Rivers exhibit large longitudinal variations, which may range from continuous predictable gradients (e.g. discharge) to small-scale anomalies, such as bedrock outcrops, fault zones, and sediment point sources. Understanding the gradient and distribution of channel types and geomorphic characteristics such as substrate, pool density, and gradient, among many others, is integral to interpreting results from studies which sample point data along the stream channel (e.g. salmonid habitat surveys and fish counts). Interpreting the results of such surveys, which are conducted over short (~200 m) reaches may be difficult, especially when determining the influence of local factors influencing physical habitat (e.g. point source runoff, land use) versus shifts occurring naturally as a result of longitudinal stream variations (e.g. increased discharge) and/or short-duration differences, such as a tributary junction).
A fluvial audit is conducted in two phases, (A) a desktop-based GIS exercise and (B) a rapid field assessment.
DESKTOP GIS: Relevant geospatial data are obtained for the study watershed. This spatial information is then synthesized and used to delineate physiographic provinces occurring in the study watershed. Ccombining these relevant sources of information is used to produce a first-order approximation of the geomorphic reach breaks that may be encountered in the field.
RAPID FIELD ASSESMENT: Crews visit the study streams with minimally the geospatial data used in the desktop GIS exercise. Notes were taken at GPS-collected data points and described geomorphic characteristics about the channels being surveyed. Additionally, a comprehensive photo record along the course of the channel was obtained using a digital camera with geotagging capability. Field crews walked the entire course of the study streams and recorded datapoints which described channel attributes including gradient, substrate, sinuosity, valley confinement, pool density, LWD density, and Montgomery/Buffington channel type. With the downstream progression of the survey, noticeable shifts in a number of these attributes results in the delineation of a new geomorphic reach.
The developed morphodynamic model will be driven using a two-dimensional hydraulic scheme (currently the CFD-driven Hydro2de model). Following calculation of hydraulics, sediment transport will be driven using a new step-length based component. In contrast to calculating whether sufficient shear stress exists for particle entrainment continuously downstream, volumes of eroded sediment will be distributed according to a specified step-length distribution, which has been demonstrated in gravel-bed channels by previous studies (Habersack, 2001; Pyrce and Ashmore, 2003). The major advantage in using a step-length based scheme is the reduction in computational overhead achieved by simplifying downstream sediment transport, allowing for greater spatiotemporal modeling range and a higher resolution. Following sediment mobilization downstream, the channel bed elevation is updated at each computational node (i.e. Exner equation), hydraulics are recomputed using the 2D CFD model and the process is repeated through time._____________________________________________________________________________________