A case study of local floods in Urumea area (Basque Country) Jaizkibel Zarautz Oiartzun J. Egaña 1 , 2 , S. Gaztelumendi 1 , 2 , V. Palacio 1 , 2 , M. Martija 1 , 2 Lesaka San Anton Lasarte 1 - Basque Meteorology Agency (EUSKALMET). Parque tecnológico de Álava. Avda. Einstein 44 Ed. 6 Of. 303, 01510 Miñano, Álava, Spain 2 - TECNALIA, Meteorology Area. Parque tecnológico de Álava. Avda. Albert Einstein 28, 01510 Miñano, Álava, Spain. Andoain Central Arrambide Artikutza Goizueta Abstract. Ameraun In this study we present an analysis of a flood episode affecting Urumea river basin on 25-27 February 2015. During those Figure 1. Urumea basin and Berastegi days a moderate and persistent rainfall episode affects this area of Gipuzkoa region in Basque Country (see Fig.1), a very rainy surroundings. Leitza area during cold season (October-April) where these situations occurs with some frequency. URUMEA Areso The synoptic environment is characterized by a powerful anticyclone centered in the Atlantic, creating a northwesterly flow where it runs different active fronts. This succession of fronts generates abundant rainfall that does not have high intensities , but are persistent . The accumulated precipitation on 24-hour exceeds 120 mm in the study area, and around 300 mm for the whole time period event. This situation causes rivers to overflow at different points, generating local floods. In addition to rain, certain contribution from melting are produced. The Urumea river overflows in different points, promoting numerous problems specially in populated areas around Donostia. Many people need to be evacuated, and water causes considerable damages and problems all around 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 meteorological characteristics are analyzed including datasets coming from the Basque Country Automatic Weather Station (AWS) Mesonetwork and other sources (MSG, Radar, etc) available during this episode. Introduction. In 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 a result of which flooding are caused with certain regularity. Nevertheless, the rivers of the Mediterranean slope have less incidence rate 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 zonal circulation descends from latitude and the areas of low pressure are deeper, with very active fronts associated. Sometimes rainfall, together with fallen melting snow, can cause significant floods. During the warm season, storms can also leave significant precipitation or in singular situations, mesoscale convective systems could leave higher amounts than the ones produced in cold periods, with higher intensities of precipitation, but these situations are unusual. One such situation was that caused one of the major floods that have hit the Basque Country in August 1983. Figure.2: See level pressure and geopotential The analyzed episode belongs to an event of cold season, when very significant amounts of precipitation are registered. and isotherms at 850 hPa Figure.3: MSG images: IR 10.8 µm channel from and 500 hPa.. Accumulated rainfall exceeds 120 mm in 24 hours and reaches 300 mm in some stations in two days (see Fig.6 and Table.1). 2015/02/24 to 2015/02/26 (00 and 12). Furthermore, we have to consider extra river flow increases because of melted snow. 4 3,5 Environment 3 2,5 A powerful anticyclone is located over the Azores and sends strong, wet and cold northwest wind to the Cantabrian coast 2 during most of the time of this episode. At the end of it, high pressures adopt wedge form in the Cantabrian zone. On the other 1,5 River Level (m) Level River Figure 6. RHI product for day 25 at 00:48 and 9:48 (left to right). hand, a depression is situated in the Tyrrhenian Sea and these low pressures prevail over the British Isles and Central Europe, 1 Txomin Enea with its center in Northern Europe (see Fig.2). The frontal systems, associated with the depression, pass through the Gulf of 0,5 Table.1: Daily and total accumulated precipitation in Urumea Biscay and, in its wake, rainfall intensify. On Tuesday 24, in addition, there is cold air in high layers, which causes instability and 0 basin and surroundings stations 6 some storms. That cold air at 500 hPa level is withdrawn during the first half of Wednesday 25 and instability in height starts to Station Day 24 Day 25 Day 26 Day 27 Total disappear. Intense and more or less short-lived showers give way to frontal rainfall and, therefore, more persistent. It is there 5 Lesaka San Anton 44.9 48.9 176.4 99.5 369.7 Central Arrambide 42.3 68.2 159.8 88.2 358.5 where the largest amounts of precipitation are accumulated, specially due to the passage of a warm quasi-stationary front (see 4 Artikutza 51.1 79.2 190.2 - 320.5 Fig.2 and Fig.3). 3 Añarbe 53.6 132.5 113.1 20 319.2 Leitza 32 69 129 80 310 In the Meteosat images (see Fig.3) we can see different large-scale cloud systems and particularly ho w it affects us a line of 2 Goizueta 44 81.5 117 63 305.5 instability at the beginning and a warm front during the 25th and 26th. In Fig.4 we can observe different types of precipitation in (m) Level River Areso 29.7 72.2 91.5 67 260.4 1 Martutene Berastegi 40.5 86 71.7 32.8 231 the Radar images. Showers take place at the beginning of the event, they are quite intense, but not widespread. However, rainfall Ameraun 38.1 85.3 71.4 29.1 223.9 become more widespread, persistent and with stratiform character due to the arrival of the warm front, when more precipitation 0 Andoain 36.5 64.5 45.8 19.2 166 3,5 5 Ereñozu 33.2 72.1 43.3 9.1 157.7 are registered. Lasarte 34.5 56.5 47.7 16.1 154.8 Precip (l/m2) 4,5 Miramon 38.1 54.8 45.8 7.7 146.4 3 Ereñozu Oiartzun 6.5 36.9 38.2 5.5 87.1 River level (m) 4 Zarautz 19.7 24 19.6 4.5 67.8 Episode description 2,5 3,5 3 2 Some rivers reach flood level and overflow in different basins during the 25th and 26th. The most affected basins is Urumea. The Urumea 2,5 river overflows in different points (see Fig.5), creating numerous problems in a densely populated area, around Donostia (Gipuzkoa capital). Many 1,5 2 people are evacuated and, besides, water generates a power failure and numerous damages in shops, stores, houses, industries, streets, etc River Level (m) Level River (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, snow 1 1,5 fallen in the previous days increases the contribution of water to the rivers. 1 0,5 Day 24 : The episode begins with snowfall around 600 meters and instability in height, which cause some storms. These particularly affect the 0,5 Cantabrian watershed. This day, rainfall is in the shape of showers mainly; thus, their intensity is occasionally moderate, but their frequency is 0 0 occasional. The highest accumulated precipitation occurs in the central area of Basque Country (see Fig.6) (Arrasate, 54.9 l/m2; Mañaria, 45.6 2,5 5 l/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, Añarbe 4,5 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, 2 4 Abetxuko in the Zadorra basin, etc.). 3,5 Day 25: As cold air retires in high layers in the early hours on Wednesday 25, part of instability also disappears. However, a warm front 1,5 3 provides persistent character to the precipitation, so accumulated amounts are much higher than the previous day. The most affected areas are 2,5 the 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 mountain 1 2 valleys 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). There 1,5 are some problems in the riverbeds from Gipuzkoa, Alava and east of Bizkaia, due to the persistent character of the precipitation principally. 24 25 0,5 1 River Level (m) Level River Day 26: In general, rainfall is less abundant, especially in the central area, since precipitation abates early in the afternoon. Even so, 0,5 accumulated amount exceeds again 100 l/m2, for example in Añarbe station (113.1 l/m2). Also noteworthy data are registered in nearby stations, 0 0 though slightly lower than the 25th (Berastegi, 71.7 l/m2; Ameraun, 71.4 l/m2) (see Fig.6 and Table.1).