Build­ing in a flood zone

Ar­chi­tect Ju­lian Owen takes a look at what de­sign el­e­ments can help to re­duce the risk of dam­age caused by flood­ing

Build It - - CONTENTS -

e right de­sign el­e­ments could fend off flood­ing and pro­tect your prop­erty when na­ture does its worst. Ju­lian Owen shares his top tips for re­duc­ing the like­li­hood of prop­erty dam­age

Many peo­ple would like to live by the sea or a river, at­tracted by the scenic views and nearby op­por­tu­ni­ties for boat­ing, fish­ing and walk­ing. How­ever, one down­side to re­sid­ing in these lo­ca­tions is the threat of wa­ter ingress. Some of the UK’S most pic­turesque towns and vil­lages have ap­peared on the news as pos­ses­sions are lost, homes ru­ined and lives put in dan­ger by flood­ing. And events like this aren’t one-offs, as it looks like the fre­quency of flood­ing is set to in­crease in the com­ing years due to cli­mate change. This means fu­ture dan­gers have to be con­sid­ered if you’re plan­ning to self-build or ex­tend in an area that’s at risk.

Plan­ning hur­dles

About 5.5 mil­lion homes in the UK are in dan­ger of be­ing flooded. That’s ap­prox­i­mately 15% of prop­er­ties built near to rivers, the sea or ar­eas of ex­treme sur­face wa­ter run-off caused by lo­cal ground con­di­tions – the lat­ter of which are of­ten in places that aren’t ac­tu­ally near to open wa­ter.

If you’re think­ing of self-build­ing or al­ter­ing a house in an area iden­ti­fied as a flood zone then the risks need to be con­fronted early. This is likely to be an in­flu­en­tial is­sue for your lo­cal plan­ning depart­ment when they con­sider your ap­pli­ca­tion, and fail­ure to com­ply with the stip­u­la­tions of the Environment Agency usu­ally re­sults in a re­fusal.

There are ways of ad­dress­ing the con­cerns of plan­ners, espe­cially if you’re ex­tend­ing a house in a lower risk area. In these cases, keep­ing the floor level the same as the ex­ist­ing build­ing and in­clud­ing mea­sures that will min­imise po­ten­tial wa­ter dam­age to the struc­ture should suf­fice.

Get­ting ap­proval for a com­pletely new house, how­ever, can be more chal­leng­ing. As well as tack­ling the risk of the build­ing flood­ing in the de­sign, you may also have to prove that the lo­cal flood plain will not be com­pro­mised by the new prop­erty. These are ar­eas of land that are de­signed to col­lect flood­wa­ter and pre­vent it from flow­ing onto higher ground, and their stor­age ca­pac­ity can be re­duced if they’re heav­ily de­vel­oped and cov­ered in build­ings.

Re­sis­tance & re­silience

If you can gain plan­ning per­mis­sion and are able to buy ad­e­quate in­sur­ance to build in a flood risk zone, there are two steps to tack­ling the prob­lem. Firstly, work to pre­vent the wa­ter from en­ter­ing the house and, se­condly, min­imise any re­sult­ing dam­age if it does. Ex­perts call these flood re­sis­tance and re­silience, and the mea­sures taken are known as wa­ter ex­clu­sion and en­try strate­gies.

The As­so­ci­a­tion of Bri­tish In­sur­ers says that pro­tect­ing a house against shal­low flash floods costs around £2,000£6,000, but keep­ing wa­ter out dur­ing pro­longed flood­ing re­quires more work and can come in be­tween £20,000 and £40,000. If you can af­ford it, re­sis­tance and wa­ter ex­clu­sion are the best long-term strate­gies.

Fit­ting an early warn­ing sys­tem in the form of a sen­sor and alarm is a sen­si­ble pre­cau­tion. Prefer­ably, these

should be po­si­tioned out­side at a lower level than the ground floor. They work to alert you if a flood is ex­pected, thus giv­ing you enough time to put tem­po­rary mea­sures in place and move some pos­ses­sions up­stairs. But, ob­vi­ously, you’ll need to be in the house at the time for this to work.

De­sign op­tions

The sim­plest wa­ter ex­clu­sion mea­sure is to build the ground floor above the max­i­mum level that any flood wa­ter is likely to reach. How­ever, this is of­ten im­prac­ti­cal and plan­ners may not ap­prove of the build­ing be­ing dis­pro­por­tion­ately higher than neigh­bour­ing prop­er­ties.

Where the po­ten­tial flood level is be­low 600mm, there are a num­ber of rel­a­tively sim­ple ways of seal­ing off the in­side and keep­ing a new house more or less dry. A com­mon op­tion is to use tank­ing; a con­tin­u­ous wa­ter­proof layer buried within the wall and floor con­struc­tion that pre­vents flood­wa­ter from seep­ing into rooms. How­ever, there are still plenty of routes for wa­ter to get in, not least through a home’s doors and win­dows. Fen­es­tra­tion can be sealed at a low level us­ing hinged flood bar­ri­ers that can be quickly swung or slot­ted in place in the event of a flood warn­ing. Air­bricks are po­ten­tially vul­ner­a­ble, but they can be fit­ted with tem­po­rary cov­ers, au­to­matic seals that drop into place, or per­ma­nent snorkels that will al­low air to flow via an out­let at a higher level.

One of the most un­pleas­ant side ef­fects of a flood is the po­ten­tial re­lease and cir­cu­la­tion of raw sewage in­side the house; this can be avoided by fit­ting non-re­turn valves to all the drainage routes. Like many of these pre­ven­ta­tive mea­sures, this is a sim­ple DIY job pro­vided the drains are eas­ily ac­ces­si­ble by man­holes. All po­ten­tial en­try points via ser­vice pipes be­low the flood level have to be checked and tightly sealed. Bear in mind that elec­tric­ity and wa­ter are not a good mix, so ex­ter­nal me­ters will need to have the pro­tec­tion of wa­ter­proof cab­i­nets.

High risk ar­eas

Where the flood level could reach above 600mm, it’s more dif­fi­cult to pro­vide ef­fec­tive pro­tec­tion be­cause the deeper the wa­ter, the more force­ful the pressure push­ing onto the prop­erty. Win­dows and doors can be spe­cially re­in­forced us­ing strong ma­te­ri­als and seals, but even these have height lim­i­ta­tions above which they will fail.

An­other more ex­treme so­lu­tion is to con­struct the house on stilts, lift­ing it above the pre­dicted flood level. A re­cent in­no­va­tion is to make the stilts ex­tend­able to al­low the house to rise and float when the wa­ter is high, drop­ping back down as it re­cedes. This tech­nique has been very suc­cess­ful in other coun­tries, such as Hol­land, but it hasn’t been used on many new houses in the UK.

Above: Baca Ar­chi­tects de­signed and built The Am­phibi­ous House, which can rise up in its dock and float if flood­ing oc­curs

This flood gate can be pulled across to pre­vent wa­ter from seep­ing into the house through the door

Be­low: The Float­ing House by Carl Turner Ar­chi­tects is a light­weight cross-lam­i­nated tim­ber (CLT) de­sign that can be sup­ported on a va­ri­ety of sub­struc­tures to suit flood-risk lo­ca­tions, in­clud­ing piles or even a fully float­ing pon­toon

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