Need help with the elements of a spillway dam wall

Hi there I'm trying to help my landlord to increase the hight of a spillway dam wall. Its very small, around 2m long, and we just want to increase the hight of it so we can retain slightly more water in the dam.

I was wondering if anyone could help me out with links or an explanation of materials and technique that we would need to use to make it nice and leak proof.

I really appreciate you taking the time to help.

Thanks a lot