Many county waterways unmapped for flood risk
Lack of analysis leaves thousands in the dark about possible danger
Nearly half of Harris County’s 2,500 miles of rivers, bayous and drainage ditches do not have mapped flood plains, according to data from the county, leaving an inaccurate picture of flood risk for thousands of residents.
In most cases, the unmapped channels are smaller than prominent waterways such as Houston’s well-known bayous and rivers. Instead they are drainage ditches and narrow canals dividing homes and subdivisions spreading all across the county — the legacy of decades of development, where exceptionally flat topography required the construction of man-made channels to drain stormwater into Galveston Bay and eventually the Gulf of Mexico.
They largely have evaded the eye of authorities seeking to study and communicate flood-risk around Houston. Officials point to several reasons they have not been examined: tight funding and technology that has ham-
pered effective mapping.
As a result, thousands of people may be living in areas at a higher risk of flooding without knowing it. While there is no way to accurately gauge the impact of the lack of flood plain maps, or what potential maps would look like, an analysis of homes flooded during Hurricane Harvey shows that nearly 6,000 homes damaged during the storm were outside any mapped flood plains, but within onetenth of a mile of an unmapped stream.
Harris County Judge Ed Emmett said the smaller waterways need to be studied.
“We can’t do things the way they were done in the past,” he said. “We’re in a new world now, where we’ve got to know more about how flooding occurs, where it’s going to occur.”
One-hundred-year flood plain maps are meant to show how water would spill out of a waterway’s banks during a 100-year storm, or one that has a 1-percent chance of occurring in any year. A 100-year storm equates to 12 to 14 inches of rainfall in a 24-hour period across the county. Flood plain maps also are drawn for more severe events: A 500-year rainfall has a 0.2 percent chance of occurring in any year and equates to between 17 and 20 inches in 24 hours.
Rebuilding again
The maps serve as critical measures of flood risk across the United States, informing decisions of where to buy or develop homes, as well as triggering a litany of regulations: New construction must be built above flood levels within the flood plains. Homeowners must have flood insurance for federally backed mortgages. Insurance rates inside the flood plain rise.
Just last week, the city of Houston joined Harris County in raising the elevation requirement for new construction in flood plains, moves prompted by Harvey’s widespread damage.
Both the 100- and 500-year maps likely will be redrawn and expanded after a National Oceanic and Atmospheric Administration rainfall study releases new data in September. Preliminary data indicated that rainfall was being significantly underestimated — the 100-year rainfall classification could rise several inches, meaning more storm water spilling out of banks, as well as wider and deeper flood plains.
Even the new maps, however, will underestimate the true risk of flooding in Harris County.
A Houston Chronicle analysis recently found that during Harvey, almost three-quarters of the 204,000 flooded homes and apartment buildings in Harris County were outside the federally regulated 100-year flood plain. More than 55 percent of the homes damaged during the 2016 Tax Day storm sat outside the flood plain, as did more than a third of those during the Memorial Day floods in 2015.
The data reflects that flooding is caused not only by how water spills out of bayous, but how it drains into them. Outdated pipes and narrow streets, water flowing across large swaths of land and subtle-to-significant changes in elevation all influence flood damage and are not captured by flood plain maps.
“Just because it’s not on the map doesn’t mean it’s not a problem,” said Wesley Highfield, a Texas A&M University Galveston professor who has studied urban flooding.
A few feet behind Tyrone and Carolyn Brown’s home near C.E. King High School in northeast Harris County, a shaded canal runs from Greens Bayou in the west nearly to Beltway 8 in the east. The canal, which drains stormwater to the bayou, does not have a mapped flood plain.
In 2001, during Tropical Storm Allison, water rose out of the canal and met water flowing toward it right at the Browns’ home. Their house was destroyed. When they rebuilt, the couple added a second story so they could weather a possible future flood in their home. The retired U.S. Postal Service workers said they were told they did not need to purchase flood insurance because their home was not in a flood zone.
Sixteen years later, Harvey hit, and the same thing happened, this time with a lot more water. The Browns are looking to repair more than $100,000 in damage to their home, largely through a Small Business Administration loan. Navigating repairs, the federal government and contractors has been exhausting, Carolyn Brown said, especially after Tyrone Brown was diagnosed with cancer in December.
“It’s not just physical,” Carolyn Brown said. “You’ve got to start all over again. It’s a mental state of ‘How are we going to do this?’ ”
New studies to come
Todd Ward, a program manager for the Harris County Flood Control District, said that until recently, the technology and methods were not available to calculate the risk around smaller channels.
A Federal Emergency Management Agency spokeswoman said in a statement that FEMA works with local agencies such as the flood control district in determining which areas are studied and classified as flood zones. She said the agency prioritizes studies based on population nearby, new development and whether flood control projects have been completed there.
Ward said the flood control district, as part of a widespread remapping effort, is planning to study the smaller channels.
“We’re going to be calculating the flows in a different way, that’s more appropriate of drainage areas of that size,” Ward said. “For those residents who live near unstudied streams, they don’t know what the risk associated with that stream is.”
That remapping also would include other ways people face flood risk, such as ponding or sheet flow.
That effort likely will take several years. The flood control district expects to finish new maps in two years or more for half of the county, Ward said. Then homeowners, developers and others can challenge the new maps through an administrative appeals process, and many do, as development and insurance costs rise in flood plains.
It is unclear how long that could take.
And it will have to be updated continually, Emmett said.
“I think it’s something we will constantly be working on and improving,” he said. “We constantly are building and changing the topography.”