|Shields Jr, Fletcher|
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 5/20/2002
Publication Date: 11/3/2002
Citation: Shields, F.D., Jr. Performance of woody debris structures for stream restoration. American Water Resources Association 2002 Annual Water Resources Conference Proceedings. 2002. Abstract p. 237.
Technical Abstract: Seventy-two large woody debris structures (LWDS) were placed along 2 km of a sand-bed stream in northwestern Mississippi for ecosystem rehabilitation. Costs for LWDS construction were about $80/m of treated bank line, which is only 19% to 49% of recorded costs for recent stone bank stabilization projects in this region. Structures performed well during the first year after construction. Baseflow water width and depth increased in the modified reach by 116% and 17% (p < 0.003) due to scour adjacent to the LWDS and associated beaver dams. Only four of the 72 structures were destroyed, despite at least three flow events with peaks exceeding 25 m3/s, depths > 2.5 m, and velocities > 1.2 m/s. Comparison of pre- and post-construction channel surveys following several high flows showed only insignificant changes in total channel volume despite about 0.3 m of thalweg degradation because of deposition of sand berms adjacent to steep, concave banks. However, high (~40 m3/s) flows during the second year triggered progressive failure of about a third of the LWDS. Damage rates were slightly higher for anchored LWDS (20/58) than for those without earth anchors (4/14/). Factors involved in failure included simplification of the LWD matrices due to breakage and decay, failure of earth anchors, scour of sediments deposited within the structures and undermining of structures by thalweg degradation. Structures located in sharp bends were most prone to fail.