Flood

A series by Stephen Baxter about an apocalyptic global flood, based on the theory that there are deep subterranean reservoirs of water locked up in the Earth's mantle. What would happen if all this water started coming up out of the planet's core?

The first novel, Flood, follows the lives of four people who were kept as hostages by terrorists for four years. They are still struggling to pick up the pieces of their lives when mysterious floods start happening all over the world. As the floods get even worse, they make a pact to always keep in contact with each other.

A second novel, named Ark, came out in 2009.

Tropes used in Flood include:

America Saves the Day: Seemingly averted. The US is portrayed as one of the last countries to not only stay intact but also have something of a functioning government. Though it too crumbles once the flood reaches the Rockies. But then there's the Ark...
Apocalypse How: Starts as a Class 0, and gradually starts ramping up through the scale as the flood reaches higher and higher. Eventually ends up as a Class 4, as the waters cover every available bit of landmass, turning the Earth into an Ocean Planet. It's implied that the few humans left over will either eventually go extinct or will eventually evolve back into an aquatic form to survive.
Apocalypse Wow: Lots of vivid descriptions of various catastrophe's, including the flooding of London after a surge of water overtops the Thames Barrier.
Apocalyptic Log: Between certain chapters are excerpts from a character's digital "scrapbook" that present blog-like entries on various world events over the course of the flood.
Cozy Catastrophe: The earliest stages of the flood were treated as this; for example, as New York City slowly starts flooding, native New Yorkers pretty much just slog through the water to continue their jobs.
Crapsack World: Pretty much what happens to the Earth as the rising sea starts mixing with all the chemicals and pollution humans normally keep on land. Not to mention the horrible conditions refugees face once they get to the overcrowded higher grounds.
Downer Ending: Humanity pretty much drowns. The main character grows old on a refugee raft, watching ignorance reign over the dwindling human population as new generations of children are born in a Crapsack World Ocean Planet, unaware of everything their ancestors lost.
Fire-Forged Friends: The main characters of the series have a Thicker Than Water connection to each other after spending four years together as terrorist hostages prior to the flood.
Giant Wall of Watery Doom: Averted. The flood slowly inches up, taking decades.
- Played straight in few isolated incidents, but justified: the weight of all that water compresses the land beneath, which sets off a calvacade of earthquakes and tsunamis. This is eventually what causes the British goverment to collapse, when a mega-tsunami takes out most the island.
Hollywood Global Warming: Accelerated, due to the flood.
- Subverted with regards to the cause of the flood. As one of the scientist characters rightly points out, there's not enough water on the surface of the planet, including the poles, to cause a flood of the proportions in the novel. It's actually due to underground reservoirs of water being released from deep within the Earth's core.
The Great Flood
Ocean Punk: What the setting gradually turns into, as the flood reaches higher and higher and more refugees start taking to rafts and boats. Becomes full-blown "Mad Max on water" towards the end of the novel.
Ruins of the Modern Age: Refugee camps survive by diving down to plunder cities that have been drowned by the floods. Quickly becomes untenable as the floods are so deep that a deep-sea diving vessel is required just to get to the flooded-out cities.
Sequel Hook: The last line in the book. "What is Ark 2?!"
Single Biome Planet: What the Earth eventually becomes.