The Coastal Hydrographic Service (HVAS) Of Oostende

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1 Afdeling Waterwegen Kust - Hydrografie - Oostende H y d ro g ra p h ie S u rvey in g B elgian users & user requirem ents The data logging and processing o f bathymetric information along the Belgian coast is carried out in different ways. Most o f the data are collected by the hydrographic vessel, Ter Streep, owned by the Ministry o f the Flemish Community and used by the Coastal Hydrographic Service in Oostende. Other parts o f the data are collected by a hoovercraft, by an amphibious car and by other vessels, which are owned by contractors, working for the local government. To each o f them, specific tasks are dedicated. The data collected by the different vessels are used for different purposes. For cach o f them, the accuracy o f the collected data (in depth and in positioning) is important: - Navigation charts, which arc established according to the IHO standards; - Dredging : for planning and for monitoring the works; - Specific tasks : e.g. wreck searching, surveys for finding suitable locations for measuring poles (these are offshore constructions, used for measuring the physical and meteorological parameters at sea, such as tides, waves, salinity, wind, temperature, etc.) - Monitoring o f the erosion o f the beaches - Localisation o f shipwrecks and determination o f the mimimum safe depth on these obstacles. Equipment of the Coastal Hydrographic Service: - Single beam : -DGPS : code based; -Echosounder: this apparatus is used to determe the normal depth o f the sea bottom; it can also give an impression o f the thickness o f the layer o f mud that covers the seabed. The last data are used for dredging works. This echosounding is corrected for heave and for roll. Single beam-techniques give the opportunity to collect real-time waterdepth-data under the ship. When the parallel lines sailed by the ship are close to one another, navigation charts can be established from the collected data, (after tidal reduction) - Multi beam : -DGPS : code based; -Echosounding, also corrected for heave, roll and pitch, by using several echosounders at the same time. This equipment gives the opportunity to get fully detailed images o f the seabed. The resolution here is very high and allows the drawing o f big scaled charts. Sailing speed is slow when using this equipment, but it offers full detail which is especially important for areas with complex patterns on the seabed such as sandbanks, special features, wrecks, etc. - Side Scan Sonar:a tool used for making scanning images o f objects on the seabed. This is particularly useful for the localisation o f wrecks. Images, coming from this sonar, offer a good qualitative view on the orientation and the position o f the wreck. Survey-frequencies and products o f the Coastal H ydrographic Service: - For the 3 navigational charts o f the Belgian co a st, part o f the seabed configuration is amended every year, and normally a new edition o f the chart is issued when the stock o f the former runs out; these amendments arc the result o f detailed surveys in the Belgian conshclf area. Weather permitting, every working day the hydrographic survey vessel is used during day-time. The necessary updates o f the charts appear in the Notices to Mariners, in order to allow people to keep their charts up to date in between two editions. - The harbours o f Zeebrugge (monthly), Oostende (2-monthly), Blankenbcrge (3-monthly) and Nieuwpoort (6-monthly) are surveyed regularly; so, it is possible to look for a time Stheme for the dredging works, that fit the needs o f the maintenance o f the harbours. - Channels are surveyed several times a year; there the surveying activities take place in close cooperation with the dredging works. - The coastal zone is surveyed twice a year: this is part o f the monitoring o f the beach erosion and the coastal protection. BABEL11.DOC

2 Afdeling Waterwegen Kust - Hydrografie- Oostende 2 E v a lu a tio n o f so m e B A S reco rd s o f 0 4 /0 1 / C o m p a riso n s w ith B elg ia n h y d ro g r a p h ic su rvey s For this evaluation we took the BAS records o f Argoss B o f the 4th o f January 1995 into consideration, bccausc we consider this run as one o f the best SAR-pictures. For 1995, the Coastal Hydrographic Service (HD), Oostcnde (Belgium) has a lot o f bathymctric records near the Kwintc- and Middelkerke bank; so we will focus our comparisons with BAS to this area alone. We subdivided this study in 3 parts: 1) A small-scale comparison: The Annex I is a valuable run o f the 4th o f January 1995, made by Argoss B, on a rather unusual scale o f 1/ , while the Annex 2 represents, on the same scalc, an extract o f our CARIS-files o f the Belgian nautical chart laamse Banken ( Flemish Banks') or D 1 1. This last is also composed, for the concerned area, o f the surveys o f 1995; here we have to add that, in the more complicated areas, the cartographer has to introduce more smoothed bathymetric lines. A visual comparison between these 2 maps can be carried out on the bathymetric lines o f 5 m, 10 m and 20 m. Here we already can notice that the lines o f 20 m fit the best on both maps, while the differences on the 5 and 10 m seem to be higher. But even, on the 20 m, we can see that the classical surveys show slightly higher depths than the satellite records. 2) A mid-scale comparison: The Annex 3 is cxactly the same area as the Annex 1, but here a scalc o f 1/ has been chosen. The reason for this choice is that in the Coastal Hydrographic Service a single beam map o f April 1995 was available in the same scale for the whole area. See also the Annex 4. Here again a visual comparison between these 2 maps can be made on the bathymetric lines o f 5 m, 10 m and 20 m. Our findings o f this comparison are the following: -for the 20 m bottom line: the accordancc is rather good, excepted when some more complex bottomfeatures occur; -for 10 m bottom line: mostoften BAS is deeper than the the HD-mcasurements. The remark for the more complicated areas, mentionned above is also valid here; - for 5 m bottom line: only for 4 lines on the southern part o f the Middelkerke Bank, we arc able to make a comparison. Here the BAS-records are deeper; that is the reason why the isolated and closed lines in this system are shorter than the HD-ones. 3) A large-scale comparison: The Annex 3 shows, somewhere in the southern part o f the Middelkerke Bank, a rectangle, where, at the last resort, some detailed quality control has been carried out on a more statistical basis. The Annexes 5 (BAS-data o f 04/01/95) and 6 (HD multi-beam survey o f HD, dated 05/10/95) show this area on a scale o f 1/ Between the Annexes 5 and 6, there is still some difference in time, but, as bottom changes are rather low, we found it reasonable to base a further study on it. With CARIS, the Coastal Hydrographic Service was able to manufacture a difference chart, where nearly figures are available. See also Annex 7. With Excel 95, it was now possible to subdivide the differences into classes or intervals o f 1 dm, so that also the table and the histogram o f the frequency distribution can be determined. The Annex 8 shows the table o f the frequency distribution o f the 9405 values (in figures and percentages), while the Annex 9 (in figures) provides the concerned histogram. On this histogram we can notice that the distribution is nearly symmetric, as the mean equals 0,016 metres and the class 0 exceeds firmly the other classes. The standard deviation, being a measure o f the scatter or the dispersion o f the values (differences), is 0,693 metres, which is rather high. As we still had some questions with this configuration, wc subdivided the conccmed area into 2 parts. We repeated the exercise o f the frequency distribution for the north eastern part, being a bit more flat and deeper than the other part. Sec the Annex 7 again. BABEL11.DOC

3 Afdeling Waterwegen Kust - Hydrografie- Oostende 3 The Annex 10 and 11 are related to the last distribution with a population size o f Here we can notice that the distribution, translated in the histrogram, has a longer tail to the left and is thus skewed to the left as well. On the other hand, here the number o f the 0-values is also significantly higher than the surrounding classes, which is rather strange... Here the mean value and the standard deviation arc -0,387 and 0,560 metres. From the first to the third o f January 1995, a rather severe and long lasting storm surge period occured in the North Sea, causing residual sea levels o f more than 1,40 metres in Oostende (Belgium). N o doubt, the bottom configuration on the 5th o f January 1995 (date o f the SAR run) is likely to be influenced by this storm event... C O N C LU SIO N S: -The quality o f the runs may differ the one from the other; -For referencing or calibrating the BAS-data, we do NOT know how many o f the classical records o f HD have been taken into account by Argoss B; -Apart from a certain offset the more smoothed deeper areas o f the BAS-measurements are comparable with the HD-data; -For some depth-levels the offsets have NOT always the same sign: this is very strange; -For the more complicated areas - even the deeper ones - BAS provides, compared to the classical records, less or no details at all; -Perhaps the value o f the run o f the 5th o f January 1995 is questionable, due to the foregoing storm event: in other words, these data are perhaps not representative enough for a common standard o f comparison, due to temporal bottom changes? -By using SAR from satellite pictures, the actual pixel-size is rather big (25 m); a trial with a high resolution radar is, in our opinion, worthwhile to carry out at a later stage. Oostende, 10/02/1999 Ing. Johan erstraeten Ing. Carlos an Cauwenberghe BABEL11.DOC

4 D i e p t e k a a r t d e p t l i m a p o t r u n r i i n - i j a n 9 5 r v d 2 m LLWS B ARGOSS B.. BAS - SUREY run 4 JANUARI 1995 KWINTE-en MIDDELKERKEBANK with surroundings scale 1/ ARGOSS B.. BAS-OPNAME run 4 JANUARI 1995 KWINTE-en MIDDELKERKEBANK en omgeving schaal 1/ ANNEX 1

5 AFDELING WATERWEGEN KUST - HYDROGRAFIE KWINTE-en MIDDELKERKEBANK with surroundings extract chart D11 - LAAMSE BANKEN scale 1/ AFDELING WATERWEGEN KUST - HYDROGRAFIE KWINTE-en MIDDELKERKEBANK en omgeving uitreksel kaart D11 - LAAMSE BANKEN schaal 1/ ANNEX 2

6 Belangrijke opmerkingen: Echoloding : uitgevoerd met een Atlas Deso 20 met frekwenties 30 khz en 210 khz (oorzien van een deiningscompensator Hippy 120) Bebakening : bijgewerkt t.e.m. BAZ. nr. 19 Editing en kartering uitgevoerd in de Hydrografische Dienst der Kust te Oostende Reductie: M2-methode (reductiekaart prog. T1JDUC) O p g e m a a k t i n h e t D a t a c e n t e r H y d r o g r a f i s c h e D i e n s t d e r K u s t O O S T E N D E d e 2 4 / 1 0 /

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