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.

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?

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?

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.

These days most websites are coded to display a little square icon in the browser tab, like this:

Riparian Houston is coded to display a tiny version of this:

But what the heck is that?

The image displays three different hydrographs.  The vertical axis displays the flow rate at a particular location at a particular moment in time. The flow rate is expressed in volume per unit time.  This might be cubic feet per second or gallons per minute.  The horizontal axis displays time in minutes, hours, or days, depending upon how large an area is draining.

When rain falls on any area of ground draining to a single point (the area of ground is called a catchment, a watershed, or a drainage area) the flow rate from the drainage area is initially zero, it increases to a peak rate, and then it declines back down to zero sometime after the rain stops.

The green hydrograph illustrates the typical flow rate variation you might see from an undeveloped area of land. The red hydrograph illustrates the typical flow rate variation you might see from development land with no mitigation. The blue hydrograph illustrates the typical flow rate variation you might see from development with a detention system used to mitigate the increase in runoff.

So the Riparian Houston icon is not just a pretty logo after all.