Environmental Engineering Reference
In-Depth Information
Fig. 13.6 Pond connectivity calculated from a digital elevation model (DEM) containing small (a) and large
(b) depressions.
directly to an adjacent pond or will flow along a
preferential pathway until it reaches another de-
pression or an inlet to the sewer network. The
connectivity algorithm used in FRMRC was de-
veloped by AUDACIOUS (2005), and utilized the
'rolling ball' technique to identify surface path-
ways and the flow between adjacent ponds. This
technique was subsequently enhanced using a
'sliding ball' technique to cope with the areas in
which 'rolling ball' comes to rest ('gets stuck'). The
analysis commences at the exit point of the up-
stream pond and identifies the preferential flood
pathway, based on terrain slope and the presence
of buildings and other features of the urban catch-
ment surface.
surface pathways). The process is illustrated in
Figure 13.7.
The process was first developed by Prodanovic
(1998) and the procedure takes into account the
variability of flow angles over different types of
cover (fabric or canopy), and the presence of arti-
ficial or man-made objects (streets, buildings).
Below a selected threshold the slope of the terrain
was considered horizontal.
Subcatchment delineation for sewered areas
and definition of undrained areas
Subcatchment delineation is also used to identify
the contributing area for each pipe in the sewer
network. Figure 13.8 illustrates the methodology
based on the use of the links (and not the nodes) to
delineate the surface. Figure 13.8 also highlights
that some areas, termed 'undrained areas', are not
directly linked to any pipe's inlet. For small storms
these areas may be considered to be unimportant
as they contribute little runoff, but in extreme
events the runoff from such areas can be signifi-
cant and hence these areas should be designated
'undrained ponds'. An algorithmwas developed in
FRMRC to do this such that it was not possible to
double count any areas common to both a pond
and a subcatchment, as detailed in Figure 13.9.
Automatic subcatchment delineation
Sub-areas that contributed flow to individual
drainage elements are established from the DTM
and are introduced to the model via the nodes and/
or links as appropriate. The sub-areas may change
according to the magnitude of the event; for ex-
ample, there may be additional runoff from per-
meable areas, or an increased depth of flow in the
pathway may result in a new route for the surface
flow, for example the formation of a bifurcation.
The catchment surface is partitioned into smal-
ler areas, using an automatic subcatchment delin-
eation routine. Each subcatchment drains to a
single network node. The procedure is based on
the elevation of the DEM and the land cover and
use, defined by the coordinates of the nodes (man-
holes and/or surface ponds) or links (pipes and/or
Estimation of pathways
and pathway geometry
The DTM is then used to establish suitable shapes
representative of the channel cross-sections that
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