It doesn't really work like that to be honest. In each module there will be a set of issues and theoretical approaches which you are working on an understanding of; you're free to choose whatever case studies you feel are appropriate to support what you write in the essays and so on, but you wouldn't really be picking your own tangents to go off on - it's rather more focused than that. So in your example of urban areas, you might be writing about, say, how the theories of political ecology apply specifically to urban sustainability, which of course requires a consideration of both physical and human factors (that's pretty much what a definition of pol. ecol. is, in fact) but it's not like you'd then decide to go and include earthquake risks in that, or something.
Hmm. I'm really not conveying things very clearly here, heh. Perhaps I mean that yes, you might very well combine physical and human approaches with some topics, but only where it was necessary/appropriate...not just out of interest, as it were.
Anyway, as I say, in the first year the modules are fixed, but in the subsequent two you can pick and chooose. And of course you have unlimited choice when it comes to your dissertation, and also some choice available in the projects and extended essays which form part of the assessment for some papers.