How Buoyant Flow Control Devices Can Reduce Pond Size Requirements

Buoyant flow control devices (BFDs) can be used to minimize the amount of storage volume that is required in stormwater ponds by maximizing the outflow rate. The devices operate on the constant head principle and have the ability to produce a constant flow rate regardless of the depth of water in the pond. These devices have been used in Washington and other states throughout the northwest and have recently been permitted for both public- and private-sector projects in Florida. This paper describes an analysis to demonstrate the volume savings made possible in a stormwater management facility by using a Thirsty Duck® BFD. Hydrologic and hydraulic models of existing stormwater ponds were analyzed based on their geometry and performance with a standard outlet control structure and with a BFD. The models were then compared to determine the pond volume required when utilizing the BFD versus a standard outlet control structure. Two different analyses were performed. The first, Analysis A, used a BFD designed to match the permitted discharge rates for the proposed ponds. The second, Analysis B, used a BFD designed to meet the existing conditions, as required by the water management districts and other agencies. Analysis A demonstrates the pond volume and area savings that would have been possible had a BFD been used in the permitted pond, as opposed to a traditional outlet control structure. Analysis B demonstrates the pond volume and area savings made possible had the original pond been completely re-designed with a BFD outlet control structure. Based on these analyses, the Thirsty Duck® BFD can reduce the required pond volume and area by an average of 36% while meeting the permitted pond's design parameters (Analysis A). The Thirsty Duck® BFD can reduce the required pond volume and area by an average of 44% while meeting the existing conditions and all stormwater requirements (Analysis B).