By Richard A. Greuel and Ronald A. Feldner

The National Pollutant Discharge Elimination System (NPDES) Phase II stormwater permits now force hundreds of communities throughout the nation to look differently at how they manage their stormwater programs. These programs will undertake various approaches to reduce nonpoint-source pollutant loadings to impaired water bodies in an effort to achieve Phase II compliance and impending total maximum daily load (TMDL) restrictions. One community taking a proactive approach to stormwater management for nonpoint-source pollution is Griffin, GA. The North Griffin Regional Detention Pond (NGRDP) accomplishes two objectives–flood control and water-quality enhancement–by incorporating constructed wetland filtration into a regional detention pond. The Consulting Engineers Council of Georgia recognized the project design and performance success with an Engineering Excellence Award in February 2000. The Georgia Environmental Protection Division and USEPA Clean Water Act (CWA) Section 319(h) Program also acknowledged the project’s achievement.

Project History

Flooding at the Waterford on Ellis subdivision

Located approximately 50 mi. south of Atlanta, Griffin was founded in the late 1800s as a crossroads between Atlanta to the north and the city of Macon to the south. Sitting atop a major topographic ridge separating the watersheds leading to the Atlantic Ocean and the Gulf of Mexico, Griffin developed as a strong railroad transportation hub that in turn led to the development of several large manufacturing plants within the city. As development grew in response to the needs of local industries, the city’s stormwater infrastructure could not keep pace with the growth of the community. Now the system is more than 100 years old and in need of extensive maintenance as well as significant improvements. Many components of the city’s existing storm sewer system are now inadequate as a result of increased stormwater runoff volume.

Significant rainfall events, such as Tropical Storm Alberto in 1994, brought about extensive flooding of numerous areas of the city. One area repeatedly inundated as a result of inadequate stormwater drainage infrastructure is the Waterford on Ellis subdivision located in the North Griffin Basin. This area of the city routinely experienced significant flooding of residential structures, inundation of residential streets, severe erosion, and other safety and environmental concerns. Following implementation of a formal stormwater management program and creation of a stormwater utility in 1997, the City of Griffin began to formulate plans to alleviate the problems faced by the citizens of Griffin and the residents of Waterford on Ellis.

Flood Control

North Griffin Regional Detention Pond aerial photo

The city evaluated the North Griffin Basin and identified an opportunity to construct a regional detention pond to service approximately 180 ac. of urbanized Griffin. The regional pond could provide the needed detention for the upstream areas as well as water-quality enhancement for a majority of the upstream basin. A project emerged to reroute the natural drainage path around the subdivision and discharge the stormwater runoff back into the existing creek downstream of the developed areas. Initially designed as a stormwater runoff quantity-control facility, the NGRDP accomplishes its primary flood-control objectives through two major components.

The first component of the facility set out to solve the initial problem of flooding within the Waterford on Ellis subdivision. This component consists of a large drainage channel that diverts and conveys stormwater runoff that previously flowed through and periodically inundated the subdivision. The channel has enough capacity to route 750 ft.³/sec. of stormwater runoff safely around the subdivision and developed areas. Key features of the channel include the use of rock check dams, erosion control matting, and large rock boulders placed within the channel to dissipate energy and prevent channel erosion from anticipated high flow velocities.

The second component of the facility consists of the main stormwater detention pond area. With a storage capacity of approximately 1.5 million ft.³ (34.4 ac.-ft.) and a pond floor area of approximately 2 ac., the pond can store and discharge stormwater runoff at a rate that minimizes flooding impacts downstream of the pond and provides filtration of pollutants present in the stormwater runoff.

Unlike most detention ponds designed to discharge water through a single outlet control device, the NGRDP features three separate stormwater discharge control devices. The primary outlet device is a large, cast-in-place concrete structure with two 10-in.-wide, vertical, rectangular weirs to control the rate of discharge of stormwater runoff from the facility. This outlet device has two large metal plates designed to slide over the weirs to control the rate of discharge as needed for flood control and mitigation of downstream erosion. Both plates fully open will restrict stormwater runoff to a discharge rate comparable to predevelopment discharge rates within the basin.

The adjustable weir concept efficiently routes pond discharge throughout the continued growth of the upstream development. Currently the basin consists of approximately 180 ac. of primarily commercial development, retail development, and associated impervious areas (e.g., major highways). As the basin continues to experience development and redevelopment along these major highways, however, the adjustable weirs will allow the city to throttle the discharge rate to maximize the storage-discharge relationship within the NGRDP, minimize downstream erosion, and maximize water-quality enhancement.

The secondary outlet device consists of a rectangular weir approximately 3 ft. wide set 11 ft. above the pond floor. This structure is designed to handle stormwater flows in excess of the 25-year design storm. Construction of this device was necessary to protect the integrity of the downstream forested wetland system. The device allows higher volumes of runoff to bypass the forested wetland system and flow directly to the stream via an underground pipe and junction box.

Finally, the third outlet includes a concrete emergency spillway to protect the integrity of the earthen dam and to convey water safely around the dam should a greater-than-100-year storm event occur.

Nonpoint-Source Pollution Control

During the design phase of the project, several conditions became apparent that would allow for incorporation of a water-quality-enhancement component into the pond design. The proposed pond design, the site conditions, and the proximity of the forested wetlands area downstream afforded the city an opportunity to evaluate the possibility of using natural filtration systems within the detention system. The city revised its design to incorporate wetland plantings within selected areas of the NGRDP and to use the downstream wetland areas to naturally filter out pollutants. Based on recommendations by water-quality professionals experienced with natural treatment systems, the city carefully selected and planted various wetland species throughout the main detention pond area. The wetland species selected and planted consisted of cattail, bulrush, pickerel weed, soft rush, wool grass, southern cutgrass, and shallow sedge. Experts chose these species based on their anticipated ability to break down and filter various pollutants commonly found in stormwater runoff.

In addition to planting of wetlands species in the pond, construction of a large rock check dam took place where the channel enters the main pond area. Placement of the rock check dam at the upstream end of the pond dissipates the flow rate/energy of the stormwater runoff, resulting in the settling out of sediments and particles within the pond rather than allowing these sediments to be transported to the downstream receiving waters.

Environmental Monitoring

Planting of various wetlands species throughout the main detention pond area

Following award of the CWA Section 319(h) grant to incorporate the constructed wetland system into the NGRDP, a monitoring program was designed and implemented in accordance with EPA guidelines. The program identified four locations within the pond system for water-quality monitoring. The selected parameters to be monitored comprised the following:

• Total suspended solids (TSS)
• Total dissolved solids (TDS)
• Nitrate nitrogen (NO₂)
• Nitrite nitrogen (NO₃)
• Total Kjeldahl nitrogen (TKN)
• Biochemical oxygen demand (BOD₅)
• Total phosphorus (P)
• Oil and grease
• Total petroleum hydrocarbons
• Fecal coliform bacteria
• Total copper
• Chemical oxygen demand (COD)
• Total lead
• Total zinc (Zn)

In addition to these laboratory-tested constituents, several in-situ parameters were taken at each monitoring site. These parameters included turbidity, dissolved oxygen (%), pH, and specific conductance.

Each location for testing within the pond system was chosen to give insight into how each of the NGRDP’s components was able to reduce nonpoint-source pollutant loadings to the downstream receiving waters.

Sample Location 1 was positioned at the uppermost portion of the main drainage channel where a majority of the contributing runoff volume enters the NGRDP system. By characterizing the stormwater runoff at this location, the city can quantify the pollutant levels present within the stormwater runoff entering the pond from the 180-ac. basin.

Sample Location 2 was positioned at the primary outlet control device of the main detention area, which routes a majority of the storm events (i.e., less than the 25-year storm). From this location the city can estimate the removal efficiency of the main drainage channel with its rock channel and the main detention pond area with its constructed wetlands.

Sample Location 3 was positioned at the outlet point of the pond system immediately downstream of the forested wetland system where the primary outlet device discharges. By characterizing stormwater runoff at this point, the city can estimate the removal efficiency for the entire system for the inflow runoff from Locations 1 and 4.

Location 4 was added to the monitoring program to characterize a small volume of runoff that enters the site between the outfall of the primary outlet control device from the pond (Sample Location 2) and the downstream discharge point for the system (Sample Location 3). The combination of stormwater runoff discharge from the NGRDP outlet and the flow from Location 4 comprise the inflow to the existing forested wetlands system and ultimately the downstream discharge point (Sample Location 3). The basin area upstream of Sample Location 4 has undergone significant development within the last 12 months. As a result, highly variable monitoring data have been reported, especially with regard to TSS, TDS, and turbidity. It will likely take several more sampling events to establish more consistent and useful data with regard to Sample Location 4. As mentioned, this flow volume represents only a small volume of flow into the NGRDP system, and its impact is yet to be fully evaluated.

Pollutant Removal Effectiveness of NGRDP System

The city collects and analyzes samples from the four locations identified above on a quarterly basis. Sampling of 10 rainfall events took place between September 1997 and December 2000. Table 1 shows average concentrations of the pollutants detected at the three primary monitoring locations (upstream, detention pond outlet, and downstream) with the NGRDP system.

Note: Several parameters being monitored are consistently reported as below laboratory detection limits by the laboratory and are not shown in the table.

By studying the results of the testing program between Sample Locations 1 and 3, one can observe that the detention pond, constructed wetlands, and forested wetlands have a positive impact on water quality through the reduction of pollutant loadings to the downstream receiving waters.

Pollutant Removal Effectiveness of Main Detention Pond

Figure 1. Rainfall/Runoff Regression

In late 1999, the city implemented another monitoring program to evaluate rainfall-runoff relationships in key areas of the city. Using one of these monitoring devices at the NGRDP, the city can accurately monitor the discharge rates leaving the main detention pond. Using the data gathered, the city developed rainfall-runoff relationships for the drainage basin.

The pollutant loading for the detention pond relative to the entire basin could be computed by using this relationship and the data gathered in the water-quality monitoring program. Table 2 summarizes the pollutant load reduction calculations for the detention pond.

The city is currently conducting water-quality and -quantity modeling studies citywide. The data compiled from these studies will be used to calculate pollutant removal efficiencies for the NGRDP system with respect to the entire watershed as well.

Project Collaboration

The NGRDP serves as a model for cooperation and collaboration between public- and private-sector entities to bring about superior results in governmental project planning, funding, and implementation. The project resulted from a collaboration of national, state, county, and city governmental agencies working in conjunction with water resource professionals.

• City of Griffin: Project leader/funding through stormwater utility
• Spalding County: Project funding
• Integrated Science & Engineering Inc.: Engineering design and construction management
• Rochester & Associates Inc.: Wetlands and biological consulting
• USEPA: Project funding through a CWA Section 319(h) grant for nonpoint-source abatement
• Georgia Department of Natural Resources Environmental Protection Division: CWA Section 319(h) grant project administration

Project Funding

An interesting aspect of the NGRDP is the funding methods used to design, construct, and monitor the project. Griffin made the decision to implement a comprehensive stormwater management program in the mid-1990s. In 1996 the citizens of Griffin and Spalding County voted to implement a Special Purpose Local Option Sales Tax (SPLOST) to fund various stormwater and transportation improvement projects. Utilizing funds from the SPLOST, the city could fund land acquisition and a majority of the construction of the main detention pond and drainage channel. In 1997 the city undertook to fund its future stormwater management program needs by implementing Georgia’s first stormwater utility. The utility generates more than $1 million per year in revenue specifically for stormwater-related projects and improvements. The utility also enables Griffin to successfully seek out and secure state and federal funds by emphasizing the city’s commitment to stormwater management. The NGRDP received a CWA Section 319(h) grant from EPA to fund the water-quality enhancement construction components and environmental monitoring tasks associated with the project. Utility revenues served as matching funds in accordance with the grant program requirements.

Summary

Throughout the state of Georgia and the nation, many water bodies are impaired as a result of point-source and nonpoint-source pollutants. Accordingly, state and federal environmental permitting agencies restricted the ability of municipal and county governments to discharge wastewater-plant effluent and untreated stormwater runoff to downstream receiving waters. Only recently have municipalities and counties begun to analyze and quantify the potential impacts of these impaired water bodies on the community’s quality of life and economic growth potential. The City of Griffin took a proactive approach to address these issues by successfully implementing stormwater management best management practices (BMPs) such as the NGRDP. The combination of the traditional stormwater runoff quantity- and quality-control BMPs into a comprehensive system makes the NGRDP an award-winning nonpoint-source pollution and flood-control facility. This project shows that incorporating a water-quality enhancement component into a traditional regional detention pond system can be accomplished cost-effectively. The City of Griffin believes this project will provide valuable data for other government entities, engineering designers, and environmental regulators to evaluate the feasibility of using regional detention ponds as a viable stormwater management BMP. The city remains confident that implementation of a comprehensive water resources management program will ensure future economic growth and quality of life for the community and its surrounding areas.

Richard A. Greuel is a project engineer, and Ronald A. Feldner, P.E., is a principal engineer within the water resources group of Integrated Science & Engineering Inc., the stormwater consultant for the City of Griffin.

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