there is the 100yr problem where you can not build buildings in bad areas but you cannot inform public not to be in the areas they will ignore warnings - even if a river is called "death from flooding is coming river" - people will ignore it day in day out

public LEGAL notice
so, fox covered that Texas thinks sirens (like japan has) for the 100yr. but schools don't line the river that's expensive
THE ONLY ADVICE I HAVE IS THAT AT THE HEAD OF THE RIVER - A WATCHER WOULD INFORM A (FIRE POLICE NEWS) HELICOPTER, WITH FUNNY LIGHTS AND LOUD SPEAKER, TO FLY THE LENGTH OF THE RIVER WARNING EVERYONE TO EVACUATE THE RIVER BED AREA. certainly they could afford that
unfortunately, they don't know while rain is falling it will be a problem necessarily. false warnings might become common (and even dei funding) (a helicopter every night with disco lights)
yet - there you go. i believe that is infact the option texas has