A crew of four employees was cleaning out storm water sediment traps using a hydraulic pump to remove the water from the gravity well and then a vac truck to remove the sand and other debris caught in the sediment traps (using a cage to surround the manhole while performing the work). The crew had finished cleaning the sediment traps and three of the employees were putting the equipment back in the truck. The injured employee was replacing the manhole cover and fell approximately five feet into the gravity well. The employee sustained broken ribs and was hospitalized.