How to cut small channel in tarmac drive

[dal5band]

My son just got a house and there is water running in under the garage door. The previous owner had the drive tarmacked. There is a good slope away from the garage as the site is on a slight hill. But near the front of the garage the tarmac is slightly higher than the garage so about 6 inches of it drains into the garage. How can i cut into the tarmac to create one or two drain channells about 12 inches long and 3 inches wide.
Hammer and bolster ? Or blowlamp and scraper ?

 

[Notch7]

9" angle grinder with diamond blade would be my guess to cut a neat line, then chip it out with a kango

 

[Alec_t]

9" angle grinder with diamond blade

Wouldn't that get instantly clogged with tar?

 

[Ian H]

Wouldn't that get instantly clogged with tar?

No, it would throw it out as dust.