Impacts of hydrograph shape on sediment transport

Building from a 2011 National Center for Earth Surface's (NCED) Visitor's Program Grant completed in collaboration with colleagues at University of Idaho's Center for Ecohydraulics Research, this project seeks to understand the impacts of hydrographs on sediment movement. 

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.  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.

A series of preliminary experiments were conducted in 2011-2012 in collaboration with U of Idaho, US Forest Service and NCED.  One set of experiments was conducted in the Outdoor Stream Lab at St. Anthony Falls Labratory, U of Minnesota to understand hydrograph influences on sand-bedded point bar morphology and a second set was conducted at the Richmond Field Station, UC Berkeley to quantify the impact of the recession rate on gravel-bedded forced bar dimensions and grain sorting.  The recession rates varied between experimental runs, and the bed topography, flow characteristics (velocity, depth), sediment transport rates and bed grain sizes were measured. Preliminary results from both sets of experiments showed that more rapid recession rates caused pool scour and preserved sediment deposited on bar tops by peak flows. Gradual recession rates decreased pool scour but redistributed sediment from the bar top to other locations.  This data was used to support a proposal for more extensive field and flume experiments funded by the National Science Foundation (NSF).

In the current NSF study, we will conduct complimentary flume, field, and numerical modeling experiments to quantify the impact of hydrographs on sediment movement. Specifically, we 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.

Researchers at CWS are leading the field experiment efforts, while colleagues at U Idaho are leading the flume experiments.  The field work is being conducted at the Caspar Creek Experimental Watershed (http://www.fs.fed.us/psw/ef/caspar_creek/), where long-term records exist of basic hydrologic and geomorphic processes spanning the downslope sequence of processes from the tree canopy to third-order channels in two primary watersheds.  Field work will focus on North Fork Caspar Creek where infrastructure exists to measure discharge and bedload transport.  We will study the impacts of various storm hydrographs on bedload transport using pit-tagged particles, in-situ bedload traps and manual event-based measurements.  Additionally we will document potential changes in topography, armor persistence and substrate composition, and explore the impacts of hydraulic variations during floods through hydrodynamic modeling.

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.

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