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.
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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
