A Study On The Calibration Of Optical Backscatter Sensors For Measurement Of Suspended Sediment Concentration And Its Application For Suspended Sediment Transport Processes

Suspended sediment transport plays a key role in estuarine and deltaic morphological evolution.Accurately obtaining suspended sediment concentration?SSC?is an important prerequisite.Except the traditional water sampling and weight measurement method,Optical Backscatter Sensors?OBS?are not only convenient to observe turbidity?which can be converted into sediment concentration?,but also flexible to collect continuous data easily.The data obtained by OBS is turbidity,hence we need extra calibration to get sediment concentration.A few factors,e.g.,sediment particle size,salinity,suspended sediment concentration,would affect the calibration results.Therefore,examining the influences of these factors on the suspended sediment concentration calibration?comparing the methods of sediment concentration calibration?put forward practical calibration curves is important for accurate SSC data collection.In addition,it provides the basis for the study on suspended sediment transport processes.This project is based on field observation and laboratory data to analyze the key factors affecting sediment concentration calibration,and compare the results of in-situ calibration and laboratory calibration.Based on large-scale observations in the Changjiang estuary,sediment concentration calibration curves in different regions are achieved.Based on the above research,the spatial and temporal characteristics of suspended sediment transport in the North Passage are analyzed and discussed.The main findings are summarized as follows:?1?Sediment particle size is the prime factors that determines the accuracy of suspended sediment concentration calibration.The coarser the sediments are,the higher the corresponding SSC under the same NTU.For instance,the calibrated SSC will increase by 2-6 times with median particle size increased by 2-3 times.As the turbidity increases,the variation range of SSC for different grain sizes increases.For the same turbidity,the SSC calculated by the calibration curves when median sediment diameter(D₅₀)is31.6?m is 2.3 times larger than that with a median diameter 14.3?m.The SSC with a D₅₀ of 186.8?m is 6.2 times of that with a D₅₀ of 14.3?m.Regarding the effects of salinity,the SSC under fresh water condition is slightly higher than that under salty water.When the salinity increases from 0 to 15 PSU,the SSC increases by 22%.However,the calibration results are weakly sensitive to changes in salinity once salinity is present.Regarding the effects of suspended sediment concentration,we find that there is a significant difference in the response of turbidity to the sediment concentration.In the linear region,turbidity increases linearly with the increase of suspended sediment concentration.In the saturated region,the turbidity increases more slowly than the increase of SSC.In the grain-shielding region,the turbidity decreases with the increase of sediment concentration.?2?Using the in-situ calibration method can calculate suspended sediment concentration more accurately.The relative error between the calibrated and measured SSCs is within 12%.Optical Backscatter Sensors is very sensitive to the change of particle size,the sediment samples used in laboratory calibration are usually collected from surface sediments which are coarser than suspended sediments as that used in in-situ calibration.The mean relative error between calculated and measured SSC is 39.7%during spring tide and 65.9%during neap tide with laboratory calibration method.By contrast,the results of in-situ calibration showes that the mean relative error is 11.2%during spring tide and 11.4%during neap tide with in-situ calibration.The accuracy of the calculation of suspended sediment concentration is increased by 28.5%and 54.5%respectively.?3?We outline a set of calibration curves for different regions in the Changjiang Estuary.According to the analysis of large scale field observation,We find that there are two areas with relatively concentrated distribution in the suspended sediment concentration calibration curves.Curve A is representative for area I,including the North Passage and South Channel;the median particle size in this area is 8.99?m.Curve B is representative for area II,including the South Passage,North Channel,outer estuary,North Branch;the median particle size in this area is 5.22?m.Comparing the suspended sediment concentration curves A and B,the SSC calculated from the calibration curves in the larger particle size region is higher than that in the smaller particle size region under the same turbidity.Again it shows that the particle size plays a major role in the in-situ calibration.Under the same turbidity,the larger the particle size is,the higher the sediment concentration will be.?4?The calculation accuracy of suspended sediment flux is improved by 16-40%with the application of in-situ calibration.During neap tide,the net suspended sediment flux is 101.2 t/m based on measurements by water sampling.The net suspended sediment flux calculated by laboratory calibration was 144.3 t/m,the calculated result is 42.6%larger than the measured sediment flux.The net suspended sediment flux calculated by in-situ calibration is 99.0 t/m,the calculated results is 2.2%smaller than the measured sediment flux.Compared with laboratory calibration,the calculation accuracy of sediment transport flux has increased by 40.4%.During the spring tide,the measured net suspended sediment flux is 246.0 t/m,the net suspended sediment flux calculated by laboratory calibration is 294.7 t/m,the calculated result is 19.8%larger than the measured sediment flux.The net suspended sediment flux calculated by in-situ calibration is 236.7 t/m,the calculated results is 3.8%smaller than the measured sediment flux.Compared with laboratory calibration,the calculation accuracy of sediment transport flux has increased by 16.0%.?5?We quantify the magnitude and direction of net suspended sediment flux in the North Passage during spring and neap tides.During spring tide,the net suspended sediment flux in the upper reach of the North Passage is seaward.The magnitude is largest in the lower reach,then is the middle reach,and the upper reach is the lowest.The magnitude of suspended sediment flux in the lower reach is 2 times as much as that in the upper reach.During neap tide,the net sediment transport is seaward in the upper and middle reach of the North Passage,and landward in the lower reach.The magnitude is largest in the middle reach,then is the upper reach,and the lower reach is the lowest.The magnitude of suspended sediment flux in the middle reach is 9 times as much as the magnitude of suspended sediment flux in the lower reach.We also find that the advective transport dominates during spring and neap tides,while the tidal pumping may induce landward residual flux in the bottom layers in the lower reach of the North Passage during neap tide.