River analysis using SRTM data

[micheletramutola]

Hi all, I've some problems about modelling a river using SRTM 90m data. For this river I've also a grid DTM generated using LIDAR+bathimetry, from SRTM using HEC-GeoRAS I've exported the geometry of channel but the lower point of the SRTM channel it's too high than the the other(LIDAR) over 5/6 meters. My question is, using Arc Hydro how can I extract the "real" river bed from SRTM?

Regards.

Michele

[MarkBoucher]

Michele  ~  A 90 meter grid is a very coarse for a DEM in my experience. Unless you have very wide rivers, I'd think you would have a tough time accurately defining the rivers using Arc Hydro. If you are working in very steep terrain you will have better success, but even then, I'd think 90m would be too coarse. If you have LiDAR, I would create a reasonably fine DEM from that; say, 3 meter grid.

[micheletramutola]

Hi MBoucher21, thanks for answering, I've a very large river and using Fill it's very visible (10-15 pixels). In my study i'm trying to calibrate srtm model using lidar (2x2) data in hec-ras. So, the problem is that the river bed of SRTM is not the real bed and it's too high compared the lidar and, with it, the level of stream. The Hydro tool can find the river and catchment area but I need to know how much put down the bed and how to do it. If it's possible!

[MarkBoucher]

Michele,

If I'm understanding you right, you have LiDAR for the river. I don't believe LiDAR can give you stream bed elevations under water. The light beams end up giving you the water surface.  I am not familiar with the  SRTM 90m data. From what I found on the web (http://www.cgiar-csi.org/data/srtm-90m-digital-elevation-database-v4-1) the SRTM DEM is created using satellite data. I'm not sure what technology they used to create it. It is a worldwide dataset so I doubt their intent was to get as detailed as you want. I would think that the 90m DEM would be inadequate for HEC-RAS modeling.

I don't know what tools would be needed to get data below water (bathymetric data). You will likely need to find another data source to get the information you want.