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4.17 Spain

CataloniaIn Spain, inundation studies are the responsibility of the respective Hydrographic Confederations of each riverbasin (River Basin Authorities). The actual status of inundation studies varies from basin to basin with significantdifferences in the level of achievement.

A good example of inundation studies is the one corresponding to river basins in Catalonia, where theGovernment of Catalonia (Generalitat de Catalunya) through the Catalan water Agency has elaborated ainundation management plan, Inuncat22, where all the inundation areas corresponding to rivers in Catalonia havealready been produced.

The Catalonian Water Agency (Government of Catalonia) has evaluated for the river basins of Cataloniainundation 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 sharedwith other regions. These studies define the inundation areas for return periods of 1/50, 1/100 and 1/500 yr23

and, also delineate potential flood areas from the geomorphological standpoint. In addition to this, the study alsoincludes a database with critical points, which are defined as locations where the experience acquired during manyyears of river management has shown that they present repeating problems24.

The inundation maps for return periods of 1/50, 1/100 and 1/500 yr (Figure 4.90) are interactively available inPDF format. In the example shown here, corresponding to the Besós river (the Northern natural border of the cityof Barcelona) only a part of the total map is shown and the legend has been placed on top in order to show onlythe 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 ofBarcelona, with a part of the legend shown above. These maps are also available as PDF files directly from theinternet25. Also in this case this is only a part of the total map; the original full sheet includes information on themap and a clear indication of the location of the map area within the total province of Catalonia. In this case, thereis no indication of the return period that is represented in the map, because they delineate potential flood areasfrom the geomorphological standpoint using historical information (areas already subjected to floods) or geologicevidences. Use is made of signs in green, orange and red to indicate level of low, medium and high risk (seelegend).

22 http://mediambient.gencat.net/aca/ca//planificacio/inundabilitat/inici.jsp23 http://mediambient.gencat.net/aca/ca/planificacio/inundabilitat/delimitacio/pl_periode.jsp24 http://mediambient.gencat.net/aca/documents/ca/planificacio/inuncat/conquesinternes/punts_critics.pdf25 http://mediambient.gencat.net/aca/ca/planificacio/inundabilitat/delimitacio/pl_potencial.jsp

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

Figure 4.91 Flood hazard map for Besós river basin (N of Barcelona) for 3 return periods

Comment on the mapsThe 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 returnperiod of 1/50 yr, which is shown both with a bordering line as well as with a hatched surface. The use of red forthe 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 outlineof such a map, which when printed on a larger scale result very clear and easy to read.

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Figure 4.92 Full image of a flood hazard map in Catalonia

Obrir el Pànol de delimitació hidràulica

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English translation of legends

Type/level of hazard Level of affectation

Low effects on an area

Medium effects on a stretch

High 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

Northern Spain

In the Spanish northern basin, 38 384 km2 of total area, 2 900 129 inhabitants use intensively the scarce plainsurfaces, mostly associated to floodplains. The geographic and geological characteristics of the Cantabrian Rangeprovide an environment where rivers typically have high gradients with straight, short and incised channels, and itsdischarges are high in amount, velocity, erosive power and load of sediments. Another important characteristic isthe quick response of these rivers after rainfall.An approach to river activity in its floodplain can be obtained by geomorphic evidences studied by theGeomorphology as the science of landscape forms. As a result, the floodplain is divided in different terrace levelsassociated to different flood frequency, mostly restricted by steep banks and cliffs, and also defined by the floodshistoric analysis. This method, based in real evidences caused by floods, is especially useful in fluvial systems whererivers 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 withtopographic criteria and helped by aerial photos. An analysis of a series of historic photographs could help tounderstand the river behaviour during the recent past. It is necessary to take note of fluvial system properties aschannel width, margin height, steep or gradual margins, granulometric measures, etc. Furthermore, the main pointis to map the geomorphic elements of the alluvial landscape that are mentioned later. GIS software is an essentialtool 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 localresidents. The former allow the identification of the main locations with flood problems, the latter provide moreaccurate data about the extent and characteristics of the events. In bibliographical literature and newspapers aninventory of sites historically affected by floods was collected for the time period 1522-2007 in an intensiverevision of nearly 7300 newspapers. The low precision of the historic floods data obtained in the previouscompilation forced to realize 2000 field interviews to local residents distributed in 340 km2 of floodplains. Dates,extent, damage (agriculture, buildings, roads links, etc.), water depth, grain size and sedimentation areas andoverflow zones were recorded. All this information was stored in a database, including photographs and videos ofsome floods and data of gauged flow and rainfall of every event when the information is available.

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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)

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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 identifiedand described: human-made conflictive points, canalizations, reservoirs, others.

All field parameters are analysed by searching overflow points and its relationship with the observed geomorphicevidences. Also, zones with different fluvial activity, based on the geomorphic analysis, are linked to anapproximate 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 checkif the followed criteria were homogeneous in the entire basin and also to compare present floods.

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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 ofgeomorphic 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 hundredyears 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 artificialelements 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 anddrainage 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 intoaccount 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 thepossible 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: verylow, low, medium, high and very high, which are displayed in a 1:5.000 scale topographic map.

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

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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)

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