Environmental Engineering Reference
In-Depth Information
referred to as 'bare earth model', and these show
the ground-level elevation without structures,
buildings and vegetation, whereas the DEM in-
cludes these data (Leitao et al. 2006). HenceDTMs
give a good representation of the contours and the
natural flowpaths of the major system whereas
the DEM may be used to highlight the surface
features - buildings, walls, hedges, trees, etc. -
that will interfere with the natural surface flow-
paths. Hence DEMs have a better definition of any
man-made flowpaths.
Surface Flow Model
As a typical example, let us consider pluvial flood-
ing of the urban surface. This is caused by the fact
that the rainfall runoff on the catchment surface
exceeds the capacity of the below-ground minor
drainage system. This results in an overland flow
that follows the urban surface flood pathways,
which form part of the major system. Typically
these pathways comprise roads and surface
pathways that are linked to natural ground depres-
sions and small watercourses. The approach to
the modelling of overland flow in the urban
environment caused by extreme rainfall was
originally described by Prodanovic (1998) and
Djordjevic (2001), but it is recognized that such
pathways can transfer flow over significant dis-
tances with the consequence that flooding may
occur at remote locationswell away fromthe point
at which the minor system capacity is exceeded.
Similarly, surface runoff from adjacent areas that
surround the urban area and that have no direct
connection to the minor drainage system, may
also result in urban flooding. The latest develop-
ments of this methodology are presented in
Maksimovic et al. (2009). Guidelines to manage
such overland flows are outlined in CIRIA C635,
Designing for Exceedance in Urban Drainage Sys-
tems (Balmforth et al. 2006), and it is clear that to
accurately describe the path and the destination of
the excess overland surface flows requires that
there is a detailed representation of the nature and
characteristics of the catchment surface such
that the overland flood route and flood volume
and velocity may be reliably represented. Within
the context of urban flooding, this usually in-
volves the identification of flood-vulnerable areas
(mainly ponds), together with a definition of the
preferential pathways linking the ponds. The
overland flow may then be coupled with the
below-ground drainage network, using a physically
basedmodel, to assess the interaction between the
above- and below-ground flowcomponents, as first
suggested by Maksimovic and Radojkovic (1986).
Today the usual approach is to make use of a
GIS Digital Terrain Model (DTM) and/or a Digital
Elevation Model (DEM). DTMs are frequently
Preparation of DTM and DEM
An accurate description of urban flood risk re-
quires accurate representation of the physical pro-
cesses of overland surface flow, surface retention,
and surface conveyance along preferential path-
ways. These require a high quality of terrain data,
particularly in urban areas, where urban features
such as buildings and streets need to be accurately
described, both in plan view (x,y coordinates) and
in the vertical presentation (z coordinate). Hence,
the performance and reliability of the surface
overland flow model are highly dependent on
quality and resolution of the DTM/DEM. This is
a function of the accuracy of the type of survey that
is used to record the elevation data and the details
of the catchment surface. Data may be obtained
from many sources.
Maps
The UK's Ordnance SurveyMastermap data give a
good representation of the surface features and
provide the opportunity for visual inspection of
potential flood flowpaths and ponds.
Light detection and ranging (LiDAR) data
Data are obtained by aerial survey andmay be used
to create a DTMor a DEM. There is a requirement
to calibrate and validate the data, but for the
purposes of urban flood modelling a vertical accu-
racy of
50 mm to
150 mm is desirable at a
horizontal grid spacing of 0.5-1.0m. Drive-by
LiDAR, using instruments
located in road
