Research On The Formation Of Phreatic Surface In An Artificial Coral Reef Island

On coral islands far from the mainland,fresh water from rainfall infiltration can form lenticular,thin and floating freshwater bodies on the surface of the infiltrated seawater.This part of freshwater resources is vividly called freshwater lens.The formation of freshwater lenses in coral islands is mainly influenced by the geological conditions and natural rainfall-evaporation conditions.This paper takes an artificial coral reef island as the research object.Based on the existing theories,the possible extension range and morphology of freshwater lenses in coral islands are estimated from two aspects of geological conditions and rainfall recharge,meanwhile,the formation of water surface is explored,which lays a foundation for further study of freshwater lens.Starting from geological conditions,a large number of stratified screening experiments have been carried out on the core of geological boreholes with multiple surveying lines of the island,and the grain size variation law of the island's strata has been obtained.The loose layer above the reef limestone is mainly divided into nearsurface layer and original reef flat layer.Further,indoor permeation experiments of calcareous sands with different original gradations after stratified sampling have been carried out,and roughly the island's geology has been obtained.The distribution characteristics of permeability are analyzed comprehensively.The coarse-fine interface of upper loose layer and the top interface of reef limestone are the key boundaries to judge the extension range of freshwater body.The approximate boundary and morphological characteristics of freshwater lens are estimated.The main conclusions are as follows: on the whole,the top boundary of the reef limestone is shallow in the south-west margin,deeper in the north-east margin,and then in the middle,that is,gradually deepening from south to North in the transverse direction.The permeability coefficient in the axial section of the island is generally small,the difference of permeability level above sea level is small,the permeability in the northern part of the loose layer of the former reef flat is relatively strong,the permeability in the central and southwestern parts is weak,and the permeability in the southwestern part is the weakest.Generally speaking,the freshwater lens is the thickest in the middle,thicker in the southwest and thinner in the Northeast in the axial direction.The transition zone of freshwater lens is wider in the northeast and narrower in the southwest.Based on the theory of unsaturated seepage,the theoretical model of rainfall infiltration and evaporation is deduced.The main parameters of unsaturated seepage model,soil-water characteristic curve and unsaturated hydraulic conductivity,are obtained through indoor experiments.The scatter data of matric suction and volumetric water content of three characteristic graded calcareous sands are obtained by pressure plate test,and the soil-water characteristic curves are fitted by different classical theoretical models.Then,according to Childs & Collis-Geroge theoretical formula,the unsaturated water conductivity is predicted by using the results of soil-water characteristic curves,and based on the principle of transient profile method.A set of laboratory experimental device was designed to measure unsaturated hydraulic conductivity in laboratory,and the results of calculation were compared.The conclusions are as follows: Fredlund-Xing,Zhong Fangjie and Van Genuchten models are used to fit the soil-water characteristic curves.The Fredlund-Xing and Zhong Fangjie formulas can be used to fit the three gradations of sand.The Van Genuchten model is not suitable for fitting the experimental data of gravel sand.The variation trend of unsaturated water conductivity measured by the experiment is the same as that predicted by the theory,but it is better than that predicted by the theory.The value is much larger,which is consistent with previous research results.Finally,based on the measured rainfall evaporation data and the experimental seepage model parameters,the software SEEP is used to simulate the unsaturated infiltration-evaporation.The annual actual infiltration recharge per unit area of the coral island under the current rainfall evaporation conditions is about 1072 mm.According to the principle of water balance,that is,the rainfall infiltration recharge balances to the island's own drainage,the estimated recharge is obtained.The maximum groundwater level above sea level of coral islands studied is about 1.84 m.Finally,the range of freshwater lenses is calculated by Ghyben-Herzberg theory.It is found that the value has exceeded the lower bound of freshwater lenses revealed by stratigraphic information.The reason is that the theoretical value idealizes the interface between freshwater and seawater as a sudden change interface,which is obviously inconsistent with the actual situation and indicates further researches should be focused on the transition zone of freshwater and seawater.