Dunkirk: Where History Happened. We visit 10 places connected with the remarkable evacuation of the British Expeditionary Force from northern France in 1940. This feature was first published in the April 2010 edition of BBC History Magazine. Tuesday 25th May 2010.
The seepage velocity is less than the actual flow rate of the liquid or gas in the pores because the fluid moves only through the portion of the cross-sectional area S that is occupied by the pores.
Terminal velocity is the speed where drag equals thrust. Drag is proportional to speed while thrust typically isn't, so as you go faster, drag increases until it cancels out all the thrust and you can't go any faster. Drag is also proportional to atmospheric density, so terminal velocity varies with altitude.