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FAQ/ Setting of Building Occupancy Ratio in Accordance with the Flood Control Economics Survey Manual

We use DioVISTA to calculate flood in flood control economic surveys.

The flood control economic survey manual (draft) (April 17) states that the building occupancy rate should be set.

Is there a way to set the building occupancy in DioVISTA?


The Flood Control Economic Survey Manual (draft) (April 17) contains the following requests.

Flood Control Economic Survey Manual (Draft) (April 17) p. 34

(7) Roughness
The judgment shall be made comprehensively based on the calculation model, land use status of the river basin, past flood performance, etc. In addition, refer to the method described in the "Flood Simulation Manual (draft)" to express roughness as a function of water depth and building occupancy.

When using the basic formula of the flood simulation manual (draft) (February 8) in DioVISTA, please [project] set [property] > [Flood equation] of . [PWRI (Ver.2.6 compatible)]

For building occupancy,[Calculation area] set > [Porosity] . However
Porosity = 1 - Building occupancy

The valid range for building occupancy is 0 - 0.6. In other words, give the porosity a number between 1 and 0.4. If you give a number less than 0.4, it will be 0.4. This is done in order to avoid large building occupancy (= small porosity) that makes the calculation unstable.

The effective maximum value of the building occupancy ratio of 0.6 is determined based on the following description in the flood simulation manual (draft) (February 8).

Flood Simulation Manual (Draft) (February 8) p. 91

If the building occupancy rate is not so large, the method of considering the building as a resistance is effective, but when the building occupancy exceeds a certain value, a method of not only viewing the building as a resistance element, but also evaluating the road as a communication element is required. However, the building occupancy rate in normal urban areas is only about 40%, and there are many floodplains that are not densely populated, such as the Sueyoshi district (total THETA = 33%, THETA = about 60% in densely populated areas), so this method is not considered to be much of a problem.

Last update: 2023-03-20