[Tagging] tagging floodplain

[Warin]

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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