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With Texas Building Boom Comes Higher Flood Risk

Over the years, planners, engineers, water quality experts and others have come to recognize how urban development can drastically alter the landscape and exacerbate flooding.

by Vianna Davila and Brendan Gibbons, San Antonio Express-News / November 29, 2016

(TNS) - The workers watched as a construction crane lowered the concrete beams, 41,000 pounds each, and laid them one-by-one across the bridge bearings that span French Creek at Prue Road, for years a problematic low-water crossing on San Antonio’s booming Northwest Side.

By day’s end, all 45 beams would be in place, part of a $4.4 million Bexar County, Texas, flood control project to raise the bridge 14 feet, widen it by another 15, and dramatically increase the size of the channel below in order to convey more rushing water during storms.

Over the years, planners, engineers, water quality experts and others have come to recognize how urban development can drastically alter the landscape and exacerbate flooding.

The culprit, many experts believe, is impervious cover — the massive buildings, commercial strips and houses in addition to the asphalt and concrete that we walk and drive on every day. By stopping rain from absorbing into the ground, impervious cover increases the volume and speed of runoff from heavy downpours, pushing it in different directions.

Impervious cover is expanding every day, especially in San Antonio, one of the fastest-growing large cities in the U.S. The region’s population is expected to increase by 1.1 million over the next 25 years. Part of Flash Flood Alley, the area sits in the path of intense storms, exposing more people to flooding hazards and destruction.

As development spreads upstream, particularly in the Hill Country, and trees and native grasslands are removed, the water flows downstream are only going to get bigger, said Simon Saenz, superintendent for Capital Excavation, the company building the Prue Road project.

“As long as San Antonio keeps growing, this is gonna continue,” Saenz said, referring to the project. “It will never stop.”

The new bridge is the usual way of dealing with more floodwater.

But now, many experts are looking at different ways to mitigate impervious cover, not only by trying to limit it but also by adopting new landscape and design techniques known as low-impact development, or LID. These projects help mimic natural drainage to get water moving down into the soil and not downstream on the surface.

The city adopted its first LID ordinance in February — but the rules are entirely voluntary. So far, few builders have embraced the method.

Like many cities, San Antonio’s building codes allow developers and property owners to build as much impervious cover as they want in most places, as long as they design structures meant to keep their runoff from affecting their neighbors downstream.

Currently, impervious cover is only limited over the Edwards Aquifer Recharge Zone. Efforts to expand those limits were met with fierce resistance from developers and some City Council members earlier this year.

Still, floods keep happening, damaging private property and roads, and taxpayers end up with the bill. Since 2007, San Antonio and Bexar County combined have spent at least $850 million on drainage: $500 million by the county; $280 million from two city bonds; and another $70 million funded from other city revenue.

That’s only a small fraction of the $2 billion worth of flood control projects needed in the city alone, according to the San Antonio’s Transportation and Capital Improvements Department.

People don’t often think about drainage or stormwater control until the day floodwater comes through their front door or their car is swept away after driving through a low-water crossing. Bexar County has at least 177 such crossings, county data show.

More people died or were injured in floods in Texas than any other state between 1959 and 2008, according to a study co-authored by University of Texas at San Antonio civil engineering Professor Hatim Sharif. In Bexar County, 68 people died in flood-related incidents during that time, more than any other county in the state, the study found.

The volume of water running off from even a small rainstorm can be staggering. A 1-inch rain falling on a typical two-story, 2,500-square-foot house would generate 935 gallons of runoff, estimates from the National Association of Realtors and Texas A&M University show.

It’s only in the past two decades that San Antonio has had drainage regulations; before that, developers were required to do very little to address excess stormwater runoff caused by their projects.

Meanwhile, there’s evidence that flooding is increasing in San Antonio.

“There is a better way to do stormwater management, there is a better way to do planning,” said Troy Dorman, director with the San Antonio office of Tetra Tech, an engineering firm that helped the city develop its low-impact development code. “To be able to effectively add 1 million people in the next 25 years to this community, we’re going to have to be able to do it smarter.”

Since the 1970s, cities and counties have been designing to accommodate 100-year floods — flood events that have a 1 percent chance of happening every year —“and we still have a tremendous amount of flooding that’s occurring,” Dorman said. “The question now becomes, have we adequately defined the risk that we’re designing to?”

Because, Dorman said, there’s no real way to know how all of the development could affect or potentially increase runoff.

“We don’t have the capability to do that,” he said.

What is impervious cover?

The concept of impervious cover is easy enough to explain to a fifth-grader, which is what San Antonio River Authority education coordinator Minna Paul does nearly every day in schools from Bexar County to the Gulf Coast.

She pulls out some simple props, a sponge and a spray bottle. The sponge symbolizes the earth’s natural surface — soil, rocks, plants and trees. She squirts a few pumps of the spray bottle, simulating a rainy day, and points out how the sponge soaks it up.

“I would like to show you another surface that rain falls on,” Paul says, pulling out a piece of purple plastic, representing asphalt and concrete. “What do you think happens when rain falls on these surfaces?”

She sprays the plastic and students watch as water pools, then drips off.

“When rain fell onto the soil, it infiltrated into the ground,” she said. “But what if we completely filled it with this? What if we had streets and parking lots and houses and malls and movie theaters? Where do you think the water is going?”

This simple facsimile is playing out all over San Antonio on a vast scale. Flooding has gotten worse in the city as the San Antonio River’s watershed has been converted from forest, field, farm and ranch land into housing developments, strip malls and parking lots, according to data from the U.S. Geological Survey, which tracks the highest stream flow each year at dozens of stream gauges around the county.

Of the 18 gauges with at least 20 years of measurements, all show an upward trend in peak floods. That trend is most obvious for stream gauges within Loop 410.

One gauge that measures Olmos Creek at Dresden Drive just south of Loop 410 on the North Side shows a steady increase since the 1970s. Peak flow at a gauge on the San Antonio River near Blue Star in Southtown shows higher and higher top flows from the 1910s until 2000, though the Mission Reach river redevelopment project, on the southern leg of the San Antonio River, has since reduced it, USGS data show.

Some think stronger storms are mostly to blame.

When floods spill over those structures designed to hold them, “it’s usually because of our crazy erratic weather,” sasid Afamia Elnakat, an engineering professor at the University of Texas at San Antonio who previously worked for Pape-Dawson Engineers and CH2M.

The county lies in so-called Flash Flood Alley, a crescent-shaped swath of land from Dallas through Austin, San Antonio and Medina County. Warm, moist air from the Gulf of Mexico rises over the Balcones Escarpment and dumps heavy rains that run off the thin soil and fill the steep canyons. Climate models do not clearly predict whether global warming will make rains in this region more severe in the coming decades, according to a study by Texas Tech Professor Katharine Hayhoe.

In a report commissioned by the city, Hayhoe, a climate modeler, wrote that the average amount of rain falling on a wet day in San Antonio has increased. So has the amount of rainfall on the wettest five days of each year. But those trends were not strong enough to be statistically significant in the formal sense, she wrote. Whether rainstorms are expected to become more intense in the coming decades also are unclear, she wrote.

San Antonio has endured three 100-year storms just in the past 18 years: 1998, 2002 and 2013, said Arthur Reinhardt, assistant director of the city’s capital improvements department and head of its stormwater management division. The October 1998 and May 2013 storms dumped roughly 11 and 10 inches of rain, respectively, on the city in a single day and caused catastrophic floods.

But flooding is not the only problem. Study after study has linked more impervious cover to reduced water quality. Stormwater runoff can carry pollutants ranging from pesticides and fertilizer to motor oil, paint solvents and other auto fluids, according to Texas A&M AgriLife Extension’s guide to stormwater management.

In 2015, biologists with the city of Austin and Baylor University looked at the relationship between impervious surfaces and the community of tiny insects and bugs, known as macroinvertebrates, that make up the bottom of a food web that supports fish, birds, reptiles, amphibians and mammals. These insects and bugs also are sensitive to pollution and used by scientists to measure the health of streams.

The researchers found that streams in Austin with more impervious cover were likely to have less diverse macroinvertebrates and fewer of them overall.

Though no one has done this exact study in San Antonio, urban runoff’s harmful effects were obvious in late July.

That’s when runoff from a rainstorm killed 12,000 fish in the Espada Acequia next to the San Antonio River. River authority biologists said they were killed by a rush of stormwater that caused oxygen levels to drop, leaving the fish gasping.

Too much impervious cover also is holding San Antonio back from having swimmable creeks and rivers, according to SARA. Of the 13 stretches of creeks and rivers in Bexar County that the U.S. Environmental Protection Agency has formally rated for human use, all of them have E. coli bacteria levels too high for swimming.

The source, SARA reports, is human, pet and wildlife feces that collect on asphalt and concrete and wash into waterways during heavy rains.

“If you’re wanting to have a quality of life where you’re able to swim, now you’re not able to do that because the water quality is impaired,” SARA watershed engineering manager Patrice Melancon said.

That quality of San Antonio’s river and creeks actually is “pretty darn good” in normal or dry conditions, said Steve Graham, SARA’s assistant general manager.

“Where it all goes to hell in a handbasket is when it rains,” he said.

Deferring flood impacts

Before the adoption of drainage regulations in 1997, San Antonio’s development code barely addressed how developers should handle stormwater or how it would affect properties downstream or rivers and streams.

Last December, the city adopted its first Storm Water Design Criteria Manual, with all of the city’s stormwater and drainage rules collected in one place.

“We filled in gaps in the policy” design requirements, said Reinhardt, with the city. The city’s Unified Development Code went from 15 pages addressing drainage to nearly 300 pages.

San Antonio now takes a wide-scale approach to managing stormwater runoff. Think of it like a traffic intersection, where only so much water is released at a time rather than all at once, Reinhardt said.

It’s a system, Reinhardt says, that lets the city mitigate the impervious cover that’s being added as the city develops.

Two decades of growth

The Interstate 10 and Loop 1604 interchange is one of the fastest-growing areas of San Antonio. See how the type of cover has changed drastically over 21 years from mostly forest and range land to neighborhoods, malls, roads and parking lots.

The regulations are supposed to keep new construction from causing any “adverse impacts” upstream and up to 2,000 feet downstream. They are meant to keep water depth and speed low enough for drivers to be able to proceed with caution in a low water crossing, even in a 100-year storm. Engineers also plan for ultimate development, meaning they must determine the potential flood impact of a new building or subdivision assuming everything downstream and upstream one day will be developed.

In a few parts of the city, developers are required to detain stormwater runoff. Those mandatory detention areas usually are upstream of “known sensitive flooding areas,” Reinhardt said.

Otherwise, the city code gives developers three options to handle stormwater runoff: build a structure on- or off-site to hold increases in runoff, typically a large, concrete basin; participate in the construction of a large-scale, off-site project to handle stormwater from the developer’s area and the surroundings; or, instead, pay a fee of 15 to 25 cents per square foot of impervious cover, depending on the type of construction — but only if the project has no adverse impact downstream and upstream.

This money, called a fee in lieu of on-site detention, or FILO, goes toward funding regional drainage projects.

Overwhelmingly, developers pay into the FILO fund, roughly 65 to 70 percent of the time, Reinhardt said.

In FY 2016, those percentages were even higher: more than 91 percent of the 578 plats and permits that the capital improvements department approved were either eligible to pay into the FILO fund, or the runoff increases the proposed projects were expected to cause were so minimal that the developers didn’t have to pay a fee at all.

Less than 7 percent of the approved development projects are slated to include an on-site, stormwater detention facility. Two percent of the projects are going to provide off-site mitigation.

“It’s easier for someone to pay FILO if they show there’s no impact,” Reinhardt said. “We wanted to encourage people to pay into regional (flood control) projects.”

The drainage review is the longest part of the developer review process, he said. Although the city’s Development Services Department handles building permits, the capital improvements department can delay a project if there are questions about the drainage plan.

Reinhardt acknowledged there is pressure to get reviews done faster but said “we do our best to work with the developers, we try to provide comments as quickly as possible.” Sometimes, other parts of a project can move forward while the drainage plan is hashed out.

“We never would compromise our comments to just approve it before their deadline,” Reinhardt said. “Stormwater design is really a balance of economics and feasibility.”

But the city’s code only requires engineers to study the potential adverse impacts of larger rainfall events, the five-, 25- and 100-year storms.

That means engineers aren’t having to design for the majority of the storms that happen here, which are the smaller ones, Dorman said. Taken cumulatively, he said, those smaller rain events can have much bigger impacts.

“They are the ones that carry the most pollution, they cause the most erosion problems, they often cause the most damage to our infrastructure because they’re more frequent,” Dorman said.

He believes the city’s modeling system simplifies how storms work. When bigger rainfall events happen than what a project has been designed for, the water is going to travel different ways.

“Water doesn’t follow what an engineer tells it to do,” Dorman said. “It follows the path of least resistance.”

A history of flooding

Little thought was given to drainage and flooding nationwide until the late 1950s. The first congressional report officially addressing floodplain management wasn’t issued until 1961, according to a report by the Association of State Floodplain Managers. The National Flood Insurance Act was created in 1969, and the Federal Emergency Management Agency started in 1979.

But in San Antonio, heavy downpours forced the city to confront the flooding in the early 20th century through massive taxpayer-funded projects.

From 1914 to 1921, the city was hit by six major floods, including one in 1921 that covered parts of downtown in 2 feet of water, according to historical documents complied by Gregg Eckhardt, a San Antonio Water System environmental scientist and amateur historian.

By 1926, the city had constructed Olmos Dam at a cost of more than $21 million in today’s dollars, according to Eckhardt’s website, The city also built the cutoff channel that connected the two legs of the horseshoe-shaped bend that’s now the River Walk.

As the city grew and farms and ranches north of downtown turned to concrete and asphalt, not even Olmos Dam was enough.

In the 1990s, the city and federal government spent $111 million drilling a 24-foot-wide tunnel 150 feet below ground. It stretches 3 miles from an intake on East Ashby Place near the Pearl to Lone Star Boulevard in Southtown, where water gushes up from the depths during floods.

But these projects only could protect downtown. San Antonio had no real drainage regulations until the late 1990s, which explains why there are so many flooding issues in older parts of the city outside downtown.

An example of one such older neighborhood is Barbara Drive and Shannon Lee, streets in central San Antonio north of the Olmos Basin, where many homes were flooded during the 2013 Memorial Day weekend flood that killed three people.

In response, the city bought nearly three dozen homes on the two streets that it demolished to make way for Bexar County-funded drainage upgrades.

The homes were built in the 1950s, and the drainage system probably was too small and wouldn’t meet today’s codes, Reinhardt said.

At that time, the low point was the street, and that was how the builders went about draining the water away from the properties, because there were no regulations to force them to account for what that water did once it left the property.

“Our grid development through the 1940s, ’50s and ’60s that’s primarily inside of Loop 410 did not respect the natural floodplain, the grid just kept going over it,” said Gene Dawson Jr., president of Pape-Dawson Engineers, a prominent San Antonio civil engineering firm. Over the next 50 years, the city had to spend money to address those past mistakes, he said.

Now, Dawson said, “We aren’t recreating the problem that we have to come back and fix again,” adding that “San Antonio already has a very robust drainage regulation that deals with water quantity.”

Dorman, the engineer with Tetra Tech, disagrees. The kinds of designs that ignore natural features still are part of cities’ flooding problems today. Streets in San Antonio still are used to convey water, and subdivision designs still are not required to follow the natural contour of the land, he said.

“We have to leave the streams,” Dorman said. “The streams are what move the flood off of our developed land without causing damage.”

Good flood control also means good land-use planning, he said. It will mean building more densely and rethinking homeowners’ desire for big lawns. Lawns without trees or underbrush will not absorb or intercept water runoff the way a natural landscape will, he said.

“When we develop property, what (does) the community get out of the development of that property? What’s the benefit for the community? As we develop property, one of the important goals is to maximize the value for the community long-term, not just when the houses are built,” Dorman said.

Trying to address problems

In the 21 years that Dave McConkey has lived on Rose Blossom Street on the Northeast Side, his backyard and an addition to his home have flooded at least a dozen times.

The problem is so bad, McConkey has had to take flood control into his own hands: he leaned rocks against his back fence. He dug a small trench around his patio to divert the water. He lifted his utility shed and set it on a flat, wooden platform. He set up bags of mulch around the back addition.

That last idea worked for a little while, but eventually he settled on a permanent solution and lined the outside walls with cinder blocks, stuck together with Portland cement. Around the garage, McConkey erected a very low wall of landscape bricks to help deflect the water.

“I could almost take my Waverunner into the backyard,” he said. “I can’t tell you how many times my garage has been full of water, and my back room is ruined.” McConkey has thought about redoing his landscaping, but, “what’s the point?”

The city now plans to overhaul an L-shaped drainage channel in an alley behind McConkey’s house. The channel, which turns at a sharp, 90-degree angle, probably was built too small to handle the speed of water flowing down the drainage system, Reinhardt said. The water moves especially fast because there’s a roughly 30-foot change in elevation between the top of the channel and the bottom, causing the walls of the ditch to erode over time as sediment has washed down the funnel.

The $1.4 million needed for that work could come from the 2017 bond, a $144 million proposal for drainage projects that will be decided by voters in May.

This project exemplifies the challenge for city and county officials, particularly when dealing with older infrastructure. Because the project is so old, built before modern-day drainage regulations, the city has virtually no recourse to force the original developer to address the problem.

Problems in the Blossom Park subdivision, as they are in many neighborhoods, also are exacerbated by development in the area off Jones Maltsberger and Starcrest Drive.

McConkey believes that Time Warner Cable, which expanded its footprint behind his house, is the real source of his flood problems.

But Reinhardt said any of the more recent development in the area could have contributed. The neighborhood is also on the downstream end of a large watershed. Time Warner Cable built detention basins, approved by the city in 2002 and 2008, which should ensure the runoff is captured and filters out more slowly.

“At no time since have we or the property owner been informed of flooding issues related to the facility,” said Brian Anderson, a spokesman for the company.

But it’s up to private developers, owners and homeowners’ associations to maintain the types of detention basins Time Warner added once they’re built. The city only will learn about faulty basins, or basins that aren’t being maintained, if Code Compliance is called.

What’s next?

Adjusting existing city ordinances to better account for impervious cover so far has been a challenge.

In 2012, the development industry representatives pushed back against the city’s proposal to revise its stormwater utility fee, so charges were based on a property’s amount of impervious cover, not just the property’s size. That’s because developers disagreed with the way the city calculated the fees and set up the fee structure, Dawson said.

“Any time there’s new regulations or an increase of cost, everybody’s going to object,” he said.

At the time, 55 of the 91 cities and counties in the state that collect a stormwater utility fee, including Austin, Dallas and Houston, based their rates on impervious cover, Reinhardt said.

So the city came up with a compromise, passed in August 2015, a so-called hybrid method for nonresidential properties that includes a base fee and a separate, impervious cover fee.

This past summer, the development industry fought hard again, against a proposal in the city’s SA Tomorrow Sustainability Plan that suggested possibly expanding the city’s impervious cover rules beyond the Edwards Aquifer recharge zone. Nothing in the plan was binding, but the San Antonio Real Estate Council lobbied against it, issuing a statement that said it is aware of “no scientific rationale” to support limiting asphalt and concrete.

In the end, city staff and City Council compromised and softened the plan language, agreeing to create a task force to study the possibility of expanding the impervious cover regulations.

The idea to reduce impervious cover is unpopular with the development community because of the financial impact, Dawson said, meaning property owners have to build less on their land.

He thinks impervious cover shouldn’t be limited in order to manage development or growth, but rather to manage and improve the region’s water quality. Instead of trying to reduce the hard surface cover on individual sites, he said the region should take a broader, watershed approach.

Dawson also thinks impervious cover limits must be enacted in combination with water quality features like LID.

Some cities have instituted strict rules on impervious cover. The regional planning agency for Lake Tahoe, for example, limits impervious cover from 30 percent of a parcel’s land area down to 1 percent, depending on the characteristics, to protect water quality of the nation’s deepest lake. In Portland, Oregon, property owners must allow as much water to infiltrate the ground on their land as possible.

Graham, the SARA assistant general manager, thinks the kind of mandatory policies used in Lake Tahoe and Portland will not be passed in San Antonio. Instead, SARA is focused on inspiring others to adopt LID practices on their own. To lead by example, it has recently converted its Euclid Avenue and Guenther Street offices to absorb stormwater through rain gardens.

“Let’s keep it voluntary,” Graham said. “If it evolves into something else, that’s fine, but let’s make it a public, transparent discussion to get there. If through that public process we keep things the same, I’d be disappointed, but that’s where we are.”

How will we know if it works?

“If you add a million people and flooding is no worse, that’s a success,” Graham said.


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