This story is called 511 because 511 km/h is the fastest recorded speed over water. This isn't set in a particular part of the Flash apart from in season 2 and is set at the end of the heroes of Olympus time line.
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"Now the Scarlet Speedster has nowhere to run." Zoom said in a mocking tone.
"Umm.. Hey, guys. A plan would be good, any time now" said Barry to Cisco.
"Dude, I don't know but Zoom...Barry?" Cisco moved over to the other monitor, Barry hadn't moved. "Guys Zoom just left." Barry voice came over the speaker, he sounded shaken and a little uncertain.

Barry looked on the roof top to where Zoom had departed in a streak about 15 seconds ago. He moved over to check Zoom wasn't still there. He leaned onto the pipe that jutted up through the roof about a meter away. His hand came to rest on something smooth. It was a scrap of Zooms suit. A scrap of Zoom's DNA. Barry ran to the nearest store grabbing some zip lock bags and leaving the exact change on the counter before speeding out onto the roof top. Why did villains always pick roof tops? Barry carefully shifted the bag so the fabric fell in without him touching it any more.

Paper went every were as Barry entered the room. "Why must you always do that?!" Catlin commented begrudgingly. But she stopped short when she saw what was in Barry's hand. "Is that...?" She started. Barry nodded.
"Yeah Baby!" Cisco voice echoed down the hall. He swaggered in carrying a milkshake "Free…" he pointed at it "…with my loyalty card" he smirked. Catlin shook her head and moved over to her lab. "I need to get back to the station" said Barry racing out the room. Leaving Cisco alone with his strawberry goodness.

Joe was at the station looking through some document. He glanced up, "Hey Bear" he greeted "I got those files for Iris. would you, give them to her for me. I think I'll be spending the night in Starling City the police there have requested back up."
"Yeah, sure and if you see Oliver whilst your there." Barry didn't need to finish.
Barry phone began to vibrate he fished it out of his pocket and answered. "Oh, hey Cisco." He said
"Meta human alert, this guy seems to have water powers. We should call him…"
"Don't say it Cisco. "Barry silently plead.
"Water boy, he's down by the water front." But Barry was already gone.

Cisco was right about the water powers thing. This meta was in the middle of a tsunami and the pavement bordering the water was cracking up. Barry sped across the water. The meta in the water was only a kid 16 or 17 maybe his head was drooped. Was he asleep? Barry knew if he ran into this wall of water it would be as bad as running into concrete but if he stopped running to think he would drown. "Cisco?" Barry asked. No response. The wave was nearing Central City. Barry had a flash back of the alternate time line, weather wizard, Iris kissing him. He ran back and forth repelling the water. Sure the wave was gone but, so was the boy. Did he just kill someone?

Sorry that this isn't really to the vibe (pun intended) of the Flash or Percy Jackson. Please R&R and tell me if you enjoy!