If you watch the 1953? film it pretty much explains the reasoning behind the
bomb, the torpedo nets plus they needed a minimum of 5000lb of high
explosives to have a chance of breaching the dams. In an interview given by
Wallace he stated that his famous 10 & 5 ton "Grandslam" an Tallboy bombs
were originally considered but would only work with a direct hit, any sort
of near miss would do no damge because of the water cushioning effect.
Interestingly the scale model dam used by wallace complete with test
blast damage still exists in the Grounds of the UK Building Research
Establishment in Hertfordshire just out side Watford in the UK.

Myc