Can a numerical model simulate observed patters of gravel dispersion based on river bed morphology and flow hydraulics?

Project ID: 5049
Principal Investigator: D. Nathan Bradley
Research Topic: Sediment Management and River Restoration
Funded Fiscal Years: 2015, 2016 and 2017
Keywords: stream restoration, gravel augmentation, sediment transport, dispersion

Research Question

The sediment supply to a river downstream of a dam is reduced because sediment inputs from upstream are blocked. Without continuous replacement by upstream material, transportable sediment below a dam is winnowed away, resulting in a river bed that is coarser and less morphologically diverse. This negatively affects fish in at two ways. First, river gravel of transportable size provides the substrate in which fish spawn. Second, mobile gravel creates and maintains the diverse channel morphology (bars, pools, riffles) that provides high quality aquatic habitat. Consequently, efforts to improve fish habitat below a dam commonly involve adding transportable sediment, typically gravel, to the river. Unfortunately, predicting how introduced gravel will move and where it will be deposited involves significant uncertainty. Unintended deposition of gravel in pools or side channels reduces the amount of high quality habitat, increasing stream restoration costs and decreasing public confidence in restoration efforts. Accurate computer simulation of the dispersion and fate of introduced gravel would lower the cost and improve the effectiveness of stream restoration projects that involve gravel augmentation. The specific question we wish to answer is:

Can a hydraulic and sediment transport model (SRH-2D, [Lai, 2010]) accurately simulate observed patterns of gravel dispersion?

We expect that the model will need improvements to accurately simulate gravel dispersion. To develop an improved gravel tracking algorithm, we will need to answer the following questions:

How does gravel in transport interact with the channel morphology?

How long does it take for introduced gravel to become incorporated into the river bed and move at the same rate as the natural sediment?

Is it necessary to simulate unsteady flow and non-equilibrium transport to predict dispersion patterns?

Are there other factors, such as flood sequencing, that affect sediment transport and what are the

Need and Benefit

Gravel augmentation is a common strategy for improving fish habitat in rivers below dams. A dam blocks all sediment inputs from upstream, reducing the supply of transportable sediment downstream of the dam. Transportable sediment provides the substrate in which fish spawn and creates and maintains the diverse channel morphology that provides fish habitat (bars, pools, riffles). To compensate for the lack of transportable gravel below a dam, efforts to improve fish habitat often include adding gravel to the river. One example is the Trinity River in Northern California where Reclamation's Trinity River Restoration Project (TRRP) has added gravel below Lewiston Dam in hopes of improving habitat for anadromous Coho and Chinook salmon and steelhead. Despite the significant costs involved, predicting how introduced gravel will move and where it will be deposited is fraught with uncertainty. Gravel deposition in deep pools used as holding habitat and thermal refugia by adult fish and the blocking of access to juvenile rearing habitat in side channels are common unintended consequences of gravel augmentation that reduce, rather than increase, the amount of quality habitat. To avoid the unintended consequences of gravel augmentation, we need tools to predict the dispersion and fate of introduced gravel.

The need to monitor the effects of gravel augmentation was recognized by S&T in the past with support of project 0295 [Gaeuman, 2013], which monitored the downstream progress of augmented gravel using a technique similar to that proposed here. That study concluded that increasing the rate of gravel augmentation above transport capacity may increase the time required for augmented gravel to improve downstream habitat. It did not address gravel dispersion specifically or attempt to simulate the behavior of the augmented gravel with a numerical model, as we propose here.

Better computer simulation of the trajectory and fate of introduced gravel based on the channel morphology and flow hydraulics would make stream restoration projects that involve gravel augmentation more effective and less expensive. An accurate gravel dispersion model would guide decisions about the grain size distribution and quantity of augmented gravel as well as where and how it should be added to the river to best achieve the desired outcome. We expect that Reclamation's existing sediment transport model (SRH-2D) will need improvements to accurately simulate gravel dispersion. A decade of gravel transport paths from the Halfmoon Creek tracer experiment will guide the development of new algorithms to more effectively simulate the dispersion of introduced gravel.

Contributing Partners

Contact the Principal Investigator for information about partners.

Research Products

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.

Can a numerical model simulate observed patterns of gravel dispersion based on river bed morphology and flow hydraulics? (final, PDF, 6.1MB)
By D. Nathan Bradley
Research Product completed on September 30, 2017

This research product summarizes the research results and potential application to Reclamation's mission.

Can a numerical model simulate observed patterns of gravel dispersion based on river bed morphology and flow hydraulics? (final, PDF, 6.1MB)
By D. Nathan Bradley
Research Product completed on September 30, 2017

This research product summarizes the research results and potential application to Reclamation's mission.


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Last Updated: 4/4/17