On 26-Sep-16 08:58 AM, Colin Smale wrote:
On 2016-09-26 00:17, Warin wrote:
On 25-Sep-16 08:48 PM, Colin Smale wrote:
Where would the boundary be? How could we describe it objectively?
In low lying areas such as the Netherlands there are sometimes so
called winter dykes which give a sharp edge to the flood plain.
Otherwise it would all be a bit vague around the edges.
//colin
Problems of defining a boundary exist with river banks too.
That is rather obvious - the area which is covered by water can be a
continuum, depending on the conditions at any given moment.
Here is a document on part of a flood plain .. only about 60 kms long.
http://www.environment.nsw.gov.au/resources/floodplains/120163JemalongCondobolinFMP.pdf
Note that the document is copyright ... Yes boundaries are a 'bit
vague', they are a 'bit vague' for scrub, wood, sand etc too.
The document gives a definition of "floodplain" as follows: "Any land
which is so designated by an order in force under section 166 (1) of
the Water Act 1912." In other words, this need have no relation to
actual flood risk but it is simply designated as such.
All this is not natural at all. A natural flood plain may or may not
flood depending on all kinds of things, and the depth of the water and
the extent of the flood are variable unpredictable. What IS fixed in
many cases is the government designation of a flood risk area
(restrictions on building, consequences for insurance etc). But that
is not what I would call a flood plain in the geographical sense.
Perhaps you could state your definition?
One definition I saw briefly said something about the maximum expected
area ... I only skimmed it .. looking for a open source map of the
floodplain (by whatever definition :) ).
There are some 'floodplains' on the NSW LPI Base Map - available in OSM
(similar availability to Bing imagery - but has addresses, parks,
forests etc).
The flood level in Forbes was supposed to peak at 10.7 metres .. over
the 'normal' level of the river? Or over the bed of the river? I don't
know .. but 10.7 is fairly high!
I would expect this to be similar to other OSM 'definitions' like stream
vs river, monument vs memorial, city vs town vs village .. a bit fuzzy
to allow for local variations.
For example in India the definitions of places looks to be swinging to
the legal/government determined rather than simply population based.
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