CPD articles

CPD: Flood mitigation solutions

1 May 2014

Floating homes, like these in Holland, could provide a solution for the UK

Flooding is rarely out of the spotlight for long these days. Anthony Lymath outlines the steps we can take to make our buildings more resistant and resilient to floods.

Barely a year seems to pass without flooding featuring in the national news – in fact, four out of the five wettest years on record have occurred since 2000. It is such an emotive subject due to the degree of devastation it can cause. Severe flooding can not only endanger life, but even flooding at moderate levels can lead to substantial damage and disruption to normal patterns of life.

Water source contamination and inhibited access are significant problems, coupled with the ease with which internal fixtures and decorations can be damaged irreparably. A flooded house can be uninhabitable for months, and subsequent insurance costs can rise considerably as a result.

But there are a number of measures that can be taken to minimise both the risks and impact of flood damage, for new development and for existing buildings.

Flood mitigation measures

Flood mitigation measures can fall into one of three strategies, as identified in the Department for Communities and Local Government publication Improving the Flood Performance of New Buildings. Depending on the depth of predicted flooding: these are termed Water Exclusion, Mitigation and Water Entry.

Table 1 (below), adapted from that document, illustrates the rationale behind each of these strategies.

The water exclusion strategy focuses on either avoidance or prevention of flooding, and is suitable for a design flood depth of up to 0.3m. This is intended to give the building’s 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.

Table 1: Flood mitigation strategies

Design water depth
Approach Mitigation measures
Greater than 0.6m

Water entry strategy
Allow water in to property to minimise risk of
(but attempt to keep low flood depth water out)

Low-permeability materials up to 0.3m
Easily-replaceable materials at higher depths
Design to drain water away after flooding
Access all voids to permit drying and cleaning
Between 0.3m and 0.6m

Mitigation strategy
High structural risk - allow water in
Low structural risk - attempt to keep water out

Low-permeability materials up to 0.3m
Flood resilient materials and designs
Access to all voids to permit drying and cleaning
Less than 0.3m
Water exclusion strategy
Attempt to keep water out
Low-permeability material
  Avoidance strategy
Remove development from flood hazard
Land raising
Landscaping to create flood retention areas
Raised door thresholds
SUDS

Flood resistance measures are appropriate for this category, which aims to keep the property interior dry during low-level and short-term flood events.

Mitigation measures, appropriate for a design flood depth of 0.3–0.6m, 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).

According to the National Planning Policy Framework (NPPF) technical guidance document: “Flood-resistant construction can prevent entry of water or minimise the amount of water that may enter a building where there is flooding outside. This form of construction should be used with caution and accompanied by resilience measures, as effective flood exclusion may depend on occupiers ensuring some elements, such as barriers to doorways, are put in place and maintained in a good state. Buildings may also be damaged by water pressure or debris being transported by flood water. This may breach flood-excluding elements of the building and permit rapid inundation”.

For resistance, barriers or bunds can be considered. These may be either permanent landscaped features or bund walls (including, for example, around fuel storage tanks), or removable products for installation, such as temporary barriers across building apertures. These include, for example, flood boards on doors, or airbricks/service ducts (although the NPPF Technical Guidance document advises that “temporary and demountable defences are not normally appropriate for new developments”). 

Similarly, temporary, freestanding barriers 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 (ie 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, including the creation of flood retention areas in places such as car parks or landscaping features.

Resistance and resilience measures can also pay a major role for:

Specialist flood protection company Aquobex and architect BACA are raising funds to build this flood-resistant house at BRE Watford

Refurbishment for resilience

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;  maintaining pre-flood dimensions (eg timber swell) and improving the speed and convenience of drying and cleaning to avoid rot or mould decay. 

The NPPF Technical Guidance document states: “Flood-resilient buildings are designed to reduce the consequences of flooding and facilitate recovery from the effects of flooding sooner than conventional buildings”, and “Resilient construction is less likely to encourage occupiers to remain in buildings that could be inundated by rapidly rising water levels”.

Floodwater is invariably contaminated in some form, and these contaminants can cause further damage to buildings and services, besides posing a threat to public health. Contaminants can include sewage, hydrocarbons, silt, salt and other biological or chemical substances. Hydrocarbons are perhaps most commonly present in this manner in the form of petroleum, although asphalt and wood preservatives such as creosote are other sources.

Water can enter buildings via many routes: the wall and floor materials themselves can be permeable; concealed voids such as wall cavities and party walls; at junctions between suspended timber floors and walls; air bricks and other ventilators; inadequate or broken seals around window and door frames (including thresholds); weepholes in facing brickwork; services entry points; cracks in mortar or render; subfloor voids; inadequate or defective damp-proof membranes or tanking; and through sanitary or washing appliances via drainage backflow.

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 the drying time of the insulation is increased considerably. Damp-proof courses and membranes should be durable (minimum 1,200 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.

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 Association.

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.

For guidance on materials specification see Table 2 and 3 below, adapted from DCLG publication Improving the Flood Performance of New Buildings.

Table 2: Flood resilience characteristics of building materials (based on laboratory tests) Table 3: Flood resilience characteristics of walls (based on laboratory tests)
Good Medium Poor  Not assessed  
Material

Resilience characteristics

Material Resilience characteristics 
 

Water penetration

Drying ability

 

Water penetration

Drying ability
Bricks
External face  
Engineering (Class A & B)
    Engineering bricks
(class A & B)

 
Facing (pressed)     Facing bricks
(pressed)

 
Facing (hand made)     Internal face 
Blocks Concrete blocks      
Concrete
(3.5N, 5N)
    Aerated concrete
(“aircrete”)
   
Aerated concrete
(“aircrete”)
    Cavity insulation  
Timber board    Mineral fibre    
OSB2
(11mm thick)
    Blown-in expanded mica    
OSB3
(18mm thick)
    Rigid PU foam    
Gypsum plasterboard    Renders/plaster  
Gypsum plasterboard
(9mm thick)
    Cement render
(external)
   
Mortars
Cement/lime render
(external)
   
Below DPC
(1:3 cement:sand)
    Gypsum
plasterboard
   
Above DPC
(1:6 cement:sand)
    Lime plaster
(young)
   

Water entry strategy

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 (ie the 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.

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.

For the property owner, help is at hand in the form of extensive guidance and standards for the safe and robust repair
of flood-damaged buildings.

Standards and guidance for the repair of flood-damaged property include:

There are also a number of standards which cover the specification of flood protection products:

Looking to the future

As the pressure on land is only likely to increase, more innovative approaches to construction may have to be considered.  Low-lying countries such as Holland have taken a more imaginative approach for a number of years now, and ‘floating’ structures are not uncommon. While tethered or anchored in position, they are able to rise and fall in response to water levels. At least one UK example is currently under construction on a riverside site in the south of England, and a prototype flood-resilient house is being developed at the Building Research Establishment (BRE).

This article was adapted from a series on Climate Change Adaptation in Buildings, originally published on theNBS.com website.

Anthony Lymath RIBA is a chartered architect and a technical author at the NBS working on its industry-standard master specification products, and writing technical articles.

Click here to take the online CPD test paper for May 2014

Comments

There are some very basic steps to combat flood waters which could be used but not mentioned here.
1. External doors that open outwards so that flood waters push the doors closed against water proof seals. (No need for sand bags!)
2. Non-return valves on all drainage and waste pipes.
3. Tanking as a standard where ever there is flood risk
4. In areas of high flood risk doors at first floor level with tanking as standard.
None of these ideas are well thought through but could be a starting point for innovative ideas.

Michael Brown Chartered Construction Manager, 8 May 2014

If developers insist on using flood plain land then why not legislate that all properties have precast waterproof basements and the excavated material is used to raise the surrounding area of each property. Basemnts with a larger footprint than the property would work best.

J Warner-Edney, 8 May 2014

Build new houses on stilts and utilise the space underneath for car parking. Beach homes at Gulf Shores in the USA are built like this as they are at risk from flooding during the hurricane season from the sea. Oil rigs are built on stilts (piles).

C J Harris, 30 May 2014

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