If you have too much watchers and you need to clear all of them, you can push them into an array and destroy every $watch in a loop.
var watchers = [];
watchers.push( $scope.$watch('watch-xxx', function(newVal){
//do something
}));
for(var i = 0; i < watchers.length; ++i){
if(typeof watchers[i] === 'function'){
watchers[i]();
}
}
watchers = [];