First, it is worth noting that water jet velocity = (Q/A) = sqrt(2 * g * d). As such, it is not dependent on the opening size of the wardrobe. The figures given appear to assume the wardrobe is 2 square meters, which is probably fairly large.
Second, there is also the matter of momentum. The water mass is ~400,000 kg/sec, at ~200 m/sec, for a net of 80,000,000 kg meter/sec. If the receiving portal is subject to conservation momentum and is another wardrobe, it will be traveling at over 99% of the water jet speed. (The reduction would actually be less than the adjustment for the correct gravitational force.) So part of what the white queen would have to worry about is this wardrobe darting all over the place at ~700 km/hour or ~440 miles/hour. Of course, the sending wardrobe will be zipping around the ocean as well, which might make "Glass Half Empty" relevant.
I have a feeling that the inconstant passage of time, between the two realms, should factor into this somehow. At the start, Lucy is in Narnia for a whole evening, and returns moments later. By the end the whole group stay for many years and are hardly away for much longer (while the 'tour' is still passing). So the amount of water that leaves Earth's oceans in a short time can probably afford to trickle out into the Lantern Wastes at a much lower volume/time. In reverse, would the passive thermal energy of Narnia (even during its snowbound period) over a long Narnian period now be exiting Earthside in a compressed form (if not in frequency, certainly in rate) making the open wardrobe door furnace, like? Or, given the above, and that nobody seems to notice any strange dilation effects during transition, and the fact that Edmund enters shortly after Lucy yet is not several days later in arriving (though possibly up to an hour or two) there may be a "time lock" facility, which means that one or more transitional areas betwixt Earth and Narnia are sealed shut behind one traveller while they start travelling further into (or out of) the furs/firs hybrid areas. We already know that the 'normal' wardrobe back can present itself at times when transit isn't required, so who knows whether it's an active compartmentalising, or just passively not wormholing betwixt the relevent domains.
And for my next essay, I shall be discussing how to automate the use of the rings made by the original Magician (the Professor's Uncle) in order to transfer spent nuclear waste to Charn, or some other, similar dead world. (And the security measures needed to ensure that Jadis does not obtain the necessary material to make radiologically-active weaponry in her conquests of other worlds brought into being by Aslan.) 126.96.36.199 16:24, 23 April 2013 (UTC)
After the loooong trip mentioned above, the owner of the wardrobe (the Magicians Nephew) explains that they should not expect to ever get back into Narnia again through the Wardrobe. It will of course only open for children, and not for anyone throwing it into the ocean or for anyone shooting a portal gun through the door. So nothing would be hitting the witch! ;-) Who by the way is dead at that time. Time passes in Narnia so it fits with what is needed for the children to be the right place at the right time. But nice spotted though that it would be a problem if the portal was indeed always open. However, if the door could be kept open and the witch was still alive... There is not a connecting wardrobe in Narnia, but just a forest - the fur skins in the wardrobe suddenly turns into threes. So there would be no wardrobe shooting around. Finally - if the earth oceans would be transferred to Narnia - the whole Narnia world would be flooded, and the heat capacity of the earths water would surely melt all snow and ice without freezing over, since new water would continue to flow through for a very long time. (See the What if - Drain the Oceans: http://what-if.xkcd.com/53/ )Kynde (talk) 19:26, 16 January 2014 (UTC)