Restoring Predevelopment Hydrology with Smart Stormwater Controls in Aiken, South Carolina

Municipalities often face difficult choices about how to allocate resources, where to build infrastructure, and with whom to partner to address complex stormwater management challenges. The City of Aiken in South Carolina is no exception as city leaders seek to restore predevelopment hydrology in the Sand River. The Sand River collects runoff from 1,109 acres of urban watershed in the City of Aiken and flows primarily through Hitchcock Woods, the largest privately-owned urban forest in the country. Hitchcock Woods and the Sand River are a haven for hikers and equestrians, and a driver of tourism in the City. What was once small enough for a person to step across, the Sand River has become a 70-ft deep canyon as a result of frequent and intense stormwater flows coming from downtown Aiken over the past 40 years [1,2]. In 2017, the newly-formed Sand River Stormwater Task Force recommended 20 capital improvement projects, sized to capture 58 ac-ft of stormwater, and costing approximately $22 million (2017 dollars) as a way to restore the river's predevelopment hydrology [3]. While this plan was based on the best available information at the time, recent developments in real-time control technology led the Task Force to reevaluate the recommended project list. In 2020, the Task Force recommended the use of forecast-based real-time control technology, also called continuous monitoring and adaptive control (CMAC), on the largest proposed stormwater project: a 24 ac-ft set of underground vaults (Figure 1). These vaults are positioned at the downstream end of Aiken's Sand River urban watershed, just upstream major erosion in Hitchcock Woods (Figure 2). CMAC was estimated to increase the effectiveness of the vaults by 2x to 3x by retaining water during and after storms, increasing infiltration, and only discharging in preparation for forecast storm events exceeding the capacity of the vaults [4]. To further quantify how the adaptively-controlled Sand River vaults perform compared to traditional passive underground storage, a modeling effort was undertaken for the period of calendar years 2001 through 2006, during which time Aiken received 295 inches of rainfall (average of 49 inches per year). The CMAC system was found to retain 71% of stormwater flow entering the site during wet weather periods, compared to only 26% wet weather capture achieved by passive storage. Modeling also showed the adaptive control vaults to exfiltrate 66% of all stormwater volume entering the system compared to 25% exfiltration by passive storage. This increase in performance will set a new baseline for stormwater flow in the Sand River, thereby mitigating the need for upstream stormwater projects and saving the City of Aiken millions of dollars in capital expenditures. This presentation will examine the decision to include CMAC on this project, discuss the design process, and present modeling results showing the performance improvement of the vaults with adaptive controls. Future work will include comparing the modeled performance to observed performance after the system is commissioned in 2022.