RUles-Based morphological Response In river Channels (RUBRIC): A decision support tool accommodating decadal-scale channel evolution in river management: proof of concept

Numerous recent studies have retrospectively reconstructed channel evolutionary trajectories in the recent Anthropocene using multiple historical data sources (review in Downs and Piégay, 2019), but how can evolutionary tendencies be projected forward to build foresight competency into the field of river management? A system of prediction is required that can simulate likely modes of channel morphological evolution to projected changes in environmental forcing conditions over management time and space scales, that is, from a decade to a century or more and over tens of miles of river. Critically, the analysis needs to be governed by, and directed towards the strategic decision-support needs of the river manager.

This report details a proof-of-concept decision support tool, RUBRIC (RUles-Based morphological Response In river Channels), designed to extend from a recently developed hydrodynamic model for channel evolution (FRAME: Future River Analysis and Management Evaluation). The theme of ‘foresight competency’ is developed for the overall process of river management in Section 1 – while FRAME focuses on the forecasting component of foresight, RUBRIC tackles the largely-neglected notion of visioning, which provides the critical decision support for translating analysis into the management realm of river projects. Section 2 details and justifies the approaches taken herein, beginning with the values and prospects for visioning channel evolution in terms of state-transition changes in river morphology over decadal timescales (2.1). The mechanics of linking the analysis to decision support through a series of channel evolution ‘metrics’ that underpin the graphical indicators in a ‘management dashboard’ is outlined (2.2), and subsequently formalized as a meta-model of FRAME-RUBRIC operation that forms the framework of current research (2.3).

Section 3 justifies the selection of seven channel foresight ‘metrics of change’ proposed to provide a broad understanding of channel evolution as it applies to common concerns in river planning, river engineering, and in river restoration and conservation. The metrics, aimed to assist in foresight over a 50–100-year period exploit the fundamental degrees of freedom in morphological adjustment that result from FRAME modeling and are drawn, where possible, from physically-based studies to provide the strongest theoretical justification. Metrics for land use planning include those for channel planform adjustment, channel cross-sectional adjustment, and the erodible river corridor. Metrics for hazard reduction and asset increases include those for floodplain hydrological connectivity and riverbank erosion rate. To support conservation management there are metrics for bedform habitat type and ecohydraulic diversity. Functional realization of two of the seven metrics, related to riverbank erosion and meander migration, await proposed developments in FRAME modeling abilities.

Channel evolution metrics are not directly suitable for communicating to the end user and, increasingly, such communication is provided in the form of a ‘dashboard’ of graphical indicators whose meaning is readily apparent to user groups. In Section 4 we thus outline the development of a prototype dashboard of graphical indicators of channel evolution. The indicators are derived from the channel metrics and are intended to convey information in a visually intuitive manner, generally relative to proximity to a relevant state-transition threshold in the metric. Prototype dashboards are designed to display (1) the spatial variation of indicators for a user-chosen year in the forecast timeframe and (2) variation through time of indicators at a user-chosen cross-section. The in-built and expanding range of user-chosen inputs within FRAME allows the analyst to choose various hypothetical management scenarios, and the indicators will evolve over time based on user feedback and the development of new capabilities within FRAME. Section 4.2 details the initial form of the indicators derived for this proof-of-concept formulation of RUBRIC and indicates the initial form of the dashboard. ‘User guidance’ is provided for setting up and running RUBRIC in its current guide in Section 4.3. RUBRIC is currently run through an Excel spreadsheet: various output data are exported from FRAME (also Excel-based) into a spreadsheet configured to automatically calculate the channel metrics. The metrics are then transferred to a further spreadsheet that translates the metrics into indicators and the resultant dashboards. This ‘semi-automated’ system was designed to allow checks on data quality and calculations: it is anticipated that in later iterations many of the calculations will be performed directly in FRAME.

Section 5 details proof-of-concept case studies on two sand-bedded rivers (the lower Mississippi and the Kankakee). Case studies for gravel bed rivers will follow once a gravel sediment transport function is added to FRAME – as many of the metrics were initially proven in gravel-bed river settings, such capability will likely provide a truer indication of the sensitivity of the metrics to different management futures. Four future scenarios are run for each river including under a 60-year continuation of recent hydroclimatic conditions and under conditions of higher flows resulting from climate change. The third scenario involves a hypothetical management intervention on each river under current hydroclimate, and the fourth combines the management intervention with climate change. Output displays are provided and discussed briefly, acknowledging that true validation of the model will require a ‘retrospective forecast’ for a river with detailed and well-constrained morphological evolution surveys, and thus the case study outputs should be treated as indicative only.

The report concludes (Section 6) by outlining near- and medium-term prospects for advancing RUBRIC beyond its current proof-of-concept status. Suggestions are provided for the integration of various RUBRIC functions within FRAME (6.1) and for future advances within FRAME itself, in the interface of FRAME with RUBRIC, for the management dashboards, and for receiving user feedback. Such suggestions are based on discussions held over the course of the project, not least on the development of width adjustment and meander migration capabilities within FRAME, and how RUBRIC might change as a result. These latter themes form the basis of two appendices suggestive of next-step developments.