NOVEL SEDIMENT PROFILER FOR PREPARING AND EVALUATING DREDGING WORKS AND DETERMINING THE NAUTICAL DEPTH

Transcription

1 NOVEL SEDIMENT PROFILER FOR PREPARING AND EVALUATING DREDGING WORKS AND DETERMINING THE NAUTICAL DEPTH By K. Geirnaert 1, P. Staelens 1, S. Deprez 1 B. Dierikx 2, M. van der Sluijs 2, G. Poot 2 ABSTRACT There is a continuous inflow of sediments in ports and access channels and therefore maintenance dredging is necessary. To determine when and how much there needs to be dredged the underwater sediment and mud layers must be monitored and analysed. This paper presents an innovative vertical profiling technique measuring the depth, thickness and density of the underwater sediment layer. The instrument uses X-ray to measure the sediment density. The data is used for two important aspects. First in the preparation of dredging works where the data is used to determine ton dry matter of the dredged material. In combination with acoustic methods like multibeam echo sounders it is used to visualize the sediment layers under a multibeam surface. Another important aspect of soft sediment is the navigability. Ships can navigate through loose mud layers if the physical characteristics of the mud stay below a critical limit. Today the measured physical characteristic in many ports is density. The proposed measurement technique allows visualization of density and enables ports to evaluate nautical depth criteria. Keywords: Mud density, Nautical depth, Maintenance dredging 1 dotocean NV, Lieven Bauwensstraat Brugge Belgium, +32 (0) , info@dotocean.eu, 2 Rijkswaterstaat Centrale Informatievoorziening (CIV) Derde Werelddreef HA Delft, +31 (0) , ben.dierikx@rws.nl,

2 1. INTRODUCTION Sediment density measurements are important for several aspects of the dredging process. Worldwide ports and waterway authorities are using density criteria to determine the navigable depth or the nautical bottom. In the Netherlands, Rijkswaterstaat is currently using a density criterion to initiate, control and evaluate dredging works. The target depth is 23.65m in the access channel to the Port of Rotterdam and the target depth is 20.5m in the access channel to the Port IJmuiden and the Port of Amsterdam. When the bulk density of the sediment above the target depth exceeds 1.2 t/m³, the sediment needs to be dredged. Therefore the accuracy of the density measurement has a direct impact on the dredging effort and cost. Besides applications such as determination of nautical bottom and preparation of dredging works, also Water Injection Dredging (WID) projects can be prepared based on sediment density profiles. 2. DENSITY MEASUREMENT BY RADIATION 2.1 X-ray as an alternative for radioactive sources Historically radioactive sources are applied in sediment density measurement systems. Attenuation of radiation provides a robust and accurate way of measuring the bulk density of sediments. The drawback of a radioactive element is that it radiates continuously and therefore requires special protection during handling and operation. The legislative restrictions on radioactive sources moves the market to look for alternative technologies like X-ray sources. X-ray sources have the advantage that they can be turned on and off. An X-ray source consists of a tube with a filament which is heated as depicted in Figure 1. Due to the high temperature, electrons are freed from the filament and they are accelerated in an electric field between the filament and a target. The field strength is expressed in kv and determines the energy of impact on the target. At impact the electrons hit electrons of the target material. The excited electrons release energy as X-ray photons. Figure 1: X-ray source With e.g. a 100kV electric field in the source, the photons sent out by the source are in the range between 5keV and 100keV as depicted in Figure 2.

3 Figure 2: X-ray spectrum When X-ray photons are applied to a medium, like a sediment section of e.g. 10 cm width, a very small fraction will be transmitted without interaction. The rest of the photons will interact with the medium. 2.2 X-ray photon interaction Depending on the energy of the photons two dominant interaction processes are possible. At low energy levels photo-electric absorption is dominant. At high energy levels the Compton scattering is dominant as depicted in Figure 3: Photon interaction. Figure 3: Photon interaction Photoelectric effect and absorption

4 At energy levels below 100KeV, the photoelectric effect is dominant. During the absorption process the photon turns its full energy to the electron and ceases to exist as depicted in Figure 4. The electron can be emitted out of its orbit or it can lead to excitation to a next orbit level and fall back by sending out a new photon (XRF). The photoelectric effect is dependent on the energy level of the photon and the chemical composition of the intruded medium Compton effect Figure 4: Photoelectric effect At energy levels higher than 100KeV, the Compton effect is dominant. The photon is scattered and deviates from its path when it interacts with the electron as depicted in Figure 5. Due to this interaction the photon is losing energy and deviates under a certain angle. The Compton effect is independent of the energy level of the photon and independent of the chemical composition of the medium. Attenuation due to the Compton Effect has a direct relationship with the density of the medium. Figure 5:Compton effect where is the original wavelength dependent on the energy of the photon is the wavelength of the scattered photon is the scattering angle

5 2.3 Measurement principle by photontransmission and scatter X-ray detector X-ray source Figure 6: X-ray density profiler The source and the detector are integrated in two legs of a profiler as depicted in Figure 6. The profiler is lowered into the loose sediment. Depending on the resistance of the sediment the profiler can intrude up to 5m in the sediment. During intrusion the density of the sediment between the legs of the profiler is measured. When photons are sent out by the source into the sediment, a portion of them reach the detector without interacting. A portion does interact with the electrons of the sediment molecules and deviates or absorbs according to the principles described above. The big advantage of using radiation scattering for measuring the density is the fact that there is physically a direct relationship between the amount of photons reaching the detector and the density of the sediment. The higher the density, the higher the number of electrons and the bigger the chance of interaction. The photons received in the detector are a measure for the density. The signal intensity received by the detector is an exponential function decreasing with the density of the mixture. The relationship between medium density D and the intensity of the signal received by the detector is: where: D = D0+ D1.Ln (Ic/Io) D is the medium density

6 Io is the signal intensity of the detector in clear water Ic is the signal intensity of the detector in mud D0 and D1 are calibration constants Figure 7: Detector output signal as a function of the density Radiation attenuation is a physical process that has a direct relationship with density. Due to the direct relation between attenuation and density, accuracies of more then 0.25% can be reached with measurement times of less then 1s. When the medium is denser, less photons reach the detector due to the increased photoelectric or Compton effect. The decrease of photons is an exponential curve as depicted in Figure 7.

7 2.4 Bulk density of sediment When measuring the density of a sediment, in fact bulk density of the sediment is measured. Every component contributes by its own atomic weight to the overall density measurement as depicted in Figure 8. Figure 8: Bulk density The contribution of each of the components is asymmetric. The density of mineral components and sand is in the range of T/m³. Carbon chains, which are the building blocks of organic material, have a density of 1 T/m³, which is a density similar to water. We are referring here to dry matter density. In a drained situation the mineral and organic components show their binding strength and generate binding forces in the sediment. So the density of a sediment measurement is not indicating the binding strength nor the displacement strength. 3. APPLICATION FIELDS There are several reasons why the density of sediment is measured. A few important fields of applications are: 1. Preparation dredging works, ton dry weight of the hopper dredger When dredging works are prepared the efficiency of the dredging process is dependent on the balance of sediment with water that is removed by the suction process. To determine upfront at which depth the suction pipe will find certain density levels, a density profile can be of value. A profile visualizing the thickness of the mud layers and the variation of the density over the depth of the mud layer is important.

8 DensX Cs-137 Figure 9: Density profile As depicted in Figure 9 the mud layer start at m and a uniform packet of loose mud is seen between m and m. From a more consolidated layer starts. When several profiles are combined a volume and mass calculation can be done. From a dredging perspective it is interesting to estimate the load of a dredging hopper when filled with a certain mud in order to estimate the dredging effort and the amount of filled hoppers in relation with a certain volume. These inputs cannot be derived from acoustic data alone since acoustics cannot directly be linked with density levels. 2. Nautical depth In muddy access channels ships can navigate close to or through a loose sediment layer. Potentially a ship can enter with 7% of it s draught inside the mud. Important is the navigatibility and controllability of a ship during navigation. Today many ports are using a mud density criterion to determine the depth inside the mud layer to where the navigability and controllability of a ship is guaranteed. This is based on the PIANC 1997 report.

9 meter DensX Cs-137 Figure 10: Density based nautical depth criterion Many waterway authorities are using a or T/m³ criterion to identify the level inside the mud layer where the nautical bottom is set as depicted in Figure Determining the gel point of sediment in preparation of WID WID is often used to liquefy sediment and remove it under a gravity flow. There are two important aspects that need to be visualized in order to determine what the dredging effort will be to mobilize the sediment layer. First the density of the sediment layer is of importance to understand how much water needs to be added to liquefy the sediment or to make it loose. Typically a water sediment emulsion with a density of T/m3 is needed to reach the gel point or the level where the sediment can start to flow. Once this point is reached the sediment can flow under gravity to a lower point or under influence of a tidal flow in a certain direction. A second important parameter to control WID works is the dredging effort of the water jet to erode the sediment layer. The water jet needs to overcome the binding strength of the sediment in order to loosen it up and to break up the binding forces of the sediment. Mud erosion resistance and mud strength are related. This type of parameters can be determined not with a density meter but with a rheology meter. As an in situ reology meter, a free fall sounding conus was used kpa Figure 11: WID erosion and sediment strenght

10 In Figure 11 a strength profile of a sediment layer is depicted. Based on the profile the amount of pressure in kpa can be determined to predict how deep the water beam will inject. 4. Follow up the consolidation process in dredging dumps and underwater cells. Underwater cells are underwater dumping sites with a significant over depth. They are used to dump dredged material and consolidate dredge material over time. To follow up the consolidation and dewatering process a density profiler can be used. In Figure 12 three profiles of a mud layer on different time intervals are depicted. From the green to the red profile the top of the mud is reducing while the density is increasing. Figure 12: Density variation over time

11 4. COMPARISON X-ray PROFILER WITH Cs-137 PROFILER Correlation exercises between an X-ray density profiler and a Cs-137 density profiler were done in 3 different ports and 3 types of sediment for hundreds of measurement locations. Both systems were operated site by site as depicted in Figure 13. The three locations are in the Port of Rotterdam in the Maasmond, The access channel to the Port of IJmuiden in the IJmond and the Port of Delfzijl on the Ems. Figure 13: Comparison tests between Cs-137 and X-ray profiler Both the Cs-137 profiler and the X-ray profiler were calibrated on mud from the region of investigation. The calibration procedure is fast and straightforward by taking a consolidated mud sample from the region of interest with a density higher then T/m³ and making a stepwise less dense emulsion by adding water to it. Both systems were calibrated on the same local mud sample.

12 DEPTH (M) DEPTH (M) DEPTH (M) The results are depicted in Figure DENSITY PROFILE DELFZIJL (NL) DENSITY PROFILE MAASMOND (NL) DENSITY (KG/M³) -27 DENSITY (KG/M³) DensX Cs-137 DensX Cs DENSITY PROFILE IJMOND DENSITY (KG/M³) DensX Cs-137 Figure 14: Comparison test between Cs-137 profiler and an X-ray profiler on three different locations

13 5. CONCLUSION A novel density profiler based on X-ray is presented. The level attenuation of radiation of Cesium or X- ray photons shows a direct relationship with the density of sediment between source and detector. The correlation between the X-ray profiler and Cs-137 profiler is significant and the reached accuracy with the X-ray profiler is above 0.25% for a 1s measurement time. An advantage of X-ray is the fact that the source can be turned off and does not suffer from strong legislation restrictions. The X-ray profiler is integrated in a fully automated survey tool which enables fast profiling and allows to easily take more than 150 profiles a day. Based on the measured profiles, high resolution grids can be made. 6. REFERENCES Kerckaert P. Malherbe B. and Bastin A. (1985). Navigation in muddy areas The Zeebrugge experience, PIANC Bulletin, No. 48, p Kerckaert P., Vandenbossche D., Malherbe B., Druyts M. And Van Craenenbroeck K. (1988). Maintenance Dredging at the Port of Zeebrugge : Procedures to Achieve an Operational Determination of the Nautical Bottom, 9th International Harbour Congress, Antwerp. Claeys S., Dierikx B., Paul S., van Reenen J. (2012). Fluid mud density determination in navigational channels, Hydro 12 Taking care of the sea proceedings, Rotterdam, Netherlands. Geirnaert, K., Staelens, P., Deprez, S., Noordijk, A., Van Hassent, A. (2013). Innovative free fall sediment profiler for preparing and evaluating dredging works and determining the nautical depth. Conference proceedings, WODCON XX: the Art of Dredging, Brussels, Belgium Kamphuis, J. (2013). Succesful approach to Keep the sediment navigable in Port of Delfzijl. Conference proceedings, SedNet PIANC-IAPH-IMPA-IALA (1997). Approach Channels: A Guide for Design. PTC II-30. Final report of the joint Working Group. PIANC (2008). Minimising harbour siltation. PIANC MarCom Report 102. Brussels, PIANC. PIANC (2013). Injection Dredging. PIANC MarCom Report 120. Brussels, PIANC Staelens P., Geirnaert K., Deprez S., Noordijk A., & Van Hassent, A. (2013). Monitoring the consolidation process of mud from different European ports in a full scale test facility. Conference Proceedings, WODCON XX: The Art of Dredging, Brussels, Belgium Delefortrie G., Vantorre M., Eloot K. (2005). Modelling navigation in muddy areas through captive modeltests. Journal of marine science and technology, Vol. 10, No. 4, p PIANC (2013) 120 Report n Injection Dredging