1 Spatial Modelling of the Terrestrial Environment - Georeferencial

86 Spatial Modelling of the Terrestrial Environment
5.2.2 Topography
Traditionally, hydraulic models have been parameterized using ground survey of crosssections
perpendicular to the channel at spacings of between 100 and 1000 m. Such data
are accurate to within a few millimetres in both horizontal and vertical levels and integrate
well with one-dimensional hydraulic models such as HEC-RAS and ISIS. These schemes
calculate flows between such cross-sections and remain the industry-standard for flood
routing and flood extent modelling studies. However, ground survey data are expensive and
time-consuming to collect and of relatively low spatial resolution. Considerable potential
therefore exists for more automated, broad-area mapping of topography from satellite and,
more importantly, airborne platforms. Two such techniques are airborne laser altimetry
(LiDAR) and airborne stereo-photogrammetry. These are now regarded as reasonably standard
methods for determining topography at the river reach scale. Of these, LiDAR has the
advantage of higher survey rates and lower cost, but with comparable accuracy to airborne
stereo-photogrammetry (Gomes Pereira and Wicherson, 1999). Accuracy is, as one would
expect, much lower than for ground survey, with rms errors for LiDAR data quoted as being
of the order of 15 cm in the vertical and 40 cm in the horizontal. The resulting datasets
are dense (typically 1 point per 4 m 2 with a 5 KHz instrument flown at 800 m) and can be
collected at sampling rates of up to 50 km 2 per hour. Initial problems with the operational
use of such systems (ellipsoid conversion, vegetation removal, etc.) have been for the most
part resolved and, in the UK at least, a national data collection programme is now producing
large amounts of high quality data. Processing techniques to extract topography from
LiDAR returns are discussed further in section 5.2.3 and the integration of LiDAR data
with hydraulic models in section 5.3.
5.2.3 Friction
Friction is usually the only unconstrained parameter in a hydraulic model. Two- and threedimensional
codes which use a zero equation turbulence closure may additionally require
specification of an ‘eddy viscosity’ parameter which describes the transport of momentum
within the flow by turbulent dispersion, however, this prerequisite disappears for most
higher-order turbulence models of practical interest. Hydraulic resistance is a lumped
term that represents the sum of a number of effects: skin friction, form drag and the
impact of acceleration and deceleration of the flow. These combine to give an overall drag
force C d , that in hydraulics is usually expressed in terms of resistance coefficients such as
Manning’s n and Chezy’s C and which are derived from uniform flow theory. The precise
effects represented by the friction coefficient for a particular model depend on the model’s
dimensionality and resolution. Thus, the extent to which form drag is represented depends
on how the channel cross-sectional shape, meanders and long profile are incorporated into
the model discretization. For example, a one-dimensional code will not include frictional
losses due to channel meandering in the same way as a two-dimensional code. In the onedimensional
code these frictional losses need to be incorporated into the hydraulic resistance
term. Similarly, a high-resolution discretization will explicitly represent a greater proportion
of the form drag component than a low-resolution discretization using the same model.
Hence, complex questions of scaling and dimensionality arise which may be somewhat