[Tagging] tagging extremely large flood control features.

[John Willis]

With the recent typhoon in Japan, I was able to see the giant river flood control systems used for the first time (since I moved here in 2011). they are the size of cities, covering many sq KM. 

There are some photos here, showing a cycling trip I took downriver to see how it works.
 https://imgur.com/gallery/9uYHFmt <https://imgur.com/gallery/9uYHFmt>

a few show the Watarase Usuichi.  https://www.openstreetmap.org/way/482421468 <https://www.openstreetmap.org/way/482421468> 

Inside, there are three “retarding basins”  (numbered 1, 2 & 3), with #1 having with a large traditional reservoir, parks, golf course, and sports grounds inside.  

As the rivers get higher, they flow into the basins via ~6m spillways upriver. eventually basins and the rivers fill up,
and the three flood control basins & the rivers merge together to form one giant lake (controlled and planned), controlled by 10M levees around the entire complex. Exit gates let the water back out into the river in a controlled rate to prevent flooding. But during the recent giant typhoon, it filled the system and flowed out of the emergency overflow spillway (~8M), seen in the background of this picture (the white line) https://i.imgur.com/z1QfYAW.jpg <https://i.imgur.com/z1QfYAW.jpg> 

The entire system worked as planned. 

These features are almost 100% man-made (levees, gates, spillways), that create large artificial lakes for very short periods  very infrequently. 

The building seen in the “lake” here https://i.imgur.com/CDU5KfE.jpg <https://i.imgur.com/CDU5KfE.jpg> is this building. It is designed to be ~1m above the flood waters, as it is in the picture. 

https://www.openstreetmap.org/way/537590387#map=16/36.2037/139.6850 <https://www.openstreetmap.org/way/537590387#map=16/36.2037/139.6850>

This complex is rarely used, perhaps once or twice every 5 years, and otherwise the area is many many square KM of golf courses, parks, sports fields, nature preserves and farmland used by people every day. 

mapping the area as an “intermittant reservoir” seems wrong, as they are merely for extreme typhoon floodwater management, and the rest of the time is is useful area for people to use. They are not “flood prone” (as I understand it), as they are *designed* to flood, and flooded in a controlled manner.

While mapping further downriver, I found a “smaller" flood control area. a giant spillway 200m across lets the river into this huge area (mostly rice fields), and control gates at the bottom of the area slowly let it back out. 

This area is confined on all sides with ~10M levees (and a few natural hills). 

https://www.openstreetmap.org/way/738164911#map=14/35.9038/139.9991 <https://www.openstreetmap.org/way/738164911#map=14/35.9038/139.9991>

(I quickly drew this way, but I will delete it). 

How should I go about tagging these kinds of huge areas - an area that is a controlled flood reservoir, used only during extreme weather? 


Javbw
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