Water Resources Research Act Program

Details for Project ID 2017TN133B

Combined Field Study of Turbulenceand Bed Morphology in Mountainous Boulder Arrayed Streams

Institute: Tennessee
Year Established: 2017 Start Date: 2017-03-01 End Date: 2019-02-28
Total Federal Funds: $5,000 Total Non-Federal Funds: $2,550

Principal Investigators: Micah Wyssmann, Thanos Papanicolaou

Abstract: The presence of roughness elements such as boulders and large particle groupings in natural coarse-grained streams, such as those in the mountainous Eastern Tennessee, plays a key role in flow routing and affects stage-discharge relationships. Understanding the stability, sediment entrapment and bedload transport characteristics within these stream reaches is critical to predicting river morphological responses, and their feedback interactions with the flow. In addition, arrays of boulders are a viable stream restoration method in order to dissipate flow energy and provide fish habitat benefits (e.g., Saldi-Caromile et al. 2004). An example restoration usage of a boulder array was carried out in the Elwha River and similar uses for energy dissipation following the removal of low head dams may be useful for stream stabilization and erosion control. With the abundance of coarse-grained mountain rivers and ubiquitous boulders in the Smoky Mountains, an enhanced understanding on the controls that these boulders exert on bedload transport is greatly needed for informing sediment transport management practices, including the design of stream restoration structures and the prediction of the life expectancy of the dams operated by the TVA. Furthermore, the proposed improvements in prediction of both magnitude and timing of bedload transport events would have specific implications for improving sediment models, particularly in the Smoky Mountain watersheds. The overarching objective of my dissertation research is the development of a mechanistic bedload transport model that can predict fluxes in mountain river reaches with ubiquitous boulders, while capturing important timescales of pulsation in bedload transport rates. In these types of streams, macro-roughness elements such as boulders play a significant role in providing additional flow resistance and energy dissipation due to the form drag that they generate (e.g., Baki et al. 2016; Papanicolaou et al. 2012). Through these effects, boulders modify bedload transport characteristics in their vicinity (Yager et al. 2007), and thereby influence stream morphology (Montgomery and Buffington 1997). Specifically, boulders tend to detain incoming mobile bedload by promoting the formation of organized particle groupings (Brayshaw 1984), which subsequently break up at high flows and supply pulses of sediment downstream (Strom et al. 2004). Current bedload transport models do not incorporate in their formulations the aforementioned boulder effects and consequently cannot fully capture either the magnitude or timing of bedload transport events within these types of channels (e.g. Yager et al. 2007; Cudden and Hoey 2003). This has important implications for the accuracy of results from sediment modeling assessments at both the stream and watershed scales, which are in turn employed for sediment management in these rivers. Based on previous research, there are two key mechanisms by which boulders affect bedload transport and deposition, namely, (1) modification of the hydrodynamic flow field within the boulder array, and (2) stimulation of particle-to-particle interactions. These key process influences are hypothesized to govern the development and destruction of particle groupings in the area surrounding boulders, whereby boulders can act as either a sink or a source of sediment and, in turn, modify the magnitude and timing of bedload transport (e.g. Strom et al. 2004; Papanicolaou and Kramer 2005; Papanicolaou et al. 2012; Heays et al. 2014). A primary objective of my dissertation research is to connect bedload movement observations with these two mechanisms in order to describe the effects of boulders on bedload transport.