Effect of Water Temperature and Salinity on the Jet Erosion Test (JET) Discharge Coefficient

Highlights The JET discharge coefficient Cd was negatively correlated to water temperature and positively correlated to salinity. The impact of water temperature was more significant than that of salinity, especially at lower temperatures. Increasing water temperatures reduced Cd by as much as 20% per 10°C increment. Low levels of salinity initially increased Cd , but its magnitude remained relatively constant for levels greater than 5 ppt. Abstract. The discharge coefficient (Cd) of the Jet Erosion Test (JET) directly affects the calculation of applied shear stresses (to) during testing and, therefore, of JET-derived erodibility parameters, namely, the soil’s critical shear stress (tc) and erodibility coefficient (kd). Typically, Cd is assumed to be constant for a given combination of JET device (e.g., original vs. mini) and setup (e.g., head tank vs. pressure gage), disregarding the effects of varying environmental conditions. The objective of this study was to quantify the impact of changing water temperature and salinity on the Cd of a mini-JET device fed by a pressure head tank. Laboratory experiments were conducted to determine the variability in Cd for ranges of water temperature between 10 and 30°C and salinity between 0 and 35 ppt. Results showed that Cd was sensitive to changes in both variables, with water temperature generating a more significant impact. Cd was negatively correlated to water temperature, decreasing by as much as 20% when temperature increased from 10 to 20°C, and positively correlated to salinity, increasing at low levels but remaining relatively constant for levels greater than 5 ppt. When extrapolated to erosion curves and JET-derived erodibility parameters, the variability in Cd significantly impacted the calculation of to and the magnitude of tc and kd, highlighting the degree of uncertainty that could be associated with these variables because of an uncalibrated Cd. Lastly, results from the laboratory experiments were used to develop a normalized predictive model and associated curves for application to other mini-JET devices with similar setups. Keywords: Discharge Coefficient, Erodibility, Jet Erosion Test, Salinity, Temperature.