Houston Drainage and Flooding Policy Options

Posted on by

Back in May of 2016, five months after he was sworn in, Mayor Turner announced that former City Council Member and mayoral candidate, Stephen Costello, P.E., would serve as the City of Houston’s Chief Resiliency Officer (CRO).  Mr. Costello’s main objective as the city’s CRO is to make some progress in addressing flooding risks in Houston.

Steve Costello and Mayor Turner. Photo: J. Evans. Reposted from Houstonia.

Since that time Mr. Costello has given out his cell phone number, met with numerous community groups and interested stakeholders. Sources close to his office have indicated that Mayor Turner has approved the appointment of a Flooding and Drainage Task Force who will consider the issues and make recommendations for improving things.  This is consistent with Mayor Turner’s original intent, announced on April 20, 2016.  Here are some ideas the Task Force should consider.

1. Gradually Increase Funding for Drainage System Improvements

Continue retiring our existing infrastructure debt using Rebuild Houston funding. Continue to move to a pay-as-you-go approach. Continue to plan, design, and construct replacements and upgrades to storm sewers to improve service levels. Continue to fix the drainage systems with the lowest level of service first – the so called “worst first.” Gradually increase funding for streets and drainage so that areas with lower levels of service can be addressed.

2. Create Tiered Detention Requirement for Redevelopment

Development in the City of Houston is currently required to provide a volume of detention that is a function of the amount of new impermeable surface area constructed. Under this scheme a 2.0 acre site with no added impermeable surface area must provide 27,150 gallons of detention storage. A 2.0 acre site with 100% added impermeable surface area must provide 325,830 gallons. This reflects the current policy of allowing redevelopment that does not change the runoff volume to not detain very much.

In areas of the city with older public storm sewers that do not meet current design standards, this policy should be adjusted. We should require property owners who are completely reconstructing their facilities to reduce the volume of runoff from their properties to help reduce the burden on these older public systems. This will benefit the private property owner or tenants by reducing the risk of flooding in the vicinity of the property.

This could be implemented using a tiered system based upon the degree to which the site is being reconstructed. For example, if a commercial strip center was repaving their parking lot and updating the center facade and signage, they might have a lower detention requirement than another project that was completely demolishing an existing building and surface parking lot and building a new mid-rise office building with underground parking.

The tiered system might be based on the capital investment per acre or a similar measure of the “degree to which the site is being reconstructed.” The detention requirement would be directly proportional to the capital investment per acre.

3. Define Pervious and Impervious Surfaces More Rigorously

Detention requirements are a function of impervious area, which is intended to account for the volume of stormwater that infiltrates into the ground rather than runoff the site. This seems rational, but we don’t consider how much infiltration any particular surface actually provides. Under the current regulatory scheme ground surfaces with very different infiltration characteristics are lumped into two categories: pervious or impervious. The reality is more nuanced, with different ground surfaces allowing stormwater to infiltrate at various and quantifiable rates. Establishing detention rates using a more nuanced consideration of infiltration rates would be helpful.

4. Create Stormwater Volume Trading Program

Many communities with “one-pipe” sewer systems installed in the late 1800’s or early 1900’s are working to capture, reuse, retain, and detain stormwater volume to reduce the severity and number of combined sewer system overflows (CSOs) under consent decrees signed with the United States Environmental Protection Agency. These communities include Chicago, St. Louis, Portland, Seattle, Philadelphia, New York, Baltimore, Washington DC, and many others (see red dots in the map below).

Cities with populations greater than 50,000 with combined sewer systems. Map from U.S. EPA.

CSOs happen when it rains a lot and the combined system can’t handle the volume of runoff and sanitary wastewater flows. CSOs allow untreated sanitary sewage to be discharged into creeks and rivers.

Every cubic foot of stormwater that can be controlled above ground in a detention basin, a cistern, a green roof, a rain barrel, a bioswale, or some other feature is one less cubic foot that needs to be managed in large, centralized, grey infrastructure facilities – think 40 ft. diameter tunnels. Any storage volume, whether provided by a public entity or a private land owner or developer, helps mitigate the overflow issue.

CSO communities are establishing stormwater volume trading schemes and using the market to create incentives to build “surplus” detention or retention volumes and then allowing the market to sell and purchase these stormwater volume credits.

We should set up this type of stormwater volume trading in Houston to reduce flood risk and flood damages.

5. Use Real-Time Controls

Every hour the Weather Prediction Center of the National Weather Service produces an estimate of the probability of precipitation and the amount of precipitation expected in upcoming 6 and 24-hour time intervals from 1 day to 7 days ahead of time.  These data can now be consumed by stormwater infrastructure control boxes connected to the internet. These real-time weather predictions (which are updated each hour) can now be used to control valves, pumps, gates, and other water system devices to enhance the level of service provided by them. Pumps can be switched on or off, valves can be opened or closed, immediately, based on real time data and decision logic programming.

For example, a cistern could be used for rainwater harvesting to reduce potable water consumption during a period of anticipated moderate to low rainfall, but the same system could be used for detention during a period of anticipated high rainfall.  This is possible by operating our drainage systems using real-time control systems.

We should start deploying these types of systems to help get more from our stormwater infrastructure.

6. Establish Hydrological Basis for Using Green Stormwater Infrastructure

The City’s drainage design criteria imposes a detention requirement that is solely a function of the change in imperviousness from pre-development to post-development. This does not allow the determination of pre-development and post-development hydrology and the direct calculation of the detention volume using a hydrological basis. See this earlier post for an illustration of what this means.

It would be helpful to allow detention volumes to be determined using hydrological principals and calculations and to be derived from the difference between the pre-development and post-development runoff volumes.  This would create more of an incentive to use green stormwater infrastructure (what I call “natural drainage” systems).

7. Try Ground Modifications to Enhance Infiltration

We don’t rely enough on infiltration. We tend to over estimate the volume of infiltration we think we are getting through prairie land (that has been historically farmed). We tend to under estimate the volume of infiltration we think we are getting through our clay soils. We almost always stabilize and compact the soils all over our development sites, which, of course reduces infiltration. Why not create more spongy areas by deep ripping some soils? Why not amend our soils in targeted locations with sand or organic material to enhance infiltration rates? Why not examine soil infiltration rates in our normal pre-design geotechnical investigations?  We could rely more on infiltration to manage stormwater volumes if we actually considered it and measured it.

8. Enhance public education on flooding risks.

We should communicate the risk of flooding more consistently, more effectively, and more loudly. This will help align public expectations with reality. I get the feeling that many people in Houston expect the risk of flooding of anything they own (car, bike, home, etc.) to be nearly zero. I get the feeling that many people in Houston are very disappointed when they find out the risk of car flooding is much, much higher than that. We design new streets to convey stormwater away from homes and businesses. That is worth repeating: our streets are designed to carry stormwater! The storm sewers below our brand new streets are sized so that they can carry a depth of rain that has a 50% chance each year of being exceeded. This means that any particular new street in Houston has a 50% of flooding every year. That occurs when we get about 3 to 4 inches of rain in 24 hours. Better risk communication would not only help our citizens with expectations and management of the impacts of high rain fall, but might help inform a discussion around increased funding for flood risk reduction.

“Harvesting the Value of Water” – New ULI Publication

Posted on by

I’ve been working to get more involved with the Urban Land Institute lately. During the 2016 Fall Meeting in Dallas I was able to connect with Katharine Burgess and Rachel MacCleery, both of ULI’s Center for Sustainability and Economic Performance. They were kind enough to allow me to help review and contribute to a report they were working on about the use of natural drainage systems in real estate that would eventually be called “Harvesting the Value of Water.”

The report, which was formally released in May of 2017, was made possible by a grant from the Kresge Foundation and was prepared with review and input from many ULI members and experts.  I was glad to have a small role in its preparation.

The report first explains why stormwater management issues have recently become more visible. In the late 1800’s and early 1900’s cities in North America were just beginning to construct urban sewer systems. There were two competing approaches: a combined sewer system, which handled both sanitary sewage and stormwater runoff, or a two-pipe system that handled each in its own pipe. Several hundred cities constructed combined sewer systems to serve at least some portion of their area.  These system can sometimes overflow during larger rains.  These overflows are known as Combined Sewer Overflows or “CSOs.”  CSOs discharge untreated sanitary sewage into area waterways, which is . . .um.. . not so good.

The Environmental Protection Agency (EPA) started working with cities with CSO issues back in 1994 and now many of them have entered into legal agreements with EPA and the U.S. Department of Justice to address them.

Initial plans to address CSOs called for large diameter tunnels to be constructed to store the mixture of water during rain events to reduce the number and severity of overflows. These projects had price tags of $4, $6, or $8 billion per city. Several cities figured out that using green stormwater infrastructure (GSI) to manage stormwater above ground at each building or site substantially reduced overall CSO mitigation costs. Some cities added the use of GSI to manage private property and public stormwater runoff volumes into their agreements with EPA. These programs require volume control to a larger degree than that typically required for merely floodplain management. So in some parts of the country, outside of Houston, the use of GSI is required.

In other parts of the country GSI is required by states and local governments to reduce stormwater pollution to estuaries or other waterbodies that don’t currently meet surface water quality standards. The Chesapeake Bay restoration effort is one of the largest examples.

The report also provides a good introduction to GSI techniques, such as green roofs, bioswales, rainwater harvesting, and rain gardens.  Chapter 4 explains how smart GSI implementation can provide private real estate developments higher operating income, faster lease rates, higher occupancy levels, greater lot yields, green marketing benefits, and reduced drainage infrastructure costs.

Chapter 5 provides eleven case studies from around the country that illustrate how GSI has been deployed in various types of real estate development projects. The publication includes a profile of Stonebrook Estates, a 50-acre Houston-area single-family residential development case study.  The development features a GSI system designed by Steve Albert, P.E., who was with Aguirre and Fields at the time.  The balance of the site civil infrastructure – the potable water distribution, the sanitary sewer system, roadway paving, and general grading was designed by R. G. Miller Engineers, Inc. before I started working there.

bioswale with overflow structures for handling larger storms at Stonebrook estates. Photo credit: m. bloom

The last chapter summarizes the stormwater policy landscape and explains the range of methods used by the local governments to either allow, encourage, or mandate the use of GSI in land development. These methods can range from merely enacting a permissive regulatory framework that allows private project sponsors to use GSI when it makes good business sense (a bottom-up, market-based approach); to offering some permitting or financial incentives; or to enacting across the board mandates for stormwater volume controls and retention to achieve CSO mitigation or surface water restoration objectives (a top-down, regulatory approach).

I encourage folks to download the publication and check it out.

The Invisible Development

Posted on by

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.

Nature Play and Natural Drainage

Posted on by

The Houston Chronicle recently published a great article by Dylan Baddour about recent work by TBG Partners at the Wetland Park in Riverstone, a master planned community in Sugar Land, between the Brazos River and State Highway 6.  It has been planned, designed, financed, and built by the Johnson Development Corporation.

The article was generally about how master planned communities are moving away from formal parks, golf courses, and playgrounds and starting to provide more natural environments for recreation. This could include things like real wood and stones for kids to climb on, mud puddles, native grasses and other plants, and direct access to water to see and touch tadpoles. The National Wildlife Federation has suggested that facilities like this provide opportunities for people to conduct “nature play.”  The Natural Learning Initiative at North Carolina State University has published guidance on how to plan and design nature play facilities.

The story highlighted how TBG Partners landscape architect Meade Mitchell designed the Wetland Park, located just east of LJ Parkway and a bit north of University Boulevard.  The Wetland Park is among the newest amenities and it provides a sharp contrast to a more traditional, hardscape amenity area called Riverstone Club, located a bit to the southwest.  The Club includes tennis courts, a dog park, pools, and gym facilities. See map below.

Aerial view of riverstone with insets and labels prepared by m. bloom
from GOOGLE EARTH image and riverstone landplan dated february 2017.

The Wetland Park includes a mud pie kitchen, rock throwing targets in ponds, a bridge with no handrails to make it easier to watch aquatic life, logs and natural wood placed in ponds for aquatic habitat, and a pathway of partly submerged rocks to encourage wet feet.

When I was a kid I played in Shortridge Memorial Park in the headwaters of the East Branch of Indian Creek just outside of Philadelphia. Building dams, catching crayfish. So I would love to live in a place with these types of amenities.

East Branch of Indian Creek, shortridge memorial park,
Wynnewood, PA. Image from Google Earth.

A close look at the 2017 aerial photography of the development from Google Earth illustrates that a traditional underground storm sewer system serves the residential lots and streets.  It also shows how engineers provided large excavated areas to hold and slowly release stormwater runoff from the development.  These are called detention basins (or ponds) and they hold some water permanently to provide lake features for residents to enjoy.  When it rains, water runs off the roofs, the yards, the driveways, and roads, and drops into concrete storm sewer pipes. The water is rushed quickly to the detention ponds, which fill up to their full capacity to store the excess water. Small discharge pipes then release the water at a rate that is less than or equal to the pre-development runoff rate.  This prevents the development from flooding downstream properties.

This traditional approach of speeding up the runoff along curbs and gutters and inside storm sewers only to slow it down inside large detention basins is not very efficient.

What if we conveyed rain water away from the homes and structures using natural topography?  If the land is too flat, what if we graded and planted the landscape to create drainage corridors that look like creeks or bayous?  What if we preserved and extended the existing wetland areas and creeks into the community so that families could interact with natural creek corridors throughout it? The natural drainage concept does this.

Natural drainage allows rain water to flow and collect in natural creek corridors more slowly. This reduces the volume of detention required to protect downstream properties. Natural drainage corridors bring nature next door.  They provide a drainage infrastructure service as well as a valuable water-based linear amenity.

The natural drainage approach can help avoid regulated wetlands and can help integrate them into the overall land plan in a way that can preserve their function and connection to other waters.  Natural drainage corridors also provide a place for hike and bike trails and enhance community connectedness.  The natural drainage approach fits perfectly with the nature play approach and visa versa.

The natural drainage approach helps enhance the economic performance of real estate projects by providing:

  • Higher operating income;
  • Faster sales;
  • Higher amenity values for more of the community;
  • Greater lot yield (through reduced detention requirements);
  • Green marketing benefits; and,
  • Reduced drainage infrastructure costs.

The Wetland Park in Riverstone is a great step in the right direction and I applaud all who were involved in its creation, but we can do even better using integrated natural drainage and nature play concepts.  We can integrate the natural world with the built environment by combining wetlands preservation, drainage, linear trails, connectivity, and recreational natural play environments all into the same system, all while enhancing the economic performance of the community.

 

Happy Earth Day

Posted on by

[This is a re-post of an essay published back in 2014 after I served as a judge for the Environmental Protection Agency’s annual Campus Rainworks Challenge.]

My eldest daughter is attending a collaborative, exciting, and dynamic high school.  When I pass through the halls of her school and see the young energy, enthusiasm, and creativity flowing from person to person and bouncing off the walls I feel a bit jealous.  It would be really fun to be back in school again.

A similar feeling welled up inside as I participated in the second round judging process for the 2013 Campus Rainworks Challenge.  Ten student teams passed round one judging in the Site Design Plan category.  They provided green infrastructure and low impact development design concepts for campus libraries, art museums, common areas, court yards, transportation links, and parking areas.  Six student teams passed round one judging in the Master Plan category.  They provided conceptual plans for larger areas of their campuses.

THE KANSAS STATE UNIVERSITY CONTEST ENTRY.

The submittals illustrated interactive art, water-powered sculptures, landscaped bioretention areas, information kiosks, and green walls.  The submissions were presented in a wide variety of video formats.  Vibrant renderings and creative video production techniques allowed me to experience a bit of the fun, collaboration, and exciting process the teams must have experienced as they developed their designs.

I watched the videos.  I reviewed the design submittals.  I felt the enthusiasm. Then in late February 2014 I hopped on a plane to Reagan International Airport and then the METRO to visit EPA Headquarters near the Federal Triangle METRO stop.  As the METRO smoothly carried me past the Netflix House of Cards posters in every station (did METRO know about what happens in Season 2 Episode 1?) I realized that I had never been to EPA Headquarter in my 23 years in the environmental engineering consulting field.  Now I was really excited.

When I arrived I marveled at high ceilings and neoclassical design of the building.  Built in the early 1930s, the building originally was the headquarters of the U.S. Post Office Department, before its name was changed to the U.S. Postal Service in 1971.

Truth be told: it was challenging to determine winners.  The American Society of Landscape Architects provided Michael Vergason and Dennis Nola as judges.  Daniel Christian and I served as judges from the Water Environment Federation, and Robert Goo and Mike Borst joined the judging team from U.S. EPA.  Led by Tamara Mittman, our EPA facilitator, we debated and discussed the merits of all submissions.  At the end of morning of heated discussion we provided a consensus list of the top three submittals in both the Master Plan and Site Design categories.  The winners were selected by Nancy Stoner, Acting Assistant Administrator for Water, U.S. EPA, and I look forward to congratulating them and all the participating teams. Meanwhile, Happy Earth Day!

Top 10 Myths About Houston Flooding

Posted on by

Myth #1: If we have three 100-yr floods in the space of 3 years, development and bad engineering must be to blame!

The 100-yr flood is the average period between rain events of certain size. There is a 1% chance of that size rain occurring EVERY year.  There is a 26% chance of that size storm occurring during a 30-yr mortgage.

Myth #2: Development makes flooding worse.

New development must include places to store water and must prove that flood levels in the area are not increased. New development does not increase flooding levels.

Myth #3: If you live somewhere for 30 years without a flood and you get flooded, something on the ground changed to cause it.

Flood waters come from rain. If you avoided flooding for 30 years it likely means that no large rain events occurred in your area.  A flood in your area probably resulted from an abnormally large rain event.

Myth #4: The streets in my area flood because of incompetence and mistakes.

Our flat terrain does not provide many places for the water to go, so we use the streets.  The streets are designed to flood above a certain size rain event to avoid flooding homes and business.

Myth #5: They are using outdated and old approaches to address drainage and flooding.

We use lasers to map the ground surface, we use advanced computer models to predict where water will flow and how high it will get.  We are now using “natural drainage” systems and “natural channel design” approaches in many projects.

Myth #6: Fill dirt used to raise a property for a new building causes flooding.

Site changes must be planned by a professional engineer. Places to move and store water must be provided. The engineer must prove that flood levels in the area are not increased. New development does not increase flooding.

Myth #7: Preserving greenspace will prevent flooding.

Preservation plays a role and can help, however new development is required to mimic the pre-development conditions. It is important to remember that Houston was flood prone prior to any development.

Myth #8: Widening and deepening all the bayous will prevent flooding.

It can help, but existing homes and businesses are frequently in the way and may be lower than the bayous banks.  Houston’s flat terrain limits how deep we can dig.  In many cases the issue is getting the water to the bayou.

Myth #9: Flooding is terrible because none of these new developments are following the rules!

Registered professional engineers must make all plans and follow regulations. Local governments must approve all plans. Inspectors check that construction is done according to these plans.

Myth #10: We can just rebuild all of the storm pipes and channels and everything will be fixed.

We are making new investments in drainage and flooding systems every year, but there are many pipes and channels that were built before the 1980s. To reduce the risk of flooding for everyone to less than 1% each year would take about $29 billion.

The Business Case for “Natural Drainage Systems” in Houston Area Development

Posted on by

Houston area developers have traditionally used concrete parking areas, concrete streets, and pre-cast concrete storm sewer systems to convey rain water quickly and efficiently to “end-of-pipe” detention basins.  From there, the collected rainwater is discharged into nearby streams or bayous at a restricted rate to avoid downstream flooding.

Developments are typically disconnected from their nearby streams in favor of locating homes and businesses around the detention basins, which are often designed with permanent pools of water and are viewed as manicured “lakes” by future residents.

There is an alternative, however: natural drainage systems – also known as “low impact development (LID)” or “green stormwater infrastructure (GSI).”

Natural drainage systems simulate natural headwater streams and bayous, more closely mimicking the natural flow of water across the landscape.  They can extend the existing bayou and stream corridors up into the development, serving as natural open-space, creek or bayou style amenities that support adjacent trails and parkland.

The use of natural drainage can replace the use of concrete storm sewers, and because the water runs off the site more slowly, the system requires less detention.  This allows the development site to accommodate a higher number of homes or commercial buildings, reduces drainage system costs, and provides for an open-space amenity, such as parks or trails.

Natural drainage systems are a marketing differentiator for developments.  They capitalize on the market demand for natural and environmentally friendly neighborhoods.  They can serve as a framework for trail systems, which are ranked among the highest in consumer-requested amenities.  They can provide a polished and manicured look along entryways and community front doors, while maintaining a wild and rustic look along paths and leading away from back doors.

Finally, natural drainage systems serve a drainage utility function under the Texas Water Code.  This means that developers can be reimbursed by a special district for the cost of their construction with the proceeds from tax free municipal bond sales.

Let’s look at a few examples.

Audubon Grove, a large-lot, single family residential subdivision in Springwoods Village near the new Exxon-Mobil, campus features 57 lots on about 24 acres.  Designed by Costello, Inc. for Taylor Morrison, the development includes trail systems along natural swales (see Figure 1).

Figure 1 – Trail Network of Audubon Grove

Image: Google Earth.

The concrete roadways do not include curbs so that stormwater runoff can drain directly into the swales.  Front yard swales have been landscaped with cobbles to create a more refined and polished look (see Figure 2).

Figure 2 – Polished Front Door Look in Audubon Grove

IMAGE: M. Bloom

Camellia, is another single-family subdivision that has used natural drainage systems.  Camellia is located in Fort Bend County and includes 323 lots on about 50 acres.  Designed by EHRA for Legend Homes, the development includes roads with cross slopes down to depressed vegetated center medians.

The outside edge of the roads are include traditional curbs while the inside edge is curbless to allow for sheet flow into the swale system.   Figure 3 illustrates the overview and Figure 4 shows the roadway and vegetated median area.

Figure 3 – Camellia Overview

IMAGE: google earth.

Figure 4 – Camellia Vegetated Median

IMAGE: M. BLOOM

The use of natural drainage systems in Camellia reduced overall project infrastructure costs by $1.6 million, increased lot yield by 99 homes, and reduced the volume of detention required to comply with floodplain regulations. [Ring, J. 2015.  Talking Dollars and Sense: LID Construction Costs. Presented at the ASCE International LID Conference. Houston, Texas. January.]

Lastly, let’s look at Stonebrook Estates in the Champions/Spring area.  Designed by R. G. Miller Engineers, Inc. and Aguirre & Fields for Terra Visions, LLC., the development includes 135 lots on about 51 acres (see Figure 5).

Figure 5 – Overview of Stonebrook Estates Under Construction

IMAGE: terra visions.

About 70% of the development is served by a natural drainage system with landscaped and manicured ditches (called “swales”) and biofiltration, which is basically a high flow rate sand filter for stormwater that removes pollutants. The rest of the development is served by traditional storm sewer.

Roadways are sloped to one side and have curbs but feature “false back inlets” that drain stormwater to vegetated swales instead of expensive underground storm sewer pipes (see Figure 6).

Figure 6 – False Back Inlet in Stonebrook Estates

IMAGE: TERRA VISIONS.

The use of natural drainage reduced the site detention requirement by 24%, which increased lot yield.

The business case for natural drainage in the Houston area is clear.  Natural drainage:

  • Reduces the volume of detention required to comply with floodplain regulations; Increases lot yield;
  • Reduces the cost of drainage infrastructure.
  • Allows for reimbursement for the cost of drainage facilities;
  • Provides an open space and natural amenity to more of the homes in the development, allowing the developer to charge higher sale prices;
  • Capitalizes on the market demand for environmentally friendly and natural communities; and
  • Differentiates the development from all the rest.

For more information about Houston-area natural drainage projects, check out the Houston-Galveston Area Council’s Designing for Impact: A Regional Guide to Low Impact Development.

Renewing Our Flood Insurance Policy

Posted on by

My family and I are fortunate not to live in a designated floodplain. Designated floodplains are areas along our bayous that have a 1% annual chance of flooding.  This diagram, adapted from the Harris County Flood Control District, illustrates the terminology we use to describe risk levels nicely:

The risks depicted in this graphic are from bayous flowing out of their banks due to large rain events. This does not illustrate the risks associated with large rainfall events exceeding the capacity of drainage systems serving local streets and neighborhoods.

In general, older drainage systems expose residents to higher risks of flooding while new drainage systems expose residents to lower risks of flooding. This is because a number of things have changed and improved over time. Our design standards have improved. Our knowledge of the location and depth of floodplains has improved. Our ability to measure ground surface elevations using laser technology has improved. Engineering practices have improved. That said, even the newest development — designed and built with the best engineering of 2017 — still has some risk of flooding from a large rain event. Current design standards accommodate the 1% annual chance rain event — about 12 inches of rain in 24 hours — anything more than that (or faster than that) may lead to structural flooding.

We have what I call legacy developments in the Houston region that were built before we had a national flood insurance program; before we mapped the 1% annual chance floodplains; and before we required flood mitigation of all new development in the form of detention basins. These are areas that have vibrant, economically productive communities; but they also have a higher risk of flooding than the current standard of practice.

We don’t have many options to address these legacy development areas and none of them are free. We could elevate all structures; buyout businesses and homes; retrofit or add new drainage systems (if the ground and flood elevations allow it); retrofit or add new detention facilities; or widen or deepen channels (again, if the ground and flood elevations allow it).  Our public agencies are taking all of these actions almost all of the time, but the general public is not generally aware of this.

Due to our high rainfall, our flat topography, the age of much of our infrastructure, and the fact that we cannot implement a zero-risk approach to design — folks living in Houston should have flood insurance — whether you live in the floodplain or not.

You can check to see if you are located in the floodplain by typing in your address on this Federal Emergency Management Agency (FEMA) map search page. After the results pop up, just click on “View Map” (the magnifying glass icon) and zoom to your location.  The legend explains all of the shading and codes.

If you are fortunate to not live the floodplain, the premiums are very affordable.  Here’s a photograph of my current premium bill:

It cost much more to insure properties inside the 1% annual chance floodplain because the underwriter is exposed to a higher risk of loss. The underwriter for those policies is the U.S. National Flood Insurance Program (NFIP).

The premium charged by NFIP does not, however, reflect the true risk of loss, because members of the United States Congress tend to keep the premium costs subsidized so they can be re-elected.  This has led to the NFIP Trust Fund to be periodically strapped for cash, as recently reported in the Houston Chronicle.  Perhaps I’ll post on this topic another day.

Are you in – or out – of the floodplain?  If you are out of the floodplain, how close to the 1% floodplain is your home located?  If you are located outside the floodplain do you purchase insurance?

Tax Day Event of 2016 vs. The “Historic” Flood of December 1935

Posted on by

After Ms. Susan Chadwick of Save Buffalo Bayou stated as “fact” that “engineers caused” the large flooding event which occurred in downtown Houston December 6-8, 1935 I though I would do some research into the rainfall on those days.

This was a tremendously damaging event that killed eight people and caused $2.5 million in property damages (in 1935 dollars).  This was according to a 1937 report submitted by the Secretary of War (transmitting a report by the U.S. Army Corps of Engineers) to the U.S. House of Representatives Rivers and Harbors Committee.

Here’s a picture of downtown Houston during the flood:

Photograph credit: Harris County Flood Control District.

In the coming weeks I’ll post more thoughts about the claim that engineers “caused” the flooding in December of 1935.  For now I just want to focus on the rainfall.

When I first saw the rainfall totals for the 2016 Tax Day event, I thought they might be similar to the December 1935 event, but of course, being an engineer and a “data geek,” I had to check.

Here is a map of the rainfall totals inside the Buffalo and Whiteoak Bayou watersheds from December 6, 1935 to December 8, 1935:

Peak rainfall over three days was estimated to be 20 inches. The smallest rainfall in the watershed was estimated at around 5 or 6 inches over three days.  The U.S. Army Corps of Engineers estimated that the average rainfall in the Buffalo and Whiteoak Bayou watersheds over the three day period was 15 and 12.7 inches respectively.

For comparison, each year Houston has a 1% chance of receiving 12 inches of rain in 24 hours; our average rainfall total is about 48 inches over an entire year; and most normal months we get about 4 inches of rain.

Here a map of the rainfall totals from the 2016 Tax Day event:

The red watershed boundary was copied over from the 1935 map and the watershed map with rain gauge totals was scaled to match.

The peak rainfall over three days was measured to be 17.6 inches at West Little York and Highway 6.  The smallest rainfall in the watershed was measured at 0 inches over Cinco Ranch.

So I’d say these two rain events were pretty similar.  What do you think?

Drafting a New Municipal Stormwater Quality Permit

Posted on by

The Texas Commission on Environmental Quality (TCEQ) held a stakeholder committee meeting today in Austin to kick-off the process of revising and reissuing their statewide permit for regulated small municipal separate storm sewer system (MS4) operators. If you are a true stormwater geek you can watch the entire meeting online at AdminMonitor.

MS4’s are the system of pipes and channels operated by your local city or county to carry rainwater away from homes and businesses and towards natural bayous and creeks.

Under the federal Clean Water Act and the Texas Water Code, certain MS4’s must obtain a permit (basically a permission slip) to discharge pollutants in stormwater runoff from their MS4 to waters of the United States.

The permit requires regulated storm sewer system operators to development and implement programs to:

  • Educate the public on stormwater pollution prevention;
  • Find and eliminate non-stormwater discharges to the storm sewer;
  • Reduce pollution from active construction sites;
  • Reduce pollution from operating new development areas;
  • Reduce pollution from city or county operations;
  • Addressing industrial sites; and,
  • Addressing discharges to impaired waters or waters with a pollution budget.

The current permit was issued in December 2013.  All Clean Water Act permits typically last for five years, so the current one will expire in December of 2018.  Due to the time required to draft the permit, issue formal public notices, take comments, and publish responses to comments — not to mention conduct negotiations with the Environmental Protection Agency (EPA) — it takes a long time to get a new permit issued.  This means the TCEQ needs to start drafting the new permit now!

The anticipated changes discussed at the meeting today breakdown into two main types:

First, the permit will be revised to implement federal digital reporting requirements, which shift paper submittals of compliance reports to a digital format.

Second, the permit language will be tightened up to better define compliance obligations that are clear, specific, and measurable. These changes will reduce or eliminate the use of words like “if practicable,” “as necessary,” “if feasible,” and similar words that can be viewed as making permit provisions optional or subject to permit holder discretion. These changes stem from a new rule known as the Remand Rule, which clarifies how states must alter their permitting process to include substantive public review (TCEQ’s process was already compliant with this) and directs states to write more explicit permit provisions.

If you have comments or questions on this post, please leave a reply below.  If you have comments on how the new TCEQ permit should be drafted, please send them to TCEQ at swgp@tceq.texas.gov by April 4, 2017.