I cant see that Leaky Dams would ever fall under CDM regs, (Projects are classified as ‘notifiable’ if they take longer than 30 business days or longer than 500 person days).
In order to check if we are bound by CDM we fill in a ‘PCI’ form (we are required to do this as we work under Shropshire Council and their contractor) See attached.
This guides you through the risks and automatically tells you if a job is CDM compliant.
CDM is generally for larger construction projects and i can see it becoming relevant if building large attenuation pools for example.
Hope you find this helpful,