water free convection

How can I calculate the folowing exemple: there it is a pipe (a circulation pipe)with water at a temperatute t1, which will give heat to a lake (at a temperature t2). beetween the water and the pipe it is forced convection (i know how to calculate the transfer coefficient ), and beetween the pipe and the lake I supposse that it is free convection. How can I calculate the transfer coefficient beetween the pipe and the lake ? Should I used the eqauation for laminar movement, and the velocity to be 0 ?