Quantification Of Flow Resistance In Mussel Raft Aquaculture Facilities And Its Ecological Effects

Mussel aquaculture in Shengsi County of Zhejiang Province is well known at home and abroad.In recent years,improper aquaculture strategies have led to many problems,which have affected the development of aquaculture.In this paper,the observation data and numerical model simulation methods were used to study the environmental problems of mussel culture area in Gouqi Island.The logarithmic method and the drag method were used to quantify the drag effect of mussel strings on the water flow,and the drag coefficient was used as the input parameter to establish the three-dimensional hydrodynamic model.Based on the hydrodynamic model verified by the measured data,coupled with the tracer model,the ecological effect caused by the change of hydrodynamic conditions in the aquaculture sea area was simulated,and the conclusions were as follows:ADCP observation results show that the flow field in the boundary layer of the depth of the three(Z_s surface,Z_minminimum depth,average flow velocity Z_max:the maximum average flow velocity was depth),Z_s depth of flow rate is about 1.5 times that of Z_minvelocity Z_max depth velocity about Z_mindepth velocity of 4.5 to 5 times,culture zones the vertical change of velocity of flow is significant in outside culture zones,and the velocity of the vertical change rate is higher than during neap tide period;According to the vertical profile of the velocity,the vertical velocity of the aquaculture area showed a gradient decline distribution structure,which conforms to the logarithmic law.In addition,the aquaculture activities changed while the hydrodynamic conditions were changed,the environmental parameters of the aquaculture area also changed,which was strongly proved by the measured results of chlorophyll,dissolved oxygen and turbidity of CTD.According to the logarithmic law of flow velocity in vertical direction,the friction velocity(_*),roughness length((5₀)and drag coefficient were calculated by logarithmic method and regression analysis,respectively,0.087±0.027,1.718±0.280,0.146±0.038 during high tide,and the mean values of three parameters were0.104±0.025,1.766±0.199,0.151±0.030 during low tide.During high tide,the mean values of the three parameters around the breeding area were 0.129±0.041,1.633±0.377,0.136±0.042,respectively.During low tide,the mean values of the three parameters around the breeding area were 0.132±0.037,1.561±0.318,0.124±0.037,respectively.The results showed that the_*in the inner and outer areas of the breeding area during high tide were always smaller than that during low tide.The second method to quantify the resistance effect of mussel strings on the flow is the resistance method,which mainly considers the momentum generated by the resistance effect of mussel strings on the water flow from the perspective of momentum accumulation,and adds this part of momentum into the momentum equation of the three-dimensional numerical model,so as to realize the use of numerical model to simulate the hydrodynamic effect of the mussel strings.The numerical models were established and verified by the two methods respectively.The verification results showed that the resistance method was more accurate in simulating the surface velocity of the mussel culture area in Gouqi Island.Therefore,the tracer module was coupled on the basis of the resistance method to achieve the simulation of the concentration,diffusion and transport of phytoplankton in the culture area.The difference of current velocity and flow direction in the aquaculture sea area at typical time with and without resistance was calculated by means of comparative numerical experiment.The current ellipse was obtained by using the calculation results of model velocity and harmonic analysis.The unit time flow and total flow were obtained by selecting 7 sections inside and outside the aquaculture area.The Euler residual current was calculated by using 5 sub-tides as the driving conditions of open boundary.According to the hydrodynamic model with and without resistance result under the condition of breeding changed aquaculture waters flow velocity and flow resistance,surface to 10 m,there is no resistance has a resistance when the velocity is0.1 times that of be series choke role is not limited in aquaculture facilities in depth and scope,the scope of its influence on the vertical across the water,in a horizontal direction increases the flow velocity and flow of natural waters,especially in the most pronounced when sharp.The anomalization of the flow field makes the amplitude of tidal component M2,S2,K1 and O1 in the aquaculture area decrease when there is resistance,and the amplitude of tidal component K1 and O1 in some peripheral aquaculture areas increase.In addition,the calculation results of water flux showed that the decrease of the flow velocity reduced the flux of the section in the culture area,and the difference of the flow of the section parallel to the flow direction was small,while the difference of the flow of the section perpendicular to the long axis of the tidal current ellipse was large,especially in the section in the northwest to southeast direction.Based on the hydrodynamic model,Lagrange particle tracking and tracer simulation were used to investigate the characteristics of precipitation of suspended particulate matter（debris,plankton）and concentration attenuation of phytoplankton in aquaculture waters.The sedimentation rate of suspended particles was promoted by the resistance of shellfish to water flow,and the maximum sedimentation rate was about 10.31%during neap tide.Tracer simulation results show that the distribution of sea surface resistance of dissolved substances and concentration influence is bigger,the concentration of dissolved substances in the breeding zone is about 0.7-0.8 times that of natural waters,which presents the field in the vertical layer is lower than the outside,and concentration difference is greater than during neap tide,investigate its root cause lies in the weak hydrodynamic characteristics of farming activities to bring the breeding sea nutrients transport and diffusion are affected.The simulation of the hydrodynamic conditions and ecological effects in the mussel culture area on Gouqi Island by using the numerical model can provide guidance for the formulation of the mussel culture strategy on Gouqi Island.The overstocked culture scheme is not conducive to the development of the aquaculture industry,but produces a series of ecological problems due to the weak hydrodynamic conditions.