You could try using a picture as a displacement map. When I did this, I drew a picture with black and white color and then blurred the picture. The blur created the blending between the wet and dry sand.

You can create wet sand by lowering the diffusion and ambient settings, and add a touch of reflection if you feel daring (adds to render time).

There are some tutorials you can check out about using displacement maps:

Kutter's tutorial:
http://www.renderosity.com/tut.ez?Form.ViewPages=291

My tutorial:
http://sittingblue.50megs.com/tutorials/terrainmap/index.htm

Charles

Charles