Hi:
I've consulted a colleague expert in hydrology, and he told me about
these fosil rivers:
"The term fosil rivers is not widely used "scientifically", they are
rather called Paleo channels and it is for rivers that have dried up
since 1000s of years and most of these paleo rivers are buried or filled
with sediments.
If a river gets dry for any reason (let say for some decades ) but u
could still see the river course (which is only possible in very rare
occasions) u could still call it a river just to maintain the landmark
notation and geography."
I am not an expert, and I would do what Arne suggest if I was mapping
without discussing this with anybody, but still, like Arne, I see the
lack of a tag for these kind of rivers.
As for the Lagh Dara, this colleague has sent me two images of the
rivers passing by Afmadow town.
One shows the river as we can see it normally: https://flic.kr/p/2iNyA5a
But the second shows how it was during the 2018 devastating floodings:
https://flic.kr/p/2iNyA7z
He has been on UN missions in Somalia many times, so he has too on the
ground knowledge.
By the way: the Lagh Dera river geometry is right now of very poor
geometry in OSM, while the UNSOS one is very good in comparison:
https://flic.kr/p/2iNyQ6G
Cheers,
Rafael.
O 09/04/20 ás 10:47, Arne Kimmig via Imports escribiu:
Hi Christoph,
Thank you very much for the examples. I think Lagh Dera is a good example of a
fossil river at least inside Somalia. I wouldn’t map it as a river because I
don’t see any erosion features nor any tributaries like gullies or creeks.
If and how these fossil rivers should be mapped is an other discussion and
might be beyond OSM philosophy (I actually don’t know very well) and for the
planned project I would recommend not to map them or add an annotation to the
already mapped ones.
Regards
Arne
Am 09.04.2020 um 00:33 schrieb Christoph Hormann <[email protected]>:
On Wednesday 08 April 2020, Arne Kimmig via Imports wrote:
- how can we distinguish a 'drainage shaped landscape' from a stream?
Reliably only through local knowledge or through imagery depicting
actual waterflow. There are occasionally good indications of recent
waterflow even in dry situation images - like a recently newly built
river crossing of a road next to an old and destroyed one. Bridges or
culverts are also a clear indicator.
Actually, what do you mean with this term? - same question for
'fossil river'. Is that something like a dry valley?
As mentioned throughout subtropical Africa - both in the north and in
the south - there are a lot of valleys evidently shaped by water that
have not seen any more recent surface water flow.
On images you can sometimes see that for example from visible artificial
or natural structures of significant age that would not withstand
significant waterflow:
https://mc.bbbike.org/mc/?lon=0.130422&lat=17.486138&zoom=15&num=2&mt0=bing-satellite&mt1=mapnik
or dunes across the riverbed:
https://mc.bbbike.org/mc/?lon=0.340901&lat=17.834483&zoom=14&num=2&mt0=bing-satellite&mt1=mapnik
https://mc.bbbike.org/mc/?lon=31.690906&lat=17.633554&zoom=16&num=2&mt0=bing-satellite&mt1=mapnik
In most cases however this is very hard to reliably see (the problem of
proving a negative - what i mentioned in my reply to Rafael) -
especially in relatively flat areas like much of Somalia where
scrubland covers much of what could be useful hints.
Famous example of a fossile river is the Molopo River in South Africa
where it is generally said that it has not carried water over its whole
length for at least a hundred years:
https://www.openstreetmap.org/relation/7370966
I could imagine that this similarly applies today to the Lagh Dera:
https://www.openstreetmap.org/relation/5967248
--
Christoph Hormann
http://www.imagico.de/
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