21 September 2014

Rain Intensity, land use change may have led to early September floods

 - Sunday, 16 September 2012, 00:00

by Annaliza Borg

The heavy flooding of two weeks ago caused irreparable damage to several vehicles besides people being rescued from the surging waters.

But was it the volume of rainfall or other factors that led to such consequences?

Speaking to The Malta Independent on Sunday, hydrologist Marco Cremona points out that contrary to public perception, it is not the total amount of rainfall (over 24 hours) that determines whether a particular storm will result in record flooding, but the intensity of the storm (in mm/min or mm/hour) together with the nature of the surface of the catchment area, essentially the size of the built up area, including roads. The latter has to be corrected with houses and commercial premises having wells or reservoirs capable of coping with flash floods, as well as roads with culverts.

In reply to questions made, a spokesman for the Malta Airport MetOffice said the rainfall registered during the storms on 2 and 3 September was not a record for a 24-hour period in the month of September. On 22 September 1997, 106.8 mm of rain fell, whereas on 1 September 2003 the rainfall measured 100.8 mm. Over the span of two days (2 and 3 September) – a period of 48 hours – the highest rainfall registered in Malta was in Birkirkara with 127.4 mm. In Xewkija, Gozo, 62.6 mm were measured.

He pointed out that the data for Birkirkara includes the Fleur de Lys, Santa Venera and Msida areas, which were heavily affected on those two days.

Replying to the question whether this was a record, the MetOffice said this was one of the highest recorded levels in the Birkirkara area over the last five years.

Perit Philip Grech, who analysed the rainfall, said that from data provided by the MetOffice, the rain was so heavy in Luqa at a particular hour on the afternoon of Monday 3 September, that over 80mm was registered, which is a lot, but in the next hour it went down to 20mm per hour.

This shows an extremely heavy rainfall in a short time.

The intensity of rain in other places, like Valletta, was less but in Birkirkara it might have even heavier than in Luqa, although he could not cite the exact figures.

His analysis shows that rainfall in different localities varies, even on our small islands.

Ing. Cremona noted that apart from intensity, changes in land use contribute to make a rain event of moderate to high intensity result in flooding.

Over the last 20 -30 years we have been building like there is no tomorrow, with more than 30 per cent of the country’s surface area now consisting of impervious surfaces like roofs and roads. It is a known fact that this transformation of porous surfaces (for example, fields and garigue) has not been complemented with adequate rainwater storage, as required by law, with the result that today even a moderate rain event results in flash floods in most areas of Malta, he explained.

It is no coincidence that the Msida − Birkirkara catchment area has the highest degree of flooding, even though it is not the largest catchment area. More than 80 per cent of this catchment area is built up.

“In my opinion, flooding should be addressed at source, with wells constructed in houses, commercial premises, cisterns in fields, dams in valleys and then moving on to regional solutions like roadside soakaways and reservoirs along roads,” he said. “I would entertain the idea of requisitioning strategically-located disused quarries to work as soakaways, provided that this would not risk contaminating the aquifer.”

He went on to comment about the widespread belief that urban storm water is contaminated and therefore should not be used to recharge the aquifer.

“But has the capability of the geology to filter/absorb pollutants ever been assessed?” he asks.

Likewise, most people assume that the running water in valleys is “clean” and should be harnessed to recharge the aquifer. But he asked whether anyone has tested valley water for soluble pollutants such as nitrate.

Ing. Cremona also believes the country needs a properly-calibrated storm water model that will be able to predict flooding for different storm events, which will also help in identifying resilient and sustainable solutions. The model should be complemented by a nation-wide survey on the location, capacity and status of all cisterns/reservoirs in Malta. “Only then can we start to address flooding and groundwater recharge in a holistic manner,” he concluded.

While several Parliamentary Questions on the rehabilitation of valleys, and water catchment have been made over time, replies never give a holistic picture. It is well known that valley cleansing does take place to some extent but a true assessment of the resources available and the necessary work to keep them functioning is not clear.

A typical reply was given to a PQ made by PL MP Anthony Agius Decelis who, last February, asked Resources and Rural affairs Minister George Pullicino to give details on the number of valleys that were cleaned and those that were still to be cleaned.

In his reply, Mr Pullicino had explained that valley cleansing is an ongoing process and such work is never complete. The process is dictated by the need for work and its urgency, according to a cyclical plan, he said. For this reason, some valleys are cleaned every year while others are cleaned less regularly.

The valleys that need most work are the ones where debris carried by rainwater accumulates. These include Wied il- Kbir, Wied is- Sewda, Wied Ħesri, Wied il- Luq, Wied Qannotta, Wied Għajn Rihana and Wied il- Qlejgħa. These major valleys are cleaned every year because they have a higher volume of water gushing through so the risk of flooding in low-lying areas like Qormi, Marsa and Burmarrad is reduced.

The decision on which valleys are to be cleaned is taken after inspections and consultations with the Malta Environment and Planning Authority (Mepa), so that the ecology and biodiversity of the valleys is protected.

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