Deformable Boulder Cascade Design

In recent years unnatural fish migration barriers, commonly dams, have been targeted for removal. In the instance when dams cannot be removed due to contaminated materials behind them or need for permanent pool or flowing water surface elevations, the bed elevation downstream of the barrier must be transitioned up to the required upstream elevation.  To do this with a nature-like fishway a flow constrictor/step pool system (FC/SP) or roughened rock ramp are typically selected. At slopes greater than 3% roughened rock ramps tend to lose their effectiveness and FC/SP are favored.   There is typically only a half foot drop between weirs in a FC/SP to ensure submergence and streaming flow, conditions most suitable for fish passage. When the total hydraulic drop is significant the structure will elongate, for example an 18 feet hydraulic drop would require 36 weirs.  The abundance of structures in this example may be contrary to project goals when a natural system is to be maximized.  A potential solution to this issue was derived based on observations of a natural system.  In this system natural weirs were found in a scattered pattern across a steep cobble/bolder channel.  This randomness is difficult to model, but if it were developed within a framework of designed weirs then the total system could create a nature-like fishway with fewer structures and be modeled.  The idea is to have two types of weirs, one is a designed grade control weir with specific fish passage criteria achieved, while the other is a habitat weir which will provide some grade control but will deform with natural flow conditions.  The total number of weirs would still be 36 but the grade control weir would be spaced at every third weir and therefore have 1.5 feet drop from each successive grade control weir.  Between the grade control weirs will be two habitat weirs, each one a half foot lower than the upstream weir.  The stones making these weirs will be sized and placed to slightly move and adjust in typical storm flow conditions making a more natural system. By allowing the habitat weir to deform wood could also be worked into the system and it will reduce the appearance of structures to the observer.  This system has not yet been built but has the potential to bridge the gap between a nature boulder cascade and the abundance of structures in FC/SP solutions.