Georgia Concludes There's No Reason to Tolerate Mud

By Jeri Gray, Technology Transfer Specialist and Editor Water Resources Research Institute of The University of North Carolina; and the North Carolina Sedimentation Control Commission Sediments Newsletter Dec 2002

Since 1993, the State of Georgia has been examining the impacts of sedimentation on its waterways and conducting studies to determine if it is technically feasible to protect its waterways from sediment pollution in a cost effective manner.

Last year the Georgia Environmental Protection Division published a number of reports resulting from studies funded by a $400,000 grant to support the work of the Erosion and Sedimentation Control Technical Study Committee (known as "DirtII"). The committee sponsored cutting-edge technical research and policy analysis by the National Academy of Public Administration and reached the conclusion that it is possible to prevent sediment pollution without imposing huge costs on development by changing the "mud happens" attitude of developers, regulators, and citizens alike. According to the Dirt II panel, "we now know how to build homes and schools, roads and utility corridors, shopping centers, offices, hospitals, and churches without significant erosion, without loosing mud-choked water on downstream homeowners and without generating the enormous financial and environmental costs that spread with sedimentation." Based on the results of actual construction projects, the panel concluded that simply by using proven state-of-practice techniques, developers can conduct construction that "does virtually nothing to accelerate the natural process of erosion."

Big Creek School Project

One of the projects that led the Dirt II Panel to its conclusions was the Big Creek School Project in Fulton County near Atlanta. Building the elementary school would require disturbing the soil on 22.5 areas of a 50-acre lot in an affluent suburban area abutted by residential properties. The neighbors were protective of property values and would be on guard against muddy runoff. The site was fairly steep and wooded, and clearing and grading would risk sending tons of mud downhill toward the neighbors and adjacent streams. The Fulton County School Board wanted to manage a serious business risk and at the same time keep design and construction costs as low as possible. The Fulton School Board decided to collaborate with the Dirt II Panel as the test site for demonstrating state-of-practice erosion prevention and sediment control measures in Georgia.

The team put together to design and execute the demonstration project included landscape architect Mike Breedlove of Breedlove Land Planning and Dr. Richard Warner of the University of Kentucky, creator of the SEDCAD4* erosion and sedimentation control modeling software. Breedlove and Warner designed a comprehensive erosion prevention and stormwater and sedimentation control system to achieve a performance goal both in the short and long term. The system used standard, economical erosion prevention and sedimentation control practices with some innovative modifications. The contractor Beers-Moody, became a critical part of the system by committing to daily site walk-throughs with attention to runoff paths changed during the course of the day's construction and quick attention to potential problems.

The Big Creek School erosion prevention and sediment control system was designed to ensure that the hydrology of the site would replicate pre-development conditions and take advantage of the functioning forested stream buffers. Rather than determining the volume of water that needed to be managed and designing structures to manage that volume, the designers evaluated overall flows and infiltration capacity of the site and used energy reduction devices and materials to reduce runoff and promote infiltration.

From the very beginning of the project, erosion prevention was a goal. Salable timber was removed from the site and all remaining woody material (about 60% of the total removed wood) was fed through a tub grinder, generating two 30 to 35-foot-high piles of roughly ground mulch. Mulch was used generously throughout the site for immediate erosion prevention, dissipating rainfall energy and reducing velocity of overland flow. As a primary erosion prevention measure, the designers paid a great deal of attention to exposed slopes, successfully using the simple techniques of top-of-slope berms and temporary slope drains feeding to sediment basins.

Before grading began, an 800-foot rock construction entrance was completed and perimeter control measures were stabilized. One of the newer system components that went in early and proved extremely effective was a seep berm-a 1,275-foot-long channel with check dams and multiple side outlets encircling one quarter of the site. A seep berm extends the functionality of a simple, everyday diversion used to carry runoff to a basin. Check dams along the diversion back up runoff, allowing sediment to settle. Runoff from smaller storms was completely contained and seeped through the berm to the down gradient forest buffer, which provided secondary treatment. The seep berm vastly reduced the quality and timing of runoff going to the site's largest sediment basin. The seep berm was sized to be wide enough for a trackhoe and small dump truck for ease of cleanout.

The basin fed by the seep berm was a multiple-treatment system. It consisted of a partially cemented riprap plunge pool and an inlet channel, a 950-cubic-yard first-flush sediment basin, an internal earthen dike with a rock drain, a 7,800-cubic-yard primary sediment basin with a floating siphon (skimmer), a drop-inlet combination principal and emergency spillway, a rock riprap outlet channel and level spreader, and a sand filtration system. The bottom of the first sediment basin was two feet higher than the principal basin, affecting complete dewatering and leaving dry sediment that was easy to remove. The first basin was sized for easy cleanout.

All four sediment basins on the side were designed to completely contain the 1-1/2 year design storm (about 3 inches). Three of the four basins on site had external sand filters to provide secondary treatment. The sand filters were simple controls fabricated from rock and sand, and perforated pipes to distribute and collect water. They were designed to further remove fine sediment.

Extensive monitoring of volume, sediment concentration, and turbidity of stormwater was performed during construction. A series of monitors tracked the progression of stormwater flow through a series of system components. The performance of the system was exemplified by a high intensity storm that occurred while the site was near its peak level of disturbance. Total precipitation was 1.07 inches, with 0.70 inches occurring in 27 minutes near the end of the storm. Peak sediment concentration monitored at the plunge-pool energy dissipater of the largest basin was measured at 160,000 mg/L. By the time the water emerged from the sand filter at the end of the treatment trains, sediment concentration had been reduced to 168 mg/L. Modeling of the storm event for this basin showed that all flow exiting the site infiltrated into the riparian buffer zone. Therefore no sediment reached surface waters from this basin, which drained over half the site. Monitoring of larger storms showed comparable performance for all controls.

Cost data provided by the contractor showed that erosion prevention and sedimentation control on the site accounted for $265,000 of the $3,015,000 sitework package, which works out to about 8.5%. This is more that the average 3-5% spent on traditional erosion and sediment control in Georgia. However, a number of features- including stabilization with coir logs of a severely eroded stream onsite, construction of a permanent level-spreader, and special level-spreader, and special accommodations for monitoring equipment-would not be part of a typical project. When these costs are subtracted, the total for erosion and sediment control is reduced to $175,000 or 5.8% of the total sitework cost.

The Georgia Dirt II Panel says its research shows that "the people of the Atlanta metropolitan area need no longer tolerate the kind of building practices that have allowed uncounted tons of mud to be scoured from the landscape and flushed into our streams.

"There is no reason to tolerate the status quo when we already know what practical steps to take to greatly reduce our costs and clean up our water."

Resources

Building Metro Atlanta's Economy by Building Systems to Prevent Erosion. ND. Dirt 2: The Erosion and Sedimentation Control Technical Study Committee, P.O. Box 1600, Franklin, GA 30217.
Warner, Richard C. and Francis X. Collins. 2001. Erosion Prevention and Sediment Control Computer modeling Project presented to the Chattahoochee-Flink Regional Development Center Dirt II Committee.
>National Academy of Public Administration. January 2001. Policies to Prevent Erosion in Atlanta's
Watersheds: Accelerating the Transition to Performance.
http://www.gaepd.org/Documents/techguide_wpb.html#es.
*SEDCAD4 for Windows was developed specifically for the design and evaluation of alternative erosion prevention and sediment control systems with a focus on earth-disturbing activities. It is a comprehensive program that includes hydrology, hydraulics, and design and evaluation of the effectiveness of both individual and an integrated system of erosion prevention and sediment control measures with respect to sediment trapping efficiency and effluent sediment concentration. Based on research conducted at the University of Kentucky, SEDCAD4 can also predict the effectiveness of sediment basins, sediment traps, silt fences, porous rock silt checks (check dams), and grass filters. Evaluation of SEDCAD4 predictive abilities on the Big Creek project is included in the modeling paper referenced above. For information about SEDCAD contact Dr. Richard Warner at the University of Kentucky (859-257-3000 ext 217 or warnerhofc@aol.com)