2022
may be the year that brought home to many on the Australian east coast what it really means in a
changing climate to have built, purchased or rented a freestanding
house, townhouse, unit or flat on flood prone land or on floodplain.
Right
now, ten months into the third year of a triple La Niña event,
individuals
and couples may well be wishing that the real estate agent, local
council, individual who did the property conveyancing, a neighbour, friend
or family member, had been a little more forthcoming about what
moving to a particular street, town, local government area or region
actually meant when it came to hazard risks from storms, heavy
rainfalls and local or widespread flooding.
Whether
it is your first home, your retirement dream home or just an
affordable rental in which you are happily settled, for literally
thousands of people the limitations of the dwelling in which they currently live is becoming apparent.
While devastated souls in catastrophically affected areas are trying to come to grips with trauma and loss as they assess their options.
Where to start with looking at your home with fresh eyes, before deciding if it will withstand the worst floods or whether you need to modify the dwelling, move the house to higher ground or look for a brand new home on land in a safer area? Big decisions.
In
mid-2021 a report was published looking at certain options
available for flood prone buildings.
Bushfire
& Natural HAZARDS
CRC,
COST-EFFECTIVEMITIGATION STRATEGY DEVELOPMENT FOR FLOOD PRONE BUILDINGS,
Final
project report,
July
2021, excerpt:
Globally,
floods cause widespread impacts with loss of life and damage to
property.
An analysis of global statistics conducted by Jonkman (2005) showed
that floods (including coastal flooding) caused 175,000 fatalities
and affected more than 2.2 billion people between 1975 and 2002. In
Australia floods cause more damage on an average annual cost basis
than any other natural hazard (HNFMSC, 2006). The fundamental cause
of this level of damage and the key factor contributing to flood
risk, in general, is the presence of vulnerable buildings constructed
within floodplains due to ineffective land use planning.
Retrospective
analysis show large benefits from disaster risk reduction (DRR) in
the contexts of many developed and developing countries. A study
conducted by the U.S. Federal Emergency Management Agency (FEMA)
found an overall benefit-cost ratio of four suggesting that DRR can
be highly effective in future loss reduction (MMC, 2005). However, in
spite of potentially high returns, there is limited research in
Australia on assessing benefits of different mitigation strategies
with consequential reduced investment made in loss reduction measures
by individuals and governments. This is true not only at an
individual level but also at national and international levels.
According to an estimate, international donor agencies allocate 98%
of their disaster management funds for relief and reconstruction
activities and just 2% is allocated to reduce future losses (Mechler,
2011).
The
Bushfire and Natural Hazards Collaborative Research Centre project
entitled Cost-effective mitigation strategy development for flood
prone buildings is examining the opportunities for reducing the
vulnerability of Australian residential buildings to riverine floods.
It addresses the need for an evidence base to inform decision making
on the mitigation of the flood risk posed by the most vulnerable Australian
building types and complements parallel CRC projects for earthquake
and severe wind.
This
project investigates methods for the upgrading of the existing
residential building stock in floodplains to increase their
resilience in future flood events. It aims to identify economically
optimal upgrading solutions so the finite resources available can be
best used to minimise losses, decrease human suffering, improve
safety and ensure amenity for communities.
This
report describes the research methods, project activities, outcomes
and their potential for utilisation.
Flood
mitigation strategies mentioned
in the report
Elevation
Elevation
of a structure is one of the most common mitigation strategies where
the aim is to raise the lowest habitable floor of a building above
the expected level of flooding. This can be achieved by extending the
walls of an existing structure and raising the floor level; by
constructing a new floor above the existing
one; or through raising the whole structure on new foundations
(walls, piers, columns or piles)as
shown in Figure 3.
Technical
considerations that need to be taken into account in raising
buildings are structure type, construction material, foundation type,
building size, flood characteristics and other hazards. Other factors
to take into consideration when elevating
existing structures are additional loading on foundations, additional
wind forces on wall and roof systems and any seismic forces (FEMA,
2012).
Generally
the least expensive and easiest building to elevate is a low-set
single storey timber frame structure (USACE, 2000). The procedure
becomes complicated and more expensive when other factors are
included such as slab on-grade construction, walls of masonry or
concrete or application to a multistorey building (USACE, 1993).
Elevation is one of the strategies which currently can result in
incentives from the insurance industry in the form of reductions in
annual premiums for flood insurance (Bartzis, 2013).
Relocation
Relocation
of a building is a dependable flood mitigation technique. However, it
is generally the most expensive as well (USACE, 1993). Relocation
involves moving a structure to a location that is less prone to
flooding. Relocation normally involves
placing the structure on a wheeled vehicle, as
shown in Figure 4.
The structure is then transported to a new location and set on a new
foundation (FEMA, 2012). Relocation is much easier and cost effective
for low-set timber frame structures. The relocation of slab-on-grade
structures is more complicated and expensive.
Relocation
is most appropriate in areas where flood conditions are severe such
as a high likelihood of deep flooding, or where there is high flow
velocity with short warning time and a significant quantity of
debris. Technical considerations for
relocation include the structure type, size and condition. Light
weight timber structures are easy to transport compared to heavy
masonry and concrete buildings. Similarly, the relocation of single
storey compact size structures is far easier
than for large multi-storey structures.
Dry
floodproofing
Dry
floodproofing essentially attempts to keep floodwaters out of the
house. The portion of a structure that is below the expected flood
level is sealed to make it substantially impermeable to floodwaters.
This is achieved by using sealant systems which include wall
coatings, waterproofing compounds, impervious sheeting over doors and
windows and a supplementary leaf of masonry (FEMA, 2012). The
expected duration of flooding is critical when deciding which sealant
systems to use because seepage can increase with time making flood
proofing ineffective (USACE, 1993). Preventing sewer backflow by
using backwater valves is
also important in making dry floodproofing effective (Kreibich et al.
2005; FEMA, 2007).
Dry
floodproofing is generally not recommended in flood depths exceeding
one metre based on tests carried out by the US Army Corps of
Engineers as the stability of the building becomes an issue over this
threshold depth (USACE, 1988; Kreibich et al. 2005). Dry
floodproofing is also not recommended for lightweight low-set
structures or structures with a basement. These types of structure
can be susceptible to significant lateral and uplift (buoyancy)
forces. Dry floodproofing may also be inappropriate for light timber
frame structures and structures that are not in good condition and
may not be able to withstand the forces exerted by the floodwater
(FEMA, 2012).
Wet
floodproofing
In
this measure floodwater is allowed to enter the building to equalise
the hydrostatic pressure on the interior and exterior of the
building, thus reducing the chance of building failure due to a
pressure differential on components. As all the
building components below the flood level are wetted, all
construction material and fit-outs should be water-resistant and/or
can be easily cleaned following a flood. Flood resistant materials
can help reduce flood damage and facilitate cleanup to allow
buildings to be restored to service as quickly as possible.
FEMA (2008) provides a detailed list of building materials classified
as acceptable or unacceptable for wet floodproofing based on
cleanability and water resistance.
Wet
floodproofing involves raising utilities (heating, ventilation, and
air conditioning (HVAC), electrical systems etc.) and important
contents above the expected flood level.
Wet
floodproofing may not be suitable in floods with duration
of more than a day as longer duration can lead to damage to
structural components of the building and can also result in the
growth of algae and mould (FEMA, 2007). Also wet floodproofing can
only reduce loss from floods but cannot eliminate
loss as some amount of cleanup and cosmetic repair will always be
necessary (USACE, 1984). Although using flood damage resistant
materials can reduce the amount and severity of water damage, it does
not protect buildings from
other flood hazards, such as the impact of flood borne debris.
Flood
barriers
Flood
barriers considered here are those built around a single building and
are normally placed some distance away from it to avoid any
structural modifications to the building. There are two kinds of
barriers: permanent and temporary.
An
example of a permanent barrier is a floodwall which is quite
effective because it requires little maintenance and can be easily
constructed and inspected. Generally, it is made of reinforced
masonry or concrete and has one or more passageways that are closed
by gates. An example of a floodwall is shown in Figure 5.
There
are also several types of temporary flood barriers available on the
market which can be moved, stored and reused. There are a number of
considerations with regard to the use of these barriers such as the
need for prior warning and enough time to be set up in order to be
effective (Kreibich et al. 2011). They also require periodic
inspection and maintenance to address any repair required. Further,
access to the building could be difficult (FEMA, 2007).
A
number of vendors make temporary flood barriers that can be assembled
relatively easily and moved into place. Some of the temporary flood
barrier options are presented below and shown in Figure 6.
Sandbags:
This is a traditional and less expensive way to construct a barrier
up to 1m high in front of a building and its openings. However, it
requires considerable time and effort to set up.
PVC
tubes: These consist of two flexible tubes laid side by side and
joined permanently to form a twin element with high stability. They
can be made ready quite quickly, generally in less than 15 minutes,
and are available in 1m height and 10m length units.
Metal
boards/fence: This fence system consists of two boards in compact
flat packs that are lifted into place after transportation to the
site and the system is stabilised by water pressure.
Flexible
barriers: These barriers are able to dam or redirect flowing water up
to 1m high and can be set up very quickly on almost all surfaces.
Box
wall: A freestanding flood barrier for use on smooth surfaces. These
can be attached and placed next to each other to build a 0.5m high
wall around a building.
Box
barrier: An effective temporary flood barrier (0.5m high) that can be
aligned easily and rapidly. After positioning, the box can be filled
with water to hold it in place.
The
report looks at vulnerability to flood risks of various types of
housing from: Timber Frame (raised floor); Cavity Masonry - Victorian
Terrace (raised floor); Cavity Masonry (raised floor); Brick
Veneer (raised floor); and
Brick
Veneer (slab-on-grade).
The
report also examines the strength of selected building components and generally the cost effectiveness of building material for use in flood prone buildings.
The
full report can be downloaded at:
https://www.bnhcrc.com.au/file/13042/download?token=2Iqm6aEk
