Gravel deposit produced by a flash paleoflood in a succession of Quaternary terraces in the Plain of Vic (NE Iberian Peninsula)

In contrast with the abundance of studies of fluvial terraces, caused by river dynamics, there are very few descriptions of alluvial deposits produced by flash floods and mass movements. This study describes a late Pleistocene sedimentary deposit produced by a flash paleoflood and attempts to explain its genesis and its source areas. The Plain of Vic, drained by the river Ter and its tributaries, is one of the eastern erosive basins bordering the sedimentary Ebre basin (NE Iberian Peninsula). This plain has a length of 35 km and an average width of 8 km with a N-S direction and lies mainly on the Marls of Vic Fm. These materials are the less resistant lithologic members of the monocline Paleogene stratigraphic succession that dips to the west. The basal resistant bed that forms the eastern cuesta is the Sandstones of Folgueroles Fm. On the top, the resistant lithologic beds that form the scarp face are the sandstones of La Noguera in the Vidrà Fm. On the scarp face, various coalescent alluvial bays have been developed, which have accumulated up to eight levels of alluvial terraces. In one of them, formed by the river Mèder and the Muntanyola stream, a gravel deposit up to 5 m thick formed in a single episode outcrops, in a position T4,. A dating of the river Ter T5 has obtained an age of 117.9 9.5 Ky. The accumulation of gravel erodes another level of metric thickness of the same lithological characteristics and texture. The deposit does not have any internal structure or organization of pebbles. At its base, there are several metric blocks coming directly from the slopes. The accumulation of gravel is block-supported with a sandy matrix. The pebbles size is centimetric to decimetric (90%). Its texture is subrounded. Lithologically, the deposit consists mostly of sandstone and limestone from the top of the series. On the ground, the accumulation of gravel is elongated, with a maximum length and width of 550 m by 160 m and a slope surface of 2.54%. With an area of 56,500 m2, this totals to a volume of 282,500 m3. It is separated 400 m from the inner slope formed by two tributaries, and 100 m from the slope on the right side. The width of the right floodplain is 60 m, and the left one is 20 m wide. The deposit is eroded laterally by several meanders of later episodes which uncovered the accumulation of gravel. This accumulation can be considered as a proximal deposition of a flash paleoflood event in the bay formed by two alluvial tributaries. It cannot be ruled out that higher and older gravel deposits, belonging to T8, have the same genesis. Distally, at a distance of 2,600 m, in the same stratigraphic position (T4) and with an average gradient of 1.26%, a lutitic layer of metric thickness that corresponds to a distal flashflood accumulation outcrops. Understanding the processes that formed this deposit can help gain insight on flash floods and their role in the evolution of geomorphic features and landscapes.Postprint (published version

Analysis of the convective timescale during the major floods in the NE Iberian Peninsula since 1871

Floods are the most severe natural hazard in the western Mediterranean basin. They cause most of the damages and most of the victims. Some of the selected floods caused more than one hundred casualties each and a large quantity of damages in infrastructures. In a previous work (Balasch, et al., 2015), using the PREDIFLOOD database (Barriendos et al., 2014) we studied the atmospheric conditions that occurred during some of the most important floods occurred in the north-east of the Iberian Peninsula in the last centuries: 1874, 1875, 1894, 1897, 1898, 1901, 1907, 1913, 1919, 1932, 1937, 1940, 1962, 1963, 1977, 1994, 1996, and 2000. We analyzed the atmospheric synoptic situations at the time of each flood from the data provided by NOAA 20th Century Reanalysis and we compared it to the rainfall spatial distributions obtained with the hydrological modeling. In this work we enlarge the previous investigation by analyzing the evolution of a convective index proposed by Done et al. (2006) and modified by Molini et al. (2011). This index, called convective time scale, is obtained from the evolution of CAPE and is used to separate equilibrium and non-equilibrium convection. In the former, CAPE generated by large-scale processes is balanced by the consumption due to convection. In the second case, CAPE is created by large-scale processes over a long time and is rapidly consumed during outbreaks of convection. Both situations produced a totally different evolution of CAPE with low and approximately constant values in the first case and large and variable values in the second. Additionally, from this index it can be estimated the rainfall rate. We use data provided by NOAA 20th Century Reanalysis, to calculate the convective time scale and to analyze its evolution and horizontal distribution. We study the correspondence between the convective timescale, the season when the flood occurred, duration of the rainfall, and the specific peak flow rate of the flood. Finally, for the most recent episodes rainfall rate estimation from the convective timescale is compared with the observations. Balasch, J. C., Ruiz-Bellet, J. L., Tuset, J., Barriendos, M., Mazón, J., Pino, D. and Castelltort, X.: Transdisciplinary and multiscale reconstruction of the major flash floods in NE Iberian Peninsula. EGU General Assembly, 2015. Barriendos, M., Ruiz–Bellet, J. L., Tuset, J., Mazon, J., Balasch, J. C., Pino, D., Ayala, J. L.: ThePeer ReviewedPostprint (published version

The 2-3 November 2015 flood of the Sió River (NE Iberian Peninsula): a flash flood that turns into a mudflow downstream

Historical and recent evidence shows that many floods in the interior of Catalonia (NE Iberian Peninsula) usually have such a great sediment load that can even alter the hydraulic behaviour of the flow. This is especially true in catchments with a great proportion of agricultural soils, which are the main source of sediment. The night of 2-3 November 2015 torrential rains fell on the headwaters of the Sió River catchment (508 km2); the subsequent flood caused four deaths and many damages along the stream. The hydrological, hydraulic and sedimentary characteristics of this recent flood have been analysed in order to gain a better insight on the characteristics of the major historical floods in the same catchment. The rainfall height on the headwaters was between 139 and 146 mm in ten hours, with a maximum intensity of about 50 mm h-1. In the rest of the catchment it rained much less (22-71 mm). The agricultural soils in the headwaters show evidence of intense erosion by laminar and concentrated Hortonian overland flow in their superficial layer (Ap1; 10 cm), which uncovered the more compact underlying layer (Ap2). The peak flow in the headwaters (Oluges) was 90 m3 s-1 (that is, a specific peak flow near 1 m3 s-1 km-2) and it diminished downstream: 40 m3 s-1 in the centre of the catchment (Oluges + 27 km) and 15 m3 s-1 in the outlet (Oluges + 54 km). The suspended sediment load was 10-15% in volume in the headwaters and, judging from recorded images and eyewitnesses, it increased as the flow moved downstream, turning the flash flood into a mudflow. This concentration gain was most probably caused by the flood wave’s water loss due to the dryness of the riverbed and translated in an increased viscosity that ultimately altered the hydraulic behaviour of the flow, slowing it down. This process of water loss has been observed in flash floods in dry riverbeds in arid and semiarid areas such as Negev (Israel) and Atacama (Chile). Historical floods in neighbouring catchments (Ondara and Corb Rivers) are known to have had hyperconcetrated flows.Peer ReviewedPostprint (published version

Middle Triassic evaporite sedimentation in the Catalan basin: implications for the paleogeographic evolution in the NE Iberian platform

The eastern sector of the epicontinental Iberian platform underwent restriction after the sedimentation of the lower Muschelkalk carbonates (Middle Triassic) under extensional regime. This resulted in the accumulation of the marine evaporites and the alluvial siliciclastics of the middle Muschelkalk facies in the Triassic Catalan basin, which varied between 100 and 120 m in thickness. This facies consists of three lithostratigraphic units sedimented at basin scale (Lower, Middle and Upper), each of which includes a distinct evaporite unit. In the Lower Unit, the evaporitic sedimentation started as a transgressive sulfate lagoon (Paüls Gypsum unit). During the Middle Unit time, a regressive evaporitic mudflat, made up of a mosaic of shallow gypsum salinas surrounded by anhydrite sabkhas (Arbolí Gypsum unit) developed; in the northeastern half of the basin, an alluvial plain was formed by siliciclastics (Guanta Sandstone unit) of a west and northwest provenance (Lleida High). During the Upper Unit time, a new transgressive sulfate lagoon occupied the southern half of the basin (Camposines Gypsum), whereas an evaporitic mudflat of red-to-variegated mudstones, marls, and lacustrine carbonates developed in the northern half. Cyclic sedimentation was mainly recorded in the evaporitic mudflat-alluvial plain complex of the Middle Unit. The sulfur isotopic values of gypsum in the three evaporite units show a decrease in d34S with time and also a clear distinction from the values of the Keuper facies in the basin. A division of the lithostratigraphic succession into two third-order depositional sequences is proposed. The middle Muschelkalk succession in the Catalan basin is compared with the equivalent one in the subsurface of the adjacent Triassic Maestrat basin, which was filled with >600 m of marine evaporites.Peer ReviewedPostprint (author's final draft

The backwater effect as a tool to assess formative long-term flood regimes

The Ter River drains the south-eastern Pyrenees and flows into the Mediterranean Sea. A lithological constriction affects the normal water flow in the La Plana de Vic area. As a consequence of this disturbance in the flow, the shrinkage in the bedrock activates a backwater effect. Systematic water retention during extreme events has formative consequences. The process involves the creation of a helical flow for the structural misalignment of the channel at the point of narrowness. The backwater effect transmits the secondary currents backwards, resulting in the creation of a sinuous pattern upstream from the shrinkage. A tributary, the Gurri River, flows into the main river just before the constriction, and this too has been affected by the process of water storage and channel pattern change. A two-dimensional numerical flow model (Iber) has modelled several hypothetical cases of flooding. This modelling aims to test the reach of the hydraulic influence upstream from the constriction, both in the main river and its tributary, due to the backwater effect. Moreover, it sought to find the best balance of discharge between the two streams. The upstream reach of the backwater effect was considered as its endpoint. During the flooding, the system reached hydraulic equilibrium between the constriction and the two endpoints when both were at the same water level, and the flow regime was subcritical everywhere. The hydraulic conditions that drove the water flow to the equilibrium are thought to be the ones that promoted formative processes in a sinuous pattern on a long-term basis. The water discharge values obtained from this procedure are, in general terms, 50% above those considered to be a peak flood with a recurrence time of 500 years (Q500), and they accomplish the conditions of extreme events. Thus classified, the calculated discharges can be helpful for comparison with those measured in historical and systematic records, because a water discharge like the one calculated has never been measured at the Ter River.Peer ReviewedPostprint (author's final draft

Drainage basins evolution during the Neogene-Quaternary. Ebro Basin eastern margin

The monocline layout of the sedimentary pile of the Ebro Basin in its Eastern margin determines the generation and emptying of the adjacent erosive basins. It controls the drainage changes in the original sedimentary basin. A model of emptying erosive basins corresponding to a drainage architecture and sediment production is proposed. The emptying of erosive basins is achieved by two vectors: A) A drainage basin area growth due to anaclinal streams eroding into the resistant layers of the monocline stratigraphic succession. These streams empty and link small depressions generated at the expense of the lateral extension on the less resistant lithologic member. B) The drainage basin outlet point base level controls the drainage network entrenchment facilitated by the gradients created by Neogene extensional faults from the Valencia Trough. A model of the growth and entrenchment of erosive basins as well as the generated landforms and sediment production is described and analyze

The Upper Garonne flood series from discharge gauging data (Bossost, Vald’Aran, Spain) and reconstruction of historical floods (Saint-Béat, France)

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