VLAAMSE OVERHEID DEPARTEMENT MOBILITEIT EN OPENBARE WERKEN WATERBOUWKUNDIG LABORATORIUM Langdurige monitoring van zout/zoet-verdeling in de haven van Zeebrugge en monitoring van zoutconcentratie, slibconcentratie en hooggeconcentreerde slibsuspensies in de Belgische kustzone
Colofon Photo coversheet: Left: Navitracker; Right: Rheotune International Marine & Dredging Consultants Address: Coveliersstraat 15, 2600 Antwerp, Belgium : + 32 3 270 92 95 : + 32 3 235 67 11 Email: info@imdc.be Website: www.imdc.be I/RA/11292/09.017/ABR page I
Table of Contents 1. INTRODUCTION... 1 1.1. THE ASSIGNMENT... 1 1.2. AIM OF THE STUDY... 1 1.3. OVERVIEW OF THE STUDY... 1 1.4. STRUCTURE OF THE REPORT... 1 2. DESCRIPTION OF THE STUDY... 2 2.1. DESCRIPTION OF THE INSTRUMENTS... 2 2.2. DESCRIPTION OF THE MEASUREMENTS... 2 3. COMPARISON OF THE RECORDED DENSITY PROFILES... 4 4. COMPARISON OF THE MASS BALANCE OBTAINED FROM THE DIFFERENT MEASUREMENTS... 10 4.1. ZEEBRUGGE... 10 4.2. DEURGANCKDOK... 17 5. CONCLUSION... 20 6. REFERENCES... 21 Annexes ANNEX A MAPS OF THE MEASUREMENT SITES... 22 ANNEX B PROFILE PLOTS ZEEBRUGGE... 25 ANNEX C PROFILE PLOTS DEURGANCKDOK... 61 List of Tables TABLE 4-1 STATISTICAL PROPERTIES OF THE PROFILING DEPTH AND TOTAL DRY MASS (PER UNIT AREA) OBTAINED FROM THE NAVITRACKER AND RHEOTUNE MEASUREMENTS IN THE HARBOUR OF ZEEBRUGGE, CONSIDERING ONLY THAT PART OF THE PROFILE WERE DATA FROM BOTH INSTRUMENTS WERE AVAILABLE.... 11 TABLE 4-2 STATISTICAL PROPERTIES OF THE PROFILING DEPTH AND TOTAL DRY MASS (PER UNIT AREA) OBTAINED FROM THE NAVITRACKER AND RHEOTUNE MEASUREMENTS IN THE HARBOUR OF ZEEBRUGGE, CONSIDERING THE PROFILES UP TO THE LOWEST MEASUREMENT POINT OF THE LOWER PROFILE.... 15 TABLE 4-3 MEASURED DRY MASS (TON/M²) PER ZONE IN THE DEURGANCKDOK FOR THE MEASUREMENTS USING NAVITRACKER (26-AUG-08) AND RHEOTUNE (11-SEP-08).... 17 List of figures FIGURE 3-1 DEFINITION SKETCH OF THE HEIGHT OF THE WATER-SILT INTERFACE NAVITRACKER MINUS THE HEIGHT OF THE WATER-SILT INTERFACE RHEOTUNE ( H)... 4 I/RA/11292/09.017/ABR page II
FIGURE 3-2 CUMULATIVE PROBABILITY DENSITY FUNCTION OF THE DIFFERENCE BETWEEN THE HEIGHT OF THE WATER-SILT INTERFACE MEASURED BY NAVITRACKER AND BY RHEOTUNE FOR THE HARBOUR OF ZEEBRUGGE.... 5 FIGURE 3-3 LEFT: AVERAGE CONCENTRATION AND STANDARD DEVIATION (ERROR BARS) OF THE DENSITY PROFILES MEASURED WITH EITHER RHEOTUNE OR NAVITRACKER; RIGHT: AVERAGE AND STANDARD DEVIATION (ERROR BARS) OF THE DIFFERENCE BETWEEN RHEOTUNE AND NAVITRACKER DATA.... 7 FIGURE 3-4 STANDARD DEVIATION OF THE NAVITRACKER AND RHEOTUNE DATASETS.... 8 FIGURE 3-5 CROSS CORRELATION BETWEEN THE NAVITRACKER AND RHEOTUNE DATA.... 8 FIGURE 4-1 DEFINITION SKETCH FOR THE PART OF THE PROFILE THAT IS USED FOR THE COMPARISON. THE COMPARISON OCCURS FROM THE WATER-SILT INTERFACE UP TO THE MAXIMUM DEPTH, WHERE DATA IS AVAILABLE FOR BOTH PROFILES.... 10 FIGURE 4-2 PROBABILITY DENSITY FUNCTION OF THE MEASURED TOTAL DRY MASS PER UNIT OF AREA FOR THE RHEOTUNE AND NAVITRACKER MEASURED IN THE HARBOUR OF ZEEBRUGGE, CONSIDERING ONLY THAT PART OF THE PROFILE WERE DATA FROM BOTH INSTRUMENTS WERE AVAILABLE.... 12 FIGURE 4-3 PROBABILITY DENSITY FUNCTION OF THE MAXIMUM DEPTH OBTAINED DURING TACKING A PROFILE USING RHEOTUNE AND NAVITRACKER IN THE HARBOUR OF ZEEBRUGGE, CONSIDERING ONLY THAT PART OF THE PROFILE WERE DATA FROM BOTH INSTRUMENTS WERE AVAILABLE.... 12 FIGURE 4-4 PROBABILITY DENSITY FUNCTION OF THE DIFFERENCE BETWEEN THE PROFILING DEPTH USING NAVITRACKER AND DENSITY AT A GIVEN LOCATION IN THE HARBOUR OF ZEEBRUGGE, CONSIDERING ONLY THAT PART OF THE PROFILE WERE DATA FROM BOTH INSTRUMENTS WERE AVAILABLE.... 13 FIGURE 4-5 CUMULATIVE PROBABILITY DENSITY FUNCTION OF THE DIFFERENCE AT A GIVEN LOCATION BETWEEN THE TOTAL DRY MASS PER UNIT AREA MEASURED USING NAVITRACKER AND RHEOTUNE IN THE HARBOUR OF ZEEBRUGGE, CONSIDERING ONLY THAT PART OF THE PROFILE WERE DATA FROM BOTH INSTRUMENTS WERE AVAILABLE.... 13 FIGURE 4-6 DEFINITION SKETCH OF THE AREA USED FOR COMPARSON IN CASE EXTRAPOLATION IS USED. THE COMPARISON IS PERFORMED FROM THE WATER-SILT INTERFACE UP TO THE DEPTH WHERE AT LEAST ONE OF THE TWO MEASUREMENT INSTRUMENTS PROVIDE DATA. EXTRAPOLATION IS USED FOR THE OTHER INSTRUMENT.... 14 FIGURE 4-7 PROBABILITY DENSITY FUNCTION OF THE TOTAL DRY MASS MEASURED USING NAVITRACKER AND RHEOTUNE, FOR THE HARBOUR OF ZEEBRUGGE, CONSIDERING THE PROFILE UP TO THE LOWEST POINT OF THE DEEPEST PROFILE.... 15 FIGURE 4-8 CUMULATIVE PROBABILITY DENSITY FUNCTION OF THE DIFFERENCE IN THE TOTAL DRY MASS MEASURED USING NAVITRACKER AND RHEOTUNE, FOR THE HARBOUR OF ZEEBRUGGE, CONSIDERING THE PROFILE UP TO THE LOWEST POINT OF THE DEEPEST PROFILE.... 16 FIGURE 4-9 DIFFERENCE IN THE DRY MASS MEASURED USING NAVITRACKER AND RHEOTUNE AS FUNCTION OF THE DIFFERENCE IN THE PROFILING DEPTH BETWEEN NAVITRACKER AND RHEOTUNE, FOR THE HARBOUR OF ZEEBRUGGE... 16 FIGURE 4-10 TOTAL DRY MASS MEASURED USING RHEOTUNE VS. THE TOTAL DRY MASS MEASURED USING NAVITRACKER IN THE HARBOUR OF ZEEBRUGGE, FOR THE DATA EXTENDING TO THE LOWEST POINT OF THE DEEPER PROFILE.... 17 FIGURE 4-11 DIFFERENT ZONES IN THE DEURGANCKDOK... 19 I/RA/11292/09.017/ABR page III
1. INTRODUCTION 1.1. The assignment IMDC N.V. was asked by Afdeling Maritime Toegang to perform an additional assignment in the project Long term monitoring of salinity gradients in the harbour of Zeebrugge and monitoring of salinity, suspended sediment concentration and high concentration benthic suspensions in the Belgian coastal zone, (Tender 16EB/05/10). This extra assignment comprises the comparison of two measurement instruments (viz. Navitracker and Rheotune), using data of the bottom density from measurements that were performed in the harbour of Zeebrugge and in the Deurganckdok. 1.2. Aim of the study The aim of this study is to compare two different instruments (Navitracker and Rheotune) for the measurement of bottom density profiles, with the main focus of the comparison on the determination of a mass balance from the measurement data. 1.3. Overview of the study In this study, two different measurement instruments are compared, the Navitracker, which measures bottom density profiles using the attenuation of gamma radiation, and Rheotune, which uses the tuning fork principle to measure bottom density profiles. For this comparison, data from two different measurement campaigns were used. One had been performed in the Albert II dock in Zeebrugge, where both instruments were used at the same day. The other dataset was from measurements in the Deurganckdok, where the data using Rheotune were obtained two weeks after the data obtained with Navitracker. 1.4. Structure of the report This report starts with this introduction. Then in chapter 2, an overview is given of the two measurements instruments followed by an overview of the two measurement campaigns that were used for the analysis in this report. This is followed with an analysis of the differences in the measured density profiles from the two different instruments in chapter 3. Then in chapter 4, the data from these profiles are used to obtain an estimate of the mass at a certain location by integrating these profiles. This report is ended with some conclusions in chapter 5. I/RA/11292/09.017/ABR page 1
2. DESCRIPTION OF THE STUDY 2.1. Description of the instruments The determination of the density of the silt layers is important to quantify the sedimentation and erosion rates, and to determine the nautical depth in harbours and waterways. Different techniques exist to determine the density as a function of depth. In this report the Navitracker and the Rheotune are compared. The Navitracker is an instrument, which uses the attenuation of nuclear (gamma) radiation to determine the density. It consists of a Cesium 137 gamma ray source in one rod of the instruments and a gamma ray detector in the other rod. Calibration is performed on board by measuring the absorption in clear water as well as a dolomite solution with a known density. Navitracker uses a pressure sensor to determine the depth. The Rheotune on the other hand, uses the tuning fork principle. One rod of the instrument is vibrating with a specific frequency, the other one responds with a frequency that depends on the density of the mud. Note that Rheotune is similar to Densitune, but the former also measures yield stresses of the mud. On board, only a calibration of the pressure sensor (used to determine the depth of the instrument) is done. Calibration of the densities and yields stresses is performed afterwards in the laboratory on the basis of a sample of the bed material that is taken during the survey. This calibration needs to be performed by an experienced geologist, because a special calibration solution needs to be made with a similar viscosity as the mud found in the measurement area. In the past, various comparisons were made between gamma ray absorption based instruments (such as Navitracker) and vibrating rod based instruments (such as Rheotune). However, these investigations differed from the present investigation in the aspect that the main objective of those studies was to determine the nautical depth, whereas in the present study, the main objective is to determine a mass balance from the density data. In can be expected that the former is more sensitive than the latter, as the latter essentially consists of taking an average over the profile, and this tends to reduce the influence of noise and random errors. The former on the other hand in based on performing an interpolation in order to determine the depth at which a predefined density (usually a value of 1.2 ton/m 3 is adopted) is reached. This interpolation can be quite sensitive, and large errors might be found especially if the gradients in the density profile are small. The results from the various comparisons do not give unambiguous results. Le Quillec et. al. performed a measurement campaign to determine the Nautical depth for a part of the Loire estuary using Densitune in combination with Silas (a system to use the backscatter strength from echo soundings to obtain bottom density profiles) as well as the gamma ray absorption based JTD3 from PANSN. They found that the differences between the two were substantial, and the results from Densitune/Silas were not consistent. However, RWS (2006), who compared Densitune with a barium 133 based nuclear density profiler developed by TNO, found that the differences in the nautical depth were usually quite small, and these were mostly attributed to the difference in the measurement location. However, they did notice some differences in the shape of the profiles, with somewhat lower gradients in the profiles from the nuclear instrument. Bundesansalt für Wasserbau (2004) used among other a combination of Silas and Densitune and a Cesium 137 based nuclear density profiler. They found that the differences in the nautical depth could vary from a few centimetres to 1.35 m, and they attributed the differences to the different measurement techniques, but also to the different stability to the flow of the different probes. Especially the Densitune appeared to be sensitive to conditions of high flow because of its small mass. Theoretical considerations on the differences between the two instruments were given in Claeys (2006). Claeys points out that nuclear instruments generally use a larger sample volume than tuning fork based instruments, which may lead to better reproducibility, less noise and a smaller change of bias error due to mud sticking to the probe. 2.2. Description of the measurements In this report, the results of density measurements with the Rheotune and the Navitracker are compared. This is done for two different locations: Deurganckdok in Antwerp and the Albert II dock I/RA/11292/09.017/ABR page 2
in Zeebrugge. For the Deurganckdok, the Navitracker measurements were performed by GEMS on August 26, 2008. The Rheotune measurements were performed by Stema and took two days to perform, respectively September 10 and September 11, 2008. In Zeebrugge, the measurements were performed using both instruments on August 21, 2008. However, the instruments were mounted onto two different ships. Hence there is a difference in the exact time of the measurement at the two locations. The measurement times for the Navitracker measurement were not available in the delivered datasets, so no exact time lag can be determined. However, for the objective of the present study, the difference in measurement time of a couple of hours (as the measurements were performed at the same day) is considered unimportant. The measurement locations of Deurganckdok are shown in Annex-Figure A-1. The measurement locations of Albert II dock are shown in Annex-Figure A-2. I/RA/11292/09.017/ABR page 3
3. COMPARISON OF THE RECORDED DENSITY PROFILES The recorded density profiles with the Navitracker and Rheotune are shown in Annex B for Zeebrugge and in Annex C for Deurganckdok. In the profiles for Zeebrugge, the yield stresses measured by the Rheotune are visualized simultaneously. No such data were available for the Deurganckdok. Because the yields stresses cannot be measured using the Navitracker, no comparison can be made between both instruments of these yield stresses. Hence they will not be discussed any further. A detailed comparison will only be performed for the data recorded in Zeebrugge, because those measurements were taken the same day. In the Deurganckdok, there was a difference of two weeks in time between both measurements, which is considered too long for making a direct comparison. In the data from Zeebrugge, there appeared to be some differences in the location of the top silt layer between those recorded with the Rheotune and those recorded with Navitracker. The water-silt interface lay 0.15 m lower in the Navitracker measurements than in the Rheotune measurements (with a standard deviation of 0.25 m). This difference might be due to the calibration of the pressure sensors used to determine the depth in the profiles. However, at least the variation of this variable will be due to difference in the measurement location in combination with spatial variations of the bathymetry. A cumulative probability density function of the difference in the height of the water-silt interface between both instruments is given in Figure 3-2. A sketch of the definition used for this height is given in Figure 3-1. Height H +0.5 m -10 m LAT -10.5 m LAT Density Navitracker Density Rheotune Figure 3-1 Definition sketch of the height of the water-silt interface Navitracker minus the height of the water-silt interface Rheotune ( H) I/RA/11292/09.017/ABR page 4
Figure 3-2 Cumulative probability density function of the difference between the height of the water-silt interface measured by Navitracker and by Rheotune for the harbour of Zeebrugge. As this might complicate the comparison of the two profiles, the reference of the measurement was changed, such that the reference plane (y=0) is at the location of the water-silt interface. This interface was determined as the depth, where the density was 0.015 ton/m 3 higher that the water density. Note that different values of the water density were found in the profiles from the different instruments. Navitracker gives a water density of 1 ton/m 3, whereas Rheotune gives a density of 1.02 ton/m 3, in Zeebrugge and 1.00 ton/m 3 in Deurganckdok (though slightly higher than the value from Navitracker). The difference in the ability to measure the water density is related to the different measurement principle. It appeared that the provided Rheotune data suffered from some noise at the lower end of the profile. Here, the measurement tends to become saturated, and thus the deepest measurement points show quite high values. This noise has been eliminated by detecting the lowest occurrence of three consecutive points with the same density and removing this point and all data below this point. No such treatment was necessary for the provided Navitracker data. The vertical resolution of the provided data also differed. The data from Navitracker were delivered with a vertical resolution of 1 cm, whereas the Rheotune data for Zeebrugge were provided at irregular distances, for which the averages were in the range between 5 and 12 cm (with a standard deviation per profile between 2 and 6 cm). In order to compare the data more easily, they were divided in bins with a distance of 20 cm. For each bin, the difference between the two profiles was determined. These differences between the profiles are also plotted in Annex B. Of course, these differences were only plotted for the depths, where data from both instruments were available, as in many occasions, the Navitracker data extended deeper than the Rheotune data. These differences in depth were typically of the order of a meter, but could occasionally exceed two meters. A more quantitative analysis of the difference in the profiling depth is given in the next section. I/RA/11292/09.017/ABR page 5
Apparently, the Rheotune cannot go deeper than the start of the hard bottom (roughly the depth indicated by the 33 khz echo sounder), whereas the Navitracker can go deeper. The density at the endpoint of the profile was 1.28 ton/m 3 (with a standard deviation of 0.08 ton/m 3 ) for Rheotune and 1.30 ton/m 3 (with a standard deviation of 0.07 ton/m 3 ) for Navitracker. However, it seems that this number is biased upwards for Rheotune, because as explained before, the densities it measured tended to become saturated near the end. A closer inspection of the density profiles revealed that a density between 1.2 and 1.25 ton/m 3 is a better estimate of the maximum density measured with Rheotune. Interestingly, the Navitracker data show that after the occurrence of the hard bottom, the density decreases in many instances with increasing depth. It is probably not a bias in the instrument, because on the rare occasion that both instruments penetrate deeper (in the Deurganckdok) such a decrease can be found in the data from both instruments. A comparison between both data is made in Figure 3-3. This figure shows first the average and standard deviation of all profiles, where both instruments had recorded with either Navitracker or Rheotune. This profile is shown for an easier comparison between both instruments. Because the conditions (i.e. the thickness of the silt layer) in the harbour might vary substantially in space, the average might not be a very physically meaning-full quantity. The averages generally compare quite well for the different instruments. It appears that the densities measured with Rheotune are slightly (typically about 0.02 ton/m 3 ) higher than those measured with Navitracker (except for the two lowest measurement bins, which are not reliable, because they are based on only one measurement for the Rheotune). In order to facilitate comparison, plots are made of the average and standard deviation of both Rheotune and Navitracker data, and of the difference between both data sets in Figure 3-3. This graph of course also shows the slightly higher values measured with Rheotune than with Navitracker. However, this difference is not significant, because it is rather small compared to the standard deviation of these differences, i.e. the average bias is small compared to the variation between the data of the different instruments at an individual profiles recorded at a certain location. Thus this bias is rather small compared to the inherent uncertainty in the measurement. Figure 3-3 further shows that the standard deviations of the Navitracker data are typically higher than those of the Rheotune, except at the deepest points in the profile, where the higher standard deviations in the Rheotune data are probably due to the small amount of data that is available at these locations. A more detailed view of the standard deviation is given in Figure 3-4. It is not a priori possible to relate this difference to different instrument characteristics, because the standard deviation is not only related to the uncertainty in the measurement, but also to the spatial variation in the densities of the bed material. In order to understand which part in the variation of the measurement is related to the spatial variation of the bottom characteristics, we calculate the cross-correlation coefficient between the Navitracker and Rheotune data. The cross-correlation coefficient is defined as: Here (y) is the measured density at a depth y, and is the average density at this depth (thus for the Rheotune the average of all Rheotune measurements, and for the Navitracker the average of all Navitracker measurements for which also Rheotune data existed). The subscripts NT and RT denote the Navitracker and Rheotune respectively. The cross-correlation is a measure to indicate whether two signals (in this case the deviation of the measured density in a profile from the mean) are similar. Here, it is assumed that if at a certain profile, the deviation of the measured density compares well between the profiles measured by the different instruments, this means that this deviation is due to a difference in the density from the average profile at that location. If on the other hand, the two profiles are not similar, the difference is probably due to measurement errors. Thus, if the cross correlation coefficient is high, this means that the variation is present in both datasets, and thus is probably related to the spatial variation of the silt characteristics. If the correlation is low, than the differences between both instruments are probably related to measurement uncertainties in the I/RA/11292/09.017/ABR page 6
instruments, as it is assumed that these uncertainties are independent of each other for both instruments. The cross correlation between the two instruments are plotted in Figure 3-5. This graph clearly shows that the cross correlation between the two datasets is rather high in the upper 1.5 m. Here, the variation in the data is caused by a variation of the conditions of the bed. Then at a depth of 1.5 m, there is dip in the cross correlation. Apparently, there are stronger differences in the profiles measured using the two different instruments at this depth. The reason for this is not clear. It is might be related to the non complete filtering of the noise at the end of the shallower Rheotune profiles. At larger depths, the correlation between both instruments is once again quite high, until it decreases from depths of 2.5 m. At this depth, the standard deviation in the Rheotune data also shows a large peak. Presumably, this is related to the noise at the end of the profiles in the Rheotune data, in combination with the low amount of Rheotune that is available for this depth. Figure 3-3 Left: Average concentration and standard deviation (error bars) of the density profiles measured with either Rheotune or Navitracker; Right: average and standard deviation (error bars) of the difference between Rheotune and Navitracker data. I/RA/11292/09.017/ABR page 7
Depth refered to silt-water interafce [m] Depth refered to the water-silt interface [m] IMDC NV 0-0.02 0 0.02 0.04 0.06 0.08 0.1 0.12-0.5-1 -1.5-2 Navitracker Rheotune -2.5-3 -3.5-4 Density standard deviation [ton/m 3 ] Figure 3-4 Standard deviation of the Navitracker and Rheotune datasets. 0-1.5-1 -0.5 0 0.5 1 1.5-0.5-1 -1.5-2 -2.5-3 -3.5-4 Cross-correlation Navitracker and Rheotune data [-] Figure 3-5 Cross correlation between the Navitracker and Rheotune data. I/RA/11292/09.017/ABR page 8
The data measured in the Deurganckdok are shown in Annex C. As mentioned previously, a direct comparison between the profiles measured with both instruments is not possible, because the time interval between both measurements was too large. For that reason, the data are presented using LAT as the vertical reference level instead of the water-silt interface, as the latter approach would not have any added value here. Even though a direct comparison is not possible, some general remarks can be made from the data. First of all, the Navitracker measures here up to larger depths than Rheotune, just as was observed in Zeebrugge. In the deeper data measured with Navitracker, the decreased densities beneath the start of the hard bottom, which were sometimes observed in Zeebrugge, are observed here as well, and at least in one profile (at location 97) such a decrease seems to be present in the Rheotune data as well, which suggests that this effect is not a bias error. A clear difference between the Rheotune data here and in Zeebrugge is that the noise that was present near the lower end of the data from Zeebrugge is not found here. I/RA/11292/09.017/ABR page 9
4. COMPARISON OF THE MASS BALANCE OBTAINED FROM THE DIFFERENT MEASUREMENTS 4.1. Zeebrugge The obtained density profiles are used next in order to perform a mass balance for the harbour of Zeebrugge. The comparison in Zeebrugge was performed for each profile separately for those locations where measurements were performed using both instruments. The data that were used were referenced to the water-silt interface, and any noise near the bottom of the Rheotune files was eliminated (see chapter 3). There was a large difference between the number of profiles take using Rheotune (32 profiles) and those using Navitracker (68 profiles). Integrating these data to obtain a total mass above a given reference plane, would be influenced significantly by the interpolation over a larger area using Rheotune, and might thus bias the comparison. A further difficulty is the smaller depth over which the Rheotune had measured. This means that if the reference plane were below the lowest profile point, extrapolation of the data would be needed, which would complicate the comparison. Therefore, it was chosen to use the last position of the profile that had penetrated the least deep into the bed as the reference plane (usually Rheotune) for the determination of the dry mass. This is illustrated in Figure 4-1 Height Considered part for comparison Considered part for comparison Density Navitracker Density Rheotune Figure 4-1 Definition sketch for the part of the profile that is used for the comparison. The comparison occurs from the water-silt interface up to the maximum depth, where data is available for both profiles. The masses here are determined as the dry mass (M ds ) per unit area. This can be calculated from the measured mass (M tot ) and volume (V tot ) per unit area using the equation: The mass per unit area are calculated by integrating the measured density profile, whereas the volume per unit is just the depth over which the integration is performed: In these equations, s and w are the material densities of the solid material and the water, which are here assumed to be 2650 kg/m 3 and 1025 kg/m 3 respectively, d is the height of the reference level above which the total mass is determined, and meas (y) is the measured density profile. I/RA/11292/09.017/ABR page 10
The statistical data of this analysis are given in Table 4-1. In this table, the average, standard deviation, minimum and maximum of the profiling depth and total dry mass from either Navitracker or Rheotune are given, as well as these statistics from the difference between the data of the two. Some more detail of these statistics can be obtained from the probability density functions of these quantities, which are given in Figure 4-2 to Figure 4-5. These figures give the probability density functions (abbreviated as pdf; it is basically a histogram) of the dry mass, the maximum depth for both instruments as well as for the differences. These figures show that the measured densities are quite comparable (similar shape in the pdf) but in general (85 % of the profiles) the measured dry masses are less for Navitracker then for Rheotune. In this way, they show that the Navitracker measured quite some deep profiles (a rather broadly spread probability density function), whereas there were mainly shallow profiles measured by Rheotune (peak at shallow depths in the pdf). This leads to a profiling range that is generally similar (with a spread of one meter), but somewhat deeper for the Navitracker, but there are also some cases in which the differences where very large (the second peak near 2.5 m), and the Navitracker extended much deeper. The table and the graphs show that the calculated dry masses from the Navitracker and Rheotune compare well. On average, the difference between the two is only 11.5% (Rheotune is 11.5 % higher), and large difference between individual total dry masses do not occur (the standard deviation of the difference is 19 %). The cross-correlation coefficient for the determination of the dry mass from both data sets was calculated to be 0.93. This is very high, which means that the variation found in the dry masses can to a large extend by explained by the spatial variation of the density. However, these graphs and table also show (as already mentioned in the previous section) that there is significant difference in the depth of the profiles. On average, the profiles recorded with Navitracker extend 0.6 m deeper than those measured with Rheotune, but the maximum difference is 2.8 m. Hence it can be concluded that if the total dry mass is calculated over that part of the profile where data from both instruments are available, very similar results are obtained. However, if deeper data are required, the differences might be larger, because one or both of the datasets data will have to be extrapolated downwards. This is investigated in the following. Table 4-1 Statistical properties of the profiling depth and Total dry mass (per unit area) obtained from the Navitracker and Rheotune measurements in the harbour of Zeebrugge, considering only that part of the profile were data from both instruments were available. Average Standard deviation Minimum Maximum Profiling depth Rheotune [m] 1.49 0.82 0.61 3.51 Profiling depth Navitracker [m] 2.08 1.00 0.80 4.14 Profiling depth NT Profiling depth RT [m] 0.59 0.77-0.38 2.76 Total dry mass Rheotune [ton/m 2 ] 0.31 0.15 0.15 0.66 Total dry mass Navitracker [ton/m 2 ] 0.28 0.13 0.11 0.59 Total dry mass NT Total dry mass RT [ton/m 2 ] -0.04 0.06-0.20 0.08 I/RA/11292/09.017/ABR page 11
Figure 4-2 Probability density function of the measured total dry mass per unit of area for the Rheotune and Navitracker measured in the harbour of Zeebrugge, considering only that part of the profile were data from both instruments were available. Figure 4-3 Probability density function of the maximum depth obtained during tacking a profile using Rheotune and Navitracker in the harbour of Zeebrugge, considering only that part of the profile were data from both instruments were available. I/RA/11292/09.017/ABR page 12
Figure 4-4 Probability density function of the difference between the profiling depth using Navitracker and density at a given location in the harbour of Zeebrugge, considering only that part of the profile were data from both instruments were available. Figure 4-5 Cumulative probability density function of the difference at a given location between the total dry mass per unit area measured using Navitracker and Rheotune in the harbour of Zeebrugge, considering only that part of the profile were data from both instruments were available. In order to investigate the effect of the different profiling depths, we subsequently used the deepest point of the lower measurement profile (usually the Navitracker) as reference plane for the mass I/RA/11292/09.017/ABR page 13
balance and assumed this to be approximately equal to the design depth (see Figure 4-6). For the instrument, for which no data were available in the lower part, the lowest measurement point was used to extrapolate the profile downward, using a nearest neighbour extrapolation. In other words, the value of the lowest profiled point was used as a constant value for the part of the profile beneath the lowest measurement point. This type of extrapolation was chosen, because it is a conservative approach, and on the other hand, because it is less sensitive to noise in the measurements than for example linear extrapolation. Height Water-slib interface Considered part for comparison Considered part for comparison Density Navitracker Density Rheotune Figure 4-6 Definition sketch of the area used for comparson in case extrapolation is used. the comparison is performed from the water-silt interface up to the depth where at least one of the two measurement instruments provide data. extrapolation is used for the other instrument. This is an important test, because it corresponds better to the present practise for determining the mass balances in the Deurganckdok (IMDC 2008b), where the design depth is used as a fixed reference plane, and the mass balance is performed above this reference plane using extrapolation of the measured profiles if data is missing above this plane. In Zeebrugge, we used the lowest part of the lower instrument as reference plane, assuming that this depth corresponds approximately with the design depth. The results of these calculations are shown in Table 4-2 and in Figure 4-7 and Figure 4-8. Of course, the total dry masses measured in this way are larger than those of Table 4-1, because the considered profiling depth is larger. We find that here too, the measured total dry mass using Navitracker is lower than using Rheotune, and that the average difference in the total dry mass increased from 11.5% to 14.4%. Also the variation in the difference between the two measurements increased (the standard deviation of the difference increased from 19 % to 27 %). This increase in the differences is obviously related to the use of extrapolation in the lower part of the Rheotune data. This can be seen easily in Figure 4-9. Here the difference between the dry mass determined from Navitracker and from Rheotune is plotted as function of the difference in the profiling depth between the two instruments. There is a clear correlation (the correlation coefficient is 0.67), and this figure shows that the largest difference occurs when the difference in profiling depth (i.e. the distance over which the Rheotune needs to be extrapolated) is largest. Nevertheless, the differences are not that large considering that the density profiles measured in Zeebrugge show a decrease of the density with increasing depth in a significant number of profiles (see Annex B). Apparently, the extrapolated densities compare on average quite well with the measured ones. Thus, even if accounting for the different profiling depths by extrapolating the data from Rheotune, the total dry masses measured using Navitracker and Rheotune compare quite well in the Albert II I/RA/11292/09.017/ABR page 14
Dok of the harbour of Zeebrugge (Figure 4-10). However, note that the difference in the location of the water-silt interface (Figure 3-2) leads to an additional difference in the total dry mass, if the comparison is made with respect to a fixed reference plane, which is estimated to be about 10 %. Table 4-2 Statistical properties of the profiling depth and Total dry mass (per unit area) obtained from the Navitracker and Rheotune measurements in the harbour of Zeebrugge, considering the profiles up to the lowest measurement point of the lower profile. Average Standard deviation Minimum Maximum Total dry mass Rheotune [ton/m 2 ] 0.58 0.36 0.20 1.73 Total dry mass Navitracker [ton/m 2 ] 0.49 0.24 0.19 1.29 Total dry mass NT Total dry mass RT [ton/m 2 ] -0.08 0.16-0.66 0.13 Figure 4-7 Probability density function of the total dry mass measured using Navitracker and Rheotune, for the harbour of Zeebrugge, considering the profile up to the lowest point of the deepest profile. I/RA/11292/09.017/ABR page 15
Figure 4-8 Cumulative probability density function of the difference in the total dry mass measured using Navitracker and Rheotune, for the harbour of Zeebrugge, considering the profile up to the lowest point of the deepest profile. Figure 4-9 Difference in the dry mass measured using Navitracker and Rheotune as function of the difference in the profiling depth between Navitracker and Rheotune, for the harbour of Zeebrugge. I/RA/11292/09.017/ABR page 16
Figure 4-10 Total dry mass measured using Rheotune vs. the total dry mass measured using Navitracker in the harbour of Zeebrugge, for the data extending to the lowest point of the deeper profile. 4.2. Deurganckdok For the measurement in the Deurganckdok, there was a large difference (two weeks) in the moments when the measurements with Navitracker and Rheotune were performed. However, for all zones in the Deurganckdok (Figure 4-11), the total dry sediment mass was calculated for both measurements. These values are given in Table 4-3 (see IMDC 2008b). It is clear that the values do not compare. This is to be expected, because of the effects of siltation that occurred in the Deurganckdok in the period between both measurements. Hence, it is not possible to assess the influence of the different instruments on these results. Table 4-3 Measured Dry Mass (Ton/m²) per zone in the Deurganckdok for the measurements using Navitracker (26-Aug-08) and Rheotune (11-Sep-08). 26-Aug-08 11-Sep-08 1 - - 2 - - 3a 0.88 1.20 3b 0.83 1.07 3c 0.82 1.01 3d 0.82 0.93 3e - 0.99 4Na 0.60 0.80 I/RA/11292/09.017/ABR page 17
26-Aug-08 11-Sep-08 4Nb 0.59 0.69 4Nc 0.59 0.54 4Nd 0.60 0.43 4Ne - 0.83 4Za 0.56 0.92 4Zb 0.52 0.74 4Zc 0.62 0.68 4Zd 0.52 0.57 4Ze - 0.55 5Na - 0.76 5Nb - 0.79 5Nc 0.84 0.52 5Nd - 0.20 5Ne - - 5Za - - 5Zb 0.87 0.77 5Zc 0.73 0.71 5Zd - 0.54 5Ze - - Trench area mean 0.83 1.04 I/RA/11292/09.017/ABR page 18
Figure 4-11 Different zones in the Deurganckdok I/RA/11292/09.017/ABR page 19
5. CONCLUSION In this report, data from two measurement campaigns, one in the harbour of Zeebrugge and one in the Deurganckdok, are used to compare the bottom density profiles and the mass balances obtained from these profiles from two different instruments: the Navitracker and the Rheotune. Navitracker measures the density of the bed using the attenuation of gamma radiation. Rheotune measures the density of the bed (and the yield stresses) using the tuning fork principle. This study differs from other comparisons between these kind of instruments in that the focus was on the determination of the mass balance, whereas most studies focused on the determination of the nautical depth, which is normally defined as the depth at which the density is equal to 1.2 ton/m 3. From the comparison, it can be concluded that the Rheotune and Navitracker give comparable density profiles (although slightly higher values were recorded using Rheotune) over the measurement where both instruments had recorded during the simultaneous measurements in the harbour of Zeebrugge, provided that noise that was present at the lower end of the Rheotune data is removed. However, the profiles measured with Navitracker extend on average 0.6 m deeper, but this difference can be as large as 2.8 m. This difference is related to the larger mass of the Navitracker, which allows it to penetrate denser mud layers than Rheotune. In general, the Rheotune profiles extend roughly to the hard bottom as indicated by the 33 khz echo sounding profile. The density at the endpoint of the profile was 1.28 ton/m 3 (with a standard deviation of 0.08 ton/m 3 ) for Rheotune and 1.30 ton/m 3 (with a standard deviation of 0.07 ton/m 3 ) for Navitracker. However, it seems that this number is biased upwards for Rheotune, because the densities it measured tended to become saturated near the end. Therefore a density between 1.2 and 1.25 ton/m 3 seems to be a better estimate of the maximum density measured with Rheotune. It was also noticed that there was a difference in the location of the water-silt interface of 0.15 m (with a standard deviation of 0.25 m) between the two instruments. The water-silt interface was located at a lower position in the Navitracker than in the Densitune measurements. The reason for the difference might be related to difference in the measurement location or to the calibration of the pressure sensors. A calculation of the total dry mass per unit area (for the part of the profile, where data from both instruments were available) showed that the Rheotune data were on average 11.5 % higher than the Navitracker results. The standard deviation of the difference between the results from the two instruments was 19%. In case a datum is required that lays below the lowest profiling point of the Rheotune data, the differences were larger (14.3%) and the standard deviation of the differences was higher as well (27 %). It appears that the largest differences occur in those cases, where the largest part of the Rheotune profile had to be extrapolated. In these cases, a constant value equal to the lowest measurement value was used for the extrapolation. However, the difference is actually quite low considering that in a significant number of cases, the Navitracker shows a decreasing density, rather than a constant one. This makes extrapolating the Rheotune data downward quite awkward. For the Deurganckdok, the difference in the measurement time (about two weeks) was too large to perform a detailed comparison. In general, it is quite difficult to compare the present results to the results from previous studies. The reason is that, previous studies focussed on the determination of the nautical depth, whereas the present study focuses on the determination of a mass balance from the data. It can be expected that the present results are less sensitive to noise than the previous results, because the determination of a mass from the profiles is mathematically equivalent to integrating the data, which is a process that removes noise from the data. In general, both instruments compare well in the present study. This agrees with the results from the study from RWS (2006), who attributed differences in the data from a gamma absorption probe and Densitune mainly due to the difference in the exact location where the profiles had been taken. However, Le Quillec et. al. finds substantial differences in the nautical depths determined from a gamma ray probe and Densitune. The difference in profiling depth between both instruments that was found in the present study, with the Navitracker extending deeper than Rheotune, was also mentioned in the publication by Claeys (2006). I/RA/11292/09.017/ABR page 20
6. REFERENCES Bundesanstalt für Wasserbau (2004), Ermittlung der Nautischen Sohle Ems km 47.3 bis km 48.1,Stellungnahme zu den geotechnischen, rheologischen und Echolot Vergleichungsuntersuchungen vom 08. bis 10. September 2003, BAW-Nr. 0250110107 Claeys S. (2006), Evaluation and combination of techniques used to determine the Nautical bottom, A call for rheology based instruments, Hydro 06. IMDC (2005) Kritische kwaliteitsanalyse peilplannen Havenbedrijf Antwerpen, gegenereerd met multibeam (I/RA/11272/05.038/MSA) IMDC (2008a) Feasibility study of echo sounding to determine fluid mud density profiles (I/NO/11283/08.001/BOB) IMDC (2008b) Langdurige metingen Deurganckdok: Opvolging en analyse aanslibbing Deelrapport 1.21: Sediment Balance: Three monthly report 1/7/2008 30/09/2008 (I/RA/11283/08.077/MSA) Le Quillec R., J.P. Helard and P. Terrier (N.D.), Essais des systems de mesure de densite de sediments Densitune et du logiciel d acquisition et de traitements des donnees acoustiques silas de la societe Stema RWS, (2006) Navitracker vs DensiTune, Unpublished manuscript I/RA/11292/09.017/ABR page 21
Annex A MAPS OF THE MEASUREMENT SITES I/RA/11292/09.017/ABR page 22
Annex-Figure A-1 Overview of the measurement locations in the Deurganckdok I/RA/11292/09.017/ABR page 23
Annex-Figure A-2 Overview of the measurement locations in the Albert II dock, Zeebrugge. Coordinates in ED 50. I/RA/11292/09.017/ABR page 24
Annex B PROFILE PLOTS ZEEBRUGGE I/RA/11292/09.017/ABR page 25
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