Getting Flood Depth Grid (BASIC)

[jakc]

I am a GIS consultant. Done a fair bit of work with modellers outputs before.
Im sure, usually, a modeller would give me a height model (LIDAR) and a bunch of cross sections which had a water level assigned to them. This level would range from 0-5m (for example) which would signify the water level. From there, I could extend these x-sections out to the bluffs of the floodplain, interpolate a water level surface. Subtract it from the Lidar (DTM model), remove any negative values and hey presto = Flood depth grid (with maybe a few tweaks here and there).

Started a new job with someone who has only done a few HecRAS runs. She has provided me with survey data (heights), 1m contour data, and cross sections with what she says is water levels. However these values are relative to the elevation - water levels range from 238 to 222 mAOD

Is this normal? Where would you advise going from here in terms of getting a flood depth grid out of the data I have?

(Some screenies in attachment)