W,
The gradient will get you before the pigs! They are quite happy on slopes providing they have somewhere level(ish) to sleep, and the whole thing is not a mountainside!.
What I would watch is run-off in to the stream or indeed letting them have access to the stream. You don't want to be
a)done by the Environment agency for pollution of the waters, or
b) would not I hope want to kill the fish or other aquatic life or
c) pollute whoever is downstream or
d) let the pigs start to erode the bank of the stream.
RPA recommended distance of organic matter from streams is 10 metres for organic matter that is spread (and indeed a requirement for SPS), for pigs spreading themselves (!), you should use your discretion, but I would go a minimum of 2 metres from the bank, and more if it slopes a lot.