Tag Archives: Houston

Buffalo Bayou and Tributaries Study (Again)

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I posted about this study back in June of 2019, when the United States Army Corps of Engineers (USACE) was asking for up front public input on the study before they got too far along. I helped the Houston Branch of the American Society of Civil Engineers (ASCE) provide some input.

Back then, many regional stakeholders chimed in, most supportive of additional federal investment to update the original federally authorized project that built the Addicks and Barker Dams. Many stakeholders encouraged USACE to be creative and to evaluate nature-based systems to reduce flood risks.

At the urging of the Harris County Flood Control District (the most likely local sponsor of any federally funded project to update the Buffalo Bayou federal project) last month the USACE released an interim version of the report for public input. An executive summary of the report is available here. For more detail, the whole report is here.



The main stakeholder reaction? Disappointment.

There are two main reasons for the public’s disappointment.

First, the report includes very traditional detention and conveyance alternatives along with buy-out options. Nothing in the report could be described as creative. There is not much in the way of nature-based solutions. The use of tunnel conveyance facilities were ruled out due to cost.

Second, most of the presented alternatives have very low benefit cost ratios (BCRs). This is very important to note because in order to attract federal support (congressional authorization) and funding (congressional appropriation), projects must have BCRs of much more than 1.0. This is embedded in laws and policies governing the White House Office of Management and Budget (OMB). Limited federal dollars most compete with scores of other projects across the country, each with calculated BCRs. Only the projects with the highest BCRs have a chance at federal funding.

So why did USACE publish a report with these two problems?

Here is my theory.

After Hurricane / Tropical Storm Harvey dropped its unprecedented rain amounts on the Buffalo Bayou system, many upstream and downstream homes and businesses were flooded. This triggered a public outcry and litigation over the federal government’s use of private property to store and convey water. It also triggered support for the resilience study and created expectations that the study would find a “silver-bullet” that would “protect” everyone from a Harvey type event in the future.

Well, the situation is not favorable to identifying projects with high benefit cost ratios (BCRs). The vast majority of homes and businesses in the Buffalo Bayou system have a very, very low likelihood of flooding during any particular time period, even with Atlas 14 rainfall used to map floodplains. This low existing risk is a direct result of the large federal investment to build the existing Addicks and Barker system.

Yes, there are homes in the inundation pools of Addicks and Barker. Yes, there are homes in the floodplain of Buffalo Bayou downstream. Yes, the Cypress Creek overflow does make drainage and land use challenging upstream. But even counting them in, it is still very challenging to identify any infrastructure investment in the Buffalo Bayou watershed that would generate a high enough monetary value of avoided damage — the benefit part (the numerator) in the BCR — to justify the required costs.

To explain this further, let’s dive into how the monetary value of avoided damage is estimated. The monetary value of avoided damage is determined by the difference between the value of avoided damages from a particular flood after a new project is built, call it post-project conditions (ADPost) minus the value of avoided damages from a particular flood before a new project is built, call it pre project conditions (ADPre) with the result multiplied by the likelihood of that particular extreme flood occurring during the study time period. If the avoided damage arising from a particular flood with a new project is not much different from the “no new project” alternative, the difference won’t be very large. This is the numerator in the BCR. If the project costs are high (Cost), the BCR denominator will be high, thus reducing the BCR. Shown as an equation, it looks like this:

Where: BCR is the benefit cost ratio; ADPost is the monetary value of avoided damages after any proposed new project; ADPre is the monetary value of avoided damages today, without any new investment in the existing Addicks and Barker system; P is the probability of the modeled extreme storm occurring during the study time period; and Cost is the cost to design, build, operate, and maintain the project during the study time period.

So let’s pretend we can devise a project that generates $8 billion in avoided damages (ADPost) from a particular extreme, but rare, storm event. Because of the prior investment in the Addicks and Barker system, the ADPre is also a pretty high number, easily $6 billion. This means the difference is about $2 billion in this hypothetical.

To account for the probability of the rare extreme event occurring during the study time period, we need to multiply the damage estimate by the likelihood of that extreme storm actually occurring during the study time period. Let’s assume that over a 100 year period the rare and extreme event has a 10% chance of occurring. That means we would have to multiply the difference in damage estimates, $2 billion, by 0.10 to compute the benefit portion of the BCR fraction. Ten percent of $2 billion is $200 million.

So to obtain a BCR of 1 or more, if the benefit portion of the fraction (the numerator) is $200 million, we need the total cost to design, permit, build, and operate the project for 100 years (the denominator) to be $200 million or less. If you’ve read the report’s executive summary, or know anything about how much infrastructure costs, you know that is pretty much impossible. (The dam safety projects – spillway fixes – should obviously be done as soon as possible. Those smaller projects cost less and yield high benefits.)

Folks who flooded and who live in the Buffalo Bayou watershed, who really want an additional federal project investment to further reduce flood risks in their area may be upset with this post. But they should know that the 1940 “Definite Plan” was a very large federal investment that reduced their flood risks when it was built. The prior federal risk reduction investment makes it harder to justify additional risk reduction investments.

Unrelated to the watershed-specific study discussed in this post, additional local, state, and federal investments should go to areas of Harris County with many more homes, structures, and businesses exposed to higher inundation risks than those in the Buffalo Bayou watershed. Think Greens Bayou, Halls Bayou, and others.

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Review of Harvest Green

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I recently took a drive through Harvest Green, a master planned community located in Richmond, Texas. Currently being developed by the Johnson Development Corporation it features agriculture and gardening as a central theme. Amenities include a working farm; optional backyard raised bed gardens; a trail system; herb gardens in some common areas; lakes; and a fitness center with pool, splash pad, and playground. I toured the community to review its drainage system, and amenities, and to see how the two were integrated.

The master planned community will eventually occupy 1,300 acres; however, only the first few sections have been completed thus far. This is an illustration of the complete master plan.

Masterplan obtained from online development literature. https://www.harvestgreentexas.com/masterplan

In some locations the four lane main roads drain to depressed areas planted with native grasses and shrubs that provide some detention and retention of stormwater (see below). I was unable to tell if these areas subsequently drain directly to underground storm sewers and then to a detention basin or if the drainage flows through a bioretention system first. I was also unable to tell if including these areas allowed the development to proceed with a lower volume of centralized detention. [If anyone involved in the design of this project knows, please leave a comment.]

Photograph by M. Bloom.

Each of these low areas has educational signage identifying it as a “Native Meadow” (see below). This helps residents and visitors, who may be used to a more formal and manicured type of landscaping, know that the tall prairie grass and wildflowers are intentional and are part of the “natural” and “wild” Harvest Green experience and brand.

Photograph by M. Bloom.

Many of the homes have backyards adjacent to trail or creek corridors. The photograph below shows a trail along Oyster Creek. Homes along this corridor have an open fence design along back lot lines. I did not notice if gates were provided for easy home owner access to the trail system. Landscaping includes a mix of both mowed turf grass and unmowed “wild” areas with native grasses and flowers.

Photograph by M. Bloom.

Some creek corridors don’t currently include trails and are not served by backyard gates. The photograph below shows an example of this. This is a bit of a missed opportunity to integrate both natural amenities and natural drainage into these corridors.

Photograph by M. Bloom.

While some of the main roads appear to drain to natural systems, local residential streets do not. All of the local streets appear to be served by traditional curb and gutter drainage systems with underground pipes flowing to centralized detention basins. The basins were built deeper than required for detention purposes and were clay lined so that they hold some permanent water and serve as amenity lakes.  The photograph below shows the traditional raised curb and a pair of side-by-side curb inlets. As noted previously, the more extensive use of natural drainage systems can reduce the need for end-of-pipe detention systems. I was not able to tell if Harvest Green was able to take advantage of that benefit.

Photograph by M. Bloom.

A recycled water system has been added to the community’s wastewater treatment system and recycled water lines have been installed throughout the development to be used for landscape and farm irrigation. State rules require the recycled water piping system be purple and signage be installed to reduce the risk of people drinking the recycled water. The quality of the water is perfectly safe for watering food crops, but it is not intended for direct human consumption. See photograph below.

Photograph by M. Bloom.

Like most suburban communities in the region, the entry points are marked with large, illuminated, entry monuments and signage.  The photograph below shows one of the entry monuments to Harvest Green.

Photograph by M. Bloom.

But this monument is a bit unusual. It is solar powered! The photograph below shows the photovoltaic cells and what I assume is a battery system of some kind.

Photograph by M. Bloom.

The Johnson Development Corporation and their design team used natural drainage approaches to manage the runoff from some areas of the master planned community. They integrated amenities and drainage system in some areas. They deployed non-potable water reuse systems and solar systems to improve the sustainability of the development.

Their marketing of the community has focused on the natural amenities, agricultural features, and gardening elements to differentiate their offering to home buyers, some of whom are looking for a more active and engaged lifestyle and to live in a more sustainable and environmental friendly community.

The Johnson Development Corporation (and their planning and design professionals) should be commended for what they have accomplished in Harvest Green.

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.