EWP - North Chickamauga Creek

NRCS recently completed Emergency Watershed Protection (EWP) work along the banks of North Chickamauga Creek in northern Hamilton County, Tennessee. The project, consisting of 950 feet of 12’ high gabion wall and more than 7900 tons of riprap, is one of the largest gabion structures designed and built by NRCS in Tennessee. Construction of the project began October 2005 and finished in January 2006. The site is located next to the Willow Creek subdivision and Soddy-Daisy Industrial Park in Soddy-Daisy, about 17 miles north of Chattanooga.

Background

In February of 2003, the weather service recorded 8.6 inches of rain for the month- the average is usually 4.9 inches. This was not the first time the area had experienced heavy rains, but it was after a particularly heavy down pour the situation for residents and businesses along North Chick began to look dire. The attention of the local media was drawn to the Willow Creek subdivision home of Diane Halstead-Renz. The back porch of her house, precariously dangling over the creeks edge, became a symbol of the situation.

For a number of years Willow Creek residents near the creek had sought assistance through various local, state and federal means. At one point an actual proposal had been designed and presented by the U.S. Army Corps of Engineers. However due to cost issues, nothing was ever implemented. After the 2003 winter rains officials with Soddy-Daisy contacted the local USDA-NRCS office and requested assistance through the agency’s EWP program.

Congressional funding for this project was approved in September of 2004.

Design and Construction

The EWP project actually consisted of two subprojects: construction of a low-flow channel and construction of the gabion wall/placement of riprap.

§         Low Flow Channel

The low flow channel was installed with the hopes of diverting water from the construction area while at the same time establishing a “pilot” channel within the channel. The pilot channel was designed to approximately carry the rain from a two year storm event. It has a trapezoidal cross-section with a top width of 50 feet. The channel actually passes under Dayton Pike, a high traffic two lane bridge and outlets just downstream of the project area. The low flow channel is 2780 feet long.

 The low flow channel was dug starting at the downstream end and working up towards the upstream project work limits. Creek rock was removed and taken off site. Two weirs were placed in the low flow channel. Several large rocks were left in the low flow channel to aid in aquatic habitat formation. 

§         Gabion Wall and Riprap

The gabion retaining wall was designed using an approach that considered different storm events and the “worse case” for stabilty. In the end, the top elevation of the wall was determined by considering the flood level during the 25 year storm. Because of the unique features of the creek in the project area, the top elevation for the 25 year storm event is actually very close to 100 year storm event elevation. The wall was analyzed for stability under the “worse case” scenario.

The location for the bottom of the wall was determined by looking at channel cross sections, about 50 feet apart, going from the upper end of the wall towards the end. And based on previous experience with gabions in dynamic channels, the wall was located three feet below the lowest point shown on the cross sections.

Before gabion walls were selected as the stabilization method, cross-section analysis was used to determine if large riprap could be placed on the banks instead. Due to concerns over cost and the potential mobility of the computationally sized riprap, it was decided that the gabion wall would, in the long run, better withstand the forces of the channel. Rock riprap was placed above the wall, to protect the structure from over topping and to tie the wall to the banks.

Comments and Reactions

Ricky Dent, Willow Creek Resident

“During the first storm I lost 35 feet from my yard and in the next rain I lost 25 more [feet]. I was in immediate danger of loosing my house. The next storm would certainly have caused me problems.

“Time will tell…but I think it looks good! NRCS did a great job- their inspector made sure the contractor did what he said he would do.”

Janice Cagle, City Manager, Soddy-Daisy, Tennessee

“I don’t think we could have gotten through the permitting process with out the help of NRCS. The working relationship with NRCS has been great; they made everything easy for me from beginning to the end.

 “I’m sure that federal officials don’t realize how important this fund is to small communities. I know what a benefit the program has been to Soddy-Daisy, I can only imagine what a benefit it is to other communities through out the United States.”

Denise Watkins, Project Engineer, Chattanooga, Tennessee

“The project was a great success- the low flow channel served its’ main purpose and allowed us to construct the wall over the winter months without many water problems.

“ In addition, the Tennessee Department of Environment and Conservation and the Corps of Engineers were very involved and helped out by working with us to overcome obstacles that would have prevented the project from being completed.”

Quick Facts

Links of Interest

In The Wake of Ivan, Chattanooga Times Free Press
North Chickamauga Creek Conservancy