Mechanical Characteristics Of Incipient Motion Of Sediment Particles On Offshore Seabed Surface

Sediment particle initiation is one of the important basic research in offshore coastal engineering.The offshore seabed is often unstable due to sediment particle initiation,which leads to disasters,such as coastal erosion,sediment erosion and deposition evolution of beach near ports,artificial fishing reef instability,local erosion of coastal buildings,etc.Therefore,it is of great significance for the safety and stability of offshore seabed to study the dynamic characteristics of sediment particles on the surface of offshore seabed.The starting velocity of sediment particles is an important judgment condition to judge whether the sediment particles start or not.However,the existing research on deducing the starting velocity by establishing mechanical model is not perfect in the force analysis of starting particles,which affects the accuracy of the calculation of starting velocity.In this paper,a dynamic model considering both the cohesive force and seepage force is established,and a new formula of additional mass force is proposed.By constructing the mechanical equilibrium equation,a new formula for the critical starting velocity of uniform and non-uniform sediment particles is derived.By applying data and formulas that already exist,we verified the accuracy of the proposed formula.Then,a PFC3D model is established to observe the characteristics of uniform sediment particles with different particle sizes and non-uniform particles with different particle sizes under the action of small waves in a wavelength range on the shallow offshore seabed.Finally,the variation law of the starting velocity of sediment particles with its main influencing factors is obtained.The main results are as follows:(1)For uniform sediment particles,a dynamic model is established to consider the three basic forces(effective gravity,lift and drag forces),additional mass force,bonding force and seepage force.By establishing the equilibrium equation under sliding instability and rolling instability modes,the new formula for critical starting velocity of uniform sediment particles under sliding and rolling instability is derived.For non-uniform sediment particles,a dynamic model is established to consider the three basic forces(effective gravity,lift and drag forces),additional mass force,bonding force,seepage force,and additional mass force.By establishing the equilibrium equation under sliding instability and rolling instability modes,the new formula for critical starting velocity of non-uniform sediment particles under sliding and rolling instability is derived.(2)On the basis of summarizing the existing calculation formulas of additional mass force on non-uniform particles,a new calculation formula of additional mass force which can simultaneously reflect the randomness of non-uniform sediment particles at the bed surface and the hidden storm between them is proposed,which can better reflect the hidden storm of non-uniform particles.(3)The formula deduced in this paper is verified.Furthermore,we become conscious of that for uniform sediment particles,the mode of starting instability is mainly sliding instability,while for non-uniform particles,its starting instability mode is mainly rolling instability.(4)The mesoscopic fluid-structure coupling models of uniform and non-uniform sediment particles are constructed by using PFC3D.The initiation models of uniform sediment particles with different particle sizes and non-uniform sediment particles with different particle sizes in shallow coastal areas are established.The simulation results show that the initiation of uniform sediment particles under the action of small waves is characterized by stratification,while the initiation of non-uniform sediment particles under the action of small waves has no obvious stratification,and the initiation particles are scattered.At the same time,the starting velocity of sediment particles obtained by numerical simulation is compared with the calculated results of the formula deduced in this paper,and the calculation of the starting velocity of sediment particles under the wave action of offshore seabed by the formula is also accurate.(5)In order to analyze the change of the threshold velocity of uniform sediment particles with the main influencing factors,this paper calculated the change rule of the critical threshold velocity of uniform sediment particles with the particle size,dry bulk density,hydraulic slope and seepage force direction,and obtained the following results:(a)When the particle diameter gets larger,the critical starting velocity of uniform sediment particles gets smaller at first and then gets bigger.And the trend exchanges when the particle diameter is 0.4mm.(b)When the dry bulk density increase,the critical starting velocity of uniform sediment particles increases linearly,which rate decreases.(c)The critical starting velocity of uniform sediment particles has obvious periodicity and symmetry with the direction of seepage force.When the direction of seepage force is 0°～180°,the critical starting velocity of particles decreases first,then increases and then decreases with the direction of seepage force,and the numerical value is about the same as 90° axis of symmetry.When the seepage force direction is 180°～360°,the increase of the seepage force direction of the critical starting velocity increases first and then decreases.The values are equal to each other along the 270° axis of symmetry.(d)The critical starting velocity decreases with the increase of hydraulic slope,and the influence of hydraulic slope on the critical starting velocity increases with the increase of particle size.(6)In order to analyze the change of the threshold velocity of non-uniform sediment particles with the main influencing factors,this paper calculated the change rule of the critical threshold velocity of uniform sediment particles with the four factors of particle size,dry bulk density,hydraulic slope and relative exposure,and obtained the following results:(a)When the particle diameter gets larger,the critical starting velocity of non-uniform sediment particles gets smaller at first and then gets bigger.And the trend exchanges when the particle diameter is between 0.2mm and 0.4mm.(b)When the dry bulk density increase,The critical starting velocity of non-uniform sediment particles increases linearly and the dry bulk density only has a significant effect on the starting velocity of particles with particle size less than 0.2mm.(c)The incipient velocity of non-uniform sediment particles decreases with the increase of hydraulic gradient and particle relative exposure,and both have a more significant effect on the incipient velocity of sediment particles with particle size greater than 0.1 mm.