"What about ground effect? Textbooks say that the ground effect is noticeable up to a height of "a tenth of the wingspan" or similar. "
Ground effect is effective to rather more than a tenth of the span. It gives very noticeable drag reduction at 4-5m height to a 15m glider: I've flown an SZD Junior (35:1 glide ratio) in ground effect, starting at around 15ft altitude and 70 kts and, 750m later, was still at 15ft and still had 60kts on the clock, at which point I popped the airbrakes and landed, having flown most of the length of a grass runway into a light breeze.
Without ground effect and at its most efficient speed, 48kts, I'd expect a Junior to be on the ground in less than 175m from a start height of 15 ft. Above 60 kts its glide performance is rubbish, so evidently ground effect had a major effect.