[Tagging] Coastal beach definition for mapping.

Dave Swarthout daveswarthout at gmail.com
Wed Apr 4 10:58:54 UTC 2018

This is an interesting problem but it has no easy solution. Even if the
Wiki definition was clear, unless you happen to be able to measure or
otherwise determine the "mean high tide" line and other important
characteristics, what we map as beach or tidal flat is purely an
approximation, especially in areas having a large tidal range, as in Alaska
where I do the bulk of my mapping (20-30 feet). Satellite imagery may offer
a clear vision of beach and tidal flat but we cannot determine the height
of the tide when the photo was obtained. Was it at high tide, low tide, or
somewhere in between?

I agree that a beach is a place where wave action has created a relatively
steep slope. Other areas closer to the sea are flatter and are often
composed of finer particles, fine sand and clay, often referred to as mud.
Indeed, much of Alaska's coastline could be characterized as mud_flat due
to the large amount of solids Alaskan rivers transport to the ocean. In my
hometown of Homer, Alaska, spring low tides can be so extreme that the
water beyond the mud_flat is too distant to see. What you can see is mud,
lots of it.

Taginfo tells us that neither mud_flat or tidal_flat (or variations without
the underscore separator) are much in use, however, for some of my mapping
I've used the combination
wetland=tidalflat (but it could just as easily be wetland=mudflat)

I've drawn those areas the best I can based on convenient satellite imagery
knowing full well it's merely a rough approximation. There may not be a
better solution.


On Wed, Apr 4, 2018 at 4:53 PM, Warin <61sundowner at gmail.com> wrote:

> On 04/04/18 18:18, Christoph Hormann wrote:
>> 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_f
>> rom_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=1
>> 4&num=3&mt0=bing-satellite&mt1=mapnik&mt2=google-satellite
> That is a very nice example, thanks .... I'd call them 'mud flats' ... :)
> Broome, Western Australia has tides of ~10 meters  and is know for the
> 'Staircase to the Moon Festival'
> where the moon is reflected off the beach/tidal flats ripples to form a
> stair case up to the moon, very pretty ...
> But I'm not certain if that is a tidal flat area or not ... the imagery
> does not revel it ..
> https://mc.bbbike.org/mc/?lon=-4.390229&lat=51.716636&zoom=1
> 4&num=3&mt0=bing-satellite&mt1=mapnik&mt2=google-satellite
> Arrr the visitors centre says tidal flats ..
> http://www.visitbroome.com.au/discover/facts-figures/staircase-to-the-moon
> There are better photos of the staircase .. http://jksj.org/2015/06/10/bro
> ome/
> I'd still map the sand area as the beach as seen in the imagery,
> think the 'tidal flat' would have one edge as the beach edge and the rest
> be further out to sea.
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Dave Swarthout
Homer, Alaska
Chiang Mai, Thailand
Travel Blog at http://dswarthout.blogspot.com
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