The introduction of large woody debris (LWD) (e.g., wood studded revetments, root wads, flow deflector jams) to streams is receiving growing attention as a stream restoration practice due to its utility for restoring aquatic habitat and promoting fish passage (e.g., by providing refuge for fish and reducing flow velocities in the stream channel). As such, placement of LWD in the immediate vicinity of hydraulic structures (i.e., in the 50 ft. upstream or downstream of the structure, as well as within the structure itself) may significantly improve fish passage at stream crossings. Such LWD implementation could provide benefits in multiple states by minimizing habitat fragmentation and facilitating ecological connectivity at stream crossings (e.g., Forest Service Stream-Simulation Working Group, 2008). However, the placement of LWD in the immediate vicinity of hydraulic structures is not allowed by current regulations (e.g., see WSDOT, 2017) due to the unknown impacts of LWD on risks to the structure (e.g., due to scour, flooding or blockage). A critical factor hindering evaluation of the risks associated with LWD near hydraulic structures is that very few studies have documented the flow field characteristics around LWD and other porous structures. In addition, these studies have not considered the presence of a nearby hydraulic structure as part of their study design, which limits their applicability for providing assessments of the impacts to structure risk associated with LWD placement. Therefore, new observations are needed to systematically document the flow field in the vicinity of LWD structures of different type, spacing and submergence while considering the presence of a nearby hydraulic structure. This data will fill existing knowledge gaps concerning the effects of LWD structures to their surrounding flow field and will aid re-evaluation of the soundness of existing guidelines restricting LWD placement within 50 ft. of hydraulic structures (WSDOT, 2017). Accurately predicting the flow characteristics at field-scale streams and LWD or hydraulic structures with numerical models is also a necessary step, in order to ensure the success of potential LWD structure designs that are implemented at stream crossings with complex geometry and stream bathymetry (Lai, 2016b). However, there is limited guidance in the literature for utilizing hydraulic models around LWD and more work is needed on this topic. SRH-2D is a two-dimensional (2D) hydraulic model with the capability to represent the LWD geometry and to predict flow around LWD structures (Lai, 2016b), because it incorporates two model inputs that can account for the effects of LWD on both flow resistance and flow energy dissipation (Lai, 2016a). The first input is the drag coefficient that accounts for LWD flow resistance, which affects the flow depth and thus will affect flooding risk predictions. The second input is the eddy viscosity that accounts for energy dissipation, which affects the flow velocity distribution around LWD and thus will affect scour risk predictions. Thus, a new SRH-2D simulation protocol is therefore needed in order to provide guidance for adjusting the model input values depending on the LWD geometry and flow conditions.

The development of new and creative design strategies for facilitating fish passage at stream crossings is needed in order to allow for habitat restoration goals to be met while simultaneously protecting the health of roadway and hydraulic infrastructure (Forest Service Stream-Simulation Working Group, 2008). In the stream restoration community, reintroduction of LWD is a practice that has received growing and multi-state interest due to the benefits for aquatic organisms by creating pools, refuge and improving the overall ecohydraulics and fish passage in rivers. However, the overall benefit of including LWD near hydraulic structures hinges on proper design consideration of the tradeoffs between fish passage benefits with structure risks. The goal of this research is to provide a design guideline aiding selection of optimal LWD structure designs (i.e., type, location and spacing) to place near hydraulic structures for promoting suitable fish passage conditions while minimizing structure risks. This guideline will build on existing LWD guidelines (e.g., USBR and ERDC, 2016; WSDOT, 2017), which provide guidance for many components of LWD structure design but do not consider the presence of a nearby hydraulic structure. This new guideline will aid DOTs to simultaneously meet strategic goals that focus on balancing stream ecological health, economic viability and public safety. Input from the Technical Advisory Committee (TAC) will aid in selection of stream site and LWD structure characteristics to be considered. There are some key challenges hindering evaluation of potential risks associated with LWD placement near hydraulic structures that must be addressed in order to develop new guidance. These challenges are the following: (1) LWD structure design characteristics, such as structure type and porosity, vary widely (e.g., Oregon DSL, 2010; WSDOT, 2017), making assessment of the structure geometry challenging; (2) submergence of LWD varies with structure size and flow condition, making prediction of the LWD flow resistance difficult (e.g., Papanicolaou et al., 2018); (3) there are few studies that have documented the effects of LWD to the surrounding flow field; (4) there are few 2D numerical models that can adequately represent the complex flow around LWD structures in order to aid evaluations of LWD effects to a nearby hydraulic structure (e.g., Papanicolaou et al. 2011; Lai, 2016b); and, (5) the models that can account for the additional flow resistance due to LWD, such as SRH-2D (Lai, 2016a), lack guidelines and protocols for simulating the flow around field-scale LWD structures.

The study will utilize a two-fold approach, which is necessary in order to address the five challenges overviewed above (see “Purpose” section). First, observations thorough targeted laboratory experiments will provide insights into the flow field characteristics around different LWD structure types and under different submergence conditions (i.e., to address challenges 1-3 above). Second, a simulation protocol for using 2D numerical models to predict the flow around LWD structures will be developed (i.e., to address challenges 4-5) in order to use these models to expand the range of laboratory testing and to simulate the flow field at field-scale hydraulic structures. The specific objectives of this research are: 1. Identify threshold conditions for fish passage, scour and flooding in the vicinity of hydraulic structures with LWD present; 2. Determine optimal locations and design of LWD structures within 50 ft. of hydraulic structures that promote fish passage while minimizing structure risk; 3. Provide nomographs and guidelines that relate stream discharge, sediment size, stream gradient, and channel morphology to the type and number of LWD structures recommended; and, 4. Develop a protocol for accurately simulating the flow features around different LWD structure designs using SRH- 2D.

Each pooled fund partner will have one representative on the pooled fund technical advisory committee (TAC) that will oversee selection of a consultant to conduct the proposed research. The TAC will also oversee the research progress with regular status reports from the selected consultant/university. TAC meetings will be conducted by telephone. It is anticipated the research will take approximately two years to complete from the time the research agreement is signed. It is estimated that the proposed research will $300,000. The minimum funding contribution from each partner is $30,000 for one year or $15,000 for two years.