• description of Farquhar falling straight downward through the bridge and losing consciousness and "was as one already dead."

Foreshadowing in literature occurs when the author gives a hint of what is going to occur. An instance of foreshadowing occurs as Peyton Farquhar stands on the bridge in part 1, waiting to be hanged. He looks down at water below. The narrator makes clear that is it rushing at a rapid pace, the "swirling water ... racing madly." However, to Peyton, it looks like it is moving "slowly." He calls it a "sluggish stream." This difference between fast-moving reality and Peyton's slow perception of it foreshadows how time will slow for him at the moment of death, allowing him to have a long, elaborate fantasy of escape.

The idea that real time will move much more quickly than the words recording Peyton's thoughts is foreshadowed a second time when we are told,

these thoughts, which have here to be set down in words, were flashed into the doomed man's brain rather than evolved from it.

In other words, the long linear sequence of language is not exactly how Peyton will experience his last images before death.

The revelation at the end of part 2 that the man dressed in gray who has made a polite call at the plantation asking for water is a union scout foreshadows the fact that Peyton has fallen, unbeknownst to him, into the trap that will cost him his life. It is, as well, a moment of dramatic irony, in which the reader knows what characters in a story do not.

The images in part 3 that Peyton fits to his escape fantasy foreshadow that he is, in reality, at the point of death by hanging, choking, his feet dangling in the air:

His tongue was swollen with thirst; he relieved its fever by thrusting it forward from between his teeth into the cold air. How softly the turf had carpeted the untraveled avenue—he could no longer feel the roadway beneath his feet!

This is perhaps Bierce's most well known Civil War story, in part because because of a "Twilight Zone" production of it in the late 1950s or early 1960s, and it has remained popular for its surprising ending.

As your question indicates, Bierce uses several instances of foreshadowing but so skilfully directs the reader's expectations throughout the story that we can only understand the elements of foreshadowing in hindsight.

For example, when we are introduced to Peyton Farquhar, he is in the last few minutes of his life as he waits for the support to be kicked out from under his feet so that the hanging can take place.  The first element of foreshadowing occurs here when Farquhar imagines freeing himself from he rope, falling into the river, and making his way home.  The reader, of course, puts this down to wishful thinking.

As Bierce jumps to the recent past in which Farquhar, at his home, has a conversation about the bridge and the Federal troops with a Conferderate soldier (who turns out to be a Federal scout in disguise), Farquhar asks the soldier what could be done to interrupt the progress of the Federals.  As Bierce moves us back to the present, we are at the point at which Farquhar is hanged, and so we understand that Farquhar has tried and failed to burn the bridge, is caught, and his now being executed, exactly as the Federal scout described.

The entire hanging sequence and escape is rendered in such detail that it is impossible to know that this is happening in Farquhar's imagination.  An element of foreshadowing does occur in this sequence, however, that might make an astute reader pause.  When, for example, Farquhar says that "he was conscious of nothing but a feeling of fulness--of congestion" in his head, he is feeling one of the first consequences of being hanged: as the noose tightens quickly, his wind-pipe is closed and the blood in his head has nowhere to go, so his head would naturally feel full and congested.  A few seconds later, after the believes the rope has broken and he has fallen into the river, he comments that "his neck ached horribly; his brain was on fire. . . his whole body was racked and wrenched with an unsupportable anguish!"  This, again, is an element of foreshadowing because he is accurately describing the sensations a hanged person would feel if his neck did not break in the initial fall when he support is removed from his feet and he is suffocating to death.  At this point, the reader still expects that this sequence is depicting reality because what Farquhar is feeling is consistent with the trauma of even an unsuccessful hanging.

Immediately after he comes to the surface, his unusual ability to sense even minute sounds and sights around him, even to the point of seeing the color of a soldier's eyes yards away from him, should give us perhaps our first serious concern that something very unusual has occurred to him.  Also, in such a situation, an officer would not give his troops formal instructions to fire at the escapee as Farquhar hears: "Attention, company! . . . .Shoulder arms! . . . .Ready! . . . .Aim! . . . Fire!"  The soldiers would be shooting as fast as they could and without formal orders.

Lastly, when he reaches home, the greeting of his wife, as if he had just been out for a walk, seems, under the circumstances, not demonstrative enough for a greeting of a husband who has been hazarding his life for the last several hours, another instance of foreshadowing.

The author doesn't make clear whether Peyton Farquhar is dreaming that he escaped or if he really escaped, but there are hints.  For example, I started to realize that the whole escape was a dream sequence when Farquhar was able to avoid being shot by all the soldiers who were aiming at him.  He succeeded in swimming away, without being killed. 

When Peyton Farquhar is running, and running, and running, trying to make it to the safe part of the state, to get home, he is tireless, he does not stop, after swimming, he is able to run, until finally he sees his wife, who looks totally, perfectly calm and happy to see him as if nothing is wrong.

Had Farquhar really escaped, his wife would not look so fresh and calm, she would have been more emotional and greeted him with passion and joy.

I knew it was all a dream and that he was about to die because all the events of the last moments of his life amount to a beautiful dream, and unattainable dream.  Farquhar dies with the vision of his wife stamped in his memory forever.

In "Bridge," Bierce foreshadows the imaginary nature of Farquhar's escape mainly through imagery and minute detail. The protagonist's eyes are not covered in Part 1 of the story, and so the reader can "see" the setup of his execution site through Farquhar's eyes. He lets "his gaze wander to the swirling water of the stream racing madly beneath his feet" and then closes his eyes to let his life flash before his eyes. When Bierce returns to present time in Part 3 of the story, Farquhar is now falling straight down. Phrases such as "he lost consciousness," he was awakened "by the pain of a sharp pressure upon his throat," and a "sense of suffocation" all hint at Farquhar's demise by hanging.

Another example of foreshadowing quoted directly from the story comes from the third section.  The third section is a continuation of the first in which the reader finds out that Farquhar is about to be hanged.  The first sentence of this section states,

     “As Peyton Farquhar fell straight downward through the bridge     he lost consciousness and was as one already dead.”

This foreshadows for the reader that at the start of section three, Peyton Farquhar was already dead and that none of the events that are described in that third section ever really happened.

Foreshadowing in literature are clues or hints in the story of events to come later in the story.

In An Occurrence at Owl Creek Bridge, the story is a little disordered and the reader gets confused about what happens first.  The first part of the story is what is currently happening, Peyton Farquhar is going to be executed by the Union or Federal Army.  He was caught conspiring to blow up Owl Creek Bridge.  

Standing on the bridge he plotted to destroy, Farquhar, noose around his neck begins to imagine his family, he imagines that he escapes.  Except, the reader does not know whether his imaginings are real or just a dream he has as he is seconds away from death.

In my view, after Farquhar begins to imagine that he has escaped, the whole sequence of events foreshadow the end of the story.  As he escapes in his dream, he swims away, unharmed, while several soldiers are shooting at him in the water.  I found this very suspicious.

Farquhar swims and swims until he can safely leave the water and run onto land.  He then begins to run and run and run, all to get away from Federal forces and back to Southern controlled territory.  He seems to have unlimited energy and ability to travel great lengths.  This is an indication that he is experiencing a fantasy rather than a real event.

"In this section, the narrator's language is often melodramatic. For example, when Farquhar is in the river, fighting to break the rope around his wrists, the narrator declares: "What splendid effort!" and "What superhuman strength!" Additionally, the surroundings are described in the minutest detail, suggesting that Farquhar could not possibly be experiencing what is being described."

Finally, when Farquhar is within sight of his home, there is no sign on the face of his wife of any emotion regarding her husband's impending death.  She appears carefree and happy, unusual behavior on the day of your husband's execution.  The only explanation for this behavior is that she is a vision, as he wants to remember her.

As he reaches out to hug his wife, who remains smiling in a rather wooden fashion, he is literally jolted back to reality, he is swinging from the end of the rope, dead by execution for his crimes against the Union or Federal Army.

Peyton Farquhar is executed, at the end of the story we discover that he was only imagining his escape it was all a dream.