johnw at mac.com
Fri Mar 24 00:42:35 UTC 2017
The thing I am tagging is not a dam. It is a series of flood basins, one of which is a "reservoir". They are made by levees that surround the rivers, but in a very complicated way. They eventually return all the water back to the river, shortly after it is captured.
I explain that below, if you are interested,
But the TL;DR is that it is a weird combination of levees, weirs, spillways, gates (which are possibly considered a dam), channels, valves, and other things that are not properly fleshed out in OSM, and they should have tags created/expanded for them.
On Mar 24, 2017, at 7:34 AM, Richard <ricoz.osm at gmail.com> wrote:
>> earthen embankment (levee) around the entire river system - it is part of that.
> this is still a dam for me?
This thing I am tagging is really weird.
Japan is full of massive gravity dams that catch storm water and snow melt to from giant lakes. Canyons has flow blocking "dams" with weirs in the center to stop flash floods.
This thing is like something I have never seen. It is like 10x 6 km of man made structures nested together.
It is a "retardation basin" for a smaller river feeding into a larger river.
The system is not for water storage, it is temporary surge overflow for the river.
Here is the river where it meets the flood plain, after the typhoon surge in the middle of the night. Note, this is completely contained in levees from this point where it leaves this mountain to where it empties into the pacific ~ 160km later, as it crosses above Tokyo.
The brown river on the left is about 5x normal flow at this point, down 3m in height from the surge the night before, which would exceed 10x.
It is heading down to this thing I am tagging, off in the haze. The ghostly river in the upper right will take the water from it afterward.
This "retardation system" forces that surge water into a series of basins connected by channels that automatically empty themselves. Due to the basin's entrance being much larger than it's exit, a typhoon surge on this smaller river is caught and dissipated, while normally (350 days a year?) the river is 5-7m lower than the entrance to the system, and is unused.
The system is made of levees, with small control gates at the bottom. These small gates would be the "dam" - the levees wrap around this thing, like a golf ball inside a snake.
Above this retardation system, the levees narrow the river basin from 200m wide to less than 50m. On either side, the narrowing section has 10m embankments with 500m long weirs that empty into both a small and a very large open basin that used to be swamps.
The large open basin has an additional levee system around a series of 3 successively larger "reservoirs" inside it. This inter basin is tagged as a reservoir, and has a little rarer year round, fed by a couple tiny streams year round through small culverts (with control valves big and small) in the levee embankment.
Gated channels direct water from the large outer basin to the inner basins that look like a heart. There is an 1km additional spillway in the inter basin to let water rapidly in or out the inner basin as well. Below this spillway is a road in the outer basin, dry and drivable most of the year.
Having the outer basin catch a surge from a typhoon on this tributary gives the system downstream a chance to deal with the surge from the main river first.
The bottom of the inner basin has flow control gates to empty it back into the outer basin. This very tiny (50m?) section would be the dam.
The water then flows into the end of the outer basin, through another set of control gates, back into the river all the water came from to begin with. There is another 1km long spillway that connects the outer basin to the river. This is the one I marked with an area.
Being able to tag levees, control flow valves for drains and streams that empty into the inner basin, the gates for the "dam" the spillways and weirs by area, are all currently deficient in OSM.
Wrapping the inner and outer basins completely in a circular shaped dam inside another circular dam is avoiding properly fleshing out the tagging of large, detailed, and complex man-made water control systems.
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