With or without a top nav, it is very common for sites to have a sticky footer. Bootstrap has a facility to easily create fixed footers, but no such facility for creating sticky footers – there is a big difference. Googling this question will reveal that hundreds if not thousands of developers have the same question but with no good answer.
Ironically, the Bootstrap documentation page itself has a sticky footer, with custom css (and a fixed top navbar). So one obvious route might be to take and refactor their documentation styling.
(Thanks Softlayer – for the graphics)
Of course the footer should be responsive and cross-browser friendly as well…