[Talk-GB] Coastline and tidal rivers

Tony Shield tony.shield999 at gmail.com
Tue Aug 28 10:50:23 UTC 2018

I'm with Colin on this.

My experience of sailing and reading Admiralty charts is that the 
coastline is the High Water line.  Yes it looks inconvenient or 
unnatural - but tidal area as implied by coastline is so important to 
small boat users. The River Dart

    Way: 194211894 waterway= riverbank

is a really arbitrary line across the river from Dartmouth Castle, this 
offends my view of what a coastline is.

Another relevant concept is salinity - tidal coastline is saline and 
does affect plant and marine life . Which leads us into a conundrum - 
things such as salt marsh and mangrove swamps which are all inter-tidal; 
where should the coastline be?

We should also be aware that an incoming tide blocks the natural flow of 
the river and causes the river to form a type of lake which reduces as 
the tide ebbs. This effect can cause people to think a river is tidal in 
that area when it is not.

In my local area the River Ribble estuary in OSM changes from riverbank 
to coastline near Warton airfield, but wikipedia describes "The Normal 
Tidal Limit (NTL) of the river is at Fishwick Bottoms, between Preston 
andWalton-le-Dale <https://en.wikipedia.org/wiki/Walton-le-Dale>, 11 
miles (18 km) from the sea"

so where should the boundary of coastline to  riverbank be? I suggest 
where the inter-tidal range or zonal area is small - range < 1 foot, 
line of zone perpendicular to the boundary is < 1 yard (or metric 
A heuristic could be where it becomes long and thin? Ribble is almost 
acceptable - Dart is not as I write this.

But really I prefer the existing guidance.


On 28/08/2018 08:49, Colin Smale wrote:
> That old chestnut again...
> There seems to be an open discussion about how far up a river the 
> natural=coastline should go. The wiki suggests the coastline should be 
> the high water line going up to the tidal limit (often a lock or a 
> wier) but this can be a substantial distance inland. This is AIUI the 
> general scientific approach.
> There has been some discussion in the past about letting the coastline 
> cut across the river at some convenient point, possibly because it 
> "looks better" or "seems more natural" or "is less work."
> I looked at a few rivers along the south coast to see how they had 
> been tagged and it seems most have the coastline up to the tidal 
> limit. However the coastline around the mouth of the Dart has recently 
> been modified to cut across the mouth, and Salcombe Harbour is also 
> mapped this way.
> Is there a consensus for a particular definition of "coastline" in 
> tidal estuaries? Should we try to keep a consistent paradigm, or 
> doesn't it matter?
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