Potential flow paths in heavy rain
Description
For the purely topographic determination of potential flow paths in the terrain, a unidirectional D8 algorithm was used. This determines the drainage direction of each grid cell of a digital terrain model from 8 possible directions. The approach can be visually imagined as follows: From every point on the surface of the terrain, one rolls a ball one after the other and records the path taken. Where the paths of many spheres overlap, it is more likely that concentrated surface runoff will also take place in the event of heavy rain. If a ball reaches a depression from which no "rolling on" is possible, the terrain is filled up at this point in the model so that the ball can "roll out". The investigation approach assumes that the replenished area is potentially filled with water in heavy rainfall, as the drainage is also obstructed and therefore water could probably accumulate. The area is identified as a land sink. The determined flow paths are intersected with the water areas (from the ALKIS data set Actual Use). The applied methodology is load-independent. It is not based on a defined heavy rain event with a certain duration or intensity, therefore the indications cannot be assigned to a specific annuality or probability of occurrence. With regard to the potential flow paths, the methodology does not provide information about expected flow depths (water level), flow velocity and spatial expansion of a flood area (e.g. width of the flow path). The potential flow paths are classified according to the size of their connected catchment area. For a larger catchment area, it is assumed that the probability of surface runoff is also greater. In general, the indications of potential flow paths at settlement edges and on open areas are to be assessed as more reliable. Within settlements, reliability can decrease, as the flow paths are influenced by numerous small structures (marginal stones, walls parked vehicles, etc.), which, however, cannot be mapped in the digital terrain model. Hints from this layer must be verified locally with local knowledge. In principle, it cannot be ruled out that flooding may also occur outside the areas and indications shown.
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