Based on Chris Ryan's Flash Flood 2 weeks before the flood started.
The Thames Barrier needed some maintenance so the government hired a full time maintainer to make sure the barrier was always working smoothly and that there would be no malfunctions. They hired a man named Nicholas Hasaan, he said he'd been living in London for 6 years.
His job involved making sure all the gears for each of the gates were working so in an emergency they could press a button in the control station to make the barriers stop the high tide from coming into London. Nicholas Hasaan seemed to be a good worker because the barriers were working fine and the there was no leaks in the wall.
Nicholas took a weekend off and flew to his home country of Kuwait. He said he was visiting relatives but when he arrived there he got into a four wheel drive and was driven by away by a man. They crossed the border into Iraq where he was dropped off into a little town 1 and a half hours south of Baghdad where there was a terrorist camp. There he was given instructions on a terrorist plot to flood London. He was told that the next Wednesday there was going to be a king tide that the Thames barrier couldn't stop all of it entering but if the Thames Barrier was up the flood that would be caused would be very minimal but if the barrier was down it would cause a massive effect to all London. The instructions were to go to work on Tuesday and in the afternoon place a bomb with a timer so the gears wouldn't work and the gate wouldn't be able to stop the flood. He left the camp and was driven to Kuwait airport and flew back to London.
When he arrived in London on that Sunday afternoon he went to his apartment where it had looked like he had been robbed but he then went to is safe and everything in it was empty. Someone was looking for something and they had found it, they took the letter which he was sent from the terrorist camp in Iraq. While he was hoping that they didn't find it he heard sirens in the near distance and the sirens were getting louder. Quickly he left the apartment and went to a local pub trying to hide his face and look normal. He was still in the pub when his cell phone rung. When he was on the phone he was given instructions on a second plan, he was going to blow it up today.
In the MI5 HQ that phone call was being tagged and they had tracked him down and knew where he was. Straight away they sent the police to that pub not letting anyone out. After 5 minutes of searching the pub they realise that he had already left. The MI5 was unaware of what the plans where so they ordered the police to search around the pub. The MI5 decided to put one of there agents in the case to help find Nicholas. So they put agent John to the case. As soon as he was informed he straight away went to the pub and started to ask customers which direction he left in, no one noticed so went in the back room of the pub where 24 hour surveillance was being viewed. He walked in and rewinded and saw which direction he had left in. He dropped the remote and went outside in the same direction Nicholas did. John kept on stopping into the shops to look at there surveillance to make sure he was going the right way. After following Nicholas he had realised Nicholas had entered the Thames Barrier. Agent John called MI5 to tell them where he was and where he had thought Nicholas was. John started running on the edge of the barrier where he caught a glimpse of Nicholas and was chasing him. Nicholas stopped and turned around facing John ten metres away from him and pulled out a hand gun from under his jacket and shot but he missed and John fired his gun and hit Nicholas' arm. Being hit in the arm made him run slower and John eventually caught him and arrested him. That Wednesday the gear-shaft in one of the gates was already being faulted by Nicholas 2 weeks earlier and the flash flood started.