Abstract: 2nd Plenary Session
Wednesday, July 20 8:30 am (BACK)

The Evolution of Structures

Dave Rosgen, PhD
Wildland Hydrology
Fort Collins, CO

The history in the development, evolution, and application of river restoration structures will be presented.  Structures have evolved from more traditional approaches, such as hardening measures using rip-rap and gabions, to Cross-Vanes, W-Weirs, & J-Hook Vanes to meet specific objectives.  Such structures are still in use; however, recent emphasis has been placed on making structures appear more natural and be more “green.”  Large woody debris has become a major focus for structures and river restoration.  The proper use of large woody debris for river restoration and aquatic habitat enhancement has many advantages, such as:

a.  creates flow resistance and reduces accelerated streambank erosion
b.  provides instream and overhead cover for aquatic habitats
c.  adds a natural, non-structural appearance if constructed correctly
d.  provides a low risk alternative compared with other structures
e.  protects land and infrastructure
f.  decreases project costs compared to projects with rock or concrete structures

The manner in which large woody debris is incorporated into river projects can meet or defeat the advantages.  Examples and design details will be presented that highlight the proper use of large woody debris for sustainable projects, including the Toe Wood design that has been implemented on large river projects that experienced major floods.  Additionally, case examples will be presented regarding the poor use of large woody debris associated with high costs, high risk, safety issues, and limited function.  However, regardless of which structures are used, it is imperative that stream functioning related to the stable river morphology for a given landscape condition is attained to provide long-term geomorphic and ecological sustainability.