Tag Archives: flooding

Harris County Community Flood Resilience Task Force

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1972 VERSION

The original Harris County Flood Control Task Force was created in 1972. Members of the original task force included representatives of environmental conservation groups, development and real estate interests, engineering organizations, and economic development organizations. Task force membership did not include regular citizens, flood victims, housing advocacy groups, or representatives of low income or minority groups.

NEW VERSION

Back in July of 2020 Harris County Commissioners Court, at the urging of the county’s chief elected official – Harris County Judge Lina Hidalgo, initiated a process to reboot the old task force. The new version of the task force, which will be called the “Harris County Community Flood Resilience Task Force” (emphasis added), will include regular citizens, flood victims, housing advocacy groups, and representatives of low income and minority groups. The word community has been inserted into the title to signal that this group won’t consist of only experts or industry groups.

The new Task Force By-Laws outline membership qualifications. The by-laws require that each member of the Commissioners Court appoint one person. Those five appointed members must then select and vote upon the remaining 12 members by the end of 2020. The 12 members must meet certain qualification requirements. The final by-laws define slightly different membership provisions than those defined in the July proposal. I prepared a table that illustrates the similarities and differences in the composition of the task force members. As of this writing, the first five appointed members are:

The Task Force by-laws indicate the following task force objectives (paraphrasing a bit here for simplicity):

  • Evaluate overall approach to strengthening flood resilience;
  • Evaluate projects;
  • Evaluate schedules and compliance with the equity based prioritization framework;
  • Help identify funding;
  • Provide oversight to resilience planning; and
  • Assist with community engagement.

INFRASTRUCTURE RESILIENCE TEAM

On September 15, 2020, in a 3 to 2 vote, the Commissioners Court established another entity called the Infrastructure Resilience Team (IRT). The IRT includes managers and practitioners from the following Harris County Departments and entities:

The activities of the IRT will be coordinated by Dr. Paula Lorente, formerly with Texas A&M University, who was hired by the Office of the County Engineer. The September 8, 2020 letter to Commissioners Court requesting the creation of the IRT mentions the development of a 2050 Flood Resilience Plan for the county. It also outlines the budgetary needs and staffing needs for the IRT. I encourage folks interested in additional details about the IRT to read the court letter.

In general the IRT and the Task Force will work together to implement the the Harris Thrives Resolution adopted by Commissioners Court with a 3 to 2 vote on August 27, 2019.

EXPECTATIONS

I anticipate that the IRT and the Task Force will be able to effectively work together to advance flood resilience in our community, but it will take time. Flood risk reduction and resilience enhancement work is challenging, expensive, and is needed across the entire 1,777 square miles of our county. Some estimate that a total investment of $60 billion would be needed to reduce the annual risk of inundation for all structures in the county to 1% or less.

The IRT has the expertise to plan and design projects, estimate costs, and build them, however, elected officials and their agents (like the Task Force) must reach consensus on which projects to build, where to build them, when to build them, and, more broadly, how to invest limited public funds so we can reduce flood risks and realize other community benefits at the same time. Consensus building takes time, so I anticipate that the work of the IRT and Task Force will take longer than we think it will. I urge all participants and observers to listen to each other and to be patient with each other as our community does this important work.

Buffalo Bayou and Tributaries Resiliency Study

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In the aftermath of Hurricane Harvey, the federal government appropriated $6 million and authorized the U. S. Army Corps of Engineers (USACE) to conduct the Buffalo Bayou and Tributaries Resiliency Study.

According to the USACE, the study will: Identify and evaluate the feasibility of reducing flood risks on the Buffalo Bayou, both upstream and downstream of Addicks and Barker Reservoirs in Harris County, Texas, while simultaneously completing a Dam Safety Modification Evaluation (DSME) on the two dams. Three primary problems will be addressed: (1) Flooding downstream of the reservoirs on Buffalo Bayou; (2) Performance and risk issues related to flow around and over the uncontrolled spillways; and (3) Flooding upstream of the reservoirs.

Map of the study area. The Cypress Creek watershed is included only to evaluate the overflow from that watershed into Addicks. Brays Bayou will not be considered during the development of risk reduction options but it will be considered when determining potential adverse impacts.

The Corps requested public input on the scope of the study and comments were due on May 31, 2019.

I helped coordinate the development of comments on behalf of the Houston Chapter of the Environment & Water Resources Institute of the American Society of Civil Engineers. The text of the submitted comments is provided below:

The Houston Branch of the Texas Section of the American Society of Civil Engineers appreciates the opportunity to comment on the above referenced resiliency study.  Our comments are provided below.

  1. Sustainable Infrastructure: Alternatives should be evaluated using the Institute for Sustainable Infrastructure’s ENVISION rating system.  Alternatives with the highest score in the rating system should be considered further for implementation.  See sustainableinfrastructure.org for additional information about the rating system.
  2. Non-Stationary Climate: Alternatives should be developed to handle rainfall amounts that have a 1% annual chance (or greater) occurring in the year 2100.  Rainfall depths appear to be trending upwards and the 1% annual chance event will likely be larger at that time.
  3. Nature-Based Alternatives: Alternatives should be developed and evaluated that include nature-based approaches, such as land acquisition and preservation, wetland creation, natural stable channel design approaches, and similar concepts.
  4. Two-Dimensional Modeling of Non-Riverine Areas: Alternatives should be evaluated using 2-D modeling approaches, especially in areas not adjacent or near bayous or channels.
  5. Triple-Bottom-Line Net Cost/Benefit Estimations:  Alternatives should be evaluated using a more comprehensive assessment of net benefits and costs. Net costs should be estimated for traditional engineering economics inputs, such as construction costs, operations costs, maintenance costs, land acquisition costs, and labor cost.  But environmental costs should be estimated as well. These should include the value of any diminished ecosystem services, lost habitat, lost carbon sequestration, lost oxygen production, lost heat island mitigation, lost recreational opportunities, and similar well studied metrics.  Social costs should also be estimated for each alternative. These should include displaced cultural or historical features, lost recreational opportunities, lost or diminished employment opportunities, diminished views and character, light pollution impacts, diminished social equity, and similar aspects. Net economic, social, and environmental benefits should also be estimated for each alternative.  These would include the value of avoided property damage (times the likelihood of loss), the number of people benefiting from a reduced risk of inundation, the value of any increase in social values or benefits (recreation, views, safety, equity), the value of any increase in environmental values or benefits (habitat, ecosystem services, etc.).  The net present value of all economic, social, and environmental BENEFITS minus the net present value of all economic, social, and environmental COSTS should be calculated for all alternatives and the alternative with the highest net present value of total triple bottom line NET BENEFITS should be recommended for implementation.

Again, we appreciate the opportunity to comment on the scope of the study.  If there are any questions about our comments, please don’t hesitate to contact us.

Very truly yours,

AMERICAN SOCIETY OF CIVIL ENGINEERS – HOUSTON BRANCH

Leave a comment about what you or your organization thought the study should consider.

The Invisible Development

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Can we make our land development projects (hydrologically) invisible to downstream properties?  Think of Claude Rains, in the 1933 film adaptation of the H. G. Wells novel, The Invisible Man.

“If I work in the rain, the water can be
seen on my head and shoulders.
In a fog, you can see me – like a bubble.
In smoky cities, the soot settles on
me until you can see a dark outline.”

— The Invisible Man, 1933

I believe we can, using a natural drainage approach.  To illustrate this we must think through some basic — no actual math required! — hydrology.

Imagine a rectangular area of undeveloped land that has a gradual slope from one corner to another.  Imagine that all rain falling on this land drains to the low corner and rain falling outside of this area drains to some other location.  Like this:

Imagine that before we develop the site – the “predevelopment” condition – we install a flow measuring device to the low point. This allows us to record the stormwater runoff flow rate leaving the predevelopment site.

If we did this, one hour before a 10 minute rain event, for example, the runoff flow rate would be 0 gallons per minute (gpm). As the first drops of the 10 minute rain hit the ground, the flow would be 0 gpm. Minutes and hours later, the flow would reach its peak and then start to decline back down to 0, like this:

The graph above displays the predevelopment hydrograph.  All plots of stormwater runoff flow rate vs. time are known as hydrographs. If we know the history of the flow rate vs. time, we can easily determine the total volume of runoff that left the site as a result of our 10-minute rain.  The total runoff volume, if you recall your calculus, is the area under the curve, like this:

This makes sense because if we multiply the dimensions of the x-axis expressed in minutes by the flow rate expressed in gallons per minute we get gallons because the minutes cancel out:

Minutes x Gallons / Minute = Gallons

If we add some kind of development (buildings, roofs, roads, etc.) to the site, thereby increasing the site impervious, the runoff hydrograph changes.  The added smooth hard surfaces and concrete storm sewers:

  • Reduce resistance to flow;
  • Eliminate nooks and crannies for surface storage;
  • Accelerate the timing of the runoff;
  • Reduce or eliminate water infiltration; and,
  • Reduce or eliminate water transpiration (consumption and release to the atmosphere by plants).

These changes to the drainage area change the hydrograph that would be produced if the exact same 10 minute rain event fell on the post-development site.  The post-development hydrograph might look something like this:

Note the following characteristics:

  • Higher peak flow;
  • Earlier peak flow;
  • Faster decline back to zero flow; and,
  • Larger total volume of runoff.

Well that can’t be good, right?

If we developed this way the bayou receiving this runoff water would see a higher flow rate. This would result in a higher water level, which might cause downstream flooding if that higher water level was higher than the top of the bayou banks.

We mitigate the effect by sizing and constructing detention basins downstream of all new development. The detention volume is generally equal to the “excess volume” produced by the development. The “excess volume” is determined by calculating the difference between the pre- and post-development runoff volumes, like this:

The light blue is the difference between the two ares (volumes).  We can place that volume of stormwater runoff anywhere we’d like on the site. Engineers love making them into the shape of nice, regular, rectangles, like this:

Landscape architects have encouraged us to make them into more natural shapes. Regardless of their shape, they are designed to hold the excess volume and to release that water at a rate that does not exceed the predevelopment peak flow rate, like this:

This prevents downstream flooding, but its not perfect. Can you see a few of the problems with this approach?

The main problem is the volume of runoff is not reduced. With detention, the site discharges at the predevelopment peak flow rate for a longer period of time.  (Compare the horizontal distance of the blue line to the brown line.)  Compare the brown area (predevelopment runoff volume) to the blue area (post-development runoff volume), below:

So how can we deal with this extra volume?

Natural drainage systems (also known as “low impact development”) can help address this. Natural drainage systems are installed to slow the water down, infiltrate water, evaporate water, store water in small or micro scale detention areas, transpirate water, and generally to mimic the predevelopment hydrology. The same rain event falling on the site – mitigated with a natural drainage approach – might produce a hydrograph that looks more like the green hydrograph below: 

So how does the runoff volume comparison look using natural drainage? 

Pretty good, huh?

The natural drainage approach seeks match the predevelopment hydrology.  This means that the downstream folks experience no difference in the timing, rate, or volume of runoff (for a given rainfall event).

Some natural drainage proponents, like me, like to say the development is hydrologically invisible.

“Boomtown, Flood Town” Reconsidered

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Originally published on February 6, 2017 on TribTalk. Coauthored with Steve Stagner, Executive Director of the American Council of Engineering Companies, Texas.

Flooding is a terrible thing to experience. Floods destroy personal belongings and homes. They create a stinking mess that must be cleaned up. They can kill drivers in below-ground underpasses. They displace families. But modern floodplain management efforts are really much better than you probably think.

In December, the Texas Tribune and ProPublica jointly published “Boomtown, Flood Town,” an article on flooding and development in the Houston region. The engineering community, which is on the front line of stormwater management, development, wetlands, and surface water quality issues in the Houston region, has a somewhat different perspective on these issues.

Since 1989, approximately 23,000 of the 1.5 million houses in Harris County — or 1.5 percent of the homes — have flooded from rainfall (not including coastal surge). In addition, the region experienced fewer than 60 of 9,500 days of high water during the same period — representing 0.6 percent of the time. With the exception of deaths due to basement parking lot flooding during Tropical Storm Allison, all fatalities, while tragic, have resulted from people driving into flooded underpasses and not from structural flooding. The risk from underpass flooding is being addressed with enhanced warning lights, gates and signage. Below-ground parking areas have been retrofitted with flood-proofing facilities, seals, and doors.

Events such as the 2016 Tax Day and Memorial Day Floods are extremely rare. During the Tax Day flood, parts of Harris County received 17.5 inches of rainfall in 24 hours, and parts of Waller County received 23.5 inches of rainfall in less than 15 hours. Compare this to the size of a 100-year event — a 1 percent storm — of 12.4 inches in 24 hours. It’s important to note that a 100-year storm has a 1 percent probability of occurring every year. That means that a home with a finished-floor elevation an inch or two below the 100-year floodplain has a 26 percent chance of being flooded in 30 years — the length of a standard mortgage.

A brief history of flood prevention in Houston

In the early 1900’s, Houston-area drainage districts assumed that a 4-foot-deep ditch with a 4-foot-wide bottom would be sufficient to prevent flooding. Gradually, this standard began to change. In the 1940’s, drainage systems were designed to handle 4 inches of rain in 24 hours. In the 1960’s, drainage systems were designed to handle storm sizes based upon the size of the area being drained. The largest areas were designed to handle about 8 inches of rain over 24 hours — or a storm with a 4 percent chance of happening every year.

In the 1970’s, the National Flood Insurance Program (NFIP) was implemented and engineers began modeling and mapping 100-year floodplains. But Houston-area floodplains were not fully mapped until the mid-1980’s.

In 1986, the region’s engineers began designing drainage with detention systems, and new storm sewers were required to hold and restrict the release of rainwater from a developed property so that the maximum flowrate did not exceed the highest flowrate from the property before the development was built — known as the “peak” pre-development flowrate.

Drainage systems designed after 1986 work amazingly well.  They include below-ground storm sewers sized to carry the rain from two-year events (with a 50 percent chance of happening every year). Roads are designed to convey rain for 1 percent events. (That’s why our streets frequently flood — we’ve chosen to avoid flooding homes and structures by routing water in the streets.) Buildings are further protected by placing their foundation slabs at least 12-inches above the 1 percent event flood elevation.  We protect downstream properties by providing about 180,000 gallons of detention for every acre of new development. Even more detention (and retention) is required in the Overflow Area of the Cypress Creek watershed, which can more easily impact downstream properties.

Our region has also been looking at the effect of climate change on rainfall patterns. In March 2016, Harris County joined a national study by the National Oceanic and Atmospheric Administration to recalculate the 1 percent storm size for Texas using rainfall records from a longer period of time. Similar work completed for the southeast and southwest regions of the country showed that rainfall depths over time did not have statistically significant trends up or down — i.e., no climate-change effect was observed on those types of storms.

The question of land preservation

Although undeveloped prairie land can retain some volume of water over a period of days or weeks, it cannot prevent flooding, especially during extreme events. Prairie land can reduce the total volume flowing to our existing reservoirs, but many conservation-minded citizens seem to over-estimate prairie land’s absorption capacity. Undeveloped prairie will not prevent high water levels across the landscape after extreme events. Case in point: the catastrophic flooding of downtown Houston in 1929 and 1935 occurred due to rainfall on the undeveloped prairie.

Preservation of land along bayous can help reduce flood damages for new development in the urban fringe, and this is already happening. In the Cypress Creek Overflow area and in other portions of western Harris County’s urban fringe, floodplain managers, public agencies, engineers, and developers continue to implement “frontier programs” to plan out preservation corridors and to thoughtfully acquire and construct appropriate drainage facilities.

However, land preservation does nothing for existing developments. Removing development is a great idea and can help reduce flood damages, but must be evaluated using a benefit-to-cost ratio. Paying $1 million to buy and remove 10 homes from the floodplain is not as cost-effective as paying $1 million for channel improvements that shrink the floodplain and effectively move 20 homes out of it. Also, there is the unaddressed question of who should pay for these types of programs.

Protection of wetlands in the frontier areas is a good practice to absorb rain and storm-water runoff and to help reduce flood damages; however, the statutory authority to do this currently rests with the federal government. While local governments might choose to enact their own rules regarding wetlands protection, the value of those rules would have to be considered against the added administrative burden of local governments creating programs that overlap with federal statutes.

The impact of flooding in Houston

The uncomfortable fact for most people is that the Houston region is flood prone and engineering and infrastructure solutions will never reduce or prevent extreme rainfall events. Large rainfall amounts, especially those that exceed our storm design, will always occur and will always result in flooding. This is not “man-made flooding.”

In spite of its flood-prone nature, Houston has flourished and people still continue to choose to live here. In 1900, fewer than 100,000 people lived in Harris County. Today more than 4 million do. Houston has grown to be the fourth-largest city in the United States and a key economic engine for Texas. About 200 people move to Houston every day. Houston has prospered and has provided affordable homes, schooling, careers, cultural opportunities, and many other amenities while being located in a flood-prone Gulf Coast area. According to the U. S. Department of Commerce, the Houston region’s gross domestic product has grown from $241 billion to $503 billion during the period from 2001 to 2015.

Hydrologists, floodplain managers, and engineers in the private and public sectors have done a remarkable job in reducing flood damages in our region. We will continue to make progress in this area, while addressing development, changes in rainfall patterns, and population growth in a sustainable manner.