Environmental Engineering Reference
In-Depth Information
flowpaths and flood-vulnerable areas. They also
afford the opportunity to locate the gulley and
manhole access points to the below-ground drain-
age system, and their condition, together with
details of
vehicles, may also be used to improve the accuracy
of local measurement where, for example, the
elevation of kerb heights and local features are
critical.
Conventional aerial LiDAR will fail to observe
any local detail that cannot be seen fromthe air, for
example the location of drop kerbs, the presence of
covered passageways between buildings or of the
potential flood paths under bridges. Some of these
features may be identified using a fuzzy classifi-
cation of the DEM (Evans 2008). It is considered
essential that site visits are made to observe these
features, particularly at locations where the DTM
highlights surface depressions or major flood
flowpaths.
Using fixed-winged aircraft it is currently fea-
sible to record data at a resolution of 1m horizon-
tal and
150mm vertical, compared with a
resolution of 0.25m horizontal and
30mm ver-
tical for rotary-winged aircraft.
Experience has shown that it can be useful to
specify four layers of data to be delivered by the
LiDAR survey contractors:
.
the first pulse return DEM, which includes
vegetation;
.
the last pulse return DEM, which includes hard
vegetation (large tree boles and dense hedge lines),
buildings and solid artefacts;
.
the DTM with all vegetation and artefacts and
buildings removed;
.
the DEM with buildings included.
The first three layers can be used to identify
permeable and impermeable surfaces and the nat-
ural flood flowpaths, whereas the last three layers
can be used to assess the impact of buildings, other
artefacts and hard vegetation on flowpaths. How-
ever, it may be necessary at critical locations to
improve the resolution of DEMs by means of
drive-by LiDAR surveys, global positioning sys-
tem (GPS) surveys or other methods.
any walls, hedges
and 'hidden
flowpaths'.
It has to be recognized that small changes in
urban surface topography can significantly change
flowpaths, and it is recommended that site walk-
overs (Allitt et al. 2008), for example, are made
such that further detailed information may be
obtained and photographic records made. In addi-
tion to the road layout, gulley and manhole spac-
ing, kerb heights and drop kerbs, buildings, etc.,
important surface features include both small
andmajor embankments, bridges, retaining walls,
culverts and open-access flowpaths
through
buildings.
Similarly, extra care should be taken where the
catchments are flat as the plan area of flooded
water and the flow depths may be large with low
velocity. As a consequence, further increases in
depth could result in a significant change in the
flow route, for example when retaining walls are
overtopped.
The FRMRC approach to develop
the surface flow model
The approach was based on analysis using an
accurate DTM/DEM and the creation of separate
GIS layers to identify and define the flood-vulner-
able areas (mainly surface ponds) and geometric
characteristics of the preferential pathways
through which the flood waves were routed over
the catchment surface. To model urban flood
flows requires that the water movement over the
catchment surface is modelled by solving the ap-
propriate mass and momentum (or energy) con-
servation equations. On the surface this includes
the dynamics of the processes that occur in tem-
porary surface retention (ponds) and of the flow
across the urban catchment along preferential
pathways. The ponds and pathways are mutually
connected and multiple connections may exist
with inlets to the underground sewer network.
During a flood event these two networks interact,
Land and GPS surveys
These provide the most accurate method of ob-
taining both elevation and ground cover data,
together with accurate measurements of any fea-
tures that are likely to influence the surface flood
