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FAQ/ House collapse risk zone appears in the drainage zone of the drainage station

A house collapse risk zone has appeared in the drainage zone of the drainage station. That point is far from the breach point. It is incomprehensible that such a point is judged to be at risk of house collapse.


This is thought to be due to the calculation of a large flow velocity in the drainage area of the drainage station. In addition, one of the reasons why such a large flow velocity is calculated is that the drainage station behaves as follows (Reference 6 in "Manual for Drawing Flood Expected Areas (4th Edition)").

Flood Area Mapping Manual (4th Edition) p. 20

3.1. Basic concept of flood analysis
(9) Setting drainage conditions in inundated areas
For drainage stations, a water collection area is set for each drainage station and the volume calculated by "pump wastewater volume √∑number of flooded meshes in the collection area" is subtracted evenly from the flooded mesh to perform wastewater (see Figure 3.2-8).

In situations where there are few flooded cells in the drainage zone and the pump's capacity is concentrated in a small number of flooded cells, the depth variation of the cells is significant. The flow velocity per cell is determined by dividing the unit width flow rate for each cell by the water depth. This calculation is done with caution, but in rare cases, the calculated flow velocity will be large if the depth variation is large.

The house collapse risk zone determines whether the value obtained from the flow velocity and water depth can exceed the threshold even for a moment, so even if the value is calculated at a certain moment, it is decided whether the value is inside the zone or out of the zone. It is recommended that the house collapse risk zone be determined by making engineering judgments with reference to the calculation results of the simulation.

For details, please refer to "FAQ/Basis for calculating the risk zone of house collapse".

Last update: 2023-03-17