Measuring erodibility parameters of cohesive soil for variable physical properties using submersible “mini” JET device

Accurate prediction of streambank and bed erosion is essential for effective erosion control in cohesive soils, which exhibit complex detachment mechanisms driven by interparticle electrochemical forces. While empirical methods such as the plasticity index and particle size distribution are often used, they often lack the generality needed to account for the heterogeneity of cohesive sediments. In this study, a portable “mini” Jet Erosion Test (JET) device, designed for greater portability and ease of use in both laboratory and field applications, was used. It was employed to determine soil erodibility parameters, namely the erodibility coefficient ( $$\:{k}_{d}$$ ) and critical shear stress ( $$\:{\tau\:}_{c}$$ ). The experimental work involved testing two distinct cohesive soil types: Soil 1 (47% clay content) and Soil 2 (30% clay content). Eighteen tests were conducted across nine different moisture levels to quantitatively assess the impact of physical properties, specifically moisture content and density, on erodibility parameters. Scour depth readings were recorded at precise intervals over 45 to 65 min per test. The raw data were then analyzed using three analytical techniques: the Scour depth principle (SDP), the Iterative principle (IP), and the Blaisdell solution (BS) to determine $$\:{k}_{d}$$ and $$\:{\tau\:}_{c}$$ . Within the specific soils and test conditions considered here, optimal erosion resistance was observed at moisture contents of 14–19% for Soil Type 1 and 22–26% for Soil Type 2, where minimum $$\:{k}_{d}$$ and maximum $$\:{\tau\:}_{c}$$ were recorded. These ranges should be interpreted as condition-specific outcomes for the tested soils compacted to approximately 85% Standard Proctor effort, rather than as general threshold values for cohesive soils. Higher bulk and dry densities were associated with lower $$\:{k}_{d}$$ and higher $$\:{\tau\:}_{c}$$ ; for example, in Soil Type 1, increasing bulk density from 1.45 to 1.95 g/cm³ raised $$\:{\tau\:}_{c}$$ from about 1.72 Pa to 32.38 Pa . An inverse power-law relationship between τc and kd was observed in this study for both soils (R² = 0.789–0.961), which is consistent with trends reported in prior studies. Among the three analytical methods (BS, SDP, and IP), overall trends were similar, though the SDP method produced the highest median $$\:{\tau\:}_{c}$$ (~ 20 Pa), while the BS method exhibited the lowest variability. This study confirms that the “mini” JET equipment is a valuable instrument for laboratory and in situ studies and illustrates the role of moisture and soil density on erodibility.