Chapter 21, entitled "Echoing Footsteps", charts the beginning of the French Revolution and the storming of the Bastille. The sea imagery is used to describe the hundreds of discontented Parisiennes who form the group that overthrows the Bastille and wreaks vengeance on the upper class. When Defarge announces that the patriots are ready, Dickens describes the reaction in the following manner:
With a roar that sounded as if all the breath in France had been shaped into the detested word, the living sea rose, wave on wave, depth on depth, and overflowed the city to that point. Alarm-bells ringing, drums beating, the sea raging and thundering on its new beach, the attack began.
Thus the sea is used as a metaphor to picture the fury, the anger, and above all the violence of the patriots as they begin their Revolution. This imagery is continued throughout the rest of the Chapter with the description of the fall of the Bastille and the unyielding violence of the Patriots:
A white flag from within the fortress, and a partly - this dimly perceptible through the raging storm, nothing audible in it - suddenly the sea rose immeasurably wider and higher, and swept Defarge of the wine-shop over the lowered drawbridge , past the massive stone outer walls, in among the eight great towers surrendered!
So resistless was the force of the ocean bearing him on, that even to draw his breath or turn his head was as impracticable as if he had been struggling in the surf of the South Sea...
This description also reveals something key about the revolutionary force that has been unleashed. Although Defarge has been key in heading it up and leading it, now that it has been unleashed, he has no control over it whatsoever - the hunger for revenge and violence and blood is so all-consuming that he has lost all control. Note how the title of Chapter 22 continues this sea imagery: "The Sea Still Rises."