Numerical Study on Fresh and Salt Water Mixing and Waterexchange in Daliaohe Estuary
|School||Ocean University of China|
|Keywords||numerical simulation Daliaohe estuary fresh and salt watermixing water exchange|
As one of the main rivers flowing through Liaoning Province, Daliaohe Riverinjects up to38.5×108m3/a freshwater into Bahai Sea. The mouth of the tide river islocated in northeast Liaodong Bays, Yingkou City, which has important geographicaladvantages in industry, agriculture, fishery, port transportation and so on. In recentyears, the rapid industrial development and increasing demand of fresh water inDaliaohe estuary have put the estuary water quality and ecosystem into jeopardy, suchas eutrophication, fresh water resource scarcity and serious salt water intrusion. Basedon the salinity, the characteristic and law of water mixing and exchange areresearched to supply scientific theory guidance for protecting water resources andusing reasonably freshwater resources.In the present study, considering the interaction of runoff and tide, thethree-dimensional hydrodynamics-water quality coupling numerical model (FiniteVolume Coastal Ocean Model, FVCOM) is applied to simulate and compute salinitydistribution, salt water invasion, mixing and exchange time. First, the simulated andobserved water level, tidal current and salinity in2009and2010are validated. Then,the runoff conditions in the dry, medium and abundant seasons are considered toobtain the salinity space-time distribution characteristics, and the mixing ratio conceptis used to quantitatively determine mixed type. Last, the water exchange characteristicis discussed by method of conservative substance convection-diffusion.Viewing the salinity horizontal distribution, there are great difference in saltwater intrusion and fresh water expansion from different seasons. In dry season, the0.5PSU isohaline is located Tianzhuangtai river reach, and salt water intrusion isfarthest and most serious. In medium seasons, the salinity is lower than0.5PSU onfluctuation tide process near Rongxing pump station with the0.5PSU isohaline aboutDaobazi reach, so the runoff can be as a reference suppressing the tide for irrigation.At the same time, diluted water mainly expands along the west river channel of thetidal flat to open sea affected by terrain and the coriolis force. With bigger runofffrom dry season to abundant season, Daliaohe River Plume expands farther andfarther so that the water30km away from the mouth is diluted under the runoff434m3/s.Viewing vertical mixing characteristics, fresh and salt water in Daliaohe estuarystays basically in the highly mixed state, that is it is the tide race estuary, which isdetermined by property of strong tide estuary and greater breadth depth ratio, and hemixing is mainly caused by the vertical turbulent diffusion movement. Tidal rangeand tidal velocity are closely related with the mixed state. Fresh and salt water canmix more sufficiently during spring tide, and the mixing intensity has the highestvalue at the moment of the high and low tide. In the abundant and medium seasonwhen runoff is larger a partly mixing phenomenon can be observed in the main body of plume river zone near the mouth. The stratification interfaces have importantinfluence on environment effects such as the estuary sediment deposition, pollutantaccumulation and so on.For the water exchange in mixing zone, the runoff is the main dynamic factordetermining the ability of the exchange, that is the exchange proceeds faster withgreater runoff. During dry season, the dilute effect of Sea tide is dominant for waterupdate, especially for the river downstream. But when the runoff increases and theinteractions between runoff and tide are obvious, it is least favorable for middlereaches water to exchange, so that it easily causes reverberation and detention ofpollutants.