1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Flood Inundation Modelling Using LiDAR and SAR Data 85
considerable value as they potentially allow split sample validation using various combinations
of extent and hydrometric data. However, the images are still only single ‘snapshots’
of inundation extent from separate events and thus still do not properly capture dynamic
flooding processes over the course of a flood. It should also be noted that, with the exception
of the Thames event, all other floods for which data have so far been identified are of high
recurrence interval. This is actually a problem when using such data for model validation,
as the flood tends to completely fill the valley floor. The flood boundary therefore lies on
relatively high slopes and large changes in water level are required to effect a detectable
(1 pixel) change in flood boundary position. The lower the image resolution, the more
significant this problem becomes.
No class of instrument used for flood inundation monitoring is thus problem-free, and
data availability is the major constraint on validation studies, regardless of the system
employed. We do not yet possess a benchmark dataset capable of convincing validation/falsification
of the dynamic behaviour of current flood inundation models. Perhaps
more fundamentally, we have not yet even observed the development of flood for a real
river reach in a systematic manner, and thus are unsure about the basic physical mechanisms
and energy losses that we need to represent. Nevertheless, in the last few years,
available inundation data have begun to be used in the model validation process and important
preliminary conclusions can be drawn from these studies. These are explored in
section 5.3.
Flow Velocity and Free Surface Elevation. Present methods of determining water
velocities rely on making in-situ point measurements, which are necessarily limited in
spatial extent and can be dangerous to undertake. A resolution to this problem is potentially
provided by Microwave Doppler radar which offers a remote means of measuring
water-surface velocities based on the Bragg scattering from short waves produced
by turbulence. When the transmitted radar signal is scattered from a rough water surface,
a Doppler frequency shift is produced by the centimetre-length surface waves that
backscatter. The magnitude of the shift is a function of the stream current. The principle
has been used to map surface velocities of coastal currents for a number of years
(e.g. Hwang et al., 2000), and has recently been applied to the measurement of the spatial
distribution of river currents using a radar mounted on a van parked next to a river
(Costa et al., 2000). By aiming the radar across the river in two directions about 30 degrees
apart, one looking upstream and the other downstream, Costa et al. found that the difference
in the Doppler shifts of the two return signals gave the downstream velocity
component.
Experiments are also now being conducted on the use of along-track airborne radar interferometry
for measuring water-surface velocities (Srokosz, 1997). These employ aircraft
having microwave coherent real-aperture radar with two antennae aimed a few degrees
apart in the along-track direction. If the two radars are arranged to look across- rather
than along-track, it is possible to perform across-track interferometry instead, allowing a
map of water-surface elevations to be constructed remotely. Such studies are still in the
research stage and a useable technique is still some distance away. Nevertheless, the model
validation potential of such surface current measurements is considerable and deserves
further exploration.