Relative Importance of Climate and Floodplain Management On The Morphodynamic Evolution of A Gravel-Bed River: Numerical Simulations Using Mast-1d

Simulating geomorphic change on rivers over the decadal timescales relevant to river management problems often requires consideration of river discharge, sediment transport, and, critically, the channel/floodplain processes that maintain the channel. This study illustrates the interacting influences of potential climate-related discharge change and modifications to management approaches (including levee setback projects and sediment augmentation/removal) on channel morphodynamics and flood hazard over decadal times scales. In gravel-bed systems, simulating reach-scale change in channel capacity, bed elevation, and bed texture from a mass-conservation basis requires a size-specific accounting of sediment fluxes. In addition, particularly in mountainous settings, tributary water and sediment inputs can represent primary controls on downstream alluvial behavior, so these too should be included. While a range of models exist for simulating change in such systems, most focus on 2-D/3-D representations of detailed in-channel processes over relatively short space/time scales or on the scale of the entire watershed network. The 1-D models that do account for change at the scale of long river reaches (10s to 100s of km) often only consider longitudinal channel fluxes, preventing them from converging on dynamically-stable channel geometries as boundary conditions change. However, understanding the adjustment of channel geometry to changing flow and/or sediment supply regimes – if such adjustment is possible given floodplain management – is important for anticipating future flood and channel migration hazards. In this study, the Morphodynamics and Sediment Tracers in 1-D (MAST-1D) model is used to illustrate the sensitivity of a 100-km long reach of a large gravel-bed river to hypothetical decade-scale changes in climate, floodplain management, and spatially-varying tributary water and sediment supply (a new feature of MAST-1D). Python-based Jupyter Notebooks demonstrating the MAST-1D simulations are presented; these provide templates that can be modified to represent similar climate and management changes in other river systems. MAST-1D computes size-specific sediment fluxes for sediment nodes representing relatively short sections of a channel reach. Computations are performed using a simplified cross section that includes a constant-elevation floodplain. A sediment budget computation at each node accounts for sediment storage in the channel bed and in an off-channel deposit representing the floodplain. Independent models for bank erosion (which depends on mobility of bank-toe material) and vegetation-moderated point bar formation allow channel width to adjust, and a size-specific Exner equation applied to each sediment storage reservoir ensures that sediment liberated by net channel widening or stored due to narrowing or floodplain deposition can be tracked through the model. The independent models for bank erosion and vegetation encroachment facilitate computations of overall lateral change rates as the channel shifts across the channel migration zone. Simulations based on daily discharge hydrographs illustrate how channel geometry adjustment in response to high flows affects subsequent flood hazard, as well as how channel widening can represent a dominant source of sediment during and well after a large flood. Management scenarios, including a levee setback project as well as sediment augmentation and removal scenarios, are included in the simulations to examine how these techniques may mitigate or exacerbate hazard in a changing climate.