Geoscience Reference
In-Depth Information
Fig. 4 SFD concept
(Boonklong et al.
2007
)
algorithm is able to approximate the flow directions on a topographic surface, the
process of tracking ''flow'' from each pixel to one of its eight neighbour pixels
(Rivix
2008
). D8 algorithm uses eight nearest neighbours of a particular pixel to
determine which direction the water will flow from its current location or pixel to
its neighbour pixel until it reaches the final destination such as a river.
Another popular algorithm in SFD model is Rho8 which was introduced by
Fairfield and Leymarie (
1991
) and work only with DEM surface model (Lindsay
2012
). The approach is quite similar to D8 algorithm, but it will correct the
removed pixel caused by all flat areas (a stagnant state) and spurious depressions
(Lindsay
2012
). While, the grid cells that have no lower neighbours (a sink state)
are assigned a flow direction of zero.
2.2 Multiple Flow Direction
On the other hand, the Multiple Flow Direction (MFD) model allows the algorithm
result to have multiple flow directions for a certain cell, most commonly to all
downslope neighbours (Laura
2001
). The simplest MFD concept and model is
illustrated in Fig.
5
. There is a disadvantage of MFD model which is the flow from
a cell will be dispersed to all neighbours of lower elevation, resulting more diffuse
flow of water (Laura
2001
).
One of the famous algorithms of MFD is D-infinity (D?) that is widely used in
advance water flow analysis with slope element as the main factor of study or
working field. D? uses a range of 45 from each neighbour's pixel origin to the
next direction using 4 9 4 pixel window as illustrated in Fig.
6
. D-Infinity algo-
rithm is also capable handling all of the ambiguous situations that can occur in real
topography (sometimes resorting to the D8 method) while many other MFD
methods cannot provide this solution. Examples of such an application are the
landslide accident and other slope applications and models.
2.3 SFD: D8 Algorithm
The D8 algorithm uses 3 9 3 pixel windows as illustrated in Fig.
7
. The angle of
each direction to the next direction is 45 degree (45) and thus the flow direction is
limited to only eight options. Although theoretically the water can flow in 360,
