| Muddy tidal flats are submerged frequently under the influences of tides and are important transition zone between land and sea.Tidal flat is an unstable geomorphic unit with obvious erosion and accretion.Therefore,field observations on tidal flats are very difficult,expensive and usually within heavy workload.The Waterline Detection Method(WDM)with remote sensing image is helpful in detecting the changes of waterline.As the hight value of each waterline can be reconstructed with hydrodynamical model,the morphology of tidal flats can be calculated well using the Digital Elevation Model(DEM).After the Yellow River channel was switched from Qingshuigou course to Qingbacha course artificially in 1996,the erosion-accretion pattern in the Yellow River Delta changed greatly.Massive sediment deposited off the active river mouth,resulted in the seaward extension of river channel and promoted the formation of new Qingbacha delta lobe.Meanwhile,serious erosion occurred in the just abandoned Qingshuigou delta lobe without sufficient sediment supply.In order to compare tidal flats of the abandoned and active river mouth quantitatively,we computed the elevation of tidal flats in 1996 and 2014 with this inverse techniques and analyzed the morphological changes.1.We applied the WDM in the calculating the elevation of the muddy tidal flats along the Yellow River mouth whose suspended sediment concentration was very high.The works includes:(1)Using different Landsat and GF-1 images in different years to extract waterline;(2)The elevation of these waterlines were calculated by the Delft3D hydrodynamic model;(3)Converting the water boundary with elevation attributes to the water boundary point and inverting it with Kruger interpolation to get the tidal DEM of the corresponding year.2.The results show that comparing the DEM inversion results based on Landsat with the measured DEM results,the error ranged from-26 to 37 cm.It accounts for88.2%of the total tidal flat area,and the average error was-4.2 cm.Comparing the DEM inversion results based on GF-1 with the measured DEM results,the error ranged from-38 to 38 cm,and the average error was-8.1 cm.Both inversion results are well.3.The DEM obtained from the inversion of muddy tidal flats in the Yellow River Estuary in 1996 and 2014.The results show that:(1)In 1996,the elevation of tidal flats ranged from-31 to 81 cm,and the elevation of tidal flats decreased gradually from land to sea.The elevation of tidal flats in the northern part of the estuary ranged from25.9 to 55.4 cm,and the tidal flats in the southern part of the estuary ranged from-30to 80 cm;(2)The elevation of tidal flats in 2014 ranged from-32 to 69 cm,which also decreased gradually from land to sea.The elevation of tidal flats in Qing 8 river delta lobe ranged from-30 to 56 cm,and the elevation of old estuary in Qingshuigou ranged from-27 to 69 cm.4.According to the contour line of 30 cm extracted from DEM of tidal flat,it is analyzed that the whole estuary area is erosion state from 1996 to 2014,and the average annual erosion area is about 1.40 km~2.The area which is the most serious erosion is the north part of the mouth of the old estuary of Qingshuigou.The average annual erosion rate of 30 cm contour is 0.17 km/a,and the average annual erosion area is about 2.13km~2.The main reason is the rapid erosion of the area after the old mouth of Qingshuigou is abandoned.The front of the mouth which protruding to the sea is the strongest erosion.5.According to the shape and change of the tidal flat profile drawn,the southeast part of the Gudong levee and the mouth area of Qingshuigou estuary(profile Y1-Y3)is erosion state,which still in erosive state.The tianshuigou tidal flat(profile Y4)is silting type,which the underwater bank slope is accumulating towards the sea,with a silting trend. |
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