One commentator from the EA suggested that sand bags do nothing to prevent flooding, the water is simply slowed down slightly, as passes straight through them, getting filtered as it passes through.
I very much agree with that, so why do they use them?
What does work, is to put the sandbag inside a plastic bag. The weight of the sand allows the bag to conform to what ever surface it is placed upon, the plastic provides the actual seal.
No flooding issues here, but last week I did need to divert a flow of rainwater away from it usual path, to avoid it washing the cement out of some newly laid concrete. Sand in a plastic bag worked fine, to achieve that.