Additionally, how much is stored in frozen storage, which would no longer be available. Most produce also won’t last that long. If water becomes scarce, many grains and dried beans will become hard to consume. Also, remember that New York County’s residences are overwhelmingly apartments, mostly small apartments. Storage of food is an issue.
Most people do not have a month’s food.
I wasn’t really envisioning anything landing. Helicopters can pick up and drop off loads carried on a cable that can be released. The only limitations for them would be having enough clearance for the rotors and there are some streets wide enough for that if you have to use them.
Landing would be helpful because the aircraft could do double duty - supplies in, people out - but not necessary.
This seems silly. It would be no big problem to within a couple of days rig a pipeline across the Hudson River that would supply more than enough water to ensure survival.
Likewise for food: on one side of the river you have many hungry people; on the other, vast numbers of trucks and train cars full of food destined for the city. All that’s needed is some boats to carry this food about 3/4 mile.
People don’t starve in days. You can die of dehydration in a few days, but it will take weeks to die of starvation even if you have absolutely no food.
Rule of threes. You’ll die with:
3 minutes without air
3 hours without shelter (self-nitpick: depending on conditions of course)
3 days without water
3 weeks without food
The point of this rule is that people in survival situations often make poor decisions because they overestimate the necessity for food in the short term. Finding or making shelter–from the sun, from the wind, from the rain, from the cold–is almost always more important than searching for food.
Anyway, nobody will starve to death unless it takes weeks or months to reconnect the island.
Considering Manhattan is surrounded by water, how usable is it in an emergency? Granted, no one would choose a Hudson Cocktail if something better were available, but how dangerous would it be to use for a short while even if no boiling facilities were available?
Why wouldn’t the rivers be drinkable? Are they brackish? Or just filthy?
If you filtered river water (using coffee filters), then boiled it to kill the bacteria, it would be fine to drink.
I find the “one month of food” figure implausibly low… I only go grocery shopping once a fortnight or so, with a separate stop at the convenience store on the off weeks to pick up a jug of milk, and I certainly don’t turn over half of my food supply on every trip. Admittedly the menu would be getting pretty monotonous after a month, but I wouldn’t be starving. And I do in fact live in a single-occupancy apartment, so that’s not an issue, either.
The East River is a tidal strait, connecting Upper New York Bay (salt water) and the Long Island Sound (salt water). This page notes that the East River has no natural fresh water source.
The Hudson River flows from upstate NY, where it meets with tidal influences from Upper New York Bay, meaning in the vicinity of Manhattan, it would likely be quite brackish. The linked site talks about “salt wedges”, but I confess to not understanding how that pratically affects the salinity around Manhattan.
I’ve already noted that it is child’s play to come down from the north, through the Bronx, into Manhattan, which is also the direction our current water supply flows. It would be even easier than a west-to-east crossing from New Jersey. The answer to Skald’s stated scenario is wouldn’t happen; everything after that is mental gymnastics as to what might happen if the isolation of New York County is much more drastic than cutting utility and land transportation links.
Snake Plissken tells me it could take years.
I don’t know housing in Mt., but you likely have significantly more room in a single-occupancy apartment than a NY-er. New Yorkers, in general, live a much different lifestyle. Because most NY-ers (specifically Manhattanites) will pass bodegas, large delis, green markets, and grocery stores on their daily commutes (which are a combination of foot and mass transit), NY-ers tend not to stock up on foods. Instead, the majority usually only keep a few days worth of food, or what they can carry in their hands at one time. I’m sure some Manhattanites, if they own cars, drive outside of Manhattan to stock up, but that is not the norm. Going to the market isn’t an inconvenience, it isn’t out of the way, and it is hard to stock up when you have to carry everything in your hands.
If they cleared the traffic on FDR drive from 34th to 85th they sure could, or 12th Ave turning into the Henry Hudson parkway.
That would be about 1800 feet then… hmm…
Well, you could definitely get some single engine airplanes in and out of there. You could land a fully loaded Caravan, but not take off without another 700 feet. Hmmm… of course, with a short field take off and nothing aboard getting out in that distance might be feasible. You’d need smaller airplanes to take people out, but you definitely could bring stuff and people in by air. Still, food drops by military cargo planes would probably be more efficient.
Helicopters, of course, would have no problem finding a landing spot. The notion you have to limit yourself to streets is ridiculous. Parks, courtyards (with sufficient clearance) and rooftops are all suitable for helicopter operations. If the air becomes the only route in and out of Manhattan (I can’t imagine why you suddenly can’t use boats, but whatever…) then you turn Central Park into an air field. You can’t land an Airbus in Central Park (just on the Hudson, apparently) but there are plenty of other aircraft that would be able to use it as is, without requiring any modification to lawn or aircraft.
There is an airport in Manhattan. It’s the New York Skyports seaplane base.
You could probably land a C-130 on the FDR.
For the record, my last NYC apartment (in Chelsea) had five supermarkets withing a 2-block radius.
You probably don’t walk to the store, and have to carry everything back. And you probably don’t have to climb five flights of stairs to your apartment. And you’re probably not elderly or disabled.
As a matter of fact, I do walk to the store, and I climb eight flights of steps to my apartment (though I usually take the elevator when I’m carrying groceries). I’m not elderly or disabled, but then, neither are most folks.
JATO/RATO? Is it plausible to strap jets or rockets onto a Caravan and blast out without needing that 700 feet?
You could bring people out by large aircraft if you use a skyhook so none of them have to land: The person straps themselves to a balloon that goes up high in the sky. The balloon is grabbed by a slow-flying aircraft, which reels them in once it has a firm grasp on the cord. I’m assured it’s quite the experience.
Anyway, as you said, none of this is even remotely plausible. It’s easier to bring most things in and out via ferry and, once built, pontoon bridges. Anyone who has to leave now can be carried on a litter to the nearest courtyard or wide street and a medievac chopper will land and pick them up all civilized-like. It wouldn’t be that different from how things work in the real world, except that CNN and FOX News would be on the scene 24/7 making the New Yorkers real pleasant to be around. 
One month of food strikes me as impossibly optimistic. Grocery stores typically get resupplied every day, right?
There would be some people with food to last four weeks, to be sure, but I think most people would have less (particularly families with kids). There will be plenty of people who have almost none, and those would likely be disproportionately young single men – the demographic most likely to cause trouble.
The bodegas would get trashed in the first few days. I predict food riots within a week.