Is this a good concept, or trash it?

This is the first time I've tried to write about a post-apocalyptic world, and I'd like some critique. Totally honest opinions, even if you have to be rude, because if you're mean to me then I'll get pissed and want to prove you wrong. :)

The story is set thirteen years after the Break. The Break was caused by the _______ Mines (trying to think of something besides "Deep Mines"), a set of mines so deep that it took more than a week for the miners to get back to the surface. There were pockets of them all over the world, and together they weakened the Earth's crust, causing huge cracks to rip the planet apart, flooding it with lava and setting off the world's nukes, thus creating some nasty-@$$ radiation.

Well, since this version of Earth is very advanced, humans have created a series of floating cities collectively called "Olympus." The people living in these cities were the only ones who survived, and there aren't many of them left either.

The Upper World is in shambles. All the orphans, children of those who died on the day of the Break, have banded together. The rest of the Uppers Dwellers refuse to take them in because of the desperate shortage of food and water, and they consider the parentless children "useless."

Well, a little girl named Whitney (a very old-fashioned name that her mother gave her) survived the Break. So did one half of a set of twin boys, names Amoryn. They have the same birthday. Both are eighteen years old, some of the oldest of the orphans, and the younger kids look up to them. Whitney has a horrible scar that nearly blinded her left eye, and after being burned by a medley of chemicals during an explosion caused by the Break, her hair is permanently bleached white. Even though she lives with the orphans, her mother is still alive, and basically the leader of the other Upper Dwellers. She too hates the orphans and is always trying to get Whitt to return to her.

After sneaking into the Sector, the settlement created by the Upper Dwellers, Whitt and Amoryn hear that Whitt's mother and another woman have come up with a plan to get rid of the orphans, who reside among the biggest and healthiest of the orchards remaining in the shattered Olympus. What they don't know is that the orchards are nearly gone, and the children have to use advanced water purifiers to convert their own urine into drinkable water. Whitt and Amoryn return to the orchards, and after much consideration, decide to lead the children away, take them into the Low Lands, the only place they can hide from the Upper Dwellers. They find a set of Wings, flexible body suits used to glide for recreation before the Break.

But the Upper Dwellers attack sooner than planned, and several children are killed before the others can escape. They manage to get to the Low Lands, and are prompty attacked by twisted, crazed cannibals. They are rescued by a man named Jonah who turns out to be Amoryn's twin, who also survived the Break (one of the few who managed to live in the Low Lands). He's a far cry from soft-spoken Amoryn, a free-spirited womanizer who has no sense of personal space. After seeing Whitt's scar, and her apparent resistance to his "charm" he attaches himself to her (figuratively AND physically). Basically, my plan for him is to be freaking hilarious, and a really sweet love interest (a well-developed relationship between him and Whitt).

Afterward, a little girl wanders into their camp, apparently the lone survivor of an attack by the same cannibals that went after the orphans. She tells them of a well that grants wishes. Of course Whitt doesn't believe the part about the wishes, but she knows that water=plants=animals=food. When Jonah backs up her story, Whitt and Amoryn decide to try and find it. The group sets off, following a half-crazy little girl and a guy they barely even know. Yeah, desperate, right?

They face many struggles, including a pack of mutant coyotes, treacherously thin sheets of stone covering a deep chasm, lava fields, acid rain, more cannibals, starvation, power struggles, and a devastating loss, in their quest to find this oasis.

I know the summary is HORRIBLE (mine always are), but what do you think of the story itself?