LOW IMPACT DEVELOPMENT

8 September/October 2013

B irnamwood Drive located in northern Harris County, Texas, runs through a naturally forested area and connects a

residential neighborhood at the south end to a park to the north end. It is a .74-mile stretch of boulevard contained within a 100ft right of way, with a center median swale that collects water from the street on either side, through false back inlets. With-in the swales are a series of forebays that

pond stormwater behind earthen berms, at the bottoms of which are 10” pipes that allow low fl ows through and extend time of concentration for larger storm events. At the low points of each swale, prior to discharge, an advanced high performance biofi ltration system using a high fl ow rate engineered media is used to fi lter water that has made its way through the pretreatment forebays and vegetated swales, prior to dis-

charging into Spring Creek at the north end and a Harris County Flood Control Ditch at the south end.

Like many regions throughout the country, the adoption and implementation of Low Impact Development (LID) and Green Infrastructure in Houston didn’t happen overnight, but the speed at which it has become a success story has astounded many outsiders, especially given our geo-

Birnamwood Drive Showcases LID-Based Roadway

One year after installation.

graphic and climatic conditions, and the lack of a regulatory requirement for the use of LID. Th is success was mostly due to an economics based approach that uses the market, not regulatory drivers, to promote Low Impact Development. Historically, TMDLs, CSO issues and/or a severely

threatened water body were thought to be the most signifi cant drivers for Low Impact Development. Th e history of Birnamwood Drive, Texas’ fi rst Green Infrastructure based County Road, is one that can show any municipality in the country how to ef-fectively implement Low Impact Develop-

ment policy goals and how using it on their own capital projects can lead to saving mil-lions of tax payer dollars.

In February of 2010, the Houston Land/Water Sustainability Forum (HL-WSF) had just completed its nationally acclaimed Low Impact Development De-sign Competition. In that competition, a Harris County roadway that was slated to be expanded from a two lane roadway to a four lane boulevard was used as the subject property for the Green Roadway Design Challenge. Each of the three Design Chal-lenges which made up the competition re-quired design teams to answer three critical questions: 1) despite our 50+ inches of an-nual rainfall, clay soils which off er minimal infi ltration potential and fl at terrain, could Low Impact Development be adapted to keep post-development discharge curves below predevelopment rates and manage the 100-yr storm, 2) exceed current water quality standards, and perhaps most impor-tantly, 3) could Low Impact Development based projects actually be less expensive to implement than conventional designs? Twenty-two teams competed, and every one of them showed that the three goals were achievable in each Design Challenge category, Green Roadway, Urban Redevel-

LOW IMPACT DEVELOPMENT

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Roadway construction began in August 2011 and was completed in February 2012.

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opment and Suburban Residential. Much to the surprise of almost everyone involved, the LID-based projects were better by al-most any measure and less expensive, and the once-hesitant local design community had proven it to themselves.

For Harris County, the Green Road-way Design Challenge solutions, which were generated by some of the most promi-nent engineering consulting fi rms in the Houston area, provided an opportunity to lead. In concert with the HLWSF, a series of three workshops were scheduled over the course of three months. Th ese full day workshops brought together a select group of local public agencies, including key staff from Harris County, the City of Houston and the Harris County Flood Control Dis-trict, and a select group of engineers, ar-chitects, landscape architects and planners from the local design community, most of whom were veterans of the competition. Approximately sixty-fi ve participants in all focused on identifying the obstacles and the options for implementing LID in the Houston area, with an emphasis on per-formance goals rather than getting caught up in over prescription and enabling a ‘get it done now’ approach. From these work-shops, the Harris County Low Impact De-velopment & Green Infrastructure Design Criteria for Storm Water Management was born. In this manual, the path was clearly paved for both private and public development to design using LID. No tax incentives, water fee reductions or other fi nancial incentive was created to promote LID development; however indirect eco-nomic incentives were strategically built in to accelerate LID implementation. It all starts with a collaborative permitting pro-cess which eff ectively fast-tracks LID-based projects through the permitting process.

Another incentive for LID-based proj-ects is reduced detention rates. Satisfacto-rily proving up the peak fl ow reductions inherent in LID design through increased time of concentration, allows LID-based projects to reduce required detention rates by as much as 47%, from .65 acre feet per acre developed to .35 acre feet per acre de-veloped. In most cases, this results in a sig-nifi cant increase in Return on Investment for the developer, whether it be a retail de-velopment, a master planned community, or anything in between.

Th ese two benefi ts, along with reduced infrastructure costs and reduced long-term

maintenance costs, makes LID extremely attractive to the development community, but also to Harris County for its own proj-ects. Birnamwood Drive was the fi rst proof of these benefi ts. Th is fi rst eff ort initially got the County’s attention when it required no traditional detention pond, saving the county more than $350,000 on land and excavation costs. Instead, all of the volume required was contained in the center me-dian bioswale.

Even with the reduced costs for drain-age infrastructure and detention pond, the LID design for Birnamwood Drive still needed to deliver water quality treatment in a way that would not increase costs to the point of exceeding the cost of a tradi-tional roadway design. Th e fi rst consider-ation was traditional slow fl ow rate biofi l-tration medias. Th is approach would have required a bioretention system to be in-stalled throughout the entire 3/4-mile cen-ter median, meaning lots of excavation and a substantial network of underdrain pipe, running the cost upwards of $270,000. Th is would have negated most of the LID design cost savings, reducing them to per-haps 1%, but the bigger problem was with maintenance. In order to avoid compaction of the bioretention media, County mainte-nance crews would not have been able to use their typical machinery and established maintenance protocols on the entire bio-swale. Concerned about the fi nancial suc-cess of their fi rst Green Roadway, Harris

County began looking into new methods and materials.

Ultimately, an advanced High Per-formance Modular Biofi ltration System (HPBMS), also known as FocalPoint, that incorporates a high fl ow rate engineered media was the answer. Th e system is as ef-fective at pollutant removal as traditional engineered medias, but requires 1/20th of the footprint needed for traditional bio-retention to match the fl ow velocity. Th e smaller footprint biofi ltration system cost more than the traditional bioretention ap-proach, but its greatly reduced size meant signifi cant savings, roughly 50%, for the water quality treatment element of the project. Th e smaller footprint also allowed the vast majority of the vegetated swale upstream of the two small biofi lters to be maintained using traditional County road-way processes. Th is solution was a win for the County and increased their savings for the LID design to 7% versus traditional pipe and pond. Subsequent LID-based Harris County roadways have saved far more money, but Birnamwood was a learn-ing exercise that proved Harris County was on the right track.

Construction phasing had its own set of challenges. Th e contract was split into two packages, a civil package and a land-scape package. Th e civil package covered the roadway, placement of false back inlets to discharge runoff into the swale, a small amount of storm sewer to convey water

LOW IMPACT DEVELOPMENT

10 September/October 2013

Native grasses were used to provide a cost-effective solution to reduce maintenance costs.

that had been cleaned through the bio-swales into Spring Creek, and rough grad-ing of the swales. After 9 months of con-struction, the roadway was complete and the project was handed over to the land-scape contractor who was responsible for fi nal grading the swales, planting the native plants throughout the swale and the instal-lation of the biofi ltration system.

Because of the effi ciency of the high fl ow rate media and the resulting small footprint of the biofi ltration system the ex-cavation required to get the job done was greatly reduced. Once the excavation was complete, laborers hand placed the modu-lar, fl at pipe underdrain system that would be connected to the downstream storm sewer system. Once in place, a small ex-cavator carefully backfi lled 3/8” peagravel bridging stone, 12” around the sides and 6” above the underdrain, to separate the engineered soils from the underdrain. 18” of the engineered soil was then placed on top of the peagravel, followed by a 3” layer of shredded hardwood mulch.

Although the biofi ltration system was installed during the landscape phase, there were numerous challenges with keep-ing the biofi ltration system from clogging with sediment while the landscaper planted and waited for establishment of the native seeds which would stabilize the swale. Ero-sion control was a high priority and mak-ing sure there was adequate ground cover to prevent erosion was critical. Th e fi rst step in preventing contamination of the biofi ltration system was encasing it with a protective geotextile cover and surrounding it with wattles. Th is process created a per-meable fi lter for the water to run through and prevented silt and sediment from pen-etrating. Th e placement of rock fi lter dams was also necessary upstream of the biofi l-tration system to both slow the velocity and prevent erosion of the swale bottom. Finally, the use of a high performance hy-draulically applied or pinned in place ero-sion control blanket along the side slopes greatly improved the success of proper seed establishment. Th e combination of these best management practices helped keep the biofi ltration system free of sediment during the construction process.

Once the bioswales were completely landscaped and stabilized, the geotextile cover was removed from the biofi ltration system and the biofi lter was tested for hy-draulic conductivity. Th is pass/fail perfor-

LOW IMPACT DEVELOPMENT

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FocalPoint removes critical pollutants like TSS, Nitrogen and Phosphorus at levels exceeding traditional systems while consuming up to 95% less space on project sites.

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LOW IMPACT DEVELOPMENT

12 September/October 2013

mance test is specifi ed for installers of bio-retention/biofi ltration vendors in order to get fi nal approval for payment from Harris County. With a passing result on the bio-fi ltration system in hand, the plants were

planted in the media bed and the system was activated.

Harris County, Harris County Flood Control District and the City of Houston

are all making signifi cant contributions to making Low Impact Development a viable alternative to traditional design in Harris County by focusing on the economics that drive implementation. Prior to the Hous-ton LID Design Competition in 2010, be-yond a few ‘tacked on’ LID features on a handful of projects, meaningful LID-based designs didn’t exist in the Houston area. In the subsequent three years, there have been roughly 50 projects completed using a complete LID approach, hundreds more have been designed, many of which are currently in construction ranging in scale from master planned communities with thousands of acres to Fortune 5 headquar-ters campuses to multifamily and mixed use developments to suburban roads and downtown green streets, to the neighbor-hood Starbucks.

Low impact Development was origi-nally conceived in Prince George’s County, Maryland as a way to improve water qual-ity AND lower the cost of compliance

with stormwater regulations. What we’ve learned in the Houston area is the empha-sis on LID/Green Infrastructure in meeting the new regulatory paradigm is well placed. If we start with the end in mind, focusing on our performance goals rather than our penchant for prescription, we create fertile ground for market driven implementa-tion. Th e LID pioneers gave us the tools; cheaper and better always wins in the mar-ket. L&W

by David Batts, Construction EcoServices & Robert Adair, Convergent Water Technologies

For more information, contact David Batts, Construction EcoServices, 1930 Aldine Western Rd., Houston, Texas 77038, Phone: 832-456-1000, E-mail: dbatts@ecosvs.com, or Website: www.ecosvs.com.

Stormwater draining through the bioswale.

The rst step in preventing contamina-tion of the bio ltration system was encasing

it with a protective geotextile cover and surrounding it with

wattles.