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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004 
In addition, information is required about the vegetation on 
groynes, such as trees and bushes, which dissipate a great dcal 
of flow energy in case of flooding. The number of stems and 
rough height classes (0-3 m, 3-5 m, 5-7 m) are necessary 
information. We did not perform much research on this item 
because of the lack of ground truth data. However, it is obvious 
that the number of trees and bushes and their approximate 
heights can easily be measured manually in the dense laser data 
(15-20 points per m”, see fig. 4), even when the laser data is 
acquired in the leaf free period. Automation of this process is 
probably possible with image processing algorithms. Further 
tests on this topic have to be performed. 
Whereas the dry parts of groynes and the floodplain topography 
can be measured with laser altimetry, the riverbed and the 
below-water parts of groynes are usually measured with echo- 
sounder systems, in our case a multi-beam system. For the 
monitoring of the groyne state as well as for forecasting of 
water levels with hydraulic models, a continuous DEM of the 
whole watercourse between the dykes, thus of riverbed and 
floodplains is required. Fig. 5 shows a combination of both 
datasets: laser and multi-beam data. However, in spite of 
measuring the multi-beam data with high-water level and the 
laser data with low-water level, there still remain some no-data 
gaps which must be interpolated. This must be taken into 
account using the continuous DEM for water level forecasting 
models. 
    
   
y [m] 
| 
425000 - 
| 
| 
| 
42495 
   
424850 .. 
424800 
424700 
T2150 142200 142250 142300 142350 142400 142450 | 
x [m] 
Figure 5. DEM of laser altimetry data in combination with 
multi-beam echo-sounder data at the river Waal. 
Laser altimetry data in combination with digital photographs 
and echo-sounder data can also be useful to illustrate new river 
structures. For the visualization of an innovative groyne type 
an animation has been madc. Figure 6 illustrates some stills of 
this animation: a traditional groyne and two examples of the 
innovative groynes consisting of a row of pales. These 
visualizations are giving a much more realistic impression of 
the future landscape to the citizens and to policy-makers than 
technical line drawings alone. 
5. RIVERBED MORPHOLOGY 
Usually, the riverbed morphology is measured with a multi- 
beam echo-sounder system during high-water level and the 
floodplain with aerial stereo-photographs during low-water 
level. The two datasets are not connected by an overlapping 
zone, but with a no data zone over the groyne fields. Normally 
the groyne fields are mostly covered with water and cannot be 
measured with multi-beam nor with sterco-photogrammetry. 
  
  
  
  
Figure 6. Visualization of traditional groyne (a) and innovative 
groynes (b - c). 
During the summer of 2003 the water level in de main Dutch 
rivers reached the lowest level ever, see fig. 7. On 29" of 
September the minimum water level of 6.91 m NAP at Lobith 
(where the Rhine enters the Netherlands) was measured. The 
normal low-water level at Lobith is between +9 and +10 m 
NAP. During this extreme low water level period the dry groyne 
fields have been measured by laser altimetry with a point 
density of 1 point per m°, see fig. 8, with an Optech scanner 
used by Terralmaging. 
a 
Figure 7. Harbor during extreme low-water level in Waal river. 
Now the whole watercourse DEM (between both dykes) is 
available. In the past the gap between the wet and dry parts was 
much bigger than shown in figure 5. In addition the laser data 
of the dried up floodplains can contribute to the floodplain 
DEM for the normally wet ditches and lakes. However, the 
riverbed morphology, the groyne fields and the floodplain 
undergo big changes due to the water displacements. The laser 
DEM thus is a single moment representation. This 
instantaneous, actual and precise DEM of the watercourse