The Alluvial Phase Space Diagram (apsd) and Its Potential Application In The Frame-Rubric Model

As described in the earlier presentations in this session, the FRAME-RUBRIC model is being developed by an international research consortium led by CHL-ERDC, in Vicksburg, Miss to help inform management decision-making that builds resilient rivers that will function reliably over periods of decades to centuries. This is challenging because evolutionary trends, forms and functions in alluvial channels will respond to future natural disturbances and engineering actions that significantly disrupt the river’s fluvial and biological processes. As the types and timings of these disturbances are not predestined, the future is not uncertain, it’s unknowable. Accepting this, following disturbance, three trajectories of morphological response are possible: 1. short-term adjustments that cancel each other out, so that the river’s form (e.g., width, depth, velocity, etc.) remains within the range of values defining its band of dynamic, meta-stability; 2. sequential adjustments that initially shift the river’s form to outside its band of dynamic meta-stability, but subsequently return the river to its pre-disturbance condition; 3. sequential adjustments that not only shift the river’s form outside its band of dynamic meta-stability, but subsequently cause the river to evolve towards a new dynamically meta-stable condition. The Alluvial Phase Space Diagram - APSD - (created jointly at the USGS Cascades Volcano Observatory and the University of Nottingham) provides a template for visualizing, quantifying and evaluating adjustments, and sequences of adjustments, in river form: differentiating between these three evolutionary trajectories. The APSD is being applied in three elements of FRAME-RUBRIC model development: A. testing the performance of FRAME in simulating morphological responses to disturbance, B. training RUBRIC for application to individual study rivers, based on their past behaviors, C. evaluating the implications of future responses to river management for river evolution, form, and functions. In this paper, we introduce the APSD using its application to 40 years of post-eruption channel evolution in the North Fork Toutle River (NFTR) at Mount St Helens, WA. The APSD reveals that in this highly disturbed system, channel evolution has been more complex than conventional channel evolution models would predict. In summary, APSD analysis dispels the orthodoxy that early-stage adjustments involve vertical adjustments, while lateral adjustments occur only during late-stage adjustments. We then illustrate the potential application of the APSD in FRAME-RUBRIC using a case study involving a 200-mile reach of the Lower Mississippi River (LMR). In contrast to the NFTR, over the past 60 years the study reach of the LMR has been close to a meta-stable condition. Notwithstanding this, the APSD reveals that the river has made numerous adjustments to its thalweg elevation and cross-sectional area, while responding to periods of high and low water, channel changes in adjacent reaches, and engineering interventions such as the construction of dike fields. The long-term record of adjustments within the band of dynamic stability that has been generated using the APSD provides useful insights regarding why this reach of the LMR has been resilient to disturbance over the last six decades.