I think we covered much of this before, however let me just say that in the 1970s (when we first came up with the 146) turbo-fan powered swept-wing aeroplanes were the norm.
We were designing a turboprop replacement aeroplane so of course it was bigger and faster and had longer range than (say) the 748 or F27.
It was intended to operate from the same short or hot or high airfields as the 748 - which it could and still does!
Mechta has explained most of the high wing logic and I'll just add the uninterrupted wing upper surface as another benefit.
At the time we were configuring the 146 there were no engines available that would have powered a new twin-engined aeroplane of that size so we went for (and took advantage of) four ALF502 power plants.
Hope this helps.
ETA the use of turbo-fan was beneficial to the noise footprint of course (and a lot of sales depended on it), but the initial reason was an Mmo of 0.73
Last edited by Allan Lupton; 1st Apr 2015 at 22:17.
Reason: response to Mechta's edit!