Capital Cost Savings Using Passive Controls in Atlanta's Utoy Creek Basin

This paper describes the use of innovative passive hydraulic controls to mitigate SSO/CSO occurrences, to enhance hydraulic conveyance within the Clean Water Atlanta Program sewer jurisdictions, and to reduce future costs of capacity enhancement programs.The City's Consent Decree requires the capacity of the existing sewers be increased to reduce overflows and to prevent surcharge conditions during future peak flow conditions (i.e., under a 2-year, 3-hour cloud burst design storm). Even with trunk sewer rehabilitation for infiltration/inflow (I/I) reduction completed under the City's sewer system evaluation survey/ sewer rehabilitation program, implementation of capacity enhancement projects in several of the City's basins will be exorbitant, especially for projects in those basins undergoing significant growth. To create potential cost reductions, traditional hydraulic capacity expansion methods were modified to use in-system storage to trim or reduce upstream design peaks so that unacceptable surcharge and inadequate levels of sanitary sewer service in existing down stream piping systems were mitigated.Use of in-system storage will invariantly cause purposefully created surcharge upstream of the control structure, but in so doing will protect downstream areas. The tactical idea is to create short upstream pipe segments used for controlled storage (but satisfying all acceptable freeboard or level of service conditions) to eliminate long downstream segments undergoing capacity restrictions. This idea proved extremely attractive in basins where upstream areas were undergoing intensive growth and existing capacity-limited trunk sewers needed to be replaced and enlarged.Generation of in-system storage during a wet event invariably will require clog-free underflow flow throttling and rapid fail-safe weir overflow. The function of flow throttling can be generated using passive controls. A passive control throttle involves a device that automatically limits discharge under increased flow conditions or rises in water surface, regardless of system hydraulic conditions. Devices such as bending weirs, vortex throttles and Hydroslides (adjustable orifice control) were used to create in-system storage and to attenuate peak wet weather flows within existing and rehabilitated trunk systems.This alternate approach to increasing hydraulic capacity reduced potential capital expenditures and construction disturbances associated with the installation of large diameter trunk systems. For the Utoy Creek Basin, these devices were used to (1) improve the distribution of flows between two existing trunk systems that convey flows across the basin and (2) purposefully store peak wet flows along major trunk systems. Trunk sewer capacity enhancement capital expenditures were reduced by 17 million or about 31 percent of traditional approach. Long term extreme event time series imulations provided equivalent SSOs occurrences and volumes for the two different approaches.