Dear Shahnaz, it is good to use Google for gettting the initial idea about your study area and you can use it to find the lat/long and elevation. However, finding the lat/long and elevation using Google has not accurate, because the Google imagery have -4,+ 4 meters as accuracy so no accurate position can get it , same thing foe elevation . 

For your research you need accurate data to get accurate result,

my best regards

Hi

Yes you can!

1) Convert  SRTM DEM (90m) to point data, I mean grid to point, it is clear that each pixel has unique value, so you have a point data for each pixel which has elevation value of corresponding pixel!

2) Extract elevation in the form of point feature in your area of interest (i.e., in this case, river line!) using Google earth. Now, you have additional elevation points which can be merge with the above mentioned points! It is clear that distribution and number of points influence the accuracy.

3) After merging both point data series, convert it to DEM.

Now, you have more accurate DEM which is ready for hydrological and surface analysis.

Archydro in ArcGIS and PCI Geomatica can help you for hydrological analysis.

Don't forget Sink operation!!

It is undeniable that Google earth has some error but it is negligible in flood mapping. 

Good luck! 

ok thank u.

Google earth and DEMs are great tools that will help locate terraces, floodplains, bottomlands and riparian areas along streams.  Remote information may or may not be able to differentiate what type valley you are looking at or the type of detail you desire.  If you are looking for a general estimate and not a delineation, the approach suggested is fine.  When the channels are entrenched into the landscape, such as gullies, the stream has no or limited access to the floodplain and the valley surface is a terrace and the stream or river does not flood.  However, these flat areas may still become saturated or receive a degree of flooding from adjacent tributaries that flow onto the terrace surface.  With some ground based verfiication, the GIS approach using Google Earth, topographic DEMs or if you have access to LiDAR DEMs, there will be more available detail to consider.  The important point is that a flat valley surface is not necessarily a floodplain, and it is not unusual in some areas for streams to be channelized or valley surfaces to be modified and reshaped to limit some flooding effects so these areas can be farmed or developed.  

Results can be well understood by varied audiences as long as we are careful to recognize the limits of the data and qualify the work if not field checked (delineations may need to be certified by a professional).  For landscape planning purposes, generalized or preliminary flood maps are of great benefit.  However, since flood delineations can affect lives and property values and uses, planning maps should include disclaimer that it is not a delineation unless field verified. 

Dear Shahnaz,

you can create a dem from google earth by extracting a point-altitude cloud. Then you can use this point cloud in a GIS software in order to create a dem using the appropriate algorithms. This dem then, can be used for flood modelling. But instead doing this you can download free dems worldwide (either srtm as Vahid Eisavi wrote, or even better pixel size dem such as aster dem -30m).

Usually, google earth is used in order to present a flooded area...

Yes, of course you can.

https://mkgeoblog.wordpress.com/2017/07/04/the-flood-simulation-in-google-earth/

See the article below in order to learn how to do it.

remember about a drawbacks like:

- Imagery details -4,+4 m as my colleague Hayder Dibs above said (for non-3D areas)

- You can lift or drop your layer within 1 meter only (for 3D areas).

- Bear in mind the river bed decline. You may reach not detailed image for longer distance (for 3D areas).

Hopefully I have helped

dear of course

you ca use the DEM of google to do your flood simulation