A Combined Raindrop Aggregate Destruction Test-Settling Tube (RADT-ST) Approach to Identify the Settling Velocity  of Sediment

Liangang Xiao,Nikolaus Kuhn,Yaxian Hu,Philip Greenwood

doi:10.3390/hydrology2040176

Abstract

The use of sediment settling velocity based on mineral grain size distribution in erosion models ignores the effects of aggregation on settling velocity. The alternative approach, wet-sieved aggregate size distribution, on the other hand, cannot represent all destructive processes that eroded soils may experience under impacting raindrops. Therefore, without considering raindrop impact, both methods may lead to biased predictions of the redistribution of sediment and associated substances across landscapes. Rainfall simulation is an effective way to simulate natural raindrop impact under controlled laboratory conditions. However, very few methods have been developed to integrate rainfall simulation with the settling velocity of eroded sediment. This study aims to develop a new proxy, based on rainfall simulation, in order to identify the actual settling velocity distribution of aggregated sediment. A combined Raindrop Aggregate Destruction Test-Settling Tube (RADT-ST) approach was developed to (1) simulate aggregate destruction under a series of simulated rainfalls; and (2) measure the actual settling velocity distribution of destroyed aggregates. Mean Weight Settling Velocity (MWSV) of aggregates was used to investigate settling behaviors of different soils as rainfall kinetic energy increased. The results show the settling velocity of silt-rich raindrop impacted aggregates is likely to be underestimated by at least six times if based on mineral grain size distribution. The RADT-ST designed in this study effectively captures the effects of aggregation on settling behavior. The settling velocity distribution should be regarded as an evolving, rather than steady state parameter during erosion events. The combined RADT-ST approach is able to generate the quasi-natural sediment under controlled simulated rainfall conditions and is adequately sensitive to measure actual settling velocities of differently aggregated soils. This combined approach provides an effective tool to improve the parameterization of settling velocity input for erosion models.

Highlights

In order to rectify the bias of settling velocity distribution introduced by mineral grain size distribution, a combined Raindrop Aggregate Destruction Test-Settling Tube (RADT-settling tube (ST)) approach was developed to simulate the generation of sediment aggregates by raindrop impact under controlled simulated rainfall conditions, and to further determine their actual settling velocity distributions
Through comparing the Mean Weight Settling Velocity (MWSV) of raindrop-destroyed aggregates with dispersed mineral grains, our results show that the settling velocity of the four soils and their SOC deposited across landscapes would have been underestimated by at least six times, if not considering the actual aggregate settling velocity distribution
The feasibility of this approach was illustrated by the comparable settling velocity distributions between aggregates collected from the RADT-ST in this study and that of the same soil collected from a previous flume experiment after receiving similar rainfall cumulative kinetic energy

Summary

Introduction

Given that a large amount of C, N, and P is eroded and redistributed across landscapes, the potential effects of soil erosion on biogeochemical cycling and on global climate change, has drawn increasing attention [15,16,17,18,19]. The redistribution of eroded sediment as well as associated substances across landscapes is far from fully understood, and has largely impeded making an accurate estimation of the environmental impacts of soil erosion [18,19,20,21,22]. Improving our understanding of soil erosion and its impact on environment sustainability requires a better understanding of the behavior of eroded soil as sediment during transport and deposition across agricultural landscapes [18,20,23]

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Conclusion