Giuseppe Gentili 
  
which may not be acceptable in some cases. According to the TopoSys Company (oral communication, 2000) the 
average accuracy in the vertical of plus and minus 15 cm can be improved to less than 10 cm under optimal survey 
conditions, i.e. best ground positioning by differential GPS with 4 instruments, low solar activity (night survey), and 
availability of signals from 8 satellites. A laserscan survey to meet these conditions could be scheduled in advance as 
the optimal satellite configuration over an area is known . This would entail, however, higher costs which may 
condition the economic feasibility of the survey. Admittedly the stretch of river chosen here is poor for studying the 
applicability of laserscan survey for hydraulic modelling of floods. It is an extreme case where man has changed the 
natural conditions of the riverbed not only because of the presence of the bridges but because most bridges have an 
apron at their downstream end which conditions the natural bed slope of the river. The presence of the bridges also 
requires information on the bridge geometry and field surveying of their hydraulic characteristics thus diminishing the 
usefulness of the laserscan method which is that of replacing or minimizing the collection of field data. On the other 
hand, most urban streams have been surveyed at least in the stretch where they cross the city and bridges information is 
available. Indeed the bridge design must have entailed computations of design floods to assess bridge openings and thus 
the information may already exist. Where the usefulness of the laserscan system is apparent for preparing flood 
inundation maps is in the countryside at large where there are no close spaced altimetric data. It is also clear that these 
surveys should be conducted along the river channel so as to span the channel itself and the flood plain and that the 
surveys should be carried out during minimum flow and vegetational cover conditions. 
Flood levels on the Parma river 
Although it is not the purpose of this paper to show hydraulic computational methodology it is instructive to 
complete the discussion by providing a practical example of computing flood levels along the Parma river. 
The first problem is the assessment of the design flood. Fortunately, for the river stretch under study there 
existed a hydrologic measuring station at the downstream bridge with discharge data from 1956 to 1977, thus with 22 
years of record. Hydrographic services in their data compilations publish only the annual average daily floods as 
opposed to the instantaneous values. These data are shown in figure 3 as a frequency plot on Gumbel s extreme value 
logarithmic paper (Tonini, 1959), a distribution widely used which often linearizes series of extreme hydrologic data. 
The line drawn ignores the 2 values of low frequency. This line, however, has little interest for us as the maximum daily 
flood levels originate from peak flows. These are shown by the upper points fitted by a line drawn through all the points 
and by one drawn through the upper 4 points only and parallel to the bottom line. The resulting 100 year peak floods are 
extrapolated , respectively, as 1400 m/s with the steeper line and as 1120 m/s with the flatter line, almost double the 
   
$484 box 
MO 
Flood discharee 
in cubic meters per second 
TIT . 
eM irit a3 tills : es i í : ! 
RE 13 2 3 10 30 90 
Kecurrence interval in vears (inverse of probability) 
Figure 3 Extreme log data plot of the magnitude and frequency of Parma River floods at Parma City. 
Lower data are average daily values; upper data are instantaneous values. 
maximum recorded of 680 m/s. 
The estimated flood values are used with part of the data from the most downstream reach, stations 10 to 7 of 
table 1, to calculate flood levels. In applying Manning s equation the slope was computed by fitting an average line 
through the points rather than by taking actual differences in elevation between successive sections and the value of the 
  
340 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.