Environmental Engineering Reference
In-Depth Information
.
Overlandmajor system:
- the urban surface that
forms surface flowpaths at the time of rainfall
events.
.
Major system:
- the above-ground network of
preferential surface flowpaths through which
storm water is conveyed during a flood event,
including watercourses and rivers.
At the time of rainfall events these systems
interact and, from an integrated urban flood man-
agement perspective, there is a need to understand
this interaction and how the performance of
the system responds to a range of urban flood
drivers that cause different types of urban flooding.
These are complex processes but a simplified
line diagram (Fig. 13.2) highlights theway inwhich
the systems are linked, whilst Figure 13.3, high-
lights the interaction of the above-ground surface
flows (major system) and the below ground drain-
age system (minor system). The rainfall triggers
surface runoff and, in the first instance, storm
water enters the drainage network. When the
drainage system becomes surcharged, water flows
out of pipes back onto the surface, resulting in
flooding. It is clear that, dependent on the flow
conditions and time after the start of surcharge, the
flow may be into the sewer from the catchment
surface or out of the sewer onto the catchment
surface. Here the role of gullies and manholes is
particularly important. This leads to the consider-
ation that there are many types of urban flooding.
catchment surface with the consequent flooding
of properties.
.
Flooding of the urban area from surrounding
catchments:
Such flooding occurs due to a
rainfall event on the rural or peri-urban area that
surrounds or is adjacent to the intra-urban area
within the same catchment. If the flowpaths from
these surrounding areas lead directly to the intra-
urban area it is feasible for the surface runoff to
flood the intra-urban area, in a similar way to
floods caused by pluvial flooding. In the case
where the slopes of the surrounding area are steep
this type of flooding may be severe (flash floods).
.
Fluvial inundation from inland waterways:
The
performance of the sewer system in the intra-
urban area may be influenced by the performance
of the fluvial drainage system in two ways:
.
Hindered performance due to a back-up of flow
in the sewer system caused by enhanced fluvial
flows that inundate the discharge outlets of the
sewer system.
.
Inundation of the intra-urban catchment sur-
face due to the failure, overtopping or by-passing
of the flood defences of the fluvial system. This
results in an inundation of the sewer system,
which becomes full and subsequently inopera-
ble due to extremely slack hydraulic gradients,
often with ponding on the low-lying areas of the
catchment surface.
.
Inundation in coastal regions:
Similar impacts
to those observed in inland systems may be ob-
served in the intra-urban areas adjacent to coastal
and estuary environments where, similarly, the
performance of the sewer systemmay be hindered
by the height of surges or the overtopping or failure
of coastal defences.
.
Asset performance, deterioration or failure:
Sewer flooding is also caused by the performance
of assets and asset failure. In many cases such
performance is governed by the condition and
status of the assets and of the way in which they
deteriorate. The primary processes that cause such
flooding include:
.
blocked or restricted sewer outfalls and inlets;
.
sewer blockages and collapses;
.
presence of sewer sediments;
.
mechanical and electrical failure.
Types of urban flooding
There are many types of urban flooding and each
type results in a different type of surface water on
the catchment surface. These include:
.
Pluvial flooding:
Rainfall in the urban area may
cause flooding due to the fact that there are inad-
equate hydraulic access pathways to the under-
ground sewer system or due to the fact that the
pipes in the sewer system have a hydraulic capac-
ity that is less than the flows that are generated by
the rainfall runoff process. In the latter case the
sewer pipes are hydraulically inadequate and this
results in a back-up of flow and a 'surcharge' of the
system. Such surcharge may result in the internal
flooding of basements or external flooding of the
