Dear Corey,
We had a project where we had the same problem, there was virtually no bathymetry data available bar a few bridge blueprints from the 80's which showed the bottom under the bridge.
The whole area had recently been laser scanned from an airplane, which gave a DTM with very good resolution and accuracy. The laser could, however, not penetrate to the bottom of the river we were looking at. The water surface was a mish-mash of land values, interpolated points and some water values.
What we (mostly on of my colleagues) did was that:
*we found out when the scanning had been performed (that info might in the original xyz-file if you are lucky, otherwise check with whoever did the scanning)
*cross checked with the nearby water power plant owner how big a flow they had allowed through their turbines at this date (luckily the output discharge was stabile during the scanning period).
*identified areas where the water surface met the river bank, as it was possible to identify certain cross sections of the river where the water surface had been scanned with reasonable accuracy.
*Then we created a water surface profile along our stretch of river using this method.
*THEN; we used the flow given by the WPP operator and a downstream WL by another dam in order to compute the water level in MIKE 21. We had used a linear slope for the bed, and adjusted this locally where needed until we got a match for the water level we defined from the DTM raster.
*(The initial water depth, i.e. bathymetry, we estimated using the energy equation and assuming critical flow regime, thus calculating the depth to what turned out to be quite a good match.
So, this is the first step. When you think your bathymetry is reasonable. But then you should also calibrate for another event, a different flow. You can say that you need to use two calibration sets:
(1) Calibrate the bathymetry using one event (i.e. alter the slope, depth etc. until you get a good match)
(2) Calibrate other factors such as M (or n), structure coefficients, finer bathymetry details etc.
As you may guess, I think we caught a lucky break on this project. There were several factors which were in our favour; steady flow during the scanning period, defined WL downstream, high quality DTM with time info, also the bottom XS was quite flat according to the bridge blueprints which eased the calibration process.
I hope you found the above helpful. Perhaps it isn't a possible work process for your project but hopefully it might help a bit :)
(Hopefully I haven't forgotten any step we made)
Best regards,
Jonas
