Management Applications of Optical Remote Sensing in the Active River Channel - Fluvial Remote Sensing for Science and Management - page 37

(discussed in the first part of the chapter), image avail-
ability and cost; the spatial extent, coverage, scale and
resolution of imagery; the need to collect repeat mea-
surements; logistical constraints; the availability of digital
data; the availability of multispectral data; and accuracy.
In rivers, the potential to protect personnel from danger-
ous field situations can be a major consideration as well.
The tradeoffs among these various factors need to be care-
fully attended to before a project commences: managers
who assume remote sensing will meet their mapping and
monitoring needs may be disappointed and upset.
From a management perspective, logistics and cost
considerations often drive the decision as to whether to
use remote sensing. In turn, the spatial extent of the
study and the number of measurements needed over time
are typically primary factors driving logistics and cost.
Assuming that the variable in question can be mapped
with remote sensing, the general rule is that the larger the
spatial coverage and the greater the number of measure-
ments needed over time, the more attractive the remote
sensing option becomes. For example, consider the com-
mon scenario where information on channel depths is
needed for flood inundation modeling. If the model-
ing involves only one relatively short river reach at one
point in time, surveyors can often measure the neces-
sary cross sections with relative ease in just one day. In
contrast, use of remote sensing would require acquisition
of remote imagery, specialised software, and expert per-
sonnel, making this alternative cost prohibitive. However,
if the intent is to collect cross sections throughout the
entire river, then the cost of remote sensing becomes
increasingly reasonable relative to mobilising field crews
for prolonged periods of time. Even in the case of one
time, local data collection, the remote sensing approach
could be cost effective if existing imagery were available
for the appropriate date and trained personnel were at
hand. The one time application of remote sensing makes
even more sense in large or fast-flowing rivers where field
data collection could place personnel at risk.
Moreover, the preceding example undersells the poten-
tial utility of remote sensing. Field measurements typically
provide point or transect data. In contrast, remote sens-
ing imagery provides continuous measurements of the
entire channel over long distances, assuming the chan-
nel can be seen by the sensor. In the context of flood
inundation analysis, remote sensing does more than just
survey isolated cross-sections as a field crew would; it
also provides continuous depth measurements for the
entire channel at the resolution of the imagery. This
enables more detailed monitoring of depth changes and

more sophisticated modeling, which in turns enhances
the potential to develop more accurate models of flood
inundation.
Furthermore, unlike the field-based cross sections, the
imagery is more versatile in its applications. Once the
imagery has been acquired, depending on its spectral
coverage and spatial resolution, it can potentially be used
to map features such as riparian cover, macrophytes,
sediment size, wood, or biotypes (glides, riffles, etc.). The
digital image format also provides a permanent record
that is useful for validation by independent parties and
allows for the possibility of developing historic change
maps for variables that cannot be detected using present
approaches. If this sounds farfetched, consider that prior
to the year 2000, the only variables that remote sensing
imagery was used for were flood extent, depth, turbidity,
and riffle/pool delineation; this list that is now much
longer (Marcus and Fonstad, 2008).
Yet many of the positive attributes listed above are
hypothetical. Most river managers do not know if they will
need continuous coverage, monitoring of other variables,
or more sophisticated modeling at a later date. What they
do know is that they need a specific measurement now .
Furthermore, techniques for remote sensing of rivers are
still being developed; there are no standard approaches
that have been endorsed by regulatory agencies or incor-
porated into readily available software packages. Finally,
and perhaps most importantly, our personal experience
suggests that projects requiring new image data must
incorporate significant flexibility. In general, one wishes
to acquire imagery for the right place at the right time with
the right specifications (resolution, signal to noise ratio,
etc.). We have considered ourselves lucky on occasions
where we have achieved two of these three objectives.
Because the costs are realised in the present, application
of remote sensing for river management, especially if new
data must be acquired, can be risky business.
Remote sensing becomes a reasonable alternative to
ground-based field surveys when it can monitor or map
the variables of interest and when it can do so on a cost
effective or safer basis than ground-based techniques. The
cost-benefit ratio varies with the factors discussed above
and with the risk averseness of the management agency.
What is certain is that remote sensing will become an
ever more viable option in the future. Increasingly, river
management goals extend beyond the active channel to
include the floodplain and nearby landscapes, so that
multiple users contribute to the purchase of image data.
Moreover, the cost of imagery has generally decreased
over the past decade, the availability of software and