Application of transport timescales to coastal environmental assessment: A case study

•Age and arrival time in the coastal area of Guangxi, China, are studied.•A numerical model based on random walk particle tracking technique is used.•1D river flow and 2D ocean flow are coupled in hydrodynamic computations.•Effects of initial tidal phase and river discharge on contaminant transport are investigated.•The numerical model is useful in coastal environmental assessment.

A numerical model based on the random walk particle tracking technique is developed to simulate the transport of the contaminants discharged into the coastal area of Guangxi, China from rivers. The model couples a 1D river flow module and a 2D coastal circulation module. Two transport timescales, i.e., the age and the arrival time, are introduced to characterize the contaminant transport process. The age is the time for a particle taken to enter the domain of interest after it left the place where the age is initially set to zero. The arrival time, on the other hand, is the age of the particle arriving at a particular place most quickly after release. It is useful for emergency responses to such an accident as toxic substance leakage. By the numerical model, the age and the arrival time under various conditions in the coastal area of Guangxi, China are studied in details. The age distribution and the arrival time of particles at a specific location are shown to closely rely on the coastal hydrodynamic environment in addition to the distance of the location from the source where particles are released. Particles released at spring tide and at ebb tide are found to spread more quickly than those released at neap tide and at flood tide, respectively. A large carrying discharge of river reduces the ages of the contaminants from the river concerned when transported to a place within the coastal area, while it has less influence on the arrival time, particularly in a place along the coast.