Runoff axes by topographic analysis (natural and anthropogenic) on Rennes Métropole

As part of the runoff study currently being carried out by Rennes Métropole (2023-2025), a first analysis of hydraulic functioning was carried out at the global scale of its territory. On the basis of the shapes of the natural terrain, the runoff axes or water paths have been mapped, distinguishing between: => Natural runoff axes, i.e. the route of the water at more or less high speed. These axes have been traced by considering different hypotheses of size of contributing active areas feeding them upstream (from 1, 5, 20 and 100 ha). It should be borne in mind that the layout of these axes is based solely on the shapes of the bare natural terrain (the buildings, walls, hedges and vegetation elements having been filtered from the NTM): the obstacle effect of buildings and walls/hedges located on the edge of parcels, particularly in urbanised sectors, is therefore not taken into account. => Anthropogenic runoff axes integrating in a simplified way the obstacle effect of buildings (extrusion of buildings) and preferential drainage along roadsides (disbursed by 15 cm). These axes make it possible, in particular, to identify the impact of anthropogenic structuring elements on the flow path. On the other hand, some elements of urban micro-topography (particularly walls/hedges bordering on parcels) have not been taken into account. The natural runoff axes are represented according to four levels of precision depending on the minimum surface area of the upstream watershed that feeds them: > 1 ha in khaki > 5 ha in green > 20 ha in blue > 100 ha in purple This discretisation makes it possible to apprehend at different scales of the network, runoff routes in addition to the perennial hydrographic network and secondary watercourses. Limits on the use of the data: => The route of runoff axes is based solely on a criterion of the size of the active surface drained and does not take into account the influence of the nature of the subsoil on the infiltration potential, on the capacity of the latter to flow and finally on the slope. However, the impact of different types of land cover, and in particular its waterproofing rate, has been taken into account by means of runoff coefficients corresponding to the main types of land cover (urban areas, areas to be urbanised (ZAUs), agricultural areas and natural areas); => It is therefore a maximalist identification of the potential axes of water flow, without prejudging the frequency of activation of these axes or the order of magnitude of the hydraulic parameters (submersion heights and speeds) that could be observed there; Thus, it is in all cases essential to compare these analyses with the reality of the territory and in particular with the historical observations recorded. Rather, it is an invitation to deepen the knowledge of the risks on the sectors at stake.