Numerical Simulation For Transport Of Biogenic Elements In Qiantang Estuary

Biogenic elements are the key factor to maintain water ecosystem health.Hydrodynamic conditions of Qiantang Estuary are very complex,which make biogenic elements transport with the flow become changeful.Therefore,the research of biogenic elements transport in Qiantang Estuary has important ecological and social significance for water ecosystem protection and river water quality improvement.Based on the biogenic elements data of tidal reach at Qiantang River,the restriction characteristics of nitrogen and phosphorus,river nutritional status,morphological correlation and spatial similarity are analyzed.Through the analysis,it is found that phosphorus is the limiting element in the study area compared to nitrogen.The carlson trophic state index of each monitoring site in the reach is greater than 60,indicating that the tidal river is at eutrophication level.At the same time,the content of biogenic elements in the reach beneath Wenyan of Qiantang River is mainly controlled by Fuyang section of the Qiantang River by cluster analysis.There is the nutrition surplus problem of nitrogen and phosphorus in Qiantang River.And phosphorus is the limiting element in river water.It can be concluded that by controlling the concentration of phosphorus in water,the harm for aquatic ecosystems caused by eutrophication of water body can be reduced.According to the situation that the former research of biogenic elements mainly concentrates in the sediments of estuaries and coastal area,this paper uses a two-dimensional Navier Stokes equation and mass transport equation to establish a two-dimensional mathematical model coupling hydrodynamic and biogenic elements(N and P).The model is used to numerically simulate the transport of biogenic elements(N and P)in Qiantang Estuary.Through the verification of the measured point(Zhijiang Station),the concentration of nitrogen and phosphorus in any point of the whole calculation area except the observation point can be obtained.Through the analysis of the biogenic elements(N and P)concentration field in the calculating region by simulation,it is found that the concentration isoline presents a tongue shape distribution in tidal reach,and water containing N and P mainly transports through the main channel.Concave bank has larger N and P concentration amplitude than convex bank at the river bend section obviously.Numerical tests are used to analyse the response of biogenic elements(N and P)transport on runoff and tidal rang.It is found that downward transport of biogenic elements is controlled by the runoff,and stronger the runoff is,obviously faster the downward transport of biogenic elements is.While the tidal process makes biogenic elements transport cyclical back and down,having blocking effect to the downward transport,the bigger the tidal range,the stronger the blocking effect.Based on the response characteristics of the biogenic elements transport on the complex hydrodynamic conditions in the tidal reach,numerical analysis of attenuation coefficient sensitivity is carried out.It is found that the farther the downstream,concentration changes of biogenic elements(N and P)response more obviously to the variations of attenuation coefficient.Specifically,as the attenuation coefficient increases,the decreasing amplitude of biogenic elements(N and P)at downstream is greater than the upstream.It shows that increasing the river attenuation coefficient through certain means can reduce the downstream concentration of N and P.It can provide reference ideas for the treatment of nutrition surplus problem in river.