Numerical Simulation Study On Flow Blocking Effect And Ecological Impact Of Large-Scale Permeable Artificial Reef

The purpose of releasing artificial reefs is to create habitat and regulate the ecology of natural sea area,so as to conserve and increase the fishery resources.After the artificial reefs are placed in the predetermined sea area,the flow field effect is mainly manifested as the upwelling from the headwater surface and the slow flow area from the back.This artificial upwelling can transport the high concentration of nutrients in the bottom water to the true light layer near the sea surface,while promoting vertical mixing and increasing the oxygen supply to the bottom water,which can effectively increase the primary productivity of the sea area and improve the marine ecological environment.Previous studies on the hydrodynamic characteristics of artificial reefs are mostly based on numerical simulations of CFD models,and the simulations are usually based on constant flow fields,which are difficult to consider various dynamic factors in the actual ocean.Large-scale ocean numerical models can simulate nutrient transport,mixing and eco-dynamic processes.If accurate simulation of the hydrodynamic characteristics of large-scale artificial reefs becomes possible,it will provide scientific basis for their site selection and deployment,better serve the construction of marine pastures,and provide reference for the evaluation of ecological benefits of the project.There are often three methods to embody artificial reefs in large scale marine numerical models.The solid reef method directly modifies the grid water depth and treats the reef as an impermeable solid,which directly changes the bottom topography;the additional bottom friction method increases the bottom friction of the model,which imposes the resistance to water flow near the bottom boundary and is difficult to apply when simulating the hydrodynamic effect of a reef with a certain height;the hysteretic force method adds a hysteretic force term to the momentum equation of the model to reflect the hysteretic effect of a permeable reef on water flow.The hysteretic force method is to add a hysteretic force term to the model momentum equation to reflect the hysteretic effect of permeable reefs on water flow.In this paper,an idealized flume model based on the 3D marine numerical model FVCOM is established to investigate the feasibility of the hysteretic force method to simulate the hysteretic effect of reefs on water flow.Secondly,a hydrodynamic model of the artificial reef area in Dachen Island,Taizhou City,Zhejiang Province was established to simulate the hydrodynamics of the reef area,and the slope of the reef area was calculated by combining multibeam bathymetry data to evaluate the rationality of the reef site selection;two methods of embodying the reef in the model were compared and analyzed to simulate the hydrodynamic changes of the reef area in Dachen Island after reef casting and the ecological effects of the reef.The ideal flume model utilizes the solid reef method and the hysteretic force method to simulate the hydrodynamic effects of non-permeable reefs and permeable reefs,respectively.The results show that the non-permeable reefs have more obvious flow field effects.Upward flow is generated at the reef’s headwater side and downward flow is generated at the reef’s backside.The solid reef method significantly increases the flow velocity at the top and both sides of the reef,and decreases the flow velocity at the headwater and backside of the reef,which will underestimate the length and overestimate the height of the slow flow zone when applied to the study of the flow field effect of permeable reefs.The hysteresis method takes into account the permeability of the reef,and the impact of permeable reefs on surrounding water bodies is less than that of the solid reef method.The hysteresis method slightly increases the flow velocity on both sides of the reef,and decreases the flow velocity on the headwater side,back and top of the reef,which makes the method more suitable for simulating the slow flow zone of permeable reefs.In the reef top plane,the reef presence changes the flow velocity in a range of about 1-2 times the reef length.The results of the ecological model show a decrease in nutrient concentration around the reef and an increase in phytoplankton concentration,with a change in the east-west direction of about 5-6 times the reef length.The presence of vertical reefs leads to the upward transport of nutrients and promotes the increase of primary productivity in the sea around the reefs.The results for different water depths indicate that the reef body will promote upward mixing.The validation results of the hydrodynamic model of Dachen Island show that the model simulates the hydrodynamics around Dachen Island with high accuracy and can accurately reflect the hydrodynamic characteristics of Dachen Island sea area.The hydrodynamic conditions of the artificial reef area are consistent with the conditions for the construction of artificial reefs,and the flow velocity at the bottom of the reef area is no more than 0.8 m/s.The slope of the reef area is calculated by using the water depth data,and the overall terrain of the reef area is relatively flat,without large reefs,which is suitable for reef casting.From the perspective of hydrodynamics and topographic slope,the area is suitable for the construction of artificial reef area.The simulation of multiple reef combinations was added to the hydrodynamic model of Dachen Island.The results show that the hysteretic force method can effectively and reasonably realize the simulation of the flow field effect of permeable artificial reefs by reducing the dynamic power density of incoming currents to simulate the obstructing effect of reefs on water flow,avoiding the problem of overestimating the flow field effect in the reef simulation by the solid reef method,and without the defects of the additional bottom friction method which has a single parameter and is only applicable to low height reefs.The reef combination headwater surface produces upwelling,the reef backwater surface decreases the flow velocity,the length of the slow flow area formed at the time of rapid rise is more than 20 times the length of the reef combination,and the siltation thickness of the bed at the end of the year is about 0.05 m.The blocking force method can set the blocking parameters in each direction and water layer according to the shape of the permeable reef,the headwater area,the number of combinations and the deployment method and water layer,etc.It is not only applicable to the bottom-set permeable reefs,but also to the floating reefs.reefs.