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Your Property and The Rain In Spain |
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Written by Mark Paddon
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‘The Rain in Spain’
Over 300 days of sunshine every year generally means for a dry climate, but Spain is subject to more extreme climatic conditions that the UK and when the rain comes down it can be very hard indeed.
In very recent years the seasonal ‘Gota Fria’ marking the end of summer with a very heavy downpour (which can last for days) has generally not occurred, but the likelihood of heavy rain occurring again in the future is very real.
‘The most significant concern in recent years involves the building of inadequately designed houses on known flood planes and in some cases, within flash flood water courses themselves!’.
In Mediterranean Spain, rainfall tends to be concentrated in just a few days, and within this sometimes within a few hours. Figures of 100mm are common, and figures of 250 not exceptional. The absolute record is a remarkable 878mm in Javea in 1957 (that’s 878L per m2!), followed 30 years later by 817mm in Oliva on 3 November 1987, followed by 600mm in Albuñol (Granada) on 19 October 1973 and 426mm in Cofrentes (Valencia) on 20 October 1987. Another example: 119mm fell in one hour on 1 July 2003 in the village of Manuel in Valencia. * This spring has also seen some heavy downpours but most are isolated such that it’s generally someone else’s problem on the news.
Average monthly total rainfall across Spain 1930-1996 was 639.5 mm* which means that more than a whole year’s national average rainfall can bucket down in one unlucky town in just one day! Most expats here would consider 30L per m2 to be a ‘heavy’ downpour but really heavy rain may involve 25 times more water! I once talked with a 70 year old property owner in Oliva who described a day when cars that had been washed down the hill caused a dam when they became jammed at the end of his road. Fortunately, significant drainage has been added to most roads since then, but blockages can occur for many reasons and a truly heavy downpour will commonly still result in flooding for many properties.
The most significant concern in recent years involves the building of inadequately designed houses on known flood planes and in some cases, within flash flood water courses themselves! Many house buyers are fooled by the ‘bone dry’ nature of Spain and often fail to notice natural watercourses close to their chosen property. Some people are even satisfied with a comment from owner or agent like ‘it has never had any water in it’. It’s important to understand that while natural or man made water courses may be dry for many years (even after moderate rain fall), the water course is there for a purpose and in extreme weather conditions may fill with a torrent of water within seconds. In many cases the water may even arrive before the rain (e.g. from a thunder storm in the mountains) and stopping it from eventually reaching the sea, is generally an impossible task. It is common for millions of liters of water to pass by a point on a flash flood watercourse within minutes and those millions can’t simply be pumped away, plus any attempt to divert the water will need to be heavily engineered.
So what does this mean for the average house?
A house that sits on ground that is not prone to flooding can generally be supported adequately on reinforced concrete strip foundations. If however the house may have to cope with large quantities of passing water, more substantial foundations will be required, otherwise the house can literally be moved or broken up by the force of the water. Commonly an externally tanked reinforced box foundation or ‘pozo de cimentacion’ may be used as the foundation base. Alternatively piling to a more stable substrate (preferably bed rock) may be used, but importantly with both systems the house and it’s surrounding landscaped elements including retaining walls, should be designed to ensure that water flows around and away from the building. Commonly piling will be extended above ground in the form of stilts to allow free water flow under the building (as infill walls are likely to collapse under force or transfer loading to the building risking movement and collapse).
The reinforced box foundation (which is a structure comparable to a reinforced concrete swimming pool), is however more likely to tolerate heavy lateral loading BUT it should also be accompanied by good external drainage/water flow provisions. Walls that cut across the direction of flow should either be avoided or incorporate significant drainage measures and structural features in order to resist collapse. Unfortunately most retaining walls in Spain (especially many of the new ones) will not cope with exceptionally heavy rainfall and hence will one day fail. Simple design features such as avoiding underbuild living space sub ground garages etc in prone areas and always ensuring that the garage door sits at the top and not the bottom of a drive slope, can all help to eliminate future problems, but unfortunately bad design and poor orientation is actually more common in recent years than in the past.
‘Most expats here would consider 30L per m2 to be a ‘heavy’ downpour but really heavy rain may involve 25 times more water!’
Once upon a time houses were built by or for owners with local knowledge such that a builder would be told to raise the house on stilts or face the garage down hill. Nowadays an incompetent architect may have designed a house built by an unconscientious builder for an innocent foreign buyer, and the combination can result in a very problematic, even dangerous property. Furthermore the builder’s 10 year guarantee may be very difficult to call upon unless flooding can be proven to affect the stability of the house. Unfortunately the day that this becomes visually evident in the structure may be the day the house collapses, so a ‘let’s wait and see’ attitude from the insurer or builder is little comfort + the ‘guarantee clock’ is ticking.
Being located on a steep slope can add to the risk of water flow related structural damage. In addition to forces that the building may be subjected to from direct water flow, additional issues such as land slide risk and substrate erosion. Land slide generally occurs when heavy or prolonged rain creates a lower slip plane between the upper earth and lower (more dense) substrate e.g. bed rock, such that normal rain may present little risk to the slope while exceptionally heavy rainfall can result is sudden failure. Tree root structures can help to stabilize the slope while a natural gradient can also help, but full or partial urbanisation commonly involves tree removal and the cutting into the slope with steeply banked access roads, this, (when combined with inadequate retaining or boundary walls, often cutting directly across the path of likely water flow), can increase the risk of problems. While a boundary wall may resist and divert moderate rainfall, it is most likely to fail under flash flood conditions which in turn will result in large-scale water flow to the house itself, which in the worst cases may result in building failure risking the lives of occupants.
While many towns have taken general measures to improve rainwater drainage, some outlying urbanisations and many of the recently built properties within them, will not stand up to extreme levels of rainfall. It’s important that buyers and existing owners assess the property properly to ascertain if 1. Flash flood water flow is adequately directed around and away from the building and 2. If the foundations are adequately designed to cope with a failure in external drainage measures or if the site experiences excessive ground water flow.
There are many well built properties in Spain that take account of flash flood risk but unfortunately, one day, very heavy rain will hit a built up area of inadequately designed villas and the result will make headline news.
*(Statistics sourcesIberiannature.com and Ministerio de Medio Ambiente)
NB:- Information for advice purposes only. Proper safety precautions should be taken and legal procedures followed when carrying out all purchases and building works. Information provided by Mark Paddon BSc Hons Building Surveying. ICIOB. Structural Surveys in the Valencia – Alicante . www.surveysspain.com T: 962807247 M: 653733066 . Free 16 page property buyer’s guide and advice available via e-mail request to
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ã Mark Paddon 2007
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