[aerohead]

I don't know about pebbles, but the bugs should collide and adhere to the inner duct, rising towards the radiator up above.
Some racers will place 1/4-inch hardware cloth in the opening to protect against rubber 'marbles' and other foreign objects kicked up by cars in front of them, making it to the heat-exchanger.
Errant plastic trash bags and palletized cargo wrap laying in the roadway on windy days probably poses the greatest risk to a cooling system.
A good reason for temperature gauges.