by Anthony Lymath
In the last article, the background and issues concerning the problems of flooding in buildings were examined. At a whole-building level, preliminary issues such as building siting and orientation, landform/ topography and site location should all be considered, with respect to the proximity of the development to areas of potential flooding, during the earliest stages of a project. Consultation with the Environment Agency and relevant Lead Local Flood Authority’s (LLFA) extensive flood risk information services should form part of the initial site analysis and data searches, to ascertain current and future risk of flooding from the various different sources (see part one for details). This information should be used to identify the development site at lowest risk of flooding, from those available. Depending on the scale and location of the development, a Flood Risk Assessment (FRA) may be required under the National Planning Policy Framework (NPPF), to demonstrate that the development will not be at an unacceptable risk of flooding and will not increase flood risk elsewhere.
Water exclusion strategy (suitable for a design flood depth of up to 0.3m)
This focuses on either avoidance or prevention of flooding. This is intended to give the building occupants more time to relocate vulnerable contents to higher levels and should not be considered to be effective for more than a relatively short duration flood event.
At a strategic level, consider alternative locations for the building as described above. If this is not possible, set the ground floor level above the maximum predicted flood height. This is obviously going to be subject to local planning restrictions and may be a requirement due to a FRA.
Specify permeable surfaces if possible to minimise reduction of surface water run-off and hence pressure on the drainage system. ‘Soft’ landscaping materials such as vegetation are the most effective. Gravels and permeable hard pavings, such as tarmacadam, and proprietary permeable brick paving units are useful where planting is not suitable.
Install Sustainable Drainage Systems (SuDS) – including hydrobrakes, dry swales (which fill to act as balancing ponds during a deluge, releasing the water slowly into the surface water drainage system usually in conjunction with a hydrobrake), green roofs and rainwater harvesting. Reference should be made to CIRIA publication C697 The SuDS manual 20072.
Mitigation (suitable for a design flood depth of 0.3-0.6m)
Measures to be adopted for managing residual flood risk are aimed either at resistance (keeping water out), resilience (to water damage), and/ or repair (of water damage).
For resistance, barriers or bunds can be considered – either permanent landscaped features or bund walls (including, for example, around fuel storage tanks), or removable products for installation as temporary barriers across building apertures, e.g. flood boards on doors or airbricks/ service ducts. Similarly, temporary, freestanding barriers which are assembled close to, but not in contact with, buildings (such as property flood skirt systems) can be useful. Fences can be designed to include impermeable materials at the base, such as concrete planks or masonry dwarf walls. Drainage systems can incorporate double-sealed lock-down inspection chamber covers and non-return valves (to prevent sewage backing-up) to BS EN 13564. Sanitary and washing appliances should be sited above ground level, i.e. not in basements.
Although it can be argued that resistance measures such as those noted above have the undesirable effect of ‘moving the problem elsewhere’, they can of course be compensated by other measures, such as the creation of flood retention areas in places such as car parks or landscaping features.
Resilience is defined as minimising the impact that flood water has upon entry to a building, seeking to avoid permanent damage or loss of structural integrity, maintain pre-flood dimensions (i.e. timber swell), and improve the speed and convenience of drying and cleaning to avoid rot or mould decay. Resilience measures can take many forms across many areas of the building:
- Floors – use ground-bearing solid concrete slabs in preference to suspended timber; specify ceramic, stone or concrete-based tiled surfaces to floors and skirtings (with cement-based adhesive and water-resistant grout), ideally draining to a floor sump pump; paint timber skirtings on the reverse before fitting; avoid concrete screeds above insulation as drying time of the insulation is increased considerably; damp-proof courses and membranes should be durable (minimum 1200 gauge for polythene) with particular attention paid to laps, and consider double-layer protection with cavity drains to retaining walls and basements; consider loose rugs in preference to fitted carpets for ease of removal and storage, as well as drying and replacement; specify closed-cell insulation to resist water absorption (but bear in mind that floor coverings will need to counteract the buoyancy of the insulant if submerged).
- Walls – use closed-cell insulation below predicted flood level in external walls; specify water-resistant walling materials such as pressed-face or engineering brick or rendered blockwork, use extended periscope subfloor ventilators or fit removable airbrick covers; fix plasterboard sheets horizontally rather than vertically, or split sheets mid-height with a dado rail, to reduce the extent of replacement; specify lime- or cement-based renovating plasters or renders rather than gypsum-based, with water-resistant paint finishes. The use of water-proof, water-resistant or micro-porous surface coatings on masonry should be viewed with caution – these have been seen in some instances to inhibit the drying-out of the building fabric, leading to further dampness-related problems internally, and their use is currently discouraged by the Brick Development Association3.
- Kitchens – specify plastic, solid wood or stainless steel for cupboards and housings, in preference to particle board or MDF; mount appliances above the predicted flood height; fit non-return valves to drains from washing machines and dishwashers; seal between and behind cupboards to minimise water penetration; specify low-porosity materials for work surfaces.
- Doors and windows – specify PVC-U, aluminium (and aluminium-faced or foam-core door panels) or hardwood frames in preference to softwood; use loose-pin butt hinges to enable easy removal of internal doors for temporary storage above flood level; ensure that all frames are well sealed and gasketed.
- Services – avoid (or minimise) any wiring below predicted flood level; fit all switches, socket outlets, service panels, meters, etc. above predicted flood level; consider routing electrical ring main at first floor level with drops to ground floor; fit electrical cabling in surface trunking rather than chased-in to wall surfaces; install boilers and other heating or cooling equipment at first floor level (or as close to ground floor ceiling level as possible); protect communications wiring and other services with insulation within services ducts.
Resistance and resilience measures can also play a major role for:
- Water-compatible development (for example, outdoor sports and recreation, or facilities such as changing rooms), and less vulnerable development (for example, retail and restaurant buildings) where appropriate flood warning is provided and temporary disruption may be deemed acceptable.
- Refurbishment of buildings (where there is no material change of use) that are already exposed to flooding, or are likely to be in the future.
- Some material change of use proposals, where it can be demonstrated that no other flood risk management measures are practicable.
Water entry strategy (suitable for a design flood depth exceeding 0.6m)
Where predicted flood depths could exceed 0.6m above the ground floor level, a ‘water entry’ strategy needs to be adopted whereby water is allowed uninhibited access into (and out of) the building. This is because the structural integrity of cavity-walled masonry buildings in particular can be jeopardised, potentially leading to collapse, if the differential head (i.e. difference in water level between inside and outside) exceeds 0.6m. The resilience measures discussed above are applicable to this approach, the focus being on enabling drying and minimising consequential repair of the building fabric.
There are a number of standards which cover the specification of flood protection products:
- PAS 1188-1:2009 Flood Protection Products. Specification. Building Aperture Products4
- PAS 1188-2:2009 Flood Protection Products. Specification. Temporary Products5
- PAS 1188-3:2009 Flood Protection Products. Specification. Building Skirt Systems6
- PAS 1188-4:2009 Flood Protection Products. Specification. Demountable Products7.
It should be noted however that at the time of writing, these are all due to be replaced in April 2014. Standards for the repair of flood-damaged property include:
- PAS 64:2013 Mitigation and Recovery of Water Damaged Buildings. Code of Practice8
- CIRIA publication C623 Standards for the Repair of Buildings Following Flooding 20059.
Following a flood event, it is recommended that a thorough survey is carried out on the property to assess structural and services damage, as well as damage to finishes and fittings, and dimensional integrity.
Distinct from simply ‘buildings on stilts’, such ‘amphibious’ buildings are more common in low-lying countries such as Holland, although occasional examples are starting to appear in the UK, albeit largely at the theoretical level.
More innovative approaches to the unpredictability of flooding include the concept of ‘floating’ structures which, while tethered or anchored in position, are able to rise and fall in response to water levels. Distinct from simply ‘buildings on stilts’, such ‘amphibious’ buildings are more common in low-lying countries such as Holland, although occasional examples are starting to appear in the UK, albeit largely at the theoretical level. However, these require careful design in order to understand fully the ramifications, which are outside the scope of this article. Detailed consultation with both the Environment Agency and the relevant LLFA is therefore recommended from the outset.
Regardless of personal opinion on the validity or otherwise of the theory of climate change, the fact remains that during extreme weather events buildings have a tendency to fail. This can be due to a number of factors, and under certain circumstances can be due to conflicting requirements on the building envelope. However, as has been identified, there are numerous systems and products available with which to combat weather and climate extremes. In order to assist in producing buildings that are better able to withstand the vagaries of the weather over their design lives, the NBS offers a number of subscription products from which systems and products may be specified to mitigate the effects of climate change.