The first evacuation (without the dumping coefficient)

  New_d = d * (1 - 2*.1 - 4*.01) + (c+e)*.1 + (a+b+f+b)*.01

is as good as conserving momentum as the original model. (Assuming you calculate all the new values using a new matrix instead of overwriting the old ones.) It breaks energy conservation, but that simulates the friction between oposite direction currents, without going to the whorls level.

I agree that this is not 100% realistic, but I think it's a good and simple addition to the model in the article, and tweaking the coefficients it may give a good enough visual result.