Collaborative Research: A mechanistic understanding of hydrograph shape influence on temporal variations in bedload transport, grain size distributions, and armor persistence

University Of California-Davis, Davis CA

Investigators

Abstract

Natural and managed rivers throughout the world display a wide range of hydrographs, from flashy and abrupt to more gradual changes in flow. Hydrograph shape (quantified by peak magnitude and rate of discharge change) significantly influences the volume, timing, and quality of water available for anthropogenic use and aquatic ecosystems. Uncertainties remain as to how unsteady flows that characterize hydrographs impact bedload fluxes and hysteresis, grain size distributions, and the persistence of armor layers. The lack of a mechanistic and predictive understanding of hydrograph influences may also partially explain why bedload transport equations in gravel-bed rivers are often inaccurate. The research team proposes complimentary flume, field, and numerical modeling experiments to quantify the impact of hydrographs on sediment movement. Specifically, they hypothesize that hydrograph shape influences the following: (1) bed load transport rates, hysteresis, and mobile grain sizes for a given shear stress, and (2) bed grain size distributions and the persistence/degree of bed armoring. In flume experiments, temporal variability in bedload fluxes, grain size distributions, and armor persistence will be measured for a range of hydrograph shapes. Field experiments will further investigate the influence of hydrograph shape on bedload transport rates, bed grain size changes, and armor persistence, mobility and removal. The combined flume and field measurements will be used to test and validate numerical and analytical models for bedload transport. Hydrographs represent how water quantities in rivers vary over time and can be characterized by the timing, magnitude and rates of change of flow. Human modifications of these hydrograph characteristics have positively influenced economic development but negatively impacted aquatic species and channel morphology. This is important because 77% of total water discharge from large northern hemisphere rivers is moderately to severely impacted by dams and diversions. Water resource managers commonly seek to assess and mitigate these impacts through river restoration efforts, hydropower re-regulation, and in some cases dam removal. For example, gravel augmentation is commonly used to mitigate for low sediment supplies and improve fish habitat below dams but this often has mixed results because of a lack of a predictive understanding of sediment transport caused by variable flows. Managers could use the results of this research to determine flow hydrograph shapes that potentially mitigate for flow regulation influence on threatened or endangered aquatic organisms. Further, this research will result in a predictive and mechanistic understanding of the interactions between flow hydrographs, sediment transport, and temporal variations in the size of sediment on the channel bed. Such knowledge is necessary to properly design river infrastructure and restoration projects, and predict long-term channel incision rates and landscape changes. Additionally, the project will contribute towards unique educational opportunities for graduate and undergraduate students and support for a Women Outdoors with Science Camp will encourage young females to enter STEM fields.