Environmental Engineering Reference
In-Depth Information
vulnerable areas are usually located in local de-
pressions (ponds) but they may also occur within
pathways. The search algorithm examines the
elevation data to extract the lowest points in the
DEM, and these are flagged as potential ponds.
Based on theDEM, the pond boundary for each low
point is delineated and the natural exit point is
identified by a review of cell elevation, as detailed
in Figure 13.5. The exit point represents the up-
stream end of the overland flow pathway and the
hydraulic characteristics of the exit point are used
to determine the discharge capacity of the outlet
from the pond.
and when the sewer network is surcharged water
canflowboth into and out of the sewer network, as
a function of the level of surcharge.
In FRMRC the surface runoff was routed by
using the full dynamic St-Venant equations. This
had the advantage that once the model had been
calibrated, any changes in land use or physical
characteristics could be represented by changing
the relevant input files without compromising
modelling integrity.
The following steps were undertaken to
develop the surface flow components, after
Boonya-aroonnet et al. (2007).
Identification of ponds and
flood-vulnerable areas
Connectivity analysis
The DEM, which also includes urban (man-made)
features such as streets and buildings, was then
used to identify the surface pathways for overland
flow. These pathways connect the previously
identified ponds in order to form a 'surface flow
network'. The upstreamflowpaths start at the exit
point(s) of the ponds or issue onto the catchment
surface from the below-ground system at man-
holes, whilst the downstream end of the pathway
was defined as the entry to another pond, the flow
entry into the sewer via the manhole or as an
outflow from the catchment. As detailed in
Figure 13.6, overland flow may be considered to
accumulate in depressions, and once the top level
of depression is reached, it will either overflow
In FRMRC the DEM raster image was used to
analyse flood vulnerability and the entire dynamic
processes. The enhanced DTMwas firstly used to
identify the location of the depressions or ponds.
For each pond the stage, depth, volume and surface
area relationships were established. Ponds may be
isolated or mutually connected. When the ponds
are full they overflow at a location termed the exit
point (or points) and the spilled flow enters a
preferential flow path on the catchment surface.
Frequently ponds are nested within larger ponds;
for example, at shallow depths there may be two
isolated ponds but at larger depths these are mu-
tually connected to form one large pond. Flood-
Pond boundary
Pond boundary
6.0
6.0 7.0 6.0
7.0
6.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
7.0
7.0 5.0 8.0
5.0
8.0
10.0
10.0
10.0 10.0
10.0 10.0
10.0 10.0
10.0 10.0
10.0
10.0
9.0
9.0 8.0 8.0
8.0
8.0
10.0
10.0
10.0
10.0
10.0
10.0
6.0
6.0
7.0
7.0
6.0
6.0
10.0
10.0
Outputs:
￿ Volume
￿ Area
￿ Exit point
￿ Elevation
10.0
10.0
7.0
7.0
5.0
5.0
8.0
8.0
10.0
10.0
10.0
10.0
9.0
9.0
8.0
8.0
8.0
8.0
10.0
10.0
9.9
9.9
10.0
10.0
Fig. 13.5 The pond delineation
(numbers in cells are elevations).
After Boonya-aroonnet
et al. (2007).
10.0 10.0 10.0 10.0 10.0
10.0 10.0 10.0 10.0 10.0
10.0
10.0
Natural exit point
Natural exit point
Search WWH ::




Custom Search