Geography Reference
In-Depth Information
features that humans observe and map in the field. The
bulk of the following section summarises this kind of
work. However, some of the more interesting advances
in remote sensing of rivers will probably result from
abandoning these anthropic constraints and letting the
images 'speak for themselves;' we briefly discuss some
of these prospects in the conclusion of this chapter.
Likewise, many of the most exciting advances in remote
sensing of rivers are coming from devices that do not
use the visible and near-infrared wavelengths with which
people are most familiar. Radar, LiDAR, and thermal
imagery and combinations of these with optical imagery
are opening up a wide range of new remote sensing
applications for river management; the reader should
be sure to review those chapters to understand the full
spectrum of potential applications.
The following summary of optical image applications
highlights recent studies that have achieved the highest
accuracies. Reviews by Mertes (2002) and Gilvear and
Bryant (2003) provide more information on the his-
tory of different remote sensing sensors and applications
in rivers, while Marcus and Fonstad (2008) and Feurer
et al. (2008) focus on reviews of optical imagery only.
Marcus (2012) provides more detail on recent advances
in using passive and active instruments to measure the
hydraulic environment of rivers. In the following discus-
sion of potential applications we provide an overview of
how various techniques work, identify major limitations
specific to those applications, and briefly summarise theo-
retical and practical limitations generic to all applications
(e.g. image quality, resolution, shadow, logistics).
access to the river is limited and potentially dangerous.
Some of the earliest applications of satellite imagery
were for measuring flood extents (Smith, 1997), but
the 17-day repeat cycle of the Landsat satellite did not
allow monitoring of change during one flood event.
Subsequently, the advent of satellites that covered the
same location on a daily basis enabled monitoring of
floods with very coarse (1 km) spatial resolution data in
large rivers (Barton and Bathos, 1989). More recently, 1
to 5-m resolution satellite imagery provides a mapping
tool in smaller streams. In contrast to satellites, however,
aircraft can fly beneath cloud cover and capture flood
events that might be missed by satellite imagery.
Because water is generally visible to the human eye
on imagery, it is possible to trace or manually digitise
the wetted channel extent. Automated approaches for
mapping flow extent and discharge using satellite imagery
are reviewed by Smith (1997). Mapping the extent of water
is typically done with the shortwave infrared bands. Water
is a strong absorber in these wavelengths and therefore
very dark on images compared to other features. It is
our experience that automated approaches with meter
or cm resolution imagery can confuse water with dark
shadow and require manual editing to ensure reasonable
results. At coarser resolutions, however, the shadows are
mixed with other features in the pixel (e.g., trees) and
the spectral signal is distinct from water. The task of
separating water from other features is also simplified
if one has change-over-time imagery. In this case, the
difference between dry and inundated conditions for a
given pixel is sufficiently large to enable change detection
algorithms to accurately map surface inundation using
a variety of sensors, including the Landsat Thematic
Mapper (Kishi et al., 2001), AVHRR (Sheng et al., 2001),
and the EO-1 hyperspectral sensor (Ip et al., 2006). Smith
(1997, p. 1429) notes that there has been little change
since the 1970s in how inundated areas are mapped and
that the approach is 'now considered operational.'
Discharge can be estimated directly from an inundated
area if there is a nearby ground-based gauging station
and a correlation can be established between discharge
and flood extent. The highest accuracy discharge mea-
surements using this approach are achieved if 'gauging
reaches' are identified where the wetted width varies dra-
matically with changes in discharge (Brakenridge et al.,
2005), a response that characterises many braided rivers
but is more difficult to find in single thread channels. To
avoid confusion, it is worth noting that large changes in
width in response to discharge variations is the opposite
of what is considered ideal for ground-based gauging
2.3 Flood extent and discharge
Documenting areas of flood inundation is critical for
emergency response and floodplain planning. Mapping
flood extents also is useful for identifying locations where
efforts to restore or maintain stream habitat and bio-
diversity could be targeted. Finally, even when rivers
are not flooding, repeat mapping of inundated area
provides insight into discharge variability over time as
well as an early warning of flood or drought hazards
in remote regions. The importance of remote sensing
for tracking flow variability and floods is particularly
important in areas without hydrologic monitoring sys-
tems or where access to such data are limited (Brakenridge
et al., 2005).
Aerial photos have long been used to document flood
extent, especially during the peak of major floods when
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