Concrete to Cobble, Lessons Learned on Small Dams and Constructed Fishways

Oregon water law has required fish passable diversion structures even prior to Oregon’s statehood. Listing of chinook salmon, coho salmon, steelhead and bull trout as threatened or endangered species and consideration of lamprey has greatly accelerated efforts to improve fish passage past diversion dam structures.

Irrigation diversion dams generally consist of a cross channel structure that creates backwater to a sufficient head to divert water through an intake structure and to the irrigation delivery system. In the agricultural setting, the selection and construction of a diversion dam is often based on the ability of the structure to deliver water, construction complexity, cost and maintenance requirements. Fish passage, sediment transport, geomorphology and channel hydraulics are often not considered by irrigators unless these features affect the delivery of irrigation water. Many streams incise below the diversion dam that exacerbates migration barrier effects and fish passage is reduced or eliminated.

Fish passage designs have recently moved from traditional engineering methods of ladders and structural passage components to the utilization of natural materials for in-stream structure design. For irrigation diversion dams, this has lead to the incorporation of “nature like” structures that more closely mimic the hydraulic and habitat components of natural channels while meeting irrigator requirements and fish passage considerations. These structures often consist of rock weirs, step-pools or constructed riffles.

Although these applications have become more popular in recent years, consistent, reproducible engineering criteria for structure design have not been clearly identified. Adequate inventory and design analysis is a critical step in providing a consistent metric for post project appraisal, and establishing a foundation to improve the design state of the art.  This presentation will describe the hydraulic design methodology used in two projects: 1) An in-line step-pool passage system, and, 2) A small dam removal and roughened chute system.

The presentation will focus on the analysis of the pre-project and design reach scale hydraulics including:

1)      The pre-project condition hydraulic model used to evaluate depths velocities and shear stress within the project reach.

2)      Use of analytical methods for preliminary fishway design.

3)      The proposed condition hydraulic model that provides water surface profiles, velocities and shear stress results to ensure passage

         design criteria have been met.

4)      Recommendations for future design based on post project performance appraisals.