Environmental Engineering Reference
In-Depth Information
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Figure 11.1 Ninety metre spatial resolution DEM for the region around Mount Everest (From Shuttle Radar Topography Mission,
NASA).
which fall either side of the vector (the two nearest cardinal
directions in the case of DEMON and the cardinal and
diagonal in the case of D-
to calculate erosion and landsliding in many other models.
Upslope area is commonly used for mapping channels on
the basis of threshold upslope areas for channel initiation.
Channels are usually estimated on the basis of either a
constant threshold upslope area for channel initiation
(O'Callaghan and Mark, 1984; Tarboton et al ., 1992)
or a slope-dependent one (Dietrich et al ., 1993). The
threshold area can usually be obtained from a log-log plot
of upslope area versus local slope (Tarboton et al ., 1992).
Vogt et al . (2002) extend this concept to include the
geological, soil, topographic and climatic characteristics
that can affect the upslope area and local slope required
for channel initiation.
Figure 11.4 shows a set of cumulative frequency distri-
butions of area for subcatchments calculated using these
different algorithms for a random 65 535 cells of the Tam-
bito DEM. The MF and the D-
). Multiple flow-direction
algorithms tend to produce flow dispersion whilst single
flow-direction algorithms lead to flow concentration. As
the occurrence of flow dispersion and concentration is
a function of dynamic characteristics of the flow and
surface, all of these static approaches to flow routing will
therefore contain some element of inaccuracy irrespective
of the DEM used.
Different algorithms will produce different definitions
of a hydrological catchment and flow topology and thus of
many hydrologically important topographic indices such
as the upslope area, A , and the specific catchment area, a ,
of a point. Upslope area is the total catchment area above
a point or length of contour and the specific catchment
area is the upslope area per width of contour or cell size,
L (Moore et al ., 1991). Specific catchment area is used to
calculate saturation and saturation excess overland flow
in hydrological models such as TOPMODEL (Beven and
Kirkby, 1979) and, along with other topographic indices,
methods tend to produce
a peak of subcatchments in the range 1500-5100 m 2
whereas the kinematic algorithm produces a sharp peak
of sub-basins sized 1200 m 2 and the DEMON algorithm a
broader peak centred at 2100 m 2 . D8 and rho-8 methods
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