Properly mapping dry washes

The waterway analysis that @amapanda_ᚐᚋᚐᚅᚇᚐ , @watmildon, and others have been working on identified a number of closed waterway=wash features in the US. These were either bars of sediment in and alongside rivers, or the wide areas of dry desert washes – coarse to fine alluvium in either case.

Most of the features were awkwardly tagged when they were imported from the USGS National Hydrology Database (NHD). To avoid having waterway loops, @watmildon recently retagged these features as natural=wash.

For the bars of sediment in and alongside rivers, I think the established natural=sand, natural=shingle, or natural=shoal + surface=* tags are appropriate and suitable.

However, we seem to have problems mapping the wide areas of dry desert washes. The established OSM tags and conventions don’t fit well with these features. That has led mappers to some creative interpretations and adaptations as they map these areas:

  • natural=water - Obviously wrong because the wash is dry. Often the result of previous imports. E.g., Way: 47356327 | OpenStreetMap

  • natural=water + water=river + intermittent=yes - An attempt to fit the established OSM tags to a dry wash, but it makes something that is normally dry look like it is normally wet, and it fails to capture the fact that there may be many years between floods in a dry wash. E.g., Way: 46227123 | OpenStreetMap

  • natural=water + intermittent=yes - Same as above. Note that a natural=shoal feature may be underwater part of the time, but the tag distinguishes it from “water.” E.g., Way: 47356794 | OpenStreetMap

  • natural=sand - Contrary to popular portrayals, most deserts are rocky or stony, not sandy. Dry washes reflect the local soils and sediments, and so are commonly filled with pebble, gravel, or cobble sized rocks. E.g., Way: 47425436 | OpenStreetMap

  • natural=shingle - “Shingle” is specifically rock that has been rounded by the motion of water. Dry washes don’t have enough water to smooth out their rocks, so this is incorrect. E.g., Way: 47356799 | OpenStreetMap

  • natural=scree - “Scree” is specifically the broken rock that accumulates at the base of a cliff or other rock face. That’s not a dry wash. E.g., Way: 47425055 | OpenStreetMap

  • natural=wetland - A possible misunderstanding or tagging for the renderer. E.g., Way: 47356584 | OpenStreetMap

The examples above are from the US, but appropriate tags for dry desert washes could be used elsewhere in the world, particularly in the Middle East where this type of landform is also prominent.

Can we agree on tags that work for the wide areas of dry washes? Possibly natural=wash or natural=dry_wash combined with surface=gravel/sand, which would be similar to the usage for natural=beach?

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Ce sujet de discussion accompagne la publication sur https://community.openstreetmap.org/t/properly-mapping-dry-washes/108437