Using Raster Analysis for improved bathymetry

I wish to edit the lidar of the bed of a stream (which falls 10m in 100m) so that the new bed is smooth, lower (by 0.5m) and constant width (for each 100m section). The stream centre-line shp is good, and a buffer area (for stream-width) is easy to create. Can you suggest please the structure of a formula within the raster_calculator to achieve this edit. (Currently I am laboriously lowering the bed in 10m sections each to a precise aod depth and width, but it is hard work). Cheers