The Study Of The Deposition And Erosion Changes Monitoring Of Mudflats Based On Repeated TLS Observation

Tidal flats are located in the intertidal zone which is the critical transition zones between marine and terrestrial systems.Tidal flats are potential land resources and they play an important role in the geomorphological and biological dynamics.In particular on mudflats,there are evident changes of erosion and deposition patterns.The geomorphological processes of erosion and deposition on tidal flats have,for example,direct effects on coastal engineering,wetland conservation;thus it is a key protective zone during coastal development.The average land area is small in Jiangsu Province;however,the tidal flat resource is enormous.Taking full advantage of tidal flat resources has been outlined by<Jiangsu coastal tidal flat reclamation and utilization outline>.The study of tidal flat changes of erosion and deposition can contribute to mastering its characteristics and providing fundamental basis for the implementation of such activities.Traditionally,research on tidal flats includes field based measurements of the variability of sediment fluxes and morphological changes by sediment traps,sedimentation boards,rods and ultrasonic altimeters.However,the methods mentioned above mainly focus on the change of very few points,which lack information on the spatial scale.In addition,the type of tidal flat in Jiangsu is a muddy coast;it is hard to achieve the goal by the traditional method.Therefore,it makes sense to explore new methods which are low-cost,but have a high efficiency and high precision.Nowadays,the problem of global change is becoming serious,changes of environmental geomorphology are more frequent,and the need for monitoring is higher.Three-dimensional laser scanning is a relatively new technology developed since the 1990s.Three-dimensional laser scanning is also called Terrestrial laserscanning(TLS),which is highly automatic,efficient and highly accurate at measurement.In addition,the data acquired by TLS offers spatial resolution and it is easy to repeat measurements.So far,the application of TLS has achieved good results in many aspects,such as archaeology,civil engineering,hydraulic engineering geotechnical engineering,urban-rural planning,and disasterinvestigation.However,whether TLS is suitable for the study of tidal flat changes is questionable,and related articles are comparatively rare.The study is focus on the applicability of TLS on tidal flat in Dongzaogang,which is located at Haimen,Jiangsu Province.This paper is aimed at discussing a new method on tidal flat erosion or deposition changes and offering some positive advice for planning and construction of Dongzaogang.The main conclusions in this paper are as follows:(1)It is the first time to use TLS on the study of the deposition and erosion changes of mudflat.High density point cloud of tidal flat can be acquired by TLS,the geomorphic features of the tidal flat can be easily modeled.Efficiency of data acquisition has been improved sharply and uncertainty factors decreased.The results reflect that it is more beneficial to analyze mechanisms and the patterns of tidal flat:changes.(2)The spatial erosion-deposit characteristics of the tidal flat outside the dyke were erosion and the case inside was deposition.April 2013 to January 2014,the tidal flat outside the dyke was erosion and the average erosive thickness was 0.18 m;the tidal flat inside the dyke was located at the corner of the harbor basin with weakness hydrodynamic force,it showed the deposition characteristic and theaverage depositional thickness was 0.36 m.The temporal erosion-deposit characteristic inside the dyke showed that the average deposit rate was 0.61 m peryear during November 2012 to April 2013 and 0.47 m per year during April 2013 to January 2014.(3)By considering the equipment systematic error,error evaluation of pointcloud flat and consistency of repeated scanning,we can get the results that the error is less than 1 cm within 50 meter distance(scanning angle?9.60),the error less than 3 cm within 80 meter distance(scanning angle?4.9°)on the monitoring of tidal flat changes.(4)As being confined by water on tidal flat and low scanning angle,extent of required point data is limited.The radius of precise observations is about 100 m on the 10 meters dyke(including the height of tripod).While the depth of water on the tidal flat more than 1 cm,it shows no data.Point density is lower on wet tidal flat than the dry areas,but it doesn't present the data dead zone.