Variability of Erodibility Parameters from Laboratory Mini Jet Erosion Tests

Application of jet erosion tests (JETs) to study in situ erodibility is gaining popularity. New versions of the JET (original JET versus mini-JET) and new data analysis techniques have introduced questions regarding their operation and data collection procedures. One of the major issues regarding JETs is the high degree of variability of the erodibility parameters (i.e.,erodibility coefficient, kd, and critical shear stress, c). This variability has been attributed to heterogeneity in different soil properties under natural field conditions, but limited research has quantified variability under controlled laboratory conditions, especially for the newer mini-JET. This study uniquely conducted 20 mini-JETs under controlled laboratory conditions on each of two soil types of contrasting texture. Mini-JETs were conducted in situ on streambanks of these same soils in previous research. The laboratory mini-JETs predicted similar values of most parameters with much less variability than in the field. Three to five mini-JETs conducted in the laboratory estimated erodibility parameters with a precision of 25% at a 95% confidence level. Laboratory JETs on disturbed, remolded samples provided baseline estimates of in situ erodibility parameters. Additional mini-JETs were conducted at three different head settings on the two soil types. The influence of the head setting was dependent on the soil type, solution technique, and detachment model. In general, variability in derived erodibility parameters increased at larger head settings especially for the less-erodible soil. Existing mini-JET data were resampled to evaluate the effect of the initial time interval and termination time interval of data collection on derived erodibility parameters. Both initial and termination time intervals were most influential at larger head settings. When the applied pressure head is sufficient to create scouring upon test initiation, an initial time interval of at least 30s and a termination time interval of at least 300s is recommended for less-erodible soils.