HBO Films and BBC Two have joined forces to make “Tsunami,” an ensemble drama about the disastrous flood that devastated the Andaman Coast in Thailand in late 2004.Tim Roth, Chiwetel Ejiofor, Sophie Okonedo, Toni Collette, Hugh Bonneville and Gina McKee will star. Bharat Nalluri is directing. Filming has just begun in the Thailand locations of Phuket and Khaolak. The film will premiere on the pay web and BBC Two at year’s end. “Tsunami” covers the disaster and its aftermath from the vantage point of survivors, media and relief workers. Abbi Morgan (“Sex Traffic”) wrote the script and Finola Dwyer will produce. Jane Featherstone and Derek Wax will exec produce.