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Title: Critical assessment of jet erosion test methodologies for cohesive soil and sediment

item KARAMIGOLBAGHI, MALIHEH - State University Of New York (SUNY)
item GHANEEIZAD, SEYED - State University Of New York (SUNY)
item ATKINSON, JOSEPH - State University Of New York (SUNY)
item BENNETT, SEAN - State University Of New York (SUNY)
item Wells, Robert - Rob

Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/2017
Publication Date: 8/4/2017
Publication URL:
Citation: Karamigolbaghi, M.R., Ghaneeizad, S.M., Atkinson, J.F., Bennett, S.J., Wells, R.R. 2017. Critical assessment of jet erosion test methodologies for cohesive soil and sediment. Geomorphology. 295: 529-536.

Interpretive Summary: This study represents a comparison of submerged jet erosion test (JET) solutions (Blaisdell, Iterative and Scour Depth), to examine the relationship between critical shear stress and soil erodibility and the effects of jet confinement. A dataset of 1030 tests were reduced to 155 tests based on conditions of a fully developed jet (distance between jet nozzle and soil surface) and constant jet velocity (consistently diminishing shear). The Blaisdell solution yields smaller critical shear values than either the Iterative or Scour Depth solutions and, over-prediction by the Iterative and Scour Depth solutions can result in zero erosion rate (unrealistic). An alternative approach, omitting data points indicative of mass erosion (initial scour depth depth measurements, ~0-5 minutes), improves scour data fitting. The Iterative methods resulted in a better statistical fit. Jet results show increased critical shear and reduced soil erodibility by a factor of 2.4 when the jet confinement effect is considered. JET results should be used with caution, or at least a degree of uncertainty should be considered.

Technical Abstract: The submerged Jet Erosion Test (JET) is a commonly used technique to assess the erodibility of cohesive soil. Employing a linear excess shear stress equation and impinging jet theory, simple numerical methods have been developed to analyze data collected using a JET to determine the critical shear stress and erodibility coefficient of soil. These methods include the Blaisdell, Iterative, and Scour Depth Methods, and all have been organized into easy to use spreadsheet routines. These methods, however, are based on the assumption of a flat, smooth bed in unconfined boundary conditions. The main objectives of this study are to critically assess and compare these methodologies, to examine the effects of jet confinement on the erodibility indices, to consider the possible relationship between erodibility coefficient and critical shear stress, and to comment on other assumptions as topics for future work. Using a large database of JET results from 20 regions around the United States and data from literature, it is shown that each of these methods can generate an acceptable curve fit through measured scour depth as a function of time, except for cases in which mass or aggregate erosion occurs. In such cases, the Scour Depth Method gives the best fit, as it forces the erosion parameters to match the data directly. The analysis shows, however, that using the Scour Depth and Iterative Methods may result in physically meaningless values for the erosion parameters. These two methods can highly over-estimate the critical shear stress, resulting in zero erosion rates when applying the linear excess shear stress equation. The JET results show that the effect of flow confinement increases the critical shear stress and decreases the erodibility coefficient with a factor of 2.4. The fitted curve through measured scour depth data as a function of time does not change. It also is shown that there is a general lack of consensus relating the derived soil erodibility coefficient to its derived critical shear stress. Although all relationships are significant and some methods result in less-scattered data, the calculated erodibility coefficient could still have an uncertainty of several orders of magnitude. The length of time over which scour depth data are collected also seems to affect the calculation of erosion parameters. The study shows that JET test results should be applied with caution and the degree of uncertainty should be considered.