[DREAL OCCITANIA] Information mapping of flooding areas — flood channel (CIZI) — Midi-Pyrénées

This lot consists of linear objects materialising the flood channels, forms inscribed in hollow in the floodplain. Flood channels are areas of flow velocity. Currents are likely to cause obstruction destruction, scouring or accumulation of gravel and sand banks. Objectives of CIZI: This map, produced under the plan contract between the State and the Midi-Pyrénées Regional Council, aims to inform citizens and decision-makers about the risk of flooding. It has no regulatory scope and cannot replace a planning document such as the Soil Occupancy Plan (POS) or the Risk Prevention Plan (RPP). Nevertheless, it allows citizens and officials, elected or administrative, to better appreciate the extent of areas that pose a significant flood risk or promote water sprawl. It is a tool for information and decision-making. All the maps are assembled, by river basin, in a cartographic atlas of all the flooding areas of the Midi-Pyrénées region, available from the State services or the Midi-Pyrénées Regional Council. This card may be reproduced, except for commercial purposes. It traces the contours of the most frequently flooded areas as well as the boundary of the highest known waters. The approach combines hydrology (knowledge of watercourses and flow dynamics) with river geomorphology (analysis of the forms of the bottom of the valley). All available data have been taken into account and analysed: studies, archives, hydrological information... Limitations of the study: The scale adopted is 1/25 000. The accuracy is therefore in the order of 25 meters (1 mm. on the map). That is why it is illusory to search for specific information on a plot scale. Enlarged by photocopy, the card will not be more precise. Developed on a scale of 1/25 000, this map was reproduced at 1/30,000 for publishing purposes. A centimeter on the map therefore represents 300 meters in reality. Another limitation of the study is the type of flood studied: the overflow of the watercourse. Floods have not been mapped by rising water or of the type of urban storm runoff. The presence of water in a bowl bottom, e.g. a doline, is another natural hydrological phenomenon and is therefore not not on that map. The mapping is very complete but is not exhaustive: — very strong but very localised rains (thunderstorms) can cause locally powerful floods but which very quickly lose this power downstream of the basin. The phenomenon is so punctual in time and space (it can happen everywhere) that it is not possible to map it. — some streams or “rus”, usually the smallest, have not been studied. Some definitions: Flood channel: linear form inscribed in hollow in the floodplain. Flood channels are areas of flow velocity. Currents are likely to cause obstruction destruction, scouring or accumulation of gravel and sand banks. Dike, lift: linear accumulation of earth usually from materials of construction, in relief on the floodplain and to protect it or part of it from rising waters. The dikes and embankments thus reduce the width of the flooding floor, which affects the downstream floodplains. In addition, work done in the past to cross rivers, cross valleys, grow alluvial lands, or even install constructions, alter the dynamics of floods. Height at scale: this icon indicates the highest height read at the station scale whose data has been systematically recorded. The station name is indicated in the cartridge attached to the card. One-off flood information: the date of the known flood is in a black icon. e = thickness of the water blade at that point, Z = altitude NGF (General Level of France) reached by flood, R = height of flood carried over to a marker (bridge battery, wall...). The red icon is a special category of ad hoc information, PHEC or Higher Waters Known; this is the most important flood whose traces are preserved by flood markers, hydrological data...