Although much research and monitoring have been conducted nationally to evaluate the effectiveness of various stormwater quality best management practices (BMPs), most studies have focused on the effectiveness of a single installation using site-specific monitoring approaches. Because of variations in study design and reported information, this approach has resulted in a patchwork of reported efficiencies for various BMP types, complicating efforts to compare results.
To address this problem locally, the Harris County Flood Control District has initiated a program to uniformly and consistently evaluate the effectiveness of regional flood-damage-reduction facilities that incorporate water-quality enhancements. The district, which is responsible for designing, implementing, and maintaining flood-damage-reduction infrastructure in Harris County, TX, will use the information derived from these evaluations to modify and update its applicable BMP design criteria, improve stormwater quality management in the Houston area, and contribute to the national effort of assessing and quantifying BMP effectiveness.
The district is currently conducting stormwater monitoring at three sites according to site-specific quality-assurance project plans and a monitoring protocol. Quality-assured pollutant-loading data for various water-quality parameters are obtained from each of the sites to evaluate if the facilities are reducing concentrations of those pollutants entering receiving waters. Initial results from the three current studies reveal varying levels of pollutant removal at each stormwater detention basin. The district plans to continue monitoring in the future and, based on the monitoring results, will potentially adjust design criteria for its infrastructure.
2,500 Miles of Stormwater Management
Located in southeast Texas, Harris County covers approximately 1,700 square miles, including most of the Houston metropolitan area, and has a population of approximately 3.9 million people. Consisting mainly of flat, Gulf Coast land in which fine-grained soils predominate, Harris County typically has mild winters and long, hot, and humid summers. With an average annual rainfall of 48 inches, Harris County is subject to occasional tropical weather systems.
Harris County has experienced significant flooding in the past. In fact, following severe floods in the 1920s and 1930s, the Texas legislature created the district in 1937 to build projects to reduce flood damage. The district’s mission is to provide effective flood-damage-reduction projects, with appropriate regard for community and natural values. The district achieves its mission by devising flood-damage-reduction plans, implementing the plans, and maintaining the related infrastructure. Over time, the district’s existing system of natural channels has been expanded to include about 2,500 miles of open channel, as well as a vast network of local, city-operated subsurface drainage systems (Figure 1).
Until 1980, the district conveyed floodwaters almost entirely within its network of open channels. In 1980, the district began using stormwater detention basins as part of its efforts to reduce flood damage. Today, the district operates and maintains approximately 130 detention basins within Harris County. In 1998, the district received a stormwater permit issued as part of the federal National Pollutant Discharge Elimination System (NPDES). Since then, the district has developed a program to monitor water quality in its detention basins and address wet-weather runoff and water-quality concerns from its municipal separate storm sewer system.
The district has worked with an engineering consulting firm, PBS&J, to develop a stormwater-quality pond-monitoring protocol to guide the development of site-specific monitoring plans. The protocol was adapted from Urban Storm Water BMP Performance Monitoring: Guidance Manual for Meeting the National Storm Water BMP Database Requirements, a monitoring guidance published in 2002 by the Urban Water Resources Research Council of the American Society of Civil Engineers (ASCE).
Under the protocol, the district and its consultants create site-specific monitoring plans, design and maintain permanent monitoring stations at each detention basin, and conduct various monitoring activities. The protocol and associated monitoring plans define qualifying rain events as a rain event producing 0.1 inch or more of rainfall having an antecedent dry period of at least 24 hours. Instrumentation at each site collects and stores hydrologic data. Automated sampling units are used to monitor inflows and outflows multiple times on a flow-proportional basis to obtain wet-weather event mean concentration (EMC) data for most parameters. Grab samples are collected for constituents that have short holding times, such as bacteria, or restrictions regarding sample collection and preservation, such as oil and grease and total petroleum hydrocarbons. (Total petroleum hydrocarbons are sampled at one site only.) According to the protocol, composite and grab samples may be analyzed for a suite of “minimum,” “recommended,” or “additional” analytes.
Under the protocol, monitoring stations typically include devices for measuring water depth, water velocity, rainfall depth, and flow, along with hardware necessary to collect and store composite samples. All monitoring stations include refrigerated samplers that are either solar powered or connected to a power line. All stations also include a cellular modem to enable connectivity for remote-control features (sampling pacing, enabling, etc.) and data retrieval. All equipment is protected by a weather-resistant enclosure. One remote location is protected by a bulletproof enclosure made of 0.5-inch-thick carbon steel. Figure 2 illustrates a typical permanent monitoring station. In addition to specifying all details of monitoring methods and equipment used, the protocol and site-specific monitoring plans also define rigorous procedures for quality control and quality assurance. All monitoring plans are currently being updated and routed for formal approval by the Texas Clean Rivers Program, which is the statewide water-quality-monitoring program that provides data for Clean Water Act Section 303(d) listings and Section 305(b) reports. This is significant, as this will allow the district’s stormwater data to be formally used by the Texas Commission on Environmental Quality in watershed assessment efforts, total maximum daily load (TMDL) activities, and TMDL implementation effectiveness monitoring.
The district is currently monitoring three facilities to determine their effectiveness as stormwater BMPs:
- The Greens Bayou Wetland Mitigation Bank, which is an advanced wetlands treatment system for stormwater runoff
- A conventional dry stormwater detention basin, known as Armand Bayou Basin, designed to control the peak flow arising from a 100-year storm event
- An enhanced stormwater conveyance and detention basin with water-quality enhancements, known as the Mason Creek Regional Detention Basin
The Greens Bayou Site
The 226-acre Greens Bayou site consists of a surge basin and a series of polishing ponds and polishing wetland marshes that receive drainage from 88 acres. The Sam Houston Parkway is the primary source of untreated stormwater runoff entering the site. Situated about 25 miles from Galveston Bay and 50 miles from the Gulf of Mexico, the Greens Bayou site consists of 1) a stormwater culvert collecting runoff from the storm sewer system serving the Sam Houston Parkway, 2) a surge basin for initial collection and storage of stormwater, 3) and a pump station and force main to convey stormwater to the polishing ponds and polishing wetland marshes. After the polishing facilities, the stormwater enters a series of interconnected habitat wetlands and swales, including ponds, littoral marshes, and transitional wetland forest areas that make up a portion of the wetlands mitigation bank. Water is ultimately discharged from the west wetland habitat to a utility right of way in the project’s southwest corner. However, most flow-through the west wetland habitat is currently recirculated to the surge basin via the east wetland habitat.
To evaluate its effectiveness at treating stormwater runoff, the Greens Bayou site includes three monitoring stations. The first is located at the main stormwater culvert that delivers runoff from the storm sewer to the surge basin. The second monitoring station is located at the aboveground section of the force main that transfers water from the surge basin to the polishing ponds. The third monitoring station consists of two sampling stations located at two different drain weirs in the southwest corner of the project’s west habitat (Figure 3). The third monitoring station is used by field staff to conduct composite grab samples.
Sampling began in December 2004, and monitoring activities to date are summarized in Table 1. The district plans to continue monitoring in future years at this site to obtain a more robust data set.
Preliminary analyses of data collected at Greens Bayou revealed no statistically significant reduction of any parameter in stormwater passing through the surge basin. However, statistically significant increases in total lead, total zinc, and dissolved zinc were found in stormwater leaving the surge basin. The pump station and associated piping are the most likely source of these metals to the system.
Compared to runoff leaving the surge basin, stormwater passing through the west habitat area was found to have statistically significant decreases in nitrate–nitrogen, total copper, total lead, total zinc, dissolved zinc, suspended sediment concentration, and total suspended sediment. These results demonstrated that the Greens Bayou site is functioning as designed to improve water quality through its extensive network of wetlands and ponds.
While noteworthy, a statistically significant removal is not as important as the effluent quality produced by the Greens Bayou wetlands mitigation bank, because removal rates depend greatly on influent quality. Data collected to date suggest that the wetlands mitigation bank consistently produces effluent quality that is well below EMCs for solids, nutrients, and bacteria, as reported in the National Stormwater Quality Database (NSQD) developed by Robert E. Pitt of the University of Alabama. The NSQD is a statistical summary of stormwater pollutant levels as measured by numerous municipalities across the country. The median site outlet EMCs from the west habitat were compared to the NSQD median EMCs. See Table 2 for results. See Figure 4 for a nitrate–nitrogen box plot comparing influent and effluent data.
A Conventional Dry Stormwater Detention Basin
The 16-acre Armand stormwater detention basin receives runoff from a 121-acre residential watershed that has 40% to 60% impervious cover. A detention basin designed to control the peak discharge flow arising from a 1% chance storm (equivalent to a 100-year storm event), Armand was not designed to provide water-quality benefits. However, the district wished to determine whether this basin design resulted in water-quality improvements and how this basin’s performance compared to basins with water-quality enhancements.
Situated roughly 10 miles from Galveston Bay and 30 miles from the Gulf of Mexico, the Armand site consists of a detention basin that includes two inlets and one outlet. The primary inlet ties into the local storm sewer system that drains the adjoining neighborhood. A secondary inlet at the detention basin’s lower end serves a small overflow area. Because the drainage volume from the secondary inlet is insignificant compared to the volume from the primary inlet, flow from the secondary inlet is not monitored (Figure 5).
Armand’s performance is monitored with two monitoring stations. The first is located at the basin inlet, and the second is situated at the outlet. Sampling began in September 2005, and activities to date are summarized in Table 3.
Preliminary analyses of data collected at Armand revealed a statistically significant decrease in total lead, total zinc, fecal coliform, and E. coli in stormwater passing through the basin. These results demonstrated that although Armand was designed without water-quality features, it may function to improve water quality. The decreases might be explained by Armand’s wide, shallow flow path. During storm events, basin flows extended beyond the concrete pilot channel and passed through the vegetated bottom at a lower velocity. As a result, these flow patterns might have facilitated additional solids settling in the vegetated basin bottom, possibly increasing the removal of bacteria and the other constituents.
As noted earlier, a statistically significant removal is not as important as a detention basin’s effluent quality. Data collected to date suggest that the basin consistently produces effluent quality that is below NSQD EMCs for solids and bacteria and above NSQD EMCs for nutrients. See Figure 6 for the E. coli box plot comparing influent and effluent data.
Enhanced Stormwater Conveyance and Stormwater Detention Basin
Located in the western corner of Harris County, the 95-acre Mason Creek stormwater detention basin and channel is designed to control the peak flow arising from a 100-year storm event. The basin is also designed to provide water-quality benefits via a forebay, a middle basin, and a lower basin with permanent pools and with vegetation designed to provide water-quality benefits. The channel conveying flow to the basin includes riparian forest vegetation, flat side slopes, and a meandering low-flow path. The wet bottom detention basin receives drainage from approximately 1,085 acres. By monitoring the Mason Creek basin and its associated channel, the district can assess the extent to which this particular basin configuration achieves its water-quality objectives and determine the effects of the water-quality enhancements, such as the permanent pool and vegetation (Figure 7).
Situated about 48 miles from Galveston Bay and 70 miles from the Gulf of Mexico, Mason Creek receives runoff from a watershed that is partly residential with 10% to 20% impervious cover and with additional development planned. The facility consists of the upstream channel, two major inlets to the basin, and one outlet. The primary inlet receives stormwater from the upstream channel and from roadside ditches. A secondary inlet at the detention basin’s southwest corner receives flows from the storm sewer system serving the nearby Williamsburg Parish subdivision.
Construction of four permanent monitoring stations was underway at press time. Located in the upstream channel, the first monitoring station will evaluate water quality in the channel by obtaining samples for laboratory analysis and conducting in situ monitoring of certain water-quality parameters in influent stormwater. Located at the basin inlet, the second monitoring station will evaluate the quantity and quality of stormwater runoff entering the detention basin. Located at the detention basin’s southwest corner, the third monitoring station will be used to evaluate the quantity and quality of stormwater runoff entering the basin from the Williamsburg Parish subdivision. The fourth monitoring station, located at Mason Creek’s outlet, will be used to evaluate the basin’s effectiveness in removing pollutants and water-quality benefits. It also includes an instrument that obtains and records real-time measurements of pH, dissolved oxygen, conductivity, temperature, and turbidity.
Grab sampling and in situ water chemistry measurements began in January 2008, and activities to date are summarized in Table 4.
A review of the initial data collected at Mason Creek indicates that the highest levels of constituents are entering the basin from the residential neighborhood and roadside runoff at the entrance to the basin. Currently, insufficient data exists to determine statistical significance. However, box plots for E. coli, fecal coliform, and enterococci show a general reduction at the outlet of the basin compared to all other stations. Figure 8 shows graphs of selected results. Data collected to date suggest that the facility may produce effluent quality that is above NSQD EMCs for bacteria. For E. coli, the median Mason Creek outlet EMC was 2,810 MPN/100 mL compared to the median NSQD EMC of 1,750 MPN/100 mL.
Tying Everything Together
In summary, the advanced Greens Bayou Wetland Mitigation Bank performed as expected, providing substantial water-quality benefits at the outlet of a long maze of wetlands and ponds, particularly regarding concentrations of pollutants comprising heavier particulates. Although the Armand Bayou Basin was not expected to provide any water-quality benefits, effluent levels of bacteria and solids were lower than typical urban runoff values. Bacteria levels, however, exceeded the standard for freshwater streams designated for contact recreation. The Mason Creek Regional Detention Facility, which includes flood-control and water-quality features, has initially shown bacteria effluent levels greater than typical urban runoff values and the freshwater contact recreation stream standard. However, this result is preliminary due to the small number of initial results.
These findings, along with future results, will provide the district with a technical basis for developing and imposing appropriate stormwater quality requirements for drainage and flood-damage-reduction facilities, as well as for modifying current requirements as appropriate. As TMDLs are adopted for waterways within its jurisdiction, the district will benefit from the data, particularly as it implements regulatory measures and conducts subsequent monitoring.
What Next?The district, of course, is not alone in its need to track and evaluate BMP performance, as many communities throughout the country, and in other parts of the world, face the same challenge. For this reason, efforts are underway at an international level to meet these needs. In 1996, a team of experts from ASCE’s Urban Water Resources Research Council began the International Stormwater BMP Database project under a grant from the US Environmental Protection Agency. Now managed by the Water Environment Research Foundation, the project has grown considerably during the past decade.
The district implemented local monitoring methods derived from monitoring guidance developed to support the International Stormwater BMP Database. Other regional stakeholders have also begun conducting BMP evaluation studies using the same protocols. The district plans to periodically evaluate the results from its stormwater management program to learn more about local BMP effectiveness and the effects of design, soils, land use, and other factors that contribute to performance and effluent quality. The district plans to submit these data and results to the International Stormwater BMP Database to assist researchers throughout the world.