Fish-face, I was thinking along similar lines ... some areas would be heavily canal-ed, ie OQ, other places not so much, ie Auldale. If city is built on a delta, or a system of rivers, as has been suggested, than the canals would have been created in order to "manage" the flow of water, prevent flooding etc.
Mugla, we're having enough trouble mapping the city surface, never mind what's underground
But I agree that it might be useful to form some notion of what is higher and lower ground in the city (Hightowne, Downtowne maybe has a bearing on this?). It's just a matter of providing a framework, really. It is starting to come together now!