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.