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Bedload Adaptation Length for Modeling Bed Evolution in Gravel-bed Rivers

Project ID: 3054
Principal Investigator: David Gaeuman
Research Topic: Sediment Management and River Restoration
Funded Fiscal Years: 2012 and 2013
Keywords: "sediment transport" bedload "adaptation length" "bed evolution"

Research Question

The purposes of the proposed research are to develop a theoretical basis for computing the bedload adaptation length (L) used in hydraulic and morphodynamic models and to incorporate the new theory in the SRH-2D numerical model, Reclamation's 2-dimensional hydraulic and sediment transport model. L is necessary to account for nonequilibrium bedload transport rates that arise in a the spatially variable flow fields found in natural rivers. There is currently no satisfactory theory available to quantify L in terms of local hydraulic or sedimentological variables.

Need and Benefit

Recent advances in physically based numerical models have greatly improved the ability of river managers to quantitatively predict scour, fill, bar formation, and other morphological changes in stream channels. The SRH-2D 2-dimensional depth-averaged hydraulic and morphodynamic model developed at the Bureau of Reclamation's (Reclamation) Technical Services Center (Greimann et al. 2008) has been applied to a wide range of Reclamation projects. For example, SRH-2D has been used to investigate morphological changes in the lower Colorado River upstream and downstream from Palo Verde Dam (Lai 2007; Lai and Randle 2009) and to assess the potential for formation of a sediment plug in a reach of the middle Rio Grande. Most recently, the sediment transport module of SRH-2D was used to predict patterns of deposition that would result from large-scale gravel augmentations in the Trinity River of California where model results were used to determine the quantities and locations of gravel injected during a 2011 high-flow release from Lewiston Dam.

Despite their usefulness, current morphodynamic models do not represent all aspects of sediment transport accurately. In particular, all current models rely on sediment transport equations that predict equilibrium sediment transport rates (i.e., transport rates that are fully adjusted to local hydraulic conditions). In reality, shear stresses and other hydraulic parameters are spatially variable, and sediment transport rates may seldom be in equilibrium with the local flow conditions. Numerical morphodynamic models attempt to account for nonequilibrium transport by incorporating an "adaptation length" (L) that quantifies the travel distance required for a packet of sediment to reach a new equilibrium concentration when it moves into a region of higher or lower shear stress.

In the case of suspended sediment transport, L may be evaluated as a function of particle fall velocity and the shear velocity of the flow (Armanini and Di Silvio 1988; Wu 2004). However, this type of formulation is inappropriate for bedload transport (Greimann et al. 2008). For bedload, SRH-2D currently uses a constant user-specified value for L. This approach neglects the fact that L almost certainly varies with the local hydraulic conditions and with grain size and essentially reduces L to a calibration parameter with little physical significance. Experience modeling bed evolution with SRH-2D has shown that model results can be sensitive to the value chosen for L.

Developing a better method for estimating L will greatly improve the accuracy and reliability of the results of bed evolution modeling and, in particular, of the SRH-2D model developed by Reclamation.

Contributing Partners

None

Research Products

Independent Peer Review

The following documents were reviewed by qualified Bureau of Reclamation employees. The findings were determined to be achieved using valid means.

Bedload Adaptation Length in Gravel-bed Rivers Final Report (final, PDF, 359KB)
By Dr. David A Gaeuman and Dr. Yong Lai
Report completed on February 26, 2014

The travel distance required for the flow to acquire a new capacity bedload concentration upon entering a region of higher shear stress is defined as the bedload adaptation length (L). Estimates of L are needed to account for non-equilibrium transport rates in morphodynamic models. Flume experiments suggest that L increases with excess shear stress, from about 30 particle diameters at a transport stage of 1.42 to about 100 diameters at a transport stage of 1.74.
Keywords: bedload, sediment transport, adaptation length

Bureau of Reclamation Review

The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.

Bedload Adaptation Length for Modeling Bed Evolution in Gravel-Bed Rivers (Interim Report) (interim, PDF, 2.6MB)
By Dr. Yong Lai and Dr. David A Gaeuman
Report completed on March 11, 2013

Spatially refined numerical models attempt to account for non-equilibrium transport by incorporating an ‘adaptation length' that quantifies the travel distance required for a packet of sediment to reach a new equilibrium concentration when it moves into a region of higher or lower shear stress. This project seeks to develop new models for incorporating the adaptation length for bedload transport into SHR-2D, a two-dimensional geofluvial model developed at the Technical Service Center.
Keywords: morphodynamic, modeling, sediment transport, bedload, adaptation length

This information was last updated on September 19, 2014
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