Numerical Study of Transport Timescale in Daliaohe Estuary
|School||Ocean University of China|
|Keywords||Residence time Exposure time Age CART Mass transport Daliaohe estuary|
The mass transport in estuary is more related to the biogeochemistry process andenvironment process. Transport timescales, as a potent diagnostic tool for studying themechanism of mass transport, have been widely used in estuaries, lakes and gulfs in theworld. They are also applied in some estuaries in our country, such as Yangtze Estuary andYellow River Estuary. In this article, the related theories of transport timescales areapplied in Daliaohe Estuary to study the characteristics of mass transport as responses todifferent dynamic conditions.At the moment, there are a variety of concepts of transport timescales, and thecalculation method for them is also not sole. Therefore, it is important to select the conceptof timescales and the related solving method. Residence time is the time taken by thewater parcel to leave the control domain for the first time, exposure time is the total timespent by the water parcel in the control domain, and age is defined as the time elapsed sincethe water parcel leaves the source. Generally, these three timescales together coulddescribe the whole transport process in estuary. In view of numerical model,computational efficiency and the simulation of diffusion process of tracer, we employedthe Euler approach to calculate the three transport timescales mentioned above.First, the embedded grids methodology was adopted to better give the open boundaryconditions of Daliaohe Estuary. The Liaodong Bay Model and Daliaohe Estuary Modelwere established based on a three-dimensional Finite Volume Coastal Ocean Model(FVCOM). The model was calibrated and verified with the measured water level, currentand salinity. The results show that the model could well reproduce the hydrologicalcharacteristics of Daliaohe Estuary and lay the foundation for the calculation of transporttimescales.Second, residence time and exposure time were calculated for five boxes in DaliaoheEstuary in three typical runoff conditions using the remanent function method. It is ableto obtain both the time required to leave the control domain for the first time and thetotal time spent in the control domain. Then the return coefficient and the subdomain exposure time matrix, which show the interaction between boxes, were alsoinvestigated. The results show that the interaction between the runoff and tidescontrols the residence time in Daliaohe Estuary. Exposure time and residence timeshow the same the change trend, but they differ greatly in size. And exposure timesare8,3and1days more than residence times during dry, normal and rainy seasons,respectively. The return coefficient could reach0.94at the downstream end duringdry season, with the tracer returning to this area many times. The downstream boxeshave little influence on the upstream boxes in Daliaohe estuary expect for the case inthe downstream end at the maximum flood tide.Finally, based on the concept of age in CART, the age in Daliaohe Estuary wascalculated to study the spatial and temporal distribution characteristics of masstransport, in response to runoff and tide. The results show that the relationshipbetween the average age of the specific position and the runoff could be expressed bya power function approximately. In the river channel, the runoff controls the masstransport: it would take about52,27and15days for the tracer to be transported fromSanchahe to the mouth during the dry, normal and rainy season, respectively. Outsidethe mouth, the tide is dominant even though the difference between spring tide andneap tide is less than5days, and the tracer transports mainly along the northwestdirection through the West Waterway and the southeast direction through the EastWaterway. The obvious age stratification emerges in the vicinity of the mouth inwhich there exists a strong interaction between the tide and runoff, and the agedifferences between the surface and bottom could reach7days.