MEASUREMENTS OF SUSPENDED SEDIMENT FLUX AT A BAY MOUTH USING A VESSEL-MOUNTED ADCP

Transcription

1 MEASUREMENTS OF SUSPENDED SEDIMENT FLUX AT A BAY MOUTH USING A VESSEL-MOUNTED ADCP Ya Ping Wang and Shu Gao Ministry of Education Laboratory for Coast and Island Development Nanjing University Nanjing, CHINA ypwang@nju.edu.cn ABSTRACT The purpose of the present contribution is to explore the technique to use Acoustic Doppler Current Profilers (ADCPs) for suspended sediment flux measurements; which may benefit studies of sediment exchange and balance in estuaries and tidal inlets. Based upon in situ calibration from the entrance of Jiaozhou Bay, Shandong Peninsula, eastern China, relationships between the suspended sediment concentration (SSC) and ADCP echo intensity output are established. Echo intensity data obtained during an ADCP survey along two cross-sections during a spring tidal cycle were transformed into SSC data. Using the ADCP current velocity and SSC data, the flux of fine-grained sediment were derived, which shows that net sediment transport at the entrance is directed towards the open sea, with an order of magnitude of 10 3 tons per spring tidal cycle. INTRODUCTION Acoustic Doppler Current Profilers (ADCPs) represent an advanced technique, in terms of spatial resolution for measurements. They were designed originally for measuring three-dimensional current velocities, using the Doppler shift of the backscattered acoustic signals. Recent studies by Thevenot and Kraus (1993), Heywood (1996) and Holdaway et al. (1999) have shown that ADCPs have a potential to generate suspended sediment concentration (SSC) data from the backscattered signals, with accuracy comparable to a transmissometer. Combing the two measurements of current velocities and SSCs, the suspended sediment flux can be calculated directly. Such a potential of the ADCP may benefit studies of sediment exchange and balance in estuaries and tidal inlets. Because of the highly variable hydrodynamic and sedimentary conditions, the processes and mechanisms of sediment movement at the mouth of an estuary or a tidal inlet are complex (Officer, 1981; Su and Wang, 1986; Gao and Collins, 1995). As a result, the net transport of sediment in such systems is often difficult to define (Kirby, 1987; O Connor, 1987). To overcome this difficulty, a multidisciplinary approach is required, including process studies and numerical modeling; in these aspects, it is important to be able to collect the current velocity and sediment data with sufficient spatial and temporal coverage and resolution. ADCPs should be an appropriate tool to meet such needs. The present study is concerned with a survey undertaken with a DR300 Broad Band ADCP at the entrance to Jiaozhou Bay, Shandong Peninsula, to measure SSCs and fine-grained sediment fluxes in a low SSC environments (the SSC over the study area is generally bellow 30 mg L -1 ). 223

2 The ADCP was deployed on a moving vessel to measure the SSC and current velocity profiles along two cross-sections at the entrance to the embayment. Each of the data sets obtained represents a quasi-simultaneous cross-sectional measurement. These data sets, as time-series, can be used to calculate net sediment flux through the entrance during a tidal cycle. A signal emitted from an ADCP is scattered by suspended mater and the seafloor or sea surface. Directly relating the intensity of an echo to the SSC requires knowledge of several variables: the power transmitted into the water, the acoustic characteristics of the transducer and the resulting acoustic beam, the power attenuation caused by propagation losses (including absorption and beam spreading), and the properties of the receivers (Deines, 1999). In principle it is possible, but experimentally difficult, to determine these parameters through laboratory calibration (Holdaway et al., 1999). Fortunately, RDI has recently specified these factors for the 4 receiver channels of the ADCP we used, which enables us to relate directly SSCs to ADCP echo intensity outputs, and to calibrate their relationships by in situ measurements. Thus, in the present study, we intend to calculate suspended sediment fluxes within the entrance channel of Jiaozhou Bay, using current velocity and suspended sediment concentration data sets obtained from ADCP surveys. STUDY AREA Jiaozhou Bay, which covers a total area of 397 km 2, is a semi-enclosed embayment located at the southern Shandong Peninsula, China (Fig. 1A). The entrance to the Bay has a minimum width of 3.1 km and a maximum water depth of 64 m (FIO, 1984; ECCHE, 1993). Moreover, Jiaozhou Bay is characterized by rocky shorelines, with limited supply of fine-grained sediments from several small rivers; hence, the deposition rate is relatively small ( mm yr -1 ), and the SSC in the water column is low (Li, 1983; Zhang, 2000). This area has an average tidal range of 2.8 m (i.e. meso-tidal) and a spring tidal range of 4.6 m; the tides are regularly semidiurnal in character. In the coastal waters of the bay, the tidal currents are mainly rectilinear, with the tidal current velocity during the flood being stronger than during the ebb. Maximum current velocities exceeding 3.0 m s -1 have been observed during the flood phase of the tide (ECCHE, 1993). 224

3 Figure 1. Location of the study area (bathymetry in meters, on the basis of 1985 Chart). The Chart Datum is 2.43 m below the mean sea level of the study area. In situ measurements METHODS A DR300 Broad-Brand ADCP, product of RD Instruments, USA, was deployed to survey the current velocity and SSC profiles along two cross-sections (P2 and P3) within the entrance channel of the Bay. The cross-section P2 (36 o 01 N, 120 o o N, 120 o 17 ) has a length of km (the entrance width here is around 3.1 km). Over the cross-section the largest water depth is around 42 m and a deep channel with a length of about 700 m and water depth > 35 m is present near the southern side where the seabed is floored with rocky reefs. The other cross-section for the survey, P3 (36º03 N, 120º14-36º03 N, 120º17 ), has a length of around 4.2 km, which is characterized by two channels along the cross-section. The western channel is relatively deep with a maximum water depth of 52 m. The survey was undertaken on 16 July The ADCP was mounted on the right side of the front deck of R/V KEJIAO II, with the beam 3 pointing ahead. The transducer was fixed at 1.5 m below the water surface in the measurements. The ADCP data were recorded for every 10 s, with a vertical resolution of 1 m. The data included the 3-D current velocities and the echo intensity, from the four beams pointing away from the vessel. The ADCP sends and receives the signals at a frequency of khz; it has a maximum survey range of 120 m (Table 1). The cross-sections were surveyed once in every hour; and it took minutes to complete one cross-sectional survey line. During the 225

4 survey, the position of the vessel was fixed using a GPS. The speed of the vessel was maintained at 2-3 m s -1. The weather was calm, and wind waves were small (wave height < 0.3 m). Table 1. Major characteristics of DR300 Broad Band ADCP Parameters Values System Frequency khz Orientation Down Beam Angle 20º Transducer 4 Beam Janus Mode (WM) 1, Bottom Track Bin Size 1.00 m Bins 50 1st Bin 2.13 m Response Time 10 s Maximum Depth Range 120 m Water sampling and analysis SSC measurements were undertaken by filtration of water samples (Banse et al., 1963; Qin et al., 1989), to calibrate the ADCP data. Thirty water samples were collected using a NISKIN sampling bottle (Model 1080, a product of General Oceanics, Inc.) in the water column between the bottom (i.e., 5 m above the seabed) and the upper (i.e., 5 m below the water surface) layers during the ADCP surveys. For filtration analysis, 1-2 L of seawater was filtered for each sample, through a 0.45 µm filter (product of the Second Institute of Oceanography, State Oceanic Administration). Since this type of filter may be subjected to some material loss during the filtering process, an experiment was performed to derive the SSC after the filtration. The relationship between the masses of the same filter before and after filtration is W 2 = W ; ( n = 30, R = 0.983), (1) where W 2 and W 1 are the masses (mg) for the filter before and after filtration, respectively, n is sample number, and R is the linear correlation coefficient. The filters with sedimentary material were placed in an oven (temperature 60ºC) for 24 hours. The SSC by filtration, C m (mg L -1 ), can be calculated by W 3 W 2 Cm =, (2) V where W 3 is the mass of the filter with sediment (mg), W 2 is determined by Eq. (1) and V is the volume of the water sample (L). In order to identify the grain size characteristics of the suspended matter, a Cilas 940L laser granulometer (measuring range µm) was used in laboratory to obtain the particle size distribution of suspended sediment of some water samples. 226

5 Acoustic methodology The ADCP transmits sound beams at a fixed frequency (307.2 khz for this study) and records the returning sound scatters from the water column with the four transducers. These scatters or echoes are due to the presence of fine-grained suspended sediment particles, detritus or planktons that reflect the sound back to the ADCP. The intensity of the echo depends upon a number of factors, including sound absorption, beam spreading, the transmitted power and the backscatter coefficient. The relationship between the echo intensity and the various factors can be written as (Thevenot and Kraus, 1993; Gordon, 1996) 1 EI' = ( SL + SV) 20log10( D) 2αD, (3) β where E I is the echo intensity recorded by ADCP sensors (db), β is a parameter associated with the particle size in the water, S L is the source level or transmitted power (db), S V is the water-mass volume backscattering strength (db), D is the distance from the transducer to the depth cell (m) and α is the absorption coefficient with db m -1. Here, D = (H-H 0 ), where H is the depth for the cell and the H 0 is the water depth of transducer below the water surface. The transmitted power is 80 W. In order to remove the influence of power attenuation caused by propagation losses (i.e., absorption and beam spreading), the corrected echo intensity (db), E I, is expressed as EI = EI' + 20log10 ( D) + 2αD. (4) Further, a relationship between the water-mass volume backscattering strength and the SSC has been given by (Urick, 1983; Dammann et al., 1991; Thevenot and Kraus, 1993) SV = 10log ( niσ i) + 10log ( C) 10log ( C0), (5) i where n i is the number of particles of grain size class i, σ i is the backscatter cross-section of a single particle of the i-th class, C is the SSC (mg L -1 ) and C 0 is a constant associated with the local conditions. The mean grain sizes of suspended sediment were Φ during the ADCP measurement on 16 July 1999 (Fig. 2). Thus, the particle size distribution was not highly variable during each of the ADCP surveys for the study area; hence, the parameters n i and σ i are taken as constants. Combining (3), (4) and (5), we have and log 10 ( C) + = K 0.1βEI, (6) C K = log 0. 1SL. (7) 0 10 ( ) niσi i The parameters K and β are determined by using the SSC data collected in situ; an algorithm is then established to transfer the acoustic signals (i.e. the corrected echo intensity data) into SSC data. Inevitably, as with other measurement techniques, an ADCP has limitations associated with the instrument and the manner in which it works. 227

6 (1) Within the distance of 2.13 m to the ADCP transducers, a ringing effect may occur due to the influence of vessel body and other electronic instruments. The ADCP then receives only ringing signals instead of Doppler shift. Hence, data cannot be obtained during the surface blank layer, i.e., 3.63 m for this survey below the water surface. However, the surface layer of 3.63 m may represent the free flow since this layer is far away from the bottom (the water depth of survey area is mainly more than 20 m) and outside the benthic boundary layer. Hence, the current velocities and SSCs in the surface blank were assumed to be the same as those data of the uppermost cell. (2) Near the seabed, where the signal is corrupted by sidelobe echoes, no data can be obtained. This bottom blank, transparently within the boundary layer, occupies approximately 6% of the water depth for a 20º beam angle. The current velocities and SSCs in this layer were derived using, respectively, the von Karman-Prandtl equation (Dyer, 1986; Kim et al., 2000) u* z u = z ln( ) κ z, (8) 0 and the sediment suspension equation (Nichols and Biggs, 1985) Cz h z a w / * ( ) s κu =, (9) Ca z h a where the constant κ is known as von Karman s constant (= 0.4 for sea water). The shear velocity (u * ) and seabed roughness (z 0 ) were calculated according to the logarithmic velocity profile model (Collins et al., 1998), using the ADCP data of lower layers. The C a represents the known SSC at the height of a (top of the blank layer in this study), and h denotes the water depth. Thus, u z and C z were obtained within the bottom blank layer. Figure 2. Grain size variations of suspended sediment along the water depth and time series 228

7 Determination of acoustic parameters K and β RESULTS Since the acoustic parameters K and β are mainly associated with the particle size of suspended sediment and local physical environment (e.g., the water temperature and salinity), they should be determined by in situ measurements. The SSCs corresponding to the acoustic signals in different water depth (from 5 m above the seabed to 5 m below the water surface) are obtained by water samplings on 16 July, The water samples collected have a wide range of SSC (i.e. from 0.67 mg L -1 to mg L -1 ), which enables us to establish a statistical relationship between C and E I. The echo intensity data were averaged for 30 s periods, representing the mean of the values from the 4 transducers, to reduce the possible turbulence associated with the 10 s data. Based upon regression analysis, the following relationships between the SSC (C) and echo intensity (E I ) are derived (Fig. 3) log ( C) = E ( n = 30, R = 0.861). (10) 10 I The correlation between the echo intensity and the SSC is significant at the confidence level of 99%. Therefore, Equation (10) was used to transform the corresponding echo intensity data into SSC data, and to obtain SSC profiles. Figure 3. Relationships between SSC (C) and echo intensity (E I ) Comparison of the SSCs by filtration with those estimated by Equation (10) indicates a mean relative error of 32%. It shows that the relative error is less than 20% for half of the regression data, and 25% for three-fourths. Such errors are comparable to those for the optical turbidity-meter, transmissometer (Thevenot and Kraus, 1993), and acoustic suspended sediment monitor (Zhang and Li, 1998; Shi et al., 1999). Further, the data pairs are close to the line C m = C, indicating that the estimated SSCs are in high agreement with the measurements (Fig. 4). Thus, the ADCP is quite robust in terms of SSC determination in coastal waters. 229

8 SSC and velocity profiles Based upon Eq. (10), SSC data were obtained for the cross-sections. The seabed position was determined by the ADCP-detected water depth plus the bottom blank thickness. In addition, the eastern or northern component of the current velocity was set to be positive, and the western or southern components to be negative in sign. Figure 4. Comparison of ADCP-measured SSCs (C) and flitration determined SSCs The water level is shown in Figure 5 for the entrance area. It shows that the lowest water level of ebb slack occurred at pm (Beijing Time), but the highest water level of flood slack at pm. The derived SSC patterns over the P2 cross-section, from am to pm (Beijing Time) on 16 July, 1999, are shown in Figure 6 (the other part of the survey line near underwater rocky reefs was too shallow for the navigation). For the exact position of the survey line, please see Table 2. Figure 5. Water level variations in Jiaozhou Bay on 16th July, 1999 (from SOIC, 1999). The 0 m water level is the Chart Datum. The time on the figure is according to Beijing Time. 230

9 Table 2. Sediment discharges through the P2 cross-section on 16th July 1999 No. Start End Survey time (Local time) Discharge ( 10 2 kg s -1 ) 1 36 o N 36 o N o E 120 o E 2 36 o N 36 o N o E 120 o E 3 36 o N 36 o N o E 120 o E 4 36 o N 36 o N o E 120 o E 5 36 o N 36 o N o E 120 o E 6 36 o N 36 o N o E 120 o E 7 36 o N 36 o N o E 120 o E 8 36 o N 36 o N o E 120 o E 9 36 o N 36 o N o E 120 o E o N 36 o N o E 120 o E o N 36 o N o E 120 o E o N 36 o N o E 120 o E o N 36 o N o E 120 o E o N 120 o E 36 o N 120 o E The SSC is less than 8 mg L -1 in the upper water column (above 10 m water depth), and reaches > 10 mg L -1 below the water depth of 20 m in the ebb (Fig. 6). Further, the stratification in terms of concentrations is present. However, the SSC is quite homogeneous, being around 8 mg L -1 near the ebb slack ( pm), but higher near the bottom of southern edge than the northern. During the flood, the SSCs of 6 mg L -1 isoline extend towards the seabed. The water column >8 mg L -1 is smaller in the flood (Fig. 6, ) than that in the ebb, which results from that clear water (<10 mg L -1 ; Zhang, 2000) of the adjoining areas enters the Bay during the flood but turbid water (>10 mg L -1 ) flows towards the open sea during the ebb. The SSC stratification also exists during the flood. The high SSCs (10 mg L -1 ) are observed to reach the water surface in the next ebb phase (Fig. 6). 231

10 Figure 6. SSC (mg L -1 ) distribution patterns over the P2 cross-section on 16th July 1999 The velocity was more than 1.0 m s -1 in the upper part of the water column on the central section, but below 0.4 m s -1 at the northern part during the ebb maximum (Fig. 7). The current velocity is homogeneous with about 0.8 m s -1, and high velocities > 1.0 m s -1 were present only over a small part in the north during the flood maximum (Fig. 7). It suggests that the current velocities are small (<0.4 m s -1 ) during the slack phases, and less than 0.2 m s -1 on the northern section during the ebb slack. Moreover, Similar current velocity patterns are observed within the water column, due to the tidal barotropic forcing and the shape of the narrow entrance. The observed interrelationship between current velocities and SSCs may by due to the fact that there is a lack of fine-grained sediment cover on the seabed at the entrance or even hard rocks are exposed. Thus, the high concentration results mainly from advection, rather than local resuspension. Figure 7. Velocity (m s -1 ) patterns over the P2 cross-section on 16th July, 1999: (A) ebb maximum, (B) ebb slack, (C) flood maximum, and (D) flood slack. 232

11 Suspended sediment fluxes The hourly survey of current velocities and SSCs allow us to estimate the sediment flux through the entrance section. Since the two edge parts of the cross-section are too shallow for vessel navigation, data in this area also cannot be obtained and are taken as the corresponding measured values in the closest cross-section edge. The bathymetry of the two edge parts is determined by the 1985 Chart. Thus, the fluxes through the cross-section edges can be calculated in an approximate way. Suspended sediment discharges (kg s -1 ) are calculated for the P2 and P3 cross-sections on 16 July 1999 and shown in Tables 2 and 3, respectively. The maximum suspended sediment discharges through P2 and P3 cross-sections are during the flood and kg s -1 during the ebb on 16 July 1999, respectively. The difference between the P2 and P3 cross-sections results from the fact that maximum current velocities occur in different tidal phase. For the P2 cross-section, maximum velocities were observed during the flood, but on the P3 cross-section during the ebb. Table 3. Sediment discharges through the P3 cross-section on 16th July 1999 No. Start End Survey time (Local time) Discharge ( 10 2 kg s -1 ) 1 36 o N 36 o N o E 120 o E 2 36 o N 36 o N o E 120 o E 3 36 o N 36 o N o E 120 o E 4 36 o N 36 o N o E 120 o E 5 36 o N 36 o N o E 120 o E 6 36 o N 36 o N o E 120 o E 7 36 o N 36 o N o E 120 o E 8 36 o N 36 o N o E 120 o E 9 36 o N 36 o N o E 120 o E o N 36 o N o E 120 o E o N 36 o N o E 120 o E o N 36 o N o E 120 o E o N 120 o E 36 o N 120 o E

12 Table 4. Suspended sediment fluxes though the entrance of Jiaozhou Bay on 16th July 1999 Cross Suspended sediment fluxes ( 10 3 t) sections Ebb Flood Net P (seaward) P (seaward) These results show that the suspended sediment transport is directed from the inner bay towards the adjoining open sea areas, with net fluxes of t and t through the P2 and P3 cross-sections, respectively, on 16 July 1999 (Table 4). The suspended load may be moved by advection and diffusion processes in coastal environments (Nichols and Biggs, 1985). The process of advection in study area is similar during between the ebb and flood. However, the SSC of embayment water is much higher than that of adjoining seawater and then the diffusion transport should be seaward. Further, the advection process also output more suspended load from the bay due to water exchanges between the embayment and the adjoining waters. CONCLUSIONS Relationships between SSC and ADCP echo intensity output are derived, which contains two calibration coefficients. The acoustic parameters K and β are mainly associated with the particle size of suspended sediment and local physical environment for the ADCP, and hence should be determined by in situ measurements. For Jiaozhou Bay, eastern China, a DR300 Broad-Brand ADCP was deployed on a moving vessel for surveying the current velocity and acoustic signal profiles along cross-sections within the entrance to the embayment, and a significant correlation between the acoustic signal and the SSC was identified. The calibrated equations were used to convert the corresponding echo intensity data into SSC data, with a relative error of around 30%, being comparable to other SSC-measurement instruments. The SSC patterns along the cross-sections within the entrance, on the basis of ACDP measurements, show that (1) the concentration in ebb is higher than that in flood; and (2) stratification in terms of concentrations is present. The current velocity distribution patterns are similar within the water column. During a tidal cycle on spring, tons of suspended sediment are transported to the open sea through the entrance. ACKNOWLEDGMENTS This project was supported financially by the Natural Science Foundation of China (No ) and China 908 projects ( ). Chenzhong Guan, Kunye Li, Jianjun Jia, Ping Li, Peng Cheng and Yunchuan Xue (Institute of Oceanology, Chinese Academy of Sciences) helped with the deployment of the ADCP and sampling. Professor Li Yan (Hangzhou) is thanked for giving comments on the original manuscript. The constructive and helpful comments of Drs. Norbert P. Psuty, Zhongyuan Chen and an anonymous reviewer are highly appreciated. Finally, we wish to thank the crew of R/V KEJIAO II for their logistic support during the cruise. 234

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