In this video it was blowing between (14-18 knots). So I did not need to put too much sand inside the bag. But if it was blowing (20 to 30 knots) then I would fill up the bag all the way so it doesn't move at all.
Now the reason I chose this location to launch was because it was on shore and no much beach plus you had some power lines as few people saw. This makes launching and landing very dangerous. But if that is the only beach you have to kite, then you have to be very very careful. This is why you need to make sure that your lines are perfectly set up correctly before you take off.
Another think you could do is take your bag into the water at least 20 meters from the shore where it's still shallow and fill it up with lots of sand so it won't move and then sink it into the bottom. Then do the same thing you see in the video by hooking the bar on the bag and then going to the kite and putting the kite on its side. But all this will be on the water now, in case you want to be far from the power lines.
This is another video that might help you also.
