砂质底质潮汐水道均衡态模拟初探

A preliminary modeling study on morphodynamic equilibrium of tidal channels floored with sandy sediments

以过程模拟法探讨了发育在砂质堆积体上的潮汐水道的均衡态问题.在潮汐作用为主导、初始状况为平坦海底、底质粒度均一等条件下,利用修正的Bagnold推移质输沙率公式和沉积物连续方程,建立砂质潮汐水道均衡态的一维模型.数值实验结果表明,潮汐水道达到均衡态的时间尺度为10 a以上,水道形成的早期冲刷速度较快,随着时间的推移,冲刷减缓.从水道冲刷出来的物质在水道前方形成堆积体.潮汐水道的演变受控于水动力变化和沉积物冲淤的负反馈机制,水道均衡态主要与沉积物粒径、涨落潮流速、历时、流速沿水道轴向分布的组合特征有关.流场特征决定了水道均衡态的形态;在沉积物组分均一的情况下水道均衡深度与潮流流速呈正相关;达到均衡态的时间尺度与沉积物粒径、涨落潮历时、流速的变化以及水道规模有关.沉积物临界起动流速随深度的变化对水道均衡态有较大影响.

A morphodynamic equilibrium associated with tidal channels, a common feature on continental shelves or in shallow marine environments, plays an important role in the coastal engineering. The mophodynamic equilibrium and the length of time required to reach equilibrium for tidal channels floored with sandy sediments were modeled on the basis of a one-dimensional process model incorporating Bagnold＇s formula for a bedload transport rate （as modified by Hardisty）and the mass conservation equation, taking into account the effect of bed slope. It is assumed that tides are predominant hydrodynamic forcing and the seafloor is flat at the initial stage with a uniform grain-size. Numerical experiments have been carried out to investigate the impact of the various controlling factors on equilibrium, including flow field characteristics, current velocities, durations of flood and ebb tides, and the grain-size of sediment. The results show that on a time scale of 100 a equilibrium may be reached. There exist negative feedback mechanisms between flow and bathymetric changes. Any mophodynamic equilibrium state can be related to a combined condition of flow field characteristics which control the eventural equilibrium configuration of bed profile, sediment grain-size, current velocity and flood or ebb duration. Maximum scour depths associated with equilibrium are sensitive to the current velocity but insensitive to the grain-size or the flood/ebb duration. However, the time span required for the channel to reach the morphodynamic equilibrium is sensitive to the grain-size and the flood/ebb duration, in addition to current velocity.