[Qgis-user] Calculating affected regions of high-water situations

[Saber Razmjooei]

Hi Andreas/Werner,

I haven't come across the practical example of why you want to change the
water level by 1m, 2m across all your river network.

However the following scenario is the common example of how a similar method
is used for creating flood warning/flood forecasting maps:

Imaging you have got a water level gauging station at a certain location.
You would want to associate the water level readings to a flood map. As
Werner mentioned, what you need then is to set up a hydraulic model with
hydrological input to simulate flow/water levels. You probably need
calibration/verification to ensure your model represents the river network
and out-of-bank flow properly. You can then run the  model for some
hypothetical scenarios (hydrological events) to generate flood maps for
1m/2m etc intervals of the gauge readings.

Based on the type of the catchment, shape of the floodplain, number of
hydraulic structures, a 1m increase in water level at the gauge might not
even increase the flood levels elsewhere in the catchment.

Hope that helps.

Regards
Saber







-----Original Message-----
From: qgis-user-bounces at lists.osgeo.org
[mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of Werner Macho
Sent: 11 December 2012 00:58
To: qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] Calculating affected regions of high-water
situations

Hi Andreas!
> I thought that this is a common use case in GIS - as many communities 
> have to do the "Gefahrenkarten" - risk maps. But probably this work 
> has been outsourced in most cases to real hydrology experts. Then 
> these hydrology experts probably do not want to make the process too 
> easy and available to the average GIS users - otherwise they would 
> loose their jobs/contracts ;-(
It IS a use case for GIS - but in this case (I think i mentioned it twice
that I worked in this area for about 7 years) I have to say that I would
love to see an easy way to do the analysis but there is a lot of knowledge
about flowing processes necessary to 1. create the model correct 2. run the
hydraulic analysis the right way and 3. interpret the outcome correct.
Ever since I started in this area long time ago I always wished there would
be a one-click tool but I think I have to disapoint you here.
Every river I started to analyse ("Gefahrenkarte, Risikozonen") was a new
experience and sometimes reacted completely different to any river I have
had before.
Beside the fact that you need deeper knowledge in hydrology to make the
correct input assumptions (for e.g. roughness.. so just having a DEM is
definitely not enough) you also need to have a very accurate model (2m DEM
is probably also not enough) to get correct results (especially for smaller
rivers 3-10m width).
>From my experience all I can say is that it is not enough to throw a DEM
into some analysis tools and expect an outcome with cm accuracy. In my last
work it was a toolchain of 5-8 programs to get the correct output (including
GIS). And even then it was sometimes not more than a rough assumption.
(You'll never know what the next rain will bring and what influence all
these newly built buildings will have to the riverstream).
I'd just say the process now is unfortunately far away from being easy.
But as I already said .. It always depends on what you are expecting. If an
accuracy of 1-2m is enough than it's probably really just a matter of one
click in GRASS. But 1-2m could mean death or severe damage in populated
areas.

So as a conclusion: You are surely able to do calculations with GIS (see the
GRASS modules) but that will give you just an overview where water "might"
be.
If you want "waterproof" results you have to go a long way and use GIS in
combination with other tools to be able to make real
riskzone-management-maps.

But you see some people are working to integrate such tools into a QGIS
chain (We'd like to call it aQuaGIS (c) by Pete Wells) maybe some time we
end up having a one-click tool but I don't expect that in the near future.

If you are really interested in this topic we can talk about it at our next
HF in Valmiera or probably in Zurich at New years eve :)

kind regards
Werner

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Saber Razmjooei and Peter Wells trading as Lutra Consulting.