Ye, i would use styrofoam for the hill. I always cover mine with paper mache before adding Celluclay. For bushes i use rubberished horsehair covere in grass scatter.

As Bish said styrofoam is the way to go here, otherwise the thing will be really heavy. 

Wallboard will be heavy and has the added disadvantage of drawing moisture and coming apart.  Personally, I'd go with a piece of 1/2 inch plywood for the base.

As for bushes, might look in your LHS in the model railroad section for what is offered in vegetation in either HO or O scales or generic that can be cut/shaped to suit.

I didn't realise what wallboard was. I would echo goldhammer, i certainly would not use that.

Gents,

Thanks for the comments; much appreciated!

Chris

Nice project! That looks like a revetment pushed up with a dozer. In which case it would have a constant section through it, very regular. Like a bar with sloping sides.