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Is Flood Frequency Really Increasing?

By Debbie Z. Harwell, PhD, Editor of Houston History Magazine

Many claims have been made about increasing local flood frequency. They raise the question, “How accurate are those statements?” The report “Significant Houston Area Floods” by Jill F. Hasling, CCM, offers an interesting list for analysis that ranges from April 1837 through February 2019 (two more floods have since occurred in May and September 2019).

Increase in Flood Frequency

Our first flood took place in April 1837, just eight months after Houston was founded at the confluence of Buffalo and White Oak Bayous. Six months later another flood found Main Street under four feet of water. But Houstonians persisted…and so did the flooding.

In the first 100 years following Houston’s founding, it experienced 36 floods, in the next 82 years, it has seen an additional 146 floods, or four times as many as in the first century.

Broken down into 30-year periods, the trend looks like this. Note that only 3 years exist in the last column so far and it already has more floods than each of the first two 30-year periods.

*Note: Last column contains only three years and already has more floods than each of the first two thirty-year periods.*

Percentage of Tropical Vs. Non-Tropical Floods

One might think that our location along the Gulf Coast and the tropical systems that come ashore are to blame for this phenomenon, but tropical systems account for only 15% (27) of these events. The other 85% (155) were caused by rain that either fell in large quantities in a short period of time or lingered in the area for multiple days; this includes 22 winter storms.

Early Flood Mitigation Efforts

With the exception of the 1900 Storm, which hit Galveston but also caused fatalities and flooding on the mainland, the worst of Houston’s early floods occurred in 1929 and 1935 (watch video), causing multiple deaths and wiping out homes, businesses, bridges, and the main water plant. With two back to back floods of this magnitude it was time to take action.

In the midst of the New Deal, the federal government put the Army Corps of Engineers in the flood control business. Houston benefited with funding for the Barker and Addicks Reservoirs, completed in 1946 and 1948 respectively, after a delay during World War II. Although creation of two proposed drainage channels north and south of Buffalo Bayou to relieve flooding did not come to fruition, the reservoirs brought some relief along Buffalo Bayou.

Influence of Urban Growth on Flooding

As Houston grew, so did it’s flooding problem, going beyond Buffalo and White Oak Bayous to include the other 22 Harris County watersheds. By 1983, Houston floods regularly saw the number of inundated homes reach into the thousands. In 1994, ninety subdivisions, including 3,400 homes, flooded. This flood was considered the benchmark for many Kingwood residents to determine the probability that their home might flood in the future. As a result, many did not have flood insurance when Harvey hit because they believed they were safe since their home did not have water in 1994.

Today some people point fingers at others, saying their flooding problem is of their own making because they built or bought a house in the floodplain, when in fact the area where their homes are located did not have a flooding problem years earlier. Rather, development around them or upstream created issues. (View Kinder Institute’s interactive map of Houston development.)

This has been documented in Meyerland in the last four years, and the most recent floods in Kingwood’s Elm Grove in May and September 2019 also make a similar case. The Elm Grove homes had never flooded and did not flood in Harvey, but now they have flooded twice in four months with the water levels increasing. Although a definitive cause for the May flood is in litigation, homeowners believe development north of them created the flooding problem. Sadly some of these residents also did not have flood insurance because they figured they were safe after Harvey.

How Quickly We Forget!

Increasingly, these rain events leave our infrastructure overtaxed, whether trying to handle street runoff or rising water in our bayous, streams, and rivers. It seems, though, that many people have let flooding fall off their radar if they were not personally impacted or time has passed, putting flooding out of sight, out of mind.

When the Kinder Institute asked Houstonians in its annual survey what they thought was the “biggest problem in Houston” prior to Harvey but after the Memorial Day 2015 and Tax Day 2016 floods, only 1% spontaneously replied “flooding.” That number grew to 15% in 2018 when asked post-Harvey. But when surveyed in 2019, with only street flooding the year before, the number who identified flooding as our biggest problem dropped down to 7%. Traffic was the leader, going from 24% in 2017 to 36% in 2019.

The list of Houston area floods clearly shows that Houston has experienced more frequent flooding of an increasingly serious nature. Everyone thought Tropical Storm Allison was “off the charts” until Harvey hit. Imelda was not as bad as Harvey, but in just two short days it managed to be the seventh leading rain event in the nation.

Need for New Ways to Address Flooding

Major floods in four of the last five years demonstrate that the old ways of addressing flooding a little at a time, or doing nothing at all, are not adequate to protect our families, homes, and businesses and maintain our quality of life in Houston.

Debbie Z. Harwell, Ph.D.
Editor, Houston History
Instructional Assistant Faculty
University of Houston

Posted on 10/28/2019 by Debbie Z. Harwell, PhD with help from Bob Rehak

790 Days since Hurricane Harvey and 39 since Imelda

Study Suggests Large Cities Like Houston Can Intensify Rainfall and Runoff From Hurricanes

A November 2018 article appearing in the peer-reviewed scientific journal Nature found that urban growth can intensify both rainfall and runoff from hurricanes. Further, urban growth can increase the risk of flooding and shift the location of flooding. The article specifically studied the effects of Hurricane Harvey on Houston and found that urban growth increased the probability of such an extreme flood across the basin by 21X.

A sister publication, Scientific American, reviewed the article the same month and helped explain the findings in Nature.

The Nature study looks at two distinct effects of urbanization. The first is the impact of impervious surface on RUNOFF. The second is the impact of the urban landscape’s surface roughness on RAINFALL.

The Runoff Component

Numerous studies have looked at the relationship between percentage of impervious cover, runoff, and flooding – a well documented phenomenon. Impervious cover accelerates transport of rainfall from neighborhoods to rivers. That raises peak flows rather than spreading them out over time. Dr. William Dupre, professor emeritus from the University of Houston visualized the relationship this way.

*Effect of Urbanization on Peak Stream Flows” by Dr. William Dupre, professor emeritus from the University of Houston.*

Rainfall Component Much Less Studied

However, the effect of urban growth and a city’s surface topography on RAINFALL from hurricanes is much less studied. The authors say in Nature that, “Urbanization led to an amplification of the total rainfall along with a shift in the location of the maximum rainfall.” (Page 386).

“Much less is known regarding the urban effects on the organized tropical rainfall of a hurricane, in particular during one like hurricane Harvey, which stalled for several days.” They continue, “…experiments (with computer models) clearly show a large increase in rainfall arising from urbanization over the eastern part of the Houston area.”

The authors compared present and past urban landscapes and also modeled a scenario in which the entire region was cropland.

Mechanisms Responsible for Increase Rainfall

To understand the physical mechanisms responsible for the heavier rainfall, they analyzed the vertical convergence of winds and wind fields.

Kingwood Greens Evacuation During Harvey by Jay Muscat — *Evacuation During Harvey. Photo courtesy of Jay Muscat*.

“The enhanced rainfall … and the shift of rainfall … are tied to the storm system’s drag induced by large surface roughness,” say the authors.

Scientific American explains in more detail. Kerry Emanuel, an atmospheric scientist at Massachusetts Institute of Technology, who did not work on the study said, “We know cyclones are sensitive to characteristics of the surface—mountains, streams, marshland. This new twist is that cities have become big enough to tangibly alter the storm.” Said Gabriele Villarini, an environmental engineer at The University of Iowa and an author on the study, “We removed the urban areas from Houston and replaced them with cropland.”

“The presence of urban areas enhanced all the things you need to get heavy precipitation,” Villarini, one of the study’s authors says. “A stronger drag on the storm winds, associated with a larger surface roughness length” contributed to the increased rainfall.

Emanuel explained, “First, the artificial ruggedness of an urban area slows air down. Whenever air slows in a hurricane, he says, it gets shunted toward the center of the storm and up into the sky. That increases rainfall everywhere [in a metropolitan area].” He added, “A storm moves particularly slowly over downtown areas where buildings are tallest, but the winds bearing down from outside the city are still moving quickly. So, [the storm] is piling up on the city.”

Impact on and Implications for Houston

This increase in urban growth in flat terrain creates problems from a flood perspective, despite mitigation measures already in place.

Urbanization has increased the probability of an event like the flooding associated with Hurricane Harvey by about 21 times, say the authors in Nature on page 388.

The authors make several high-level recommendations.

Urban planning must take into account the compounded nature of the risk now recognized.
Flood mitigation strategies must recognize the effect of urbanization on hurricanes.
Weather and climate models must incorporate the effects of urbanization to increase forecast accuracy on local and regional levels.

“It is critical for the next generations of global climate models to be able to resolve the urban areas and their associated processes,” conclude the authors.

About the Authors and Models

The authors are:

Wei Zhang and Gabriele Villarini from the Department of Hydroscience & Engineering, The University of Iowa, Iowa City, IA
Gabriel A. Vecchi from the Department of Geosciences, Princeton University and the Princeton Environmental Institute, of Princeton, NJ
James A. Smith from the Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ.

This presentation explains the Noah Model that the authors used to calculate air/ground interactions.

Local Questions Raised by Study

To date, the role of a city in altering rainfall during tropical cyclones has received very little attention. Houston has had the largest urban growth and the fifth-largest population growth in the United States in the period from 2001–2011. Much of that growth is now on the periphery of the city. The two fastest growing parts of the region are Fort Bend and Montgomery Counties.

As the city grows, we need mitigation measures that can offset the impact of that growth. That’s why the meeting of the Montgomery County Commissions on August 27th is so important. They will vote on whether to close a loophole that allows developers to avoid building onsite detention ponds. Closing that loophole is important. It will help protect hundreds of thousands of downstream residents as well as those in Montgomery County.

Also, the new NOAA Atlas-14 (rainfall measurements updated after Harvey) does not consider forward-looking urban growth effects. The precipitation frequency data in NOAA Atlas 14 was determined by a statistical analysis of historical rainfall, a key input for FEMA Flood Insurance Rate Map (FIRM) modeling. With all that uncertainty, we need to err on the side of caution in flood planning.

For more about Atlas 14, see this link.

Posted by Bob Rehak on 8/18/2019

618 Days after Hurricane Harvey

Guide to Lake Houston Area Floodplain Regulations

Guidelines for floodplain development can bewilder even professionals. Overlapping jurisdictions often have different guidelines. And guidelines often change, as Houston’s just did. Houston now manages the 100-year and 500-year floodplains differently. Cities also have building codes that include more requirements.

Site of the proposed new marina and high rise development. Shot from over the West Fork shortly after Harvey. Note sand deposited by Harvey. 25 and 50-story high-rises would be built on the narrow strip between the lake and the Barrington at the top of frame.

Overview

People ARE generally allowed to build and place fill in floodplains. However, they must follow local floodplain guidelines and obtain permits that restrict what they can do. They must also submit environmental surveys, mitigate wetlands, and provide hydrologic and hydraulic studies. In Houston, they may move earth from one location to another within a floodplain, but not add to the total volume. The general rule of thumb: zero negative impact on the conveyance of the river.

If a development destroys wetlands, wetland credits must be purchased from a mitigation bank. Mitigation banks place conservation easements on some of our most valuable wetlands. By helping to finance conservation of those areas, destruction of less valuable wetlands elsewhere may be permitted. Generally but not always, the mitigation credits must be within the same watershed. However, this is not always the case. Extenuating circumstances may exist.

KSA once considered placing East End Park in a mitigation bank as a way to help finance its long range parks plan. The conservation easement would ensure that the character of the park never changed. And the money raised would have provided needed improvements to other parks at no cost to residents.

Federal Guidelines and How They Relate to Local

FEMA establishes minimum standards for a community to enroll in the National Flood Insurance Program (NFIP). By enrolling and administering floodplain regulations, it allows their residents the opportunity to purchase Flood Insurance through the NFIP. You must at least build at FEMA’s base flood elevation (BFE). But communities can and do set higher standards. And each may have different guidelines.

Engineers and regulators often talk about “freeboard factors.” Freeboard, a nautical term, means “the height of a ships side between the waterline and the deck.” In a flooding context, freeboard means minimum elevation above the BFE. You often see it described as “BFE + 1 ft.” Or 2 feet. Or X feet. Think of it as a safety margin. Any freeboard above the BFE is considered a local community’s higher standard.

To provide a context, below are links to some of the floodplain management orders/ordinances.

Houston Guidelines

Houston Ordinance

HOW Ordinance is Executed

Note Chapters 9 and 13. They changed on September 1, 2018. Changes address building code issues for FEMA X zones. Zone X includes the 500 year flood plain. Many such areas flooded during Harvey.

Army Corps

If a development affects a major waterway like the San Jacinto River, its wetlands, its flow, or endangered wildlife, the Army Corps will also review studies submitted as part of the permitting process. They would look at applications from the point of view of the EPA and Clean Water Act, especially Section 404. Section 404 of the Clean Water Act (CWA) establishes a program to regulate the discharge of dredged or fill material into waters of the United States, including wetlands. … For most discharges that will have only minimal adverse effects, a general permit may be suitable. This is the major focus of the permitting process now underway for the high-rise development in Kingwood.

TCEQ

The Clean Water Act also contains a section 401. It specifically focuses on how States and Tribes can use their water quality standards in Section 401 certifications to protect wetlands. States and Tribes can review and approve, condition, or deny any Federal permits or licenses that may result in a discharge to waters of United States within their borders, including wetlands. States and Tribes make their decisions to deny, certify, or condition permits or licenses primarily by ensuring the activity will comply with applicable water quality standards. In addition, States and Tribes look at whether the activity will violate effluent limitations, new source performance standards, toxic pollutants restrictions and other water resource requirements of State or Tribal law.

Jurisdictional Divides

The Houston ordinance only applies to Houston’s jurisdiction. Houston does not influence neighbors and cannot control or force their policies on other jurisdictions. That is important since Kingwood is surrounded by Humble, unincorporated Harris County (Atascocita and Huffman), and unincorporated Montgomery County.

The Key

Understand that if a developer/individual meets the requirements identified in the floodplain ordinance(s), they can develop in the floodplain (including the floodway). Floodplain administrators must follow the law. However, they try to discourage dangerous floodplain development by “working to rule.” By strictly following all rules with no wiggle room, floodplain administrators can drag permitting processes out. A knowledgeable floodplain administrator can find problems with plans, surveys, and engineering reports for years. By requesting revisions, they can make life so difficult for applicants that it affects the economics of their developments. Eventually they may decide that a project falls into that great black box called “too hard to do,” and walk away.

Words of Wisdom

A regulator told me today that the more people who protest a permit, the harder they are to ignore.

If you have concerns about the high rise development in Kingwood, make sure you register them with the Army Corps (which is currently reviewing the permitting from a CWA 404 perspective). The deadline: January 29.

Comments and requests for additional information should reference USACE file number, SWG-2016-00384, and should be submitted to:

Evaluation Branch, North Unit
Regulatory Division, CESWG-RD-E
U.S. Army Corps of Engineers
P.O. Box 1229
Galveston, Texas 77553-1229
409-766-3869 Phone
409-766-6301 Fax
swg_public_notice@usace.army.mil

Posted By Bob Rehak on January 9, 2019

498 Days Since Hurricane Harvey