Sediment transport and wind flow around hummocks

This study presents field-based observations demonstrating the relationships between vegetation density, shear stress, and sediment transport surrounding hummocks. Data collection for 120 min measured wind velocities using a sonic anemometer, grain impacts from four miniphones (MICs) deployed on and to the side (in unobstructed flow) of a hummock, trap-derived sand transport, and hummock vegetation densities between 3–26%. These data provide the parameters to estimate model-based transport rates from Bagnold, Zingg, Kawamura, and Lettau and Lettau, and to use the Bagnold slope correction equation. The average trap- and co-located MIC-based transport rates were 25.0 and 89.8 g/m2/s with an R2 of 0.39 (p < 0.01). Linear regression analysis comparing model-estimated and trap-based transport was significant (p < 0.05) using the Kawamura and Lettau and Lettau models. The highest correlation between modeled and observed transport rates from the MICs and the trap was found using the Zingg and Bagnold models, though all four models were statistically significant (p < 0.05). Correcting the Bagnold transport model using his slope correction equation did not substantially change the R2 value (p > 0.05). A positive relationship between vegetation coverage and transport rate was observed. The strongest correlation between vegetation coverage and transport rate was found when considering the two MICs placed on the hummock within the vegetation. The MIC placed adjacent to the hummock had a low R2 that was statistically insignificant (p > 0.05). These findings suggest that flow steering around the hummock side and grain projection around and over the hummock plays an important role in hummock morphology and processes.

► Transport and wind conditions were measured around a hummock with natural vegetation. ► Observations suggest vegetation was not the primary control on transport rates. ► Results suggest steering is an alternative control on transport rates.