If the sand is electrostatically charged, perhaps having a high iron or iron oxide content (either naturally, because they're a rusting WW2 battleship out in the bay, or artificially, because some super-villain made it that way deliberately), then it might 'cling' more than normal to the Flash's body, which, logically, should pick up a bit of static charge through running (and with all that speed force lightning they draw around him when he runs these days, it sure seems like that would be the case!).
I vaguely recall back in the Who's Who days, he was 300 or so pounds, because of that, but I could be misremembering, and it's hardly like anything from the eighties could be considered canon these days...
If there was even a handwaving attempt at 'science,' he'd have to weigh a hundred tons to be as strong and tough as he is, but that's obviously not the case. His power seems to come more from the solar energy in his cells, than his bodily density, and if his physical body was significantly denser than a normal dude's, he'd retain some measure of increased strength and toughness even when drained of power, under a red son, etc. which clearly isn't the case.
So, as with all things comic book, there's evidence either way you want to go.