Propagation, mixing, and turbulence characteristics of saline and turbidity currents over rough and permeable/impermeable beds

Abstract

A series of constant-flux saline and turbidity current experiments were carried out, focusing on the coupling impact of bed roughness and permeability on current propagation, mixing, and turbulence characteristics. The distinct current propagation phases on RI (rough and impermeable) and RP (rough and permeable) beds are identified respectively. Experimental results revealed that intermittently undulating bed surface breaks the strict no-slip boundary, thereby increasing local current velocity near the bed, while its roughness reduces the current peak profile velocity. Interbed pores of rough bed induce vertical fluid exchange, which synchronously decrease the current velocity, and causes the density profile no longer follow a monotonous variation trend along with water depth. The larger bed surface roughness or the interbed porosity leads to the smaller upper TKE (Turbulent Kinetic Energy) peak. On the other hand, the lower TKE peak is inversely proportional to the bed surface roughness of the RI beds but proportional to the porosity of the RP bed. Rough bed surface intensifies the asymmetry of the mean velocity distribution around peak velocity, resulting in a transfer barrier of turbulent momentum triggered by the interbed pores. For the RP bed, the cross-correlation function based on two-point statistics captures the spikes associated with pore-scale eddies locally, while on the RI condition the function only obtains the time scale characterizing the largest eddies of the current. Sediment deposition makes the turbidity current easier to separate from the RP and RI bed than the saline type, as a consequence of increasing the current height.

Hydrodynamics and Sediment Transport of Downslope Turbidity Current Through Rigid Vegetation

“…For the bare‐slope condition (Figure 7a, run 1), the TKE profile of the turbidity current without sediment deposition satisfies a bimodal distribution. In general, the upper TKE peak locates at the current‐ambient fluid interface, and the lower TKE peak is close to the bed surface, which is consistent with previous studies (Baas et al., 2004, 2005; He et al., 2022). The effect of vegetation height α on the TKE profiles is only manifested in the upper peaks (Figure 7a, runs 1, 2, and 4).…”

“…Therefore, the friction velocity of the slope surface $${u}_{z=0}^{\ast }$$ is obtained through an appropriate linear extrapolation as was done by He et al. (2022).…”

“…It must be pointed out that it is unlikely to precisely obtain the hydrodynamic parameters of the slope surface (z = 0) using the ADVP. Therefore, the friction velocity of the slope surface 𝐴𝐴 𝐴𝐴 * 𝑧𝑧=0 is obtained through an appropriate linear extrapolation as was done by He et al (2022).…”

“…Therefore, the three‐dimensional velocity obtained from the ADVP needs to be converted based on geometric relationships. The sampling volume of the ADVP for turbidity currents was set at approximately 0.12 cm 3 , which meets the accuracy requirements of the experimental data (He et al., 2022). The signal‐to‐noise ratio (SNR) and the correlation level represent the quality of the collected data and were more than 15% and 75%, respectively (Sharma et al., 2017; Yang et al., 2021).…”

Turbulence characteristics and mixing properties of gravity currents over complex topography

“…These frequent and diverse occurrences of gravity currents in the natural environment have motivated numerous investigations, of interdisciplinary nature, of these gravity flows. Several studies have shown how the propagation of dense, bottom gravity currents is affected by topographic aspects [15][16][17][18][19][20][21][22] , by sloping boundaries [23][24][25][26][27][28][29][30][31] as well as rapidly changing slopes [32][33][34][35] . Most of these studies have focused on the mean flow dynamics and the related entrainment of ambient fluid that strongly affects the flow development and fluid properties.…”