Compensatory Water Quality Treatment With Smart Stormwater Ponds

Stormwater storage assets, such as ponds, constructed wetlands, and underground vaults, are often designed to provide both water quality and flood mitigation for a range of critical storm events (e.g., 2-year, 10-year, and 100-year storms), consequently, by design, these assets do not perform optimally for any individual storm event. This is not meant as a criticism, but rather a recognition of technological limitations at the time these systems were designed. Advances in communications and control technology, cloud-computing, weather forecasting, and sensing technologies have now made it possible to optimize stormwater infrastructure for each individual storm event. One approach that leverages these technologies is continuous monitoring and adaptive control (CMAC). CMAC systems monitor the local weather forecast, compare the forecast runoff to existing field conditions by reading on-site sensors, and automatically control the timing and rate of stormwater discharge by actuating on-site valves, gates, or pumps [1]. With adaptive controls, stormwater facilities discharge water in advance of storms, creating capacity for flood mitigation. CMAC systems then hold water during and after storms to increase hydraulic residence time, settle sediment and nutrients, and improve water quality. Adaptive controls are being used across the country to optimize stormwater management for water quality and flood mitigation. For example, the City of Ormond Beach in Florida used CMAC before 2017's Hurricane Irma to create 70 acre-feet of storage capacity, protecting people and property from flooding [2]. The ease with which existing stormwater facilities can be retrofitted with CMAC lends itself to innovative project delivery models. For example, stormwater ponds owned by the Florida Department of Transportation (FDOT) are being retrofitted with adaptive controls to generate nutrient removal credits. These credits are then purchased by other entities to meet water quality goals, thereby providing compensatory treatment. Because CMAC systems collect real-time continuous data on the weather forecast, precipitation, storage volumes, discharge rates, residence time, and water quality parameters, performance is being documented to assure regulatory compliance. One example of a compensatory treatment project with CMAC technology exists in Tampa, Florida. Along State Route 45 in Tampa, a hydraulically-connected set of stormwater ponds ('SR 45 Pond 1') was retrofitted with CMAC (Figure 1). CMAC optimizes the pond storage and discharge to increase hydraulic residence time, thereby increasing sediment and nutrient settling. In the Florida regulatory environment, residence time is used to determine a stormwater pond's nutrient removal efficiencies as determined by the 'Harper Curves' [3]. Figure 2 shows modeled results of increased residence time and nitrogen treatment with CMAC for ponds similar to SR 45 Pond 1. The compensatory treatment credit generated by retrofitting SR 45 Pond 1 with CMAC is purchased by Port Tampa Bay to meet their water quality goals (Figure 3). The use of CMAC technology improves the pond's nitrogen removal efficiency by 44% and increases the flood attenuation volume by 84%. By purchasing compensatory treatment, Port Tampa Bay saved approximately $2.63 million compared to traditional stormwater management, while improving water quality in the bay. This presentation will provide a technical overview of CMAC, discuss compensatory treatment as a project delivery model, and show quantitative data from projects in Florida.