even when there is NO water (which I think is impossible to achieve) the gravel will still "shift" or slosh when carried or driven. so expect the set up to be runied. when there is very little water (I tried once with something like 1cm water, the minimum before the siphon died, and remember you cannot tip the tank to siphon because the gravel will "landslide&quot, the effect is just as disastrous because every movement especially when the car brakes is like a tsunami []

Bad Idea.