Giuseppe Gentili 
  
APPLICATION OF LASERSCAN TO FLOOD MAPPING OF AN URBAN STREAM 
A Cavazzini., G Gentili, E. Giusti, Compagnia Generale Ripreseaeree, (CGR), Parma, Italy 
Cgrit@tin.it 
Key words: Laserscan, Floods, Hydrology, Fluvial DEM, Disaster planning, Natural hazards 
ABSTRACT 
The advent of air-borne laserscan systems capable of recording land elevation with a precision of 15 cm makes 
it possible to measure rivers cross sectional area with such accuracy as to calculate flood levels of given return periods 
by hydraulic formulas. The sequence of steps for the preparation of flood maps is as follows: 
1) Estimation of a design flood discharge such as the 100 years at a given point on a river. 
2) Calculation of the height of the 100 year flood at a given section of a river (point 1) from hydraulic formulas 
such as Manning s. 
3) Application of step backwater techniques to calculate flood heights at upstream sections starting with (2) 
and including adjustments for constricted openings like bridges. 
4) Preparation of flood inundation maps of river stretches by connecting points determined by (3) 
The applicability of some of these techniques were evaluated for the Parma River within its course through the 
homonymous city taking an all purpose laserscan survey over the city as the data base. The 100 year flood discharge 
was extrapolated graphically from a Gumbel distribution plot of a 22 years record of average daily discharge available 
at the site and 6 corresponding values of maximum instantaneous floods. Flood heights were computed by trial and 
error application of the Manning formula using the river sectional data from the laserscan survey. The final simulated 
worst case condition for the 100 year flood indicates that the city can be inundated from overtopping of the river s 
banks at various points if even a small amount of clogging of bridge openings takes place. 
The conclusion about the system is that it can be very useful in flood studies in that it can reduce the field work 
to a minimum where stream s slopes are well defined. Where the low flow is limited to a small portion of the within- 
banks cross section the laserscan data can be used as-is with little field work to assess roughness coefficients. In larger 
rivers with flat slopes, where the water depth is more than a few decimeters and it occupies most of its cross section 
there is the need to measure some wetted cross sections within the length of the reach under study. Where river 
constrictions occur ( culverts, bridges etc.) the geometry of the structures must be assessed in the field. 
INTRODUCTION 
The problem of mapping maximum flood levels or flood levels with a given probability of occurrence, for 
example the 100 year flood , is of primary importance in regional planning especially in defining zoning areas, thus 
places subject to urban development. Of similar importance is the mapping of maximum water surface elevations 
downstream from dams in the event of a dam breach or, in general the application of hydrologic and hydraulic 
modelling to forecast the propagation of a flood wave through a river network. These applications are rare because of 
the great expense of surveying cross sections in the field by standard surveying methods. The recent advent on the 
market of systems that record directly the altimetry of the terrain with a decimetric precision makes it possible to 
straight forwardly apply hydrologic and hydraulic techniques to reconstruct and eventually map expected water levels. 
The paper gives an example applied to an urban stream. 
THEORETICAL ASPECTS 
The height of the water along a stretch of a river during the passage of a flood wave is a function of the 
discharge and the morphology of the cross sections of the river. If the flow is free, not restricted by the accumulation of 
debris (trees) on the river bed and if the river is not undergoing erosion the flow is considered uniform. In this condition 
the hydraulic gradient, the surface water gradient, and the stream bed gradient are parallel and the cross sectional area, 
hydraulic radius and water depth are constant; and Manning s equation, one of the many practical variations of Chezy s 
equation, is applicable (Dalrymple and Benson, 1967). The equation, written in terms of discharge is given by: 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 335