Sand Bed, Sandbed, or Sand-bed?

usually , the rule is
If used as a noun its sand bed

If used as an adjective its sand-bed

I dont recall seeing sand bed used as an adjective .......except in hydrology, like
"I have to create a sand-bed meander.

Its like surface water or ground water.

Lake Erie is a body of surface water
The surface- water basin was designed improperly