Failure to control storm water makes floods more likely

In the months after September’s epic floods, the AJC interviewed more than 60 homeowners, local and state officials, engineers and policy experts and reviewed state storm water reports for 15 metro governments. The AJC also analyzed development’s impact on metro streams using 10 years of stream gauge data from the U.S. Geological Survey.

The findings show metro streams are rising earlier and higher than in the past. Such an intense shift wasn’t inevitable. Missed opportunities during the building boom helped make the area more susceptible to washouts than ever before. For example:

● Before 2005, Cherokee County didn’t have a civil engineer perform detailed reviews of how water flows across subdivisions before approving them, potentially missing flooding problems and leaving homebuyers in the dark. In the high-growth year of 2004 alone, the county issued more than 4,000 residential building permits.

● Roswell’s backlog of major storm water projects — such as replacing corroded lines that can cause sinkholes under streets — climbed to 18 years from 13 years after September’s floods. The city has limited cash for repairs, which run $400,000 to $500,000 a year simply for pipe replacements.

● In counties such as Cobb, Fulton and Gwinnett, houses continued to rise in areas at higher risk of flooding in recent years. While some of the homes sit atop dirt fill elevating them above the flood plain, experts say that approach often just worsens flooding elsewhere.

“We tell people we’re one 4-inch rainfall away from all hell breaking loose,” said Ron Feldner, an engineer and storm water consultant in Roswell who works with local governments.

Floods carry a high cost to both homeowners and taxpayers, who subsidize aid as well as federal flood insurance. Across Georgia, flood claims have paid policy-holders more than $225 million since 1978. Nearly half of that was paid out last year. The floods in September caused 10 deaths and damaged $500 million worth of property.

Careful planning in new developments can minimize overflows from all but the most extreme storms, experts say. Well-maintained infrastructure can whisk water safely from homes.

But for decades, metro area governments’ efforts to control flooding didn’t go far enough, interviews and records show.

Planning officials grappling with the building boom weren’t required to ask whether new developments would flood others downstream. Developers won approval to build in flood-prone areas. And public works crews and subdivisions neglected rainfall-detention ponds, pipes and storm drains that desperately needed costly maintenance.

Now, metro cities and counties are scrambling to catch up.

“We know that throughout the state of Georgia there are probably billions of dollars needed for storm water repairs,” said Sam Olens, Cobb County Commission chairman and former chairman of the Atlanta Regional Commission.

In the past few years, metro governments have adopted ordinances that aim to reduce flooding downstream of developments and construction in the flood plain. Some cities and counties improved their storm water programs.

But the changes came slowly in a state where all land-use decisions are local and politicians often hesitate to restrict commerce. Controlling runoff better can add time and money to developers’ projects. Years of drought made flooding seem a distant threat.

With metro building permits peaking in 2004 and falling sharply by 2007, the bulk of the boom had passed by the time some local governments took a tougher stance.

“We were 10 years too late,” Feldner said.

Hard lesson about surfaces

The link between development and flooding is clear in data showing how streams changed as more streets, parking lots and buildings covered the ground nearby.

In metro Atlanta, the area blanketed by hard, man-made solids such as asphalt — what experts call impervious surface — grew by 20 percent over the past decade, the AJC’s analysis shows, replacing acres of absorbent grasslands and forests.

Over the past decade, regional development added 91,000 acres of impervious surface — the equivalent of nearly 69,000 football fields. The growth was most dramatic in northern suburbs in Paulding, Cherokee, Forsyth and Gwinnett counties, maps show. In one part of north Gwinnett and Forsyth along the Chattahoochee River, impervious surface grew 71 percent.

Such changes to the land had a profound effect on streams. Rainfall runs more quickly off pavement and rooftops than off natural areas. Less water evaporates or soaks into the ground when plants aren’t there to hold it.

As a result, metro streams near built-up areas carried higher volumes of water and swelled more quickly when it rained, driving up the chances of flash floods, the AJC’s analysis of Geological Survey data shows.

A tipping point emerged when impervious surface covered 30 percent of the land. Each new acre of development in such areas caused a more extreme response from streams, with even higher, earlier peaks in flow. Experts say such streams are “flashier.”

For example, Suwanee Creek in Gwinnett rose just under 2 inches in the first hour of rain one day in June 2002. A similar storm hit in March 2009, after more than 30 percent of the land nearby had been developed. The creek rose 5 inches the first hour and kept climbing.

.

A ‘river’ runs through it

One Cherokee County family learned in 2008 how storm water problems can slide by unnoticed during reviews of development plans. The Beardens were dismayed to see “a river” of water cutting across their front lawn and driveway the first time it rained on their brand-new home.

“The water was so wide that our son could not get to the school bus,” Laura Bearden said. The flooding covered the 8-year-old’s shoes. “My husband would put him in a truck and drive him down the driveway.”

Since then, the family has spent $20,000 re-grading the yard and installing drainage. Bearden said a hydrologist she hired quickly spotted flaws in the original plans approved by county. “The information was in their hands,” she said.

Will Martin, Cherokee’s storm water engineer, said the county didn’t perform a detailed review of the hydrology study the developer submitted before approving the Royal Oaks subdivision.

While the county does such reviews now, it still relies on developers’ engineers. Martin said any dispute is between the Beardens and the builder. “When we approve the plans and approve the work, we’re not putting a warranty on the engineer’s plans,” Martin said.

Builder James Cagle of Riverstone Homes and engineer Ken Trevor said the subdivision went up in dry years, and they didn’t know the yard would flood, either. The water flows from nearby properties, they said. “The county agreed everything was done that was supposed to be done,” Cagle said.

Engineers try to strike a balance between keeping developers’ costs down and not worsening flooding, Trevor said. The new storm water rules are complex, however, and difficult to put into practice, he said.

“You go with the rules you have at the time,” Trevor said, “whether it’s overkill or under-kill, who knows?”

Scrutinizing developers

Cherokee and other metro counties and cities stepped up scrutiny of development plans after the regional Metropolitan North Georgia Water Planning District required them to.

The state Legislature created the district in 2001 to tackle water-related problems such as runoff. The move was sparked by increasingly stringent federal and state mandates to decrease pollution in streams, which are conduits for rainfall dirtied by streets, houses and businesses.

The district’s storm water efforts have included creating model flood-control ordinances that nearly all local governments have adopted. Kit Dunlap, the district’s chairwoman, said it has made progress in a short amount of time.

“We do have a plan, and we know we need to get better,” said Dunlap, who is also CEO of the Greater Hall Chamber of Commerce.

Yet some say the new rules don’t go far enough.

Because local officials do not redo developers’ hydrology studies to check them, planners can’t be sure storm water control is adequate, said Stuart Teague, a Cumming attorney. He has represented builders, developers and homeowners such as the Beardens in flooding and development cases for nearly 20 years.

“Developers make mistakes all the time,” he said. “They design a lot of detention that just doesn’t work.”

Georgia environmental attorney Don Stack, whose clients have included hundreds of homeowners with flooding problems related to construction and development, said sometimes developers don’t consider options that would reduce flooding — such as leaving some land untouched — because they would reduce profits.

Designs fail and construction mistakes happen, conceded Michael E. Paris, CEO of the developers’ group the Council for Quality Growth. But, he said, developers have grown more diligent about managing storm water. The task isn’t easy.

“It’s sort of a legendary push and pull between these ordinances and the practical application,” Paris said. “There’s always trade-offs that have to be examined in terms of the cost and effectiveness.”

Complicating matters, Feldner, the engineer, said local officials sometimes don’t know what to require from a developer because they don’t understand how their streams and creeks behave when it rains.

That’s like planners not knowing where all the county’s streets are or how much traffic they carry, he said. “We time all the lights, but we don’t time all the detention ponds,” he said.

In addition, new rooftops continued to appear in the 100-year flood plain in recent years, maps show.

Owners are supposed to buy flood insurance for such homes, which have a 1 percent chance of flooding in a year. The odds rise to 26 percent over the life of a 30-year mortgage, according to the Geological Survey.

Homeowners who don’t buy insurance can find themselves out tens of thousands of dollars if a flood damages their home.

Restricting construction in flood plains would head off costly losses. But local officials haven’t always done so.

For one, the flood plain changes so rapidly in high-growth areas that federal flood maps quickly become outdated as new impervious surface makes streams more prone to flash floods, said Steve Haubner, a water resources engineer for the Atlanta regional group.

Furthermore, federally mandated regulations for the flood insurance program don’t ban development in the plain.

In recent years, most metro Atlanta cities and counties have adopted a new model ordinance that gives them more power to restrict such risky construction. The law requires local officials to determine how development will expand the flood plain and makes it harder to build there. Builders sometimes use dirt fill to elevate structures out of the plain, for instance, but that can cause flooding elsewhere, Haubner said. The new rules also allow local officials to deny projects if they are likely to worsen flooding off-site.

Regional planners hoped cities and counties would use the new information to identify properties in the greatest danger and warn residents they need flood insurance. But Haubner said he didn’t know of any metro governments that had. Many aren’t finished mapping their flood plains.

It’s impossible to prepare for a storm of the epic nature of the September floods, he added. But problems are predictable in the highest-risk areas. “There’s no excuse for why folks weren’t informed and educated about their situation, ” he said.

What now?

Though it was late coming, the metro region has been improving its storm water controls in recent years. Here are examples of what communities have done here and in other parts of the country.

Among the improvements in the region:

● New laws that require local governments to study their flood plains to better identify properties at risk.

● New laws that require developers to better control runoff downstream.

● Growing use of storm water utilities to fund repairs and upgrades of drainage features.

Other areas of the country are introducing creative measures, including:

● Laws requiring that more water be captured on site.

● The use of trees and other plants to soak up rainwater on site.

● Extensive use of porous pavement.

● Using plants on rooftops to absorb rain.

How we got the story

For this story, The Atlanta Journal-Constitution interviewed more than 60 homeowners, local and state officials, engineers and policy experts, and reviewed state storm water permits and annual reports filed by 15 metro Atlanta cities and counties.

The newspaper performed its own analysis of the effect of development on metro Atlanta streams, using monitoring data provided by the U.S. Geological Survey.

The analysis looked at the effect of impervious surface – hard surfaces such as pavement or roofs that prevent rainwater from soaking into the soil – on stream flashiness. Flashiness is the speed and height of peak stream flows during a rainstorm. The more “flashy” the stream, the more susceptible it is to flash flooding.

The AJC estimated impervious surface from Atlanta Regional Commission land use maps using a land use model that set an average impervious surface percent for different land uses. For example, single-family residential areas average 20 percent impervious surface, while commercial areas average 85 percent impervious surface.

To measure stream flashiness, the AJC used the Richards-Baker flashiness index, computed by dividing the sum of differences between daily median stream flow by the sum of daily averages for each year, 1999 through 2009. The analysis used daily stream flow data from 19 Geological Survey stream gauges in the metro area.

The analysis found that differences in impervious surface explain more than 70 percent of differences in stream flashiness among the 19 gauge sites. The analysis also found that, as impervious surface reached 30 percent or more, it had an increasingly dramatic impact on stream flashiness.