4.17 Spain. Catalonia

Transcription

1 4.17 Spain Catalonia In Spain, inundation studies are the responsibility of the respective Hydrographic Confederations of each river basin (River Basin Authorities). The actual status of inundation studies varies from basin to basin with significant differences in the level of achievement. A good example of inundation studies is the one corresponding to river basins in Catalonia, where the Government of Catalonia (Generalitat de Catalunya) through the Catalan water Agency has elaborated a inundation management plan, Inuncat 22, where all the inundation areas corresponding to rivers in Catalonia have already been produced. The Catalonian Water Agency (Government of Catalonia) has evaluated for the river basins of Catalonia inundation maps for the main river courses (Delimitació de zones inundables a les conques internes de Catalunya) as well as for the Ebro river (Delimitació de zones inundables a les conques de l'ebro) which has a basin shared with other regions. These studies define the inundation areas for return periods of 1/50, 1/100 and 1/500 yr 23 and, also delineate potential flood areas from the geomorphological standpoint. In addition to this, the study also includes a database with critical points, which are defined as locations where the experience acquired during many years of river management has shown that they present repeating problems 24. The inundation maps for return periods of 1/50, 1/100 and 1/500 yr (Figure 4.90) are interactively available in PDF format. In the example shown here, corresponding to the Besós river (the Northern natural border of the city of Barcelona) only a part of the total map is shown and the legend has been placed on top in order to show only the most relevant information. In Figure 4.91 an example is shown a flood hazard map for the Besós river basin at the northern part of the city of Barcelona, with a part of the legend shown above. These maps are also available as PDF files directly from the internet 25. Also in this case this is only a part of the total map; the original full sheet includes information on the map and a clear indication of the location of the map area within the total province of Catalonia. In this case, there is no indication of the return period that is represented in the map, because they delineate potential flood areas from the geomorphological standpoint using historical information (areas already subjected to floods) or geologic evidences. Use is made of signs in green, orange and red to indicate level of low, medium and high risk (see legend) Atlas of Flood Maps

2 Figure 4.90 Flood extension maps for the Besós river basin (N of Barcelona) for 3 return periods Atlas of Flood Maps 120

3 Figure 4.91 Flood hazard map for Besós river basin (N of Barcelona) for 3 return periods Comment on the maps The layout of these maps is very clear and it is also relatively easy to distinguish between the three return periods. From the maps it is clear though that emphasis is placed on the presentation of the flood extension for a return period of 1/50 yr, which is shown both with a bordering line as well as with a hatched surface. The use of red for the lowest return period (1/50 yr) is chosen not to indicate the highest danger, but the highest risk of occurrence (i.e. the highest probability). In Figure 4.92 a full flood hazard map is shown of a part of the Spanish coast in order to show the general outline of such a map, which when printed on a larger scale result very clear and easy to read. 121 Atlas of Flood Maps

4 Obrir el Pànol de delimitació hidràulica Figure 4.92 Full image of a flood hazard map in Catalonia

5 English translation of legends Type/level of hazard Low Medium High Level of affectation effects on an area effects on a stretch Critical point / hot spot (e.g. bridge) effects on large areas Description code AA: river or creek BB: municipality NN: number of order of hazard From top to down: Legend Geomorphology Symbols Flood hazard area Embankment area Limit of historical flood area Mark of recent movements Active cone of dejection = floodable Possible flow direction or water flow Flow deviation due to existing anthropogenic actions Flood retaining wall Mark of alluvial erosion / old meanders Former lagoon/ dried deltaic lagoon or wetland 123 Atlas of Flood Maps

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7 Northern Spain In the Spanish northern basin, km 2 of total area, inhabitants use intensively the scarce plain surfaces, mostly associated to floodplains. The geographic and geological characteristics of the Cantabrian Range provide an environment where rivers typically have high gradients with straight, short and incised channels, and its discharges are high in amount, velocity, erosive power and load of sediments. Another important characteristic is the quick response of these rivers after rainfall. An approach to river activity in its floodplain can be obtained by geomorphic evidences studied by the Geomorphology as the science of landscape forms. As a result, the floodplain is divided in different terrace levels associated to different flood frequency, mostly restricted by steep banks and cliffs, and also defined by the floods historic analysis. This method, based in real evidences caused by floods, is especially useful in fluvial systems where rivers are confined within high valley walls and where the floodplain external limit is highly abrupt. The first step is to define the study area by delineating the alluvial plain limits and the channel course with topographic criteria and helped by aerial photos. An analysis of a series of historic photographs could help to understand the river behaviour during the recent past. It is necessary to take note of fluvial system properties as channel width, margin height, steep or gradual margins, granulometric measures, etc. Furthermore, the main point is to map the geomorphic elements of the alluvial landscape that are mentioned later. GIS software is an essential tool because it allows to map and store all the information for its representation in flood maps or to be used later, for example, in emergency plans. Historical information of flood events has been obtained from documentary sources and field interviews with local residents. The former allow the identification of the main locations with flood problems, the latter provide more accurate data about the extent and characteristics of the events. In bibliographical literature and newspapers an inventory of sites historically affected by floods was collected for the time period in an intensive revision of nearly 7300 newspapers. The low precision of the historic floods data obtained in the previous compilation forced to realize 2000 field interviews to local residents distributed in 340 km 2 of floodplains. Dates, extent, damage (agriculture, buildings, roads links, etc.), water depth, grain size and sedimentation areas and overflow zones were recorded. All this information was stored in a database, including photographs and videos of some floods and data of gauged flow and rainfall of every event when the information is available. 125 Atlas of Flood Maps

8 Information obtained from documentary sources (1 and 3) and field work (2 and 4). Example of event map in a section of the Arnoia river (Galicia, NW Spain) Atlas of Flood Maps 126

9 Geomorphological evidences of floods are erosive or depositional landforms or other indicators of fluvial activity: Channel course as a sign of the different fluvial energy among straight, meandering or braided rivers. Steep banks or cliffs: linked to the main channel, to secondary channels or isolated in the floodplain. Overflow point within we can think about water course throw the floodplain. Recent deposits (characterization): levees, crevasse splays (granulometric fractions help to understand the flood energy). Microtopographies are identified as an irregular topography in the floodplain, at different scales, as a result of the combination of erosive and deposited forms (negative and positive forms). Natural narrowings or river confluences create important local variations of the fluvial energy. Evidences of different floods frequency: 1 and 2 flood deposits, 3 microtopographt, 4 crevasse and overflow point and 5 steep bank defined by a cliff. The hydrological behaviour of rivers can be altered by different anthropic elements which have to be identified and described: human-made conflictive points, canalizations, reservoirs, others. All field parameters are analysed by searching overflow points and its relationship with the observed geomorphic evidences. Also, zones with different fluvial activity, based on the geomorphic analysis, are linked to an approximate return period by comparison of event properties obtained from the historic analysis (surface occupied, speed, swept sediments, overflow points, etc.). A good practice is to carry out a regional analysis in order to check if the followed criteria were homogeneous in the entire basin and also to compare present floods. 127 Atlas of Flood Maps

10 Finally, all the information is used to distinguish different units represented in the flood map: Low Terrace: it is the most active floodplain terrace flooded at least once every 10 years so it is plentifully of geomorphic evidences. Middle Terrace: higher than the low terrace, it is associated to a flood frequency of once every 50 years. High Terrace and Very High Terrace: with a flood frequency of once every one hundred years and five hundred years respectively. They are short of geomorphic evidences and human activity is highly intense. Example of a floodplain zonification in a section of the Narcea river (Asturias, NW Spain) with fluvial and torrent floods. Other floods can be mapped as Tidal influence, mountain torrents an also drainage deficiency caused by artificial elements in the floodplain. River flood risk determination has been carried out combining flood hazards mapping and land use vulnerability. Additionally, the risk map provides supplementary information about mountain torrent hazard, tide dynamics and drainage deficiencies, and also it includes an inventory of assets at risk in the analyzed river sections. Vulnerability maps show different classes established regarding land use and a combined indicator which takes into account material loss (direct and indirect economic vulnerability, VED and VEI), loss of life (population vulnerability, VP) and the reaction capacity decrease and services provision interruption (community vulnerability, VC), plus the possible damage of Cultural Monuments protected by the regional government. Vulnerability and flood mapping are combined in a GIS in order to obtain different categories of flood risk: very low, low, medium, high and very high, which are displayed in a 1:5.000 scale topographic map. Atlas of Flood Maps 128

11 Flood hazards (left) and Land-use (right) maps of the Caudal river floodplain through the city of Mieres (Asturias, NW Spain) 129 Atlas of Flood Maps

12 Vulnerability map of the Caudal river floodplain through the city of Mieres (Asturias, NW Spain) used in the risk estimation. Flood risk map of the Caudal river floddplain through the city of Mieres (Asturias, NW Spain) Atlas of Flood Maps 130