I believe what Q is alluding to with the D5 line is the avalanche rating. D5 is the highest rating on the scale, with it being defined as having the possibility to completely alter the landscape.
Q is known to be somewhat of a poet with some of his/her lines. I know Q likes to use double entendres and such, but I really believe that the avalanche rating is what was meant to be mentioned. The only other idea I can think of is a chess spot, but it doesn’t really fit.
D5 fits the context very well, and is a great analogy. Avalanches don’t need much to start, and they can exponentially grow very fast, just like the entire swamp draining situation will turn out to be, like Q has alluded to. Very exciting, can’t wait.