J . Egaña 1 ,2 , S . Gaz te lumend i 1 ,2 , V. Pa lac io 1 ,2 , M. Mar t i j a 1 ,2

1 - Basque Meteorology Agency (EUSKALMET). Parque tecnológico de Álava. Avda. Einstein 44 Ed. 6 Of. 303, 01510 Miñano, Álava, Spain2 - TECNALIA, Meteorology Area. Parque tecnológico de Álava. Avda. Albert Einstein 28, 01510 Miñano, Álava, Spain.

Contact info:

santiago.gaztelumendi@tecnalia.comjoseba.egana@tecnalia.com

TECNALIA

Parque Tecnológico de BizkaiaC/ Geldo Edificio 700E-48160 DERIO (Bizkaia) Spainwww.tecnalia.com

Abstract.In this study we present an analysis of a flood episode affecting Urumea river basin on 25-27 February 2015. During those

days a moderate and persistent rainfall episode affects this area of Gipuzkoa region in Basque Country (see Fig.1), a very rainyarea during cold season (October-April) where these situations occurs with some frequency.

The synoptic environment is characterized by a powerful anticyclone centered in the Atlantic, creating a northwesterly flowwhere it runs different active fronts. This succession of fronts generates abundant rainfall that does not have high intensities , butare persistent . The accumulated precipitation on 24-hour exceeds 120 mm in the study area, and around 300 mm for the wholetime period event. This situation causes rivers to overflow at different points, generating local floods. In addition to rain, certaincontribution from melting are produced. The Urumea river overflows in different points, promoting numerous problems specially inpopulated areas around Donostia. Many people need to be evacuated, and water causes considerable damages and problems allaround the area including industries, shops, houses, roads, electricity distribution, etc.

In order to understand the evolution of this event, synoptic characteristics, mesoscale situation and other local meteorologicalcharacteristics are analyzed including datasets coming from the Basque Country Automatic Weather Station (AWS) Mesonetworkand other sources (MSG, Radar, etc) available during this episode.

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ConclusionsThese meteorological situations generate persistent and widespread precipitations over a long period of time, so that floods are

caused by the growth of river flow and subsequent overflow. The, most of the precipitation was stratiform type.These events havehigher predictability than warm season convective events, in which, can be generated mesoscale convective systems that dischargelarge amounts of water in a short time causing less predictable floods.

The situations as described in this paper occur relatively frequently in Urumea basin. Quasi-stationary and warm front movesslowly from west to east, and tends to be stagnating as it moves, with northwest flow in all levels. Moreover, we should mention thatin the study area northwest flow generates orographic precipitation, so that the efficiency of such fronts with northwesterly flows ishigher than in the rest of the Basque Country due to the terrain. Rainfall usually stratiform, although the intensity is higher than in therest, without exceeding 15 mm in one hour, but staying on average around 10 mm in 1 hour. Accumulated precipitation in 24 hourscan easily exceed 100 mm. So, the problems are generated due to the persistent rainfall.

References.

A case study of local floods in Urumea area (Basque Country )

AknowledgementsThe authors would like to thank the Emergencies and Meteorology Directorate - Security Department - Basque Government for public provision of data and operational service financial support. We also would like to thank all our colleagues from EUSKALMET for their daily effort in promoting valuable services for the Basque community...

EnvironmentA powerful anticyclone is located over the Azores and sends strong, wet and cold northwest wind to the Cantabrian coast

during most of the time of this episode. At the end of it, high pressures adopt wedge form in the Cantabrian zone. On the otherhand, a depression is situated in the Tyrrhenian Sea and these low pressures prevail over the British Isles and Central Europe,with its center in Northern Europe (see Fig.2). The frontal systems, associated with the depression, pass through the Gulf ofBiscay and, in its wake, rainfall intensify. On Tuesday 24, in addition, there is cold air in high layers, which causes instability andsome storms. That cold air at 500 hPa level is withdrawn during the first half of Wednesday 25 and instability in height starts todisappear. Intense and more or less short-lived showers give way to frontal rainfall and, therefore, more persistent. It is therewhere the largest amounts of precipitation are accumulated, specially due to the passage of a warm quasi-stationary front (seeFig.2 and Fig.3).

In the Meteosat images (see Fig.3) we can see different large-scale cloud systems and particularly how it affects us a line ofinstability at the beginning and a warm front during the 25th and 26th. In Fig.4 we can observe different types of precipitation inthe Radar images. Showers take place at the beginning of the event, they are quite intense, but not widespread. However, rainfallbecome more widespread, persistent and with stratiform character due to the arrival of the warm front, when more precipitationare registered.

Some rivers reach flood level and overflow in different basins during the 25th and 26th. The most affected basins is Urumea. The Urumeariver overflows in different points (see Fig.5), creating numerous problems in a densely populated area, around Donostia (Gipuzkoa capital). Manypeople are evacuated and, besides, water generates a power failure and numerous damages in shops, stores, houses, industries, streets, etc(see Fig.7). The 27th, rivers gradually begin to return to normal (see Fig.5). We have to take into account that, besides abundant rainfall, snowfallen in the previous days increases the contribution of water to the rivers.

Day 24: The episode begins with snowfall around 600 meters and instability in height, which cause some storms. These particularly affect theCantabrian watershed. This day, rainfall is in the shape of showers mainly; thus, their intensity is occasionally moderate, but their frequency isoccasional. The highest accumulated precipitation occurs in the central area of Basque Country (see Fig.6) (Arrasate, 54.9 l/m2; Mañaria, 45.6l/m2; Otxandio, 45.4 l/m2). However, very abundant rainfall is already accumulated in the Añarbe station that day (53.6 l/m2). During the evening,rivers reach the yellow level in some basins from east and south (Eibar in the Deba basin, Alegia in the Oria basin, Ibai Eder in the Urola basin,Abetxuko in the Zadorra basin, etc.).

Day 25: As cold air retires in high layers in the early hours on Wednesday 25, part of instability also disappears. However, a warm frontprovides persistent character to the precipitation, so accumulated amounts are much higher than the previous day. The most affected areas arethe same as the day before, that is, the central area of Basque Country (Otxandio, 76.2 l/m2, Arrasate, 62.1 l/m2) and, above all, the mountainvalleys in the east of Gipuzkoa (Añarbe, 132.5 l/m2; Berastegi, 85.4 l/m2; Ameraun, 85.3 l/m2; Ereñozu, 76.2 l/m2) (see Fig.6 and Table.1). Thereare some problems in the riverbeds from Gipuzkoa, Alava and east of Bizkaia, due to the persistent character of the precipitation principally.

Day 26: In general, rainfall is less abundant, especially in the central area, since precipitation abates early in the afternoon. Even so,accumulated amount exceeds again 100 l/m2, for example in Añarbe station (113.1 l/m2). Also noteworthy data are registered in nearby stations,though slightly lower than the 25th (Berastegi, 71.7 l/m2; Ameraun, 71.4 l/m2) (see Fig.6 and Table.1). Rivers remain at yellow or orange level insome Gipuzkoa basins with Urumea basin iexpetially affected where Ereñozu reachs red level (see Fig.5).

Day 27: During the first half of this day, rainfall is occasional and less intense, allowing a decrease in the rivers level, so the episode gets tothe end (see Fig.5 and Fig.6).

Episode description

IntroductionIn the Basque Country the hydrography shows two main watersheds, the Cantabric and the Mediterranean (see Fig. 1). In the

first one, there are steep slopes of around 1000 meters or more in few kilometers and there, the precipitation rates are high, as aresult of which flooding are caused with certain regularity. Nevertheless, the rivers of the Mediterranean slope have less incidencerate of floods because the precipitation that falls there and the slope of the basins are less.

The study area, Urumea area, has one of the highest records in the Iberian Peninsula and Europe, specially certain stations,such as Artikutza (see Fig.1.), where the average annual precipitation is 2600 mm.

Events when more rainfall is accumulated in the study area, occur in the cold season, between October and April. The zonalcirculation descends from latitude and the areas of low pressure are deeper, with very active fronts associated. Sometimesrainfall, together with fallen melting snow, can cause significant floods. During the warm season, storms can also leave significantprecipitation or in singular situations, mesoscale convective systems could leave higher amounts than the ones produced in coldperiods, with higher intensities of precipitation, but these situations are unusual. One such situation was that caused one of themajor floods that have hit the Basque Country in August 1983.

The analyzed episode belongs to an event of cold season, when very significant amounts of precipitation are registered.Accumulated rainfall exceeds 120 mm in 24 hours and reaches 300 mm in some stations in two days (see Fig.6 and Table.1).Furthermore, we have to consider extra river flow increases because of melted snow.

15th EMS Annual Meeting 12th European Conference on Applications of Meteorology (ECAM)

07 – 11 September 2015, Sofia, Bulgaria

Station Day 24 Day 25 Day 26 Day 27 Total

Lesaka San Anton 44.9 48.9 176.4 99.5 369.7Central Arrambide 42.3 68.2 159.8 88.2 358.5

Artikutza 51.1 79.2 190.2 - 320.5Añarbe 53.6 132.5 113.1 20 319.2Leitza 32 69 129 80 310

Goizueta 44 81.5 117 63 305.5Areso 29.7 72.2 91.5 67 260.4

Berastegi 40.5 86 71.7 32.8 231Ameraun 38.1 85.3 71.4 29.1 223.9Andoain 36.5 64.5 45.8 19.2 166Ereñozu 33.2 72.1 43.3 9.1 157.7Lasarte 34.5 56.5 47.7 16.1 154.8

Miramon 38.1 54.8 45.8 7.7 146.4Oiartzun 6.5 36.9 38.2 5.5 87.1Zarautz 19.7 24 19.6 4.5 67.8

Central Arrambide

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Figure 1. Urumea basin and surroundings.

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Figure.3: MSG images: IR 10.8 µm channel from2015/02/24 to 2015/02/26 (00 and 12).

Figure.2: See level pressure and geopotentialand isotherms at 850 hPa

and 500 hPa..

Table.1: Daily and total accumulated precipitation in Urumeabasin and surroundings stations

Figure.6: Accumulated precipitation during all event (12:00 24th to 12:00 27th)and for day 24th, 25th, 26th and 27th (from top to down and left to right).

Figure.7: Some pictures for the studied event (from Goizueta in Hernani and Martutene in Donostia).

Figure.5: River level evolution and precipitation in some stations during the event( Txomin Enea, Martutene, Ereñozu, Añarbe)

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Figure 6. RHI product for day 25 at 00:48 and 9:48 (left to right).

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