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Water: Nonpoint Source Success Stories

Georgia: Section 319 Success Stories, Vol. III

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Broad River Streambank Stabilization Project:
Tree Revetments Rescue Eroding Banks

 


Contact:
Jim Wren
Oconee River RC&D Council, Inc.
P.O. Box 247
Watkinsville, GA 30677
706-769-7922
Primary Sources of Pollution:

erosion
Primary NPS Pollutants:

sediment
Project Activities:

tree revetment
Results:

decreased sediment loads

monitoring in progress

Streambank erosion on the streams and rivers of Georgia continues to be a growing problem. Erosion is particularly evident in the Broad River Watershed District of northeastern Georgia. The accepted consensus is that it is much easier, and more cost-effective, to prevent erosion before it occurs than to restore streambanks after the damage has been done. However, because in many cases erosion already exists, new and better ways of solving the problem are being explored.

One of the methods being tried in the Broad River watershed is the technique of installing "tree revetments." New to Georgia, this technique is relatively inexpensive when compared to other types of streambank stabilization techniques currently in use.

Demonstrating the technique

The Chestatee-Chattahoochee Resource Conservation and Development Council, through a 319 grant from the Georgia Department of Natural Resources, Environmental Protection Division, is implementing a project designed to demonstrate to landowners the positive effects of tree revetments on eroding streambanks. The project calls for 15 tree revetment sites, plus additional best management practices, to be installed on selected streams throughout the Broad River watershed.

A tree revetment is a bioengineering method that uses whole trees cabled tightly together in giant bundles. These bundles are then secured to the eroded streambank in a shingling effect, just like the shingles on a roof, through an anchoring system of cables. The trees used in the installation are selected by the contractor with assistance from an Natural Resources Conservation Service specialist or by the participating landowner from the landowner's own property. The streambank height should usually be 6 feet or more, with a steep incline; revetments can't be constructed on gradually sloped streambanks.

Tree revetments have been shown to greatly slow the stream current along an eroding bank, decreasing erosion and allowing sediment to be deposited in the tree branches of the revetment. The deposited sediment forms an excellent seedbed in which the seeds of riparian trees such as sycamores and maple, as well as other plants, can sprout and grow. The resulting growth spreads roots throughout the revetment and into the existing streambank. In addition to slowing streambank erosion, tree revetments also provide excellent habitat for birds, fish, and other forms of wildlife.

Continuing efforts

As of August 2001, seven revetment demonstration sites had been installed throughout the watershed. An additional five sites will be installed through the end of 2001, as weather permits. The progress of these sites will be monitored over the next two years.
   


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North Griffin Storm Water Detention Pond Project:
Constructed Wetland System Protects Water, Wins Award

  


Contact:
Brant Keller
Public Works Director
Storm Water Utility Division
P.O. Box T
Griffin, GA 30224
770-229-6603
Primary Sources of Pollution:

urban storm water runoff
Primary NPS Pollutants:

nutrients

metals

oil

grease
Project Activities:

constructed wetland system
Results:

reduction of storm water constituents

An important function of wetlands is their role in maintaining and enhancing water quality. Urban storm water contains a variety of constituents, such as nitrogen, phosphorus, metals, oil, and grease, that can contribute to nonpoint source pollution. Because many complex chemical and biological processes that affect water quality occur in wetlands, a vegetated wetland system can incorporate and transform many of these storm water constituents through biological breakdown by microorganisms or vegetative decomposition. In addition to providing water quality-enhancing attributes, constructed wetland systems offer other potential advantages, including comparatively simple operation with low maintenance, process stability under varying environmental conditions, and low construction and operating costs when compared with traditional water treatment facilities. Additionally, the introduction of emergent wetland species not only provides several benefits for water quality enhancement but also results in improvement of wildlife habitats.

Comprehensive watershed management

In 1997 the City of Griffin began a comprehensive watershed management program by implementing a Storm Water Utility to address the city's aging infrastructure and improve the quality of storm water runoff. One of the first projects successfully completed under the management program was construction of the North Griffin Regional Detention Pond (NGRDP). This regional pond was designed for flood control and to enhance and preserve water quality in Shoal Creek and Wildcat Creek of the Flint River Basin.

The NGRDP features a drainage channel, a regional detention pond, and two constructed wetland areas for storm water filtration. The pond and wetland areas use natural filtration and other biological processes, rather than traditional mechanical means, to improve the quality of storm water runoff. The pond serves as a comprehensive storm water management system that eliminates flooding problems in a 180-acre area of North Griffin while enhancing water quality.

Evaluating the performance of the NGRDP

To determine the overall performance of the wetland system, an evaluation of water quality was performed by collecting and laboratory testing storm water samples from locations upstream of, within, and downstream of the detention pond. A baseline sampling protocol was developed to establish the initial quality of storm water runoff from the North Griffin Drainage Basin.

Review of the monitoring data for the first 21 months (between January 1999 and September 2000) indicates that the actual removal efficiencies are showing significant reductions for the constituents listed (see table). The City of Griffin anticipates that the future monitoring results for the mature site will be comparable to the theoretical removal efficiencies documented. Wetland maturation should result in utilization and transformation of these constituents through biological breakdown by microorganisms and vegetative decomposition.

The American Consulting Engineers Council awarded the City of Griffin and Integrated Science and Engineering the 2000 Engineering Excellence Award for this project.

Monitored Removal Efficiencies

This table shows the removal efficiencies for several constituents that are currently being monitored. The table represents data collected between January 1999 and September 2000 by the City of Griffin, Georgia.

Constituent Station 1 Station 3 Average removal Theoretical removal
  (influent) (effluent) efficency efficiency
Total suspended solids 42.86 mg/L 36.71 mg/L 14% 65% to 80%
Total Kjeldahl nitrogen 4.53 mg/L 1.76 mg/L 61% 60% to 80%
Total phosphorus 0.17 mg/L 0.10 mg/L 41% 25% to 50%
Chemical oxygen demand 52.00 mg/L 31.86 mg/L 39% 35%
Total lead BDL BDL 0% 50%
Total zinc 0.13 mg/L 0.07 mg/L 46% 60% to 70%
Fecal coliform bacteria 25,457 no/100 mL 8,169 no/100 mL 68% NCLI
Note: BDL - below detection limits; NCLI - no comparison level identified.

 

 

 

 

 

 

 

 

 

 

 

 

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