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View Spring 2007 Stream Restoration Networker [pdf]
LITTLE TOPASHAW CREEK STREAM CORRIDOR REHABILITATION PROJECT
Key elements
Bank stabilization, biomechanical erosion control, large woody debris structures
Location
This project focused on a reach of Little Topashaw Creek, located in Chickasaw County, North Central Mississippi. It is a tributary within the Yalobusha River watershed, which itself flows into a large US Army Corps of Engineers flood control reservoir, Grenada Lake, and then into the Yazoo River, a tributary to the Mississippi River.
Historical context
The creek is located in hilly terrain just east of the Mississippi River alluvial floodplain and has been subjected to a great deal of watershed disturbance, starting about the time of European settlement in the 1830s and ‘40s. The channels in the watershed were prone to blockage from extreme amounts of eroded sediment from cultivated lands and were repeatedly channelized by local drainage districts. In the late 1960s and early ‘70s, the federal government sponsored a comprehensive channelization project that triggered several waves of head cutting and channel incision in the system. As a result, the study reach, which has a contributing drainage area of about 15 square miles, has a channel width of about 33 meters and an average channel depth of more than three meters. Bank heights on the outside of bends are enlarged to six meters. The bed material is medium sand, with outcrops of consolidated, cohesive material.
Project description
The Little Topashaw Creek project, implemented as part of the Demonstration Erosion Control Project (DEC), was a collaborative effort between DEC members, the US Army Corps of Engineers, the USDA’s National Resources Conservation Service, and the USDA’s National Sedimentation Laboratory. One of its major objectives was to demonstrate the efficacy of using large woody debris (LWD) and other biomechanical techniques as an inexpensive alternative method of controlling bank erosion. In particular, the project hoped to show that LWD structures could stabilize banks for less than $25 per foot of bank, compared to various stone and concrete methods that can cost twice as much. This article focuses on the LWD portion of the project.
Doug Shields, Little Topashaw Creek Project Director and lead scientist on the LWD portion, notes that in addition to the specific erosion control objective, "we were very interested in in-stream habitat rehabilitation. Two aspects of this were primary: creating a stable pool habitat and increasing LWD density in the channel." Both of these, according to Shields, were seriously deficient compared to lightly disturbed reaches in the same watershed: channel incision had eroded the riparian vegetation, reducing wood loading in the channel, and channel widening meant that channels didn’t retain very well what woody debris remained. Widening channels have also caused baseflows to become shallower, thereby reducing pool habitat. The project team’s hypothesis was that by encouraging sedimentation in the reach, through the use of LWD and willow posts, they would end up with a narrower and deeper baseflow channel that had vegetation on the berms.
The Little Topashaw Creek project built and installed 72 structures using materials found either in the channel or on the floodplain away from the channel. Each structure was designed to protect about 25 meters of channel. About 65% of the key members of the structures were buried in the bank and 80% of them had earth anchors as well.
The project team did pre-project monitoring in 1999 and 2000, installed the LWD structures in late 2000, and planted willow posts in 2001. Post-project monitoring continued through 2004.
The monitoring revealed that, although positive outcomes (structure retention, habitat restoration, and fish populations) were seen over the first two years, many of these effects disappeared by 2004 as 35% of the installed structures failed.
There were several factors that played into the failure of these structures, according to Shields:
- Based on field inspections of the reach, the team expected that the reach had finished the widening phase (Stage 4 in the Channel Evolution Model) and was beginning to aggrade (Stage 5). This turned out to be too simplistic an hypothesis as the geomorphic behavior of the reach varied over both space and time. "Geomorphically," said Shields, "we misread the signs. We thought we weren’t going to get any more bed degradation, but we did."
- In hindsight, not enough of a safety factor was applied when sizing the anchors. Duckbill-type earth anchors cabled together were installed (and load-tested) at a depth of 1.2 meters, yet most of them pulled out. There may have been two factors at play here. First, the woody material dried out quite rapidly, leading to a significant increase in the buoyant forces at work on the structure (see graph below—graph courtesy of the USDA). Second, the woody material started to decay, and the first things to go were the smaller twigs and branches. As the woody decay progressed, the structure matrix simplified, flow velocities increased, and sediment that had been trapped in the matrix began to erode. All of these factors not only reduced the bank’s hold on the structure but removed the substrate for vegetation growth. "We expected that woody vegetation would colonize the sediment deposits that were triggered beside and within the large woody structures," said Shields, "and that did not happen, at least not on a large enough scale."
The short-term, positive results from the LWD experiment demonstrate that the technique has merit and warrants further study, says Shields, and the information learned from the structure failures, like all research "failures," will hopefully help the next group of researchers succeed.
Food for thought
Doug Shields shares his thoughts on the field of stream restoration on NCED’s website.
Project website
More information can be found at http://ars.usda.gov/Research/docs.htm?docid=5526. |
