KHAO LAK, Thailand — Sitting on the beach with a smile, it's easy to forget how much was lost on this stretch of sand.

On the morning of Dec. 26, 2004, supermodel Petra Nemcova took a stroll with her photographer boyfriend along the Khao Lak beach's white shore. She remembers wondering at the time why the water had retreated so much, but assumed it was because of a full moon. It was just an hour later, when she heard screams, that she discovered the real reason.

"I started seeing people frantically running," she told NBC News on Thursday from that same beach. "In the next second water came in and crushed all the windows."

A wall of water ripped through her bungalow and swept the model and her boyfriend Simon Atlee out into a churning, angry sea.

"I never knew what a tsunami was," she said, 10 years to the day from the moment her life and love were turned upside down. "I never heard the word before...Never."

Nemcova clung to a tree for eight hours, listening to screams for help and watching bodies float by mingled with debris. Her boyfriend, Atlee, was lost. He was one of more than 5,000 people on Thailand's west coast killed by the tsunami that ripped through southeast Asia.

"That day I didn't have a choice. Now I have a choice."

Ten years later, Khao Lak has been rebuilt and tourists have returned to the coastal paradise. But the lost are never far from mind — and the focus now is on building a brighter and strong future, from earthquake-proofing homes to planting mangroves to protect the coastline.

Nemcova has never forgotten the cries for help that she heard as she clung to her palm tree — and never forgotten how she was unable to heed them.

"That has given me drive to help children every day since because that day I didnt have a choice now I have choice," she said.

The Czech-born model smiles easily and often, even while discussing the tragedy that took her love and "devastated" Atlee's family and friends. The tsunami inspired her to found the Happy Hearts Charity, which has rebuilt more than 105 schools in areas impacted by natural disasters.

“The tsunami has given me many gifts and one of them has been consciousness of how lucky I am,” she said, explaining how she doesn't cry for the past and instead looks forward. "I cry when there’s no hope, when I have to choose where to build a school and which kids we will help."