OpenStreetMap

Severe flooding

Posted by Gerhardus Geldenhuis on 29 December 2012 in English (English)

I was using CD:NGI imagery to trace the Vaal River borders and thought I would double check an inlet using Bing, was amazed to see the difference. The CD:NGI imagery is much closer to normal probably winter flow if I were to hazard a guest with the Bing imagery taken during a severe flooding event by the looks of it. Interesting.

It does raise another point though. Where is a river's boundaries? What is normal flow? Should we in an ideal world map drought, "normal/average" and flood boundaries if we had the data? I think so but it would be a massive project and accuracy would certainly need to be stepped up a bit. It also becomes a 3D problem because what would normally be an island in a river now suddenly disappears beneath the water.

Location: R370, Dikgatlong Local Municipality, Frances Baard District Municipality, Northern Cape, SAR

Comment from Sanderd17 on 30 December 2012 at 14:08

The normal river boundaries is the line of "average maximum water height". Comparable to the shore line on the sea.

So we don't tag exceptional floods, but if water comes regularly to a certain point, then the river boundary is there.

Of course, regularly can be quite ambiguous, but let's say, if you don't expect it to be there on a normal day, it doesn't regularly get there.

Of course, you can tag fields and wetlands through the river. There's no problem with that. See this as example: http://www.openstreetmap.org/?lat=51.5225&lon=0.7632&zoom=13&layers=M

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Comment from Skippern on 30 December 2012 at 20:33

For verious reasons, my opinion is that the "normal" riverbank should be tagged, as this not only represent the most commonly observed river layout, but also is what is generally considered when buildingnroads, residential areas, industrials, etc, and also considered as "limits of the waterway" for shipping and boat traffic, planning of waterworks and aquaculture and more. O river bank that often dries in dry seasons can be tagged with seasonal, intermitten or similar, while areas that often floods can be tagged floodplain and flood_prone

Making several different riverbanks depending on season is very confusing, very demanding on data consumers, and IMO plain wrong

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Comment from Gerhardus Geldenhuis on 30 December 2012 at 21:21

I agree that the normal river bank should be tagged and should be the one that gets displayed. However I believe OSM is about more than just a pretty map and a potentially very powerfull future data source and in that regard it would be useful to have more river bank information.

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Comment from Skippern on 1 January 2013 at 13:20

The solution though is not to tag several riverbanks for the same river, but to find alternative tags adding the information you look for. In you case floodplain and flood_prone are probably the most correct. If there are permanent riverbeds within the floodplain, that normally are dry outside of flood season, than that should be tagged as an intermittant river, etc. This way you separate the river, as a permanent flowing waterway from the flood rivers and the flood plains, where you only would expect water during flood seasons

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