[Tagging] Coastal beach definition for mapping.

Christoph Hormann osm at imagico.de
Wed Apr 4 08:18:05 UTC 2018


On Wednesday 04 April 2018, Warin wrote:
>
> So a 'beach' may include a 'tidal flat' ... confused.

I tried to explain the difference - a beach is primarily shaped by waves 
while a tidal flat is shaped by tidal currents.

The domination of waves can usually be seen in the form of a smooth 
surface where structures (like waves in the slope) typically form 
parallel to the shore.  Like here:

https://commons.wikimedia.org/wiki/File:The_beach_11111.JPG

On tidal flats OTOH the water flow often form small or large channels 
like here:

https://commons.wikimedia.org/wiki/File:Waikaraka_Cycleway_from_Mangere_Bridge_IV.jpg

Beaches can only form from relatively coarse material (sand or 
coarser) - fine silt cannot form beaches because it does not settle 
fast enough in the fast moving water so the beach would quickly erode 
away.  Tidal flats can form both from fine silt and coarse sand.

At coasts with a significant tidal range (like in the UK) there is often 
a beach in the upper part of the tidal range with a steeper slope and 
coarse sand and a tidal flat with less slope with either sand as well 
or finer material.

Example:

https://mc.bbbike.org/mc/?lon=-4.390229&lat=51.716636&zoom=14&num=3&mt0=bing-satellite&mt1=mapnik&mt2=google-satellite

In the upper part this is clearly a beach (as visible in the Bing image 
with high water level).  In the lower part with the tidal channels 
visible in the Google image it is clearly a tidal flat.

-- 
Christoph Hormann
http://www.imagico.de/



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