johnw at mac.com
Wed Mar 22 01:44:17 UTC 2017
How do you tag an emergency spillway?
I am tagging a giant flood control reservoir in my region. The “lake” is surrounded by giant man-made embankments on all sides, surrounded by an additional set of embankments, with gates to let the water out. There is no dam per se, because there is ~200 km of this man-made 10-20m tall earthen embankment (levee) around the entire river system - it is part of that.
When this levee system fails uncontrollably (like it did downstream from me in 2015), it is really bad. https://www.youtube.com/watch?v=Hfs3OeqiqRk <https://www.youtube.com/watch?v=Hfs3OeqiqRk>
This reservoir system catches water during a typhoon from a smaller river, then releases it slowly after the peak flooding into the larger river. this reduces the chance of flooding further downstream.
However, it has 2 sets of emergency spillways (each about 1Km long) to let water out of both sets of embankments, so where it “fails" can be controlled. it is 5 m lower than rest of the embankments surrounding the reservoirs. These were recently added.
I made a polygon on one such spillway. https://www.openstreetmap.org/way/481898093#map=14/36.2115/139.6880 <https://www.openstreetmap.org/way/481898093#map=14/36.2115/139.6880> . the heart-shaped lake is the reservoir, and most of the surrounding green areas are part of the flood control basin made by the second set of levees.
These *emergency* spillways are considered safety features of most large water control projects - if the human-controlled gates o the normal spillway jam shut, the emergency spillways will keep the dam/embankments from being over-topped in unexpected places. I know these are a major mappable feature of large dams, but they are not mentioned on the Dam wiki page. Maybe I am looking at it wrong.
As these are not a weir, nor a normal path for water to go, I wouldn’t tag them as a weir nor as a object normally associated with a waterway line running through it.
is there an existing tag or another name for these features that I don’t know of? How do I tag these emergency spillways (and heck - these huge levee embankments!) correctly?
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