Washington airport project uses floating outlet to meet goals
Due to continued growth and a five-year expansion plan, Bellingham International Airport in Bellingham, Wash., faced a number of stormwater challenges in 2011, including area wildlife. As part of a solution, the airport’s consulting firm—Reid Middleton, Inc.—contacted Thirsty Duck for a drainage retrofit.
Three primary goals were immediately identified: Compensate for increased tailwater at the outfall, convert a large wet pond to become dry and add capacity for the projected build-out of the airport. These goals were complicated by the fact that the expansion plans would add several acres of impervious area to the drainage system and the increasing tailwater—caused by beavers on the creek to which the system discharges—required more storage in the pond, but its conversion to dry storage would reduce the available space.
The drainage analysis for the system was prepared to meet the Washington Department of Ecology's flow duration requirements. These standards require a continuous simulation analysis in which the post-development flow duration must not exceed pre-development flow durations for a range of flows from 50% of the two-year recurrence flow rate to the 50-year recurrence flow rate of the pre-development model.
After a study for the Bellingham International Airport was completed in the Western Washington Hydrology Model, it was forwarded to Thirsty Duck, which evaluated the data. It was determined that a Thirsty Duck patented floating outlet would address each of the primary goals. The company based its solution on the size of the required flows, with the lowest controlling flow equaling 19 cu ft per second, but it also took into consideration the tailwater situation. It determined that the most efficient product would be an ER-202 because of its dual restriction configuration that provides a constant discharge with minimal impact from tailwater.
To handle flows in excess of the lowest controlling flow, an orifice and a weir were designed and installed in a concrete riser that also connected the ER-202 to the outfall pipe. The ER-202 was placed in a round concrete structure and set in a sump in the bottom of the pond; this allowed the device to drain the pond to the bottom of storage. This configuration served the dual purpose of minimizing standing water and discouraging waterfowl from congregating near the airport. The ER-202‘s self-skimming feature also helps to minimize the amount of trash that passes through the outfall and discharges to the creek, where the beavers thrive in the project basin.
Installation of the ER-202 was initiated after the fabrication of the concrete structures and completion of the pipe assemblies. The floating outlet and skimmer, delivered to the jobsite almost completely assembled, was simply lifted by a small excavator and lowered into the housing structure for a precise fit. The holes for the anchor bolts were drilled and set, and the entire installation was completed in less that one hour.
The Thirsty Duck patented floating outlet, installed in November 2011 at Bellingham International Airport, meets the requirements of the Washington Department of Ecology's flow rate criteria and has been operating flawlessly ever since.