flood

The climate zone of the Iberian Southeast has an extremely erratic precipitation regime, combining severe and prolonged droughts with sporadic flooding, some of which are extremely severe. Sometimes jarrear and dilutive verbs, which have a common meaning to make it rain abundantly, are insufficient to describe an event where the rain does not fall in buckets or buckets, but in the seas, the dome of the sky is torn and the waterfalls open. your heaven. The harmfulness of torrential rains and water hoses largely depends on the hourly concentration of rain. In the catastrophic floods of 19 October 1973, the Almeria town of Zurgena amassed 600mm in just three hours, with the peculiarity of the 420 falling as a terrifying and terrifying waterfall between 1 and 2 am on 19 October. Admittedly, precipitation coincided with a basin of 139 km2, a total lightning of 1,974 m3/sec on the Rambla de Nogalte, and a terrible hekatomb in the Murcian town of Puerto. ; and even greater, it produced a flood of 2,489 m3/s on the boulevard itself on September 28, 2012, violating all return calculations – no less than 500 years. The Riada de Santa Teresa (14-15 October 1879), the most notorious in the Segura basin and having the greatest European resonance among the Spaniards, is, in the words of the famous potamologist Maurice Pardé, “one of the deadliest floods in the European hydrological annals”. Published by two commissioned Civil Engineers According to the report, on a farmhouse at the foot of the Vélez River, one of the main tributaries of the formidable Guadalentín, more than 600mm in an hour could be measured (for comparison, Alicante’s annual precipitation averages 355mm). It should be noted that at one point in the Guadalentín river the average precipitation for at least one hour will exceed 10 mm/min, ten times the threshold that defines this limit (1 mm/min). mm/minute), very difficult, but we dare not say impossible, to overcome. More than a century later, on November 5, 1987, a massive downpour k (Orihuela, 316 mm) Vega leveled Baja and restored it, until the eruption of the Segura spot allowed drainage, reinstatement, and a swamp look; The same thing happened with the local deluge or flood of Santa María in September 2019.

The calendar of devastating rains in southeast Iberia presents a high concentration in late summer from mid-September to similar dates in November, peaking in October, the period of maximum risk ending, the period of maximum risk ending, although flooding advances. The first fifteen days of September or the summer in between are postponed to December. The Mediterranean’s high specific heat, inertia and thermal behavior with its hygrometric effects on air in contact, combined, though not exclusively, with influx of cold air at altitude form the process that gives autumn an ​​extraordinary priority. These are hydrological disasters. Surface temperatures of sea waters are mandatory reference data warning of the degree of potential risk. In fact, the ultimate cause of the occasional fall floods of the Iberian Southeast is the Mediterranean itself, a huge reservoir of water and calories. Surface marine isotherm maps often show relatively high temperatures in the affected areas that coincide with these torrential rains. There is no shortage of winter rains; however, great historical floods do not occur frequently in that season; on the contrary, they are rare. High-intensity torrential rains mark a secondary peak in the spring, although at some distance from the main maximum in autumn. Infrequent generalized instability in summer, which can produce strong downpours or sometimes significant water hoses, is caused by the occasional intrusion of cold air at altitudes; these cases are rare at the moment, but the last two weeks of August, the beginning of autumn, are ahead of the previous month and a half. The genesis of a deluge requires suitable atmospheric conditions to settle on the surface and height; if this link does not exist, it will not be reproduced. The advection of very humid air near saturation poses an important precursor and potential risk to heavy showers and high hourly density; it is a necessary, but not sufficient, cause of them: it is by no means synonymous with them, since the harmony of other factors is essential for the downpour. Among them, the predominance of cold air in the upper levels of the troposphere occupies a habitual place; this presence translates into the formalization of a cold air pocket, the cold depression at altitude, the appearance of a depression that may or may not be able to evolve into a self-contained low or isolated depression (DANA) at high levels. It should be noted that the DANA phase was not necessary for the eruption of massive floods, which were also triggered by a sufficiently large trough, provided other necessary causes were at work. It is also notable as the pathway to potential instability and ejection involving the arrival of said Mediterranean air driven by winds from the first and second quadrants (primarily levantes and gregals; much less siroccos or jaloques). in the vertical, exaggeration of the thermal gradients in the vertical usually requires a triggering effect that applies relaxation to initiate the ascent; There may also be areas with cyclonic return of winds.

Alicante’s most frequent torrential downpours, true flooding, are by far the Marquesat or Marina Alta, the wettest in the state, outside the climate zone of the Iberian Southeast; The orography favors the penetration and triggering of westerly winds of eastern and Mediterranean origin (especially from northeast storms driven by the gregal, “llevant de dalt” in the region). But where natural flooding is more significant and dangerous is in the heart of Bajo Segura, in Vega Baja, a vast alluvial plain with almost no slope, semi-endoreic, dangerously drained and slow-drained, algadise, prone to stagnation. rain water and spills, namely into the pool.

On rare occasions, Atlantic storms stimulated by Mediterranean air cause flooding in the southeastern Iberian; Incidence and effectiveness are much higher on the eastern front, especially when an adverse phase of the North Atlantic Oscillation (NAO) has caused them, as this spring. Very heavy rains in Ribera del Júcar (300-400 mm, in less than 24 hours) on 5 November 2020 and also very heavy rains on Alto Palancia (300-400 mm) on 22 March 2021 in Valencia and the metropolitan area (200 -250 mm) on the 4th day of the currents, these extreme torrential rains constitute prototypical and invincible examples of the aforementioned ultimate causality of the Mediterranean. Episodes were triggered by extratropical cyclones; however, in the above-mentioned cases, the mainstay was the presence of very unstable Mediterranean air with high specific humidity, driven by winds from the first and second quarters.

At this point, it should be emphasized that the situations that cause heavy rains vary in terms of surface and altitude. Surface analyzes show various isobaric reliefs, but the dominance of low pressures as well as the subordinate position of Spain’s eastern front on the “anticyclone rim” should be noted; On its southern edge is a large blocking anticyclone. The circulation of the east on the eastern and southeastern fronts of the Iberian Peninsula. At the same time, it should be emphasized that the mechanisms capable of ejecting the said air upward up to the tropopause limit are different and are not limited to “cold drip” processes; Let’s add at least the forced convection imposed on a very unstable flow by retrograde and meridian troughs, other separated or isolated high-altitude lows (DANA), cyclogenetic developments, medium-scale convective systems, anabatic cold fronts, and even relief. very high load hygrometric (flood 3 November 1987 at La Safor: 817 mm at Oliva); This mechanism is worth mentioning because the explanation of the phenomenon, once again, as a necessary causation, starts from the surface, and the only requirement at altitude is that the state curve does not bring absolute stability. Therefore, the explanation and rationale for a flood requires joint consideration of the factors that motivated the impact at surface and height.

Source: Informacion

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