Van Cauwenberghe, C.

English

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Interrelationship between hydrography, sedimentology and modelling .  
by Ing. Carlos VAN CAUWENBERGHE  
Retired Hydrographer of the Coastal Hydrographic Service,  
Oostende, Belgium. 
 
Introduction  
Hydrography may be defined as the science of measuring and depicting the parameters, necessary to describe the 
precise nature and configuration of the seabed, its geographical relationship to landmass and the caracteristics and 
dynamics of the sea 1.  
Those parameters encompass different disciplines such as bathymetry, the knowledge of the horizontal and vertical 
tides, waves, the physical properties of the seawater (mainly temperature and salinity), geology, geophysics.  
The major benefits, gained from the modern hydrographic surveying, are the abilility to compile detailed and quality 
controlled graphic documents (sea charts) and to set up the "ECDIS" (Electronic Chart Display and Information 
Systems); this information is mainly used by the mariners but also by other people, concerned with the marine 
environment, such as hydraulic engineers, oceanographers, marine geologists, sedimentologists, modellists and 
environmental scientists.  
Another important application of the hydrographic know-how, since many years, is its use in the planning of the 
marine resources, i.e. the exploration and the exploitation of the seabottom.  
   
   
EVOLUTION OF HYDROGRAPHIC SURVEYING FROM 1800 TILL NOW  
1. The first very simple but rather reliable hydrographic techniques for the areas of the lower countries (Belgium and 
Holland) started in Napoleon’s time, namely from 1799 till 1811, when the French hydrographer Charles-François 
Beautemps Beaupré (1766-1854) surveyed the coasts of Flanders (Belgium), the river Scheldt and the southern part 
of The Netherlands.  
In those areas for the first time, the positionning was carried out by observing the two angles between three marks 
on land by sextants; the station pointer was used for mapping the intersection of the two angles.  
This sort of positionning provided sea charts, on which only the general shapes of the channels and the banks could 
be recognised.  
At that time, for the depth measurements, the hand lead was still the common practise. If such a lead line or later on, 
for deeper bottoms, a mechanical sounding gear was used, there was quite an interval between consecutive 
soundings, dependant on the personal factor and/or the efficiency of the machine. This means that, as a matter of 
fact, only a limited amount of soundings could be gathered; so, a complicated bottom configuration, such as the 
sandwaves on the sandbanks and in the channels, could not be identified.  
Nevertheless, from this time on (the beginning of the 19th century), these data already could be used for the 
monitoring of the general shapes of the channels and the sandbanks 2 and 3, not only in the mouth of the river Scheldt 
but also for the area of the so called Flemish Banks. By doing this, the stringent condition was that the available 
charts had to be converted to the same horizontal and vertical standards (i.e. the European Datum 50 as projection 
system and local mean lowlowwaterspring as chart datum).  
The Belgian engineer Ir. P. De Mey was in fact the first explorer, who used some older charts for making 
comparisons (but without introducing any sort of correction on the horizontal and vertical standards) 4.  
Many years later, the Dutch engineer J. van Veen did the same, but he extended the area of the Flemish banks more 
westwards to Dover Street; at the same time, he recognised the typical patterns of the ebb- and flood channels in the 
area of the Flemish Banks 5.  
2. As, some years before Worldwar II, the first versions of the single beamed echosounder became operational, the 
same van Veen examined - for the first time - the sandwave structures of some banks in the Dover Street and the 
southern North Sea 5.  
He tried to explain the general shape and the direction of these features, but, as the information of an echosounder is 
limited to one single track, no doubt, it was very difficult for him to extend his study to some other areas and to 
make more detailed conclusions concerning these configurations.  
Positionning by sextant angles was only possible to approximately 12 km from the coast line; for the areas beyond 
that distance, buoys were used and, after World War II, the Decca Navigator, as the first radio positionning system 
for shipping and surveying.  
As the last system was not very adequate for hydrographic suveying, in the late sixties, positionning systems, more 
adapted for the needs of hydrography, became operational.  
In this manner, the accuracies on the planimetric coordinates considerably increased, mainly for the remote areas 
and fortunately the dependence on the terrestrial visibility was no longer existent.  
The use of the side scan sonar in the seventies was also, among other applications, a big improvement for the 
determination of the direction of the sandwave crests.  
Also in the early sixties the existence of moving crests of sandwaves in the deep draft routes, situated in the southern 
part of the North Sea, was a big concern for the hydrographers of the national hydrographic services of France, UK 
and The Netherlands.  
Therefore, from that time on, for the safety of navigation, some critical areas were resurveyed periodically and the 
tasks were shared by the three hydrographic services concerned.  
This sort of international cooperation was adopted on the first meeting of the North Sea Hydrographic Commission 
(NSHC) at The Hague (The Netherlands) in October 1962.  
Later, on the 15th Meeting of the NSHC at Antwerp (Belgium) in September 1984, the periodical monitoring of the 
so called boxes in the Trafic Separation area at the southern part of the North Sea was adopted, which led to the Four 
Nations Agreement of 1984 (meanwhile Belgium was also involved). These check surveys delineated areas of 
national responsability, having survey intervals, varying from 1 to 5 years.  
In 1998, the Agreement of 1984 was subject to a revision; France, on a meeting of the 4 nations involved, presented 
a report (in French), where, for 4 boxes in Dover Strait and for 4 boxes at the southern entrance of the North Sea, 
some important movements of sand dunes were identified during the period from 1984/1985 till 1990/1992.  
3. For the time being, the single beam measurements with a hydrographic echosounder and with a side scan sonar 
provide still always valuable records. The earlier shore-based positionning systems are now replaced by the allround 
- code based - Differential Global Positionning System(DGPS), increasing considerably the reliability and the 
accuracy of the hydrographic surveying.  
Further, with the venue of the multi-beam systems, it is possible - specially in complicated areas- to make evidence 
of all features, such as the sand dunes and even of the smaller hazards on the sea floor.  
The "oldies" in hydrography must emphasize also the ever growing possibilities of the computer techniques, which 
allow the hydrographers to store big quantities of data and to process and represent them as well.  
The Belgian Coastal Hydrographic Service, Oostende, participated in the EC programme STARFISH ("Sediment 
transport and bedform mobility in a sandy shelf environment"), as a part of MAST II, which was set up to run over 
more than 3 years (from January 1993 to the end of April 1996).  
Part of Task 1 of this project dealed with "the chronosequential multibeam echosounding by vessel: recording and 
processing". The overall task was related to the assessment of the volumetric and the morphological evolution trends 
of the Middelkerke Bank; this is a large sand bank, off the Belgian coast, belonging to the area of the Flemish Bank 
complex.  
As it was practically impossible to resurvey the entire bank at the desired frequency of at least 3 times a year, two 
smaller areas, R2 and R4, were chosen as best reflecting the behaviour of local bedforms.  
These two areas are not the shallowest parts of the bank, but do contain a number of identifiable sandwaves.  
The multi-beam sounding tracks were spaced 40 to 50 metres, depending on the local depths.  
Nevertheless, during the summer and autumn of 1995, the Coastal Hydrographic Service was able to survey the 
whole of the Middelkerke Bank simultaneously by means of the common vertical echosouder and by the EM 950 
swath sounding system of KONGSBERG SIMRAD.  
The details of this study are given in the References 7 and 8; the general conclusions were:  
• the surface layers of the shallower R2 box only were found shifted with some 20 metres after a stormy 
period, but, later on, this shift reversed gradually towards its original situation;  
• the deeper R4 box gave evidence of a large stability, mainly in the tranversal direction of the bank;  
• once more, the high degree of an overall stability in the Flemish Bank area, found by the earlier 
comparisons of the older hydrographic maps 3, has been identified;  
• however, later re-surveys of the 2 boxes on suitable occasions took place and the continuation of those for 
the future is also highly desirable.  
4. Finally, as I already tried in an earlier paper 9, we could venture upon predictions about possible future 
developments in the hydrographic surveying.  
First, I think that very soon the DGPS, based on OTF-techniques, will come into common use in hydrography; in 
this manner, the necessary tidal corrections and the attitude parameters of the survey vessel (roll, heave and pitch) 
may be determined at the same time.  
Further, considering that I am NOT a sailor myself and probably, as a matter of fact, not influenced, biased or 
prejudiced by any sea navigational concerns or feelings, I think that hydrographic surveying by ships or similar 
vehicles remains a - sea-state dependent - business; it is also a time consuming process for the data collection, due to 
the lower speeds of the survey vessels.  
Sea surveying from planes and helicopters with laser technology is already a quick and adequate possibility in clear 
coastal waters.  
Satellite derived data may provide a cheap (less time consuming) method for mapping large near-shore areas. The 
imagery from satellites can be either passive (optical or micowave) or active (radar) 10. In the last system, the return 
radar pulse provides us with sea surface details, such as surface roughness and current information. These data are 
influenced by the bathymetry in shallow waters, so that, by tide en current modelling, they can be used to derive the 
sea bed topography.  
For our less transparant coastal waters, in the lower countries, we already did some trials with the last remote 
sensing techniques  
A contract with the European Space Agency (ESA) allowed the three contrac-tors (Argoss BV, Vollenhove, The 
Netherlands – the University of Ghent, Belgium - the Coastal Hydrographic Service, Oostende, Belgium) to make a 
- one year - study of the Kwinte- and the Middelkerke Bank (for 1998). Here, the Dutch spin-off firm Argoss BV 
was the manufacturer of the system.  
For this study, the information, coming from the ERS II-satellites, was taken into account, providing pixel-sizes of 
25x25 metres; basicly airborne data can also be used, where pixel-sizes of 3x3 metres can be obtained 11.  
So far, the results were satisfactory as far as only small-scale comparisons between the BAS records and the CARIS 
electronic chart-files of the concerned area occur. By comparing mid-scaled (e.g. 1/20.000)- and large-scaled charts 
(1/5.000) sometimes the differences are still too high; the histogram of the frequency distribution (where 6893 
values were taken int consideration) indicated also skewness to the right. This means that the frequencies of the 
differences of +1 dm and more (i.e. the conventionnal measurements being deeper than the BAS ones) are much 
higher and also more spreaded over the higher positive differences than at the other side; a further detailed 
examination gave evidence that, in the shallower part (areas, less than 10 metres), the differences were the highest; 
here also the spreading of the positive differences was bigger, which means that the system is less accurate on the 
shoals...  
An explanation for this phenomenon of the skewness cannot be given.  
These first, perhaps unsatisfactory, results do not mean the end of further endeavours, because the continuation of 
the system is assured with the start of a new ESA contract.  
Perhaps in a not to far future, better results with a modified system, perhaps airborne installed, could provide us with 
better results, allowing to reduce considerably the need for the classical sea surveying.  
   
   
CONCLUSIONS  
The complicated configurations of the sea bottom worry the hydrographers since quite a lot of time.  
Their periodic hydrographic activities in the channels, covered with sand dunes, mainly were and are still always 
intended for the safety of navigation.  
A further concern is coastal management (coastal protection, harbour and dredging works, etc;) sedimentological 
knowledge of the movements of the channels and banks is also needed; this knowledge can only can be enhanced by 
obtaining more detailed and reliable bathymetric information.  
As the standards of precision increased, specially during the last 10 years, this information is also more and more 
applicable for scientific sedimentological studies and morphological modelling.  
We explained that the use of ships is time consuming and thus expensive and that in the future – let us hope - still 
more complicated and reliable systems, will reduce or eliminate this usage of ships, providing us, at least, the same 
details in a shorter period of time.  
This possibility would also be interesting for a faster monitoring of the situation in critical or – scientificly 
interesting – areas.  
In ancient times, the Greeks made the following statement: "παντα ρει " or "everything changes as the time is 
passing by".  
Let us hope this proverb also applies to the future positive developments in hydrography!  
   
   
   
   
REFERENCES:  
1. United Nations, New York. Meeting of the Group of Experts on Hydrographic Surveying and Nautical 
Charting. December 1977.  
2. Van Cauwenberghe C. Hydrografische analyse van de Scheldemonding ten oosten van de meridiaan 3°05’ 
tot Vlissingen. Het Ingenieursblad, Jaargang 35, Nr.17, 1966.  
3. Van Cauwenberghe C. Hydrografische analyse van de Vlaamse Banken langs de Belgische kust. Het 
Ingenieursblad, 40e Jaargang 35, Nr.19, 1971.  
4. De Mey P. Etude sur l’amélioration et l’entretien des ports en plage de sable et sur le régime de la côte de 
Belgique. Paris 1894.  
5. Van Veen J. Onderzoekingen in de Hoofden in verband met de gesteldheid der Nederlandse kust. Uitgave 
van het Ministerie van Rijkswaterstaat, Algemene Landsdrukkerij, ’s Gravenhage, 1936.  
6. Service Hydrographique et Océanographique de la Marine (Garlan, Guyomard et Bouroullec). Etude 
sédimentologique des zones évolutives (Studies on southern North Sea dunes movements). Presentation on 
the 2nd BE/FR/NL/UK Expert Meeting in Taunton (UK). September 1998.  
7. Van Cauwenberghe C. Contract No. MAST II-CT92-0029. Task 1-Assessment of the volumetric and the 
morphological evolution trends on the Middelkerke Bank area - Subtask 1.2. Chronosequential multibeam 
echosounding by vessel: recording and processing. 1996.  
8. Van Cauwenberghe C. Multibeam Echosounder Technology: a Time Series Analysis of Sandbank 
Movement. The Hydrographic Journal, No.81, July 1996.  
9. Van Cauwenberghe C. "As it is". Hydro International, Volume 3, Number 4, April 1999.  
10. Shannon J. The Sea from Space–Practical applications of satellite derived data. Hydro International, 
Volume 3, Number 2, March 1999.  
11. Wensink G.J., Hesselmans G.H.F.M., Calcoen C.J. and Vogelzang. The Bathymetry Assessment 
System(BAS). Proceedings of Hydro ’96–Tenth Biennal International Symposium of The Hydrographic 
Society-Session II: New Technology. Rotterdam (The Netherlands). September 1996.  
12.Van Cauwenberghe C. Babel report – Contribution of the Coastal Hydrographic Service. December 1998.  
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Interrelationship between hydrography, sedimentology and modelling

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Interrelationship between hydrography, sedimentology and modelling

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