most counties were solely relying on drainage systems to combat runoff. Dunbar explained that Fort Bend County opted to use a calculation that assumes undeveloped land has a slower release rate from the ponds. The net result was developers are required to build ponds that could store a larger volume of water than what's demanded in Harris County.

"At the time, developers and engineers wanted to know why we were doing that — not that it was wrong, but it would most certainly scare away development," Dunbar said. "Well, clearly that didn't happen. It's booming. And you don't see the kind of urban flooding problems in Fort Bend County that you do elsewhere."

When land is covered by concrete surfaces it loses ability to act like a sponge and absorb water. The GIFs show how much Houston has grown from 1984 to 2012 using satellite images from Google Earth. Click the buttons above the image to zoom in on another section of Harris County. (John D. Harden/Houston Chronicle)

Protected by almost 100 miles of levees, Fort Bend is indeed different than its suburban Houston counterparts when it comes to flooding.

During this year's Memorial Day flood, the Brazos River jumped its banks at several locations in Fort Bend, saturating more than 1,200 homes and causing an estimated $19 million worth of damage.

The city of Sugar Land, however, saw almost no flooding — a fact the city officials have chalked up to the levees, good drainage plans and the detention ponds.

Still, it's unclear how Fort Bend County's detention ponds actually performed during that heavy rainfall in late May. Like other counties, maintenance of the ponds often is left up to the developer or subdivisions' homeowners associations.

"It is one of the things that keeps me up at night," Vogler said. "We do all we can on the front end with our requirements, but ultimately it's up to the owner to ensure they're functioning properly."

Source of friction

In neighboring Harris County, where growth also has been heavy and flooding rampant, the adequacy of detention pond regulations is a source of friction.

In May, a group of Memorial-area residents sued the city of Houston and one of its local redevelopment authorities, alleging they allowed commercial development without requiring sufficient storm-water mitigation.

The group, Residents Against Flooding, cited the 2007 widening of Bunker Hill Road north of Interstate 10 and the elevation of nearby commercial properties, among other projects, alleging they funneled water into surrounding neighborhoods. The residents' group claims the defendants promised to build five detention ponds to alleviate flooding but delivered only one.

"Obviously, it would have helped with the flooding situation, but it wouldn't have solved our problem," said Ed Browne, one of the group's leaders.

Browne said inadequate drainage and poorly functioning detention ponds are among the reasons Memorial-area neighborhoods are experiencing flooding for the first time.

"In our area, it's pretty clear what's going on," he said. "The other thing that's clear is this is a citywide problem."

Running low on land

Houston and Harris County officials dispute that the storm-water mitigation rules are insufficient. They do concede the regulations are not a panacea for flooding.

"You are not improving the problem caused by impervious surfaces," said Steve Costello, Houston's newly appointed flood czar. "You are mitigating the effect of new development, but the problem (water runoff) still exists."

It also has become increasingly difficult to find land for new detention ponds, given the development boom. Census numbers released in May show Houston is the second-fastest growing U.S. city — now home to 2.2 million residents. The area's rapid growth has caused a land crunch in Houston's surrounding suburban counties, where another 4.2 million people live.

With land in such demand, Montgomery County officials in 2014 considered revising their retention pond requirements to make them smaller, an idea they ultimately abandoned. There is little evidence that the more stringent requirements have impeded development. Between 2001 and 2011, the county saw a 33 percent increase in impervious surfaces, the second-biggest increase in the Houston area during that stretch.

Alternative solutions

Given the challenges associated with detention ponds and other structural solutions to Houston's chronic flooding, regional leaders are being asked to consider "greener" options for flood control, such as preserving more land in a region where developers have always had clout.

The Bayou Land Conservancy, which protects more than 2,000 acres in Montgomery County, wants to keep development out of the floodplain and turn that land into a community amenity, whether a butterfly garden, wildlife habitat or a place for recreation.

Stephanie Prosser, the conservancy's interim executive director, points to the Spring Creek Greenway, a forested corridor that spans the divide between Harris and Montgomery counties, as a good example of an environmentally friendly project.

"It is becoming an amazing resource for the community," Prosser said. "Most people will utilize it for some form of recreation, but it obviously has flood control value as well."

There is no doubt the vegetated landscape absorbs rainfall better than developed land. The question is how much.

A study conducted by the Harris County Flood Control District two years ago looked at how water flows from the Cypress Creek watershed to Addicks and Barker reservoirs and shows that sage grass — grass with a similar root structure of the prairie grass found in that area — does a pretty good job of absorbing water given the poor absorption of the area's clay soil.

"I have no doubt that the water the prairie held on to during the Tax Day flood would have eventually wound up in the Addicks or Barker reservoirs," Piancentini of the prairie conservancy said. "And I think anyone would tell you they certainly didn't need anymore than what they got."

Indeed, the Tax Day Flood was one for the record books. On average, communities across Harris County received 7.75 inches of rain. That's equal to about 240 billion gallons of water spread over the entire area.

In Pattison, smack dab in the middle of the Katy Prairie, more than 23 inches of rain fell during a 14-hour period on April 17 and 18, the highest amount ever recorded in the county for a historic rainfall event. That's equivalent to a one-in-a-10,000-year flood, though the definition of such events is quickly changing in a climate change-influenced world.

"Yes, it could have been worse," said Mike Talbott, head of the Harris County Flood Control District. "Still, all the water did fall on the Katy Prairie, and it did flood downstream. The prairie is certainly good at what it does, but it's not magic."

The Katy Prairie is also shrinking. Conservancy leaders estimate the original prairie was between 500,000 and 750,000 acres before the first settlers arrived in 1870. Today, about 20,000 acres have legal protection preserving it from development. The conservancy would like to save an additional 30,000 acres, but it is competing against developers with plans to bring more homes and commercial businesses to the area.

First, they'll need new roads to get there.

Officials with Harris County and the city of Houston now are discussing a long-term highway plan that calls for a 25-mile loop around the prairie.

"With new roads comes new development," Piacentini said. "And while we're not anti-development, we want to protect this land, which is the last and best shield for Houston during floods."