A novel approach for determination of fundamental physical transport processes in natural channel design restoration sites with river steering structures

River restoration projects in the United States are frequently proposed and constructed with the intention of improving water quality, yet relatively little evidence exists regarding the success of these efforts. Many projects use an approach known as natural channel design (NCD), and include river steering structures. Prior assessment of water quality improvements within NCD sites has involved hydrologic retention modeling using a non-reactive tracer, with the goal of separately identifying hyporheic and surface transient storage (STS). A comparative approach involving NCD and non-NCD sites used by the authors yielded mixed results: although physically-based assessments of STS profiles in many NCD sites support larger STS zones than non-NCD sites, these differences are not apparent when examining common transient storage metrics. Inverse modeling within nine NCD sites reveals additional obstacles, including generation of spurious lateral inflow/outflow values, limited detection of hyporheic processes due to strong surface transient storage, shear and Taylor dispersion, and divergent temporal patterns of solute flux over channel cross sections bounding structures. To overcome the obstacles encountered with 1D inverse modeling, data is presented from a new approach used in NCD reaches. This approach involves deriving a mass flux signature via pairing velocity and channel geometry with multiple electrical conductivity (EC) loggers deployed laterally at control cross sections (CCS). These CCS bound sub-reach segments (15 total across four NCD reaches) that include river steering structures and intermediate geomorphic features. Velocity and geometry measurements yield discharge values surrounding each EC logger which are used to weight a composite mass flux breakthrough curve above, within, and below each segment. Composite mass flux signatures reflect exchange processes that are not fully integrated laterally immediately below structures, and can be analyzed via inverse modeling techniques, a hybrid of forward and inverse modeling techniques, and temporal moment analysis. Results reveal strong hydrologic retention within steering structures owing to large volumetric storage, dispersion, and surface transient storage zones. This technique represents the development of a unique measurement approach for use in streams with complex geomorphology and hydraulics, as advocated by researchers in the field of transport modeling. Figure 1. Illustration of approach to defining a mass flux signature at control cross sections above, within and below a river steering structure.