So I'm in my studio trying to go through some design iterations, in an attempt to find something concrete to work with. This is the scenario so far:

Our Site:

800 HA on the outskirts of Bratislava, mostly flat (40m rise over 6km) and featureless (there is literally, one tree), save for a system of canals which handle water flowing from the mountains to the Danube (transversally across the short axis of the site, about 2km)

No overwhelming usage of the site which dictates future program (some scattered farms, but as a whole, the site is just a greenfield)

A developer who's open to whatever makes them money (which is motivation to get something worthwhile done, because no matter what, this piece of land is getting "improved" - for better or worse)

Oh yeah, a gigantic car factory that produces Touregs (sp?) and Cayennes, directly to our west.

It one of the strangest sites I've had to deal with in studio. This isn't to say that I haven't had idiosyncratic sites to deal with before, but most of those times, the idiosyncrasy was almost expected (knowing that every architecture project brief will have some crazy constraint that almost guarantees a creative solution - kind of like an architectural ambush). This site is strange for that fact that its the closest thing to tabula rasa that i've encountered ... now i know there are some features that need to be looked at, but the size of the site is so big, and the clients - both the city/developer - are so open to potential possibilities, that it almost seems to dilute whatever features/constraints we invent for ourselves. So ...

... here I am wondering about these canals.

Our group has been talking about strategies of open space that address both the public and ecological concerns of the project. That got us wondering about how much of the land is needed to retain and purge runoff of harmful contaminants before it feeds into the river. We know there are passive ways to do this using specific types of mineral leaching vegetation, but as to the amount of land needed to do so for the amount of runoff expected .... thats where we are a bit at a loss. Now we've set up a spreadsheet which lets us know the amount of increase of runoff given any sq m increase in built area and composition of said coverage (never thought excel would be my friend) but we don't know how to differentiate between amounts of grey/black water, and how much land will be needed to filter/retain said grey water given the variations of topography and resulting flow rates.

Anyone on architect with a head for this stuff know of some simple resources that we could use to at least get some rough figures for the amount of land needed? Most of what we turn up has to do with calculating the flow rates, which we have already.