ecological infrastructure and nature conservation. With respect to 
the latter the emphasis is shifting from rigid maintaining of the 
present situation to allowing dynamic processes to take place and 
even to actively restore dynamic processes in the area. Through 
integral management one tries to involve and balance the various 
interest. groups and co-ordinate their activities. Up to date 
information is required to effectively carry out the managerial task 
concerned. 
3.2 A new bathymeric surveying method 
The traditional technique to measure sea bottom morphology, 
using in-situ shipborne echo sounders, is expensive and time 
consuming. Based on nearly 10 years of fundamental and 
application research an innovative method has been developed 
using radar satellite images, hydro-dynamical models and a 
limited set of in-situ depth measurements to assess a bottom depth 
chart (Vogelzang et al., 1994). Under favourable meteorological 
and hydrodynamic conditions (moderate winds of 3 to 5 m/s and 
tidal currents of about 0.5 m/s), airborne or spaceborne Synthetic 
Aperture Radar (SAR) imagery shows features of the bottom 
topography of shallow seas to a maximum depth of approximately 
30 m. The imaging mechanism of mapping sea bottom 
topography by imaging radar consists of three stages (e.g. 
Calkoen et al., 1993): 
- Interaction between (tidal) flow and bottom topography 
results in modulations in the (surface) flow velocity. This 
relation can be described by several models with an 
increasing level of complexity (continuity equation, shallow 
water equations, and the Navier Stokes equations). 
-  Modulations in surface flow velocity cause variations in the 
surface wave spectrum. This is modelled with the help of the 
action balance equation, using a source term to simulate the 
restoring forces of wind input and wave breaking. 
- Variations in the surface wave spectrum cause modulations 
in the level of radar backscatter. To compute the backscatter 
variations a simple Bragg model can be used. 
  
  
Based on the above three physical mechanisms, a suite of models 
has been developed and operationalized. This suite of numerical 
models generates the radar backscatter given the bathymetry and 
the wind. Because the parameter of interest is the bottom depth it 
is necessary to invert this depth-radar backscatter relation. 
Therefore, a data assimilation scheme has been developed, 
minimizing the difference between the calculated and the 
measured radar backscatter by adjusting the bottom topography 
(figure 2). 
In 1995 a study has been performed to test the method on 
accuracy, reliability, cost-effectiveness in a "pseudo" operational 
situation. Test sites with various specific morphologic features, 
such as shipping lanes, intertidal flats and shallow water sand 
waves, were chosen in the Wadden Sea area and in the 
Netherlands foreshore area near Rotterdam harbour. The most 
important results are : 
-  Multi-temporal analysis of SAR imagery improves the 
accuracy of depth assessments, up to a level comparable to 
that of traditional methods, i.e. 30 cm depth accuracy for 
monitoring surveys; 
- The use of radar remote sensing techniques may reduce 
traditional monitoring survey efforts with a factor of 
approximately 50 %. 
Developing and implementing a method like this as an 
operational production process is a time- and money consuming 
business. The costs of fundamental and application research are 
estimated to be 1.5 million Dutch guilders, while the system 
development and implementation costs approach nearly 3 million 
guilders in 5 years time. So, the total investment on research and 
development approaches 5 million guilders over a period of 15 
years (1987-2001). When fully operational this system will 
reduce the government expenses on monitoring surveys from 2 
million to 1 million a year, therefore a yearly cost-reduction of 1 
million is obtained. It is expected that the return on investment of 
this system will be reached after 5-7 years. 
  
HENITIAL BOTTOM DEPTH 
   
  
    
    
  
    
    
  
  
FLOW MODEL 
SHORT WAVE MODEL 
ADJUSTED BOTTOM 
DEPTH 
  
  
  
    
RADAR BACKSCATTER 
MODEL 
  
    
    
    
    
  
SIMULATED RADAR 
IMAGE 
DATA-ASSIMILATIE 
  
  
  
  
   
ECHO-SOUNDINGS 
A 
  
Figure 2: Outline of remote sensing based bathymetric assesment system 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B6. Vienna 1996 
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