Estimation of capacity of suspended load considering effects of preservation of turbulent kinetic energy

Monday, 15 December 2014
Hajime Naruse1, Daisuke Sugawara2 and Kazuhisa Goto2, (1)Kyoto University, Kyoto, Japan, (2)Tohoku University, Sendai, Japan
Quantitative estimation of capacity of suspended sediment load is critical for reconstruction of flows such as tsunamis or turbidity currents. Capacity of suspended load is a layer-averaged concentration at which suspended sediments are saturated in flows, and it works as a threshold between erosion and deposition from suspended sediments. Capacity of suspended load varies depending on sediment grain-size, flow velocity and flow height, and therefore it is useful for reconstructing paleohydraulic conditions of suspension-rich flows.
Generally, suspension capacity has been calculated from a simple equilibrium conditions of rates of sediment entrainment and suspension fallout. Various empirical functions of sediment entrainment are available, and suspension fallout rates can be estimated from theoretical distribution. However, Goto et al. (2014) recently revealed that sediment concentration of actual run-up flows of large-scale tsunamis is far below the estimated value based on the field observations of 2011 Tohoku-Oki Tsunami. Thus, it is necessary to reconsider existing models of suspension capacity.
 Here we propose a new method to estimate capacity of suspended load considering preservation of kinetic energy of turbulence. Density stratification caused by suspended sediments expends energy of turbulence in flows, but most of previous methods did not consider this effect. We employed a model to calculate preservation of turbulent kinetic energy proposed by Parker et al. (1986). As a result, it was revealed that capacity of high-velocity flows (e.g. 10 m/s) is quite low (e.g. 2 %) although previous models predict very high-concentration (e.g. ~20 %). Our estimation is quite conformable to the result of the observation of 2011 Tohoku-Oki Tsunami. Also, our model predict that friction of flows remarkably decrease due to expended turbulence energy. Decrease of Reynolds stress causes apparently low friction coefficient of flows.
Our new method is especially important for considering high-velocity flows although the effect of density stratification of suspended sediments is not significant in cases of low-velocity flows (< 3 m/s). Thus, in future studies, our method can be expected to apply for inverse analysis of paleohydraulic conditions of flows such as tsunamis, turbidity currents or rivers on Mars.