Mitigating High-Flows and Improving Water Quality in a Stormwater Impaired Brook

Many New England communities are undergoing flow restoration efforts to improve the health of impaired waterbodies. One such waterbody is Englesby Brook, which drains an area of approximately 605 acres in Vermont's Burlington Bay watershed before flowing into Lake Champlain. Englesby Brook was designated as a stormwater-impaired watershed on the 2006 Vermont 303(d) list due to multiple impacts associated with excess stormwater runoff [1]. A Total Maximum Daily Load (TMDL) target was subsequently released, requiring an 11.2% increase in stream flows during low flow conditions and a 34.4% reduction in flows during the 1 year 24 hour storm event. To improve stream health in Englesby Brook, the City of Burlington Department of Public Works planned a number of stormwater projects. One such project was the retrofit of a stormwater basin called '08 Pond', a wet detention pond collecting runoff from 131.5 acres (21.7% of the Englesby Brook watershed). The proposed retrofit would have included expanding the existing pond footprint, excavating below the pond bottom, and creating additional storage above the existing permanent pool at an estimated 2015 cost of $400,000. This proposed project was estimated to reduce peak discharge from the pond by 63.4% for the 1 year storm [1]. However, recent advances in communications and control technology provided an alternative to this traditional expansion retrofit. To optimize flow through the 08 Pond, the City of Burlington decided to enhance the basin with continuous monitoring and adaptive control (CMAC) technology. CMAC systems leverage real-time site data, the weather forecast, cloud-based software, and flow controls (e.g., actuated valves) to predictively control the timing and rate of flow through stormwater facilities [2]. At 08 Pond, CMAC automatically draws down the pond below the normal pool in advance of storms, allows the pond to fill up targeting zero discharge during events, and performs a post-storm release. Model results show an up-to 80% reduction in peak discharge during the 1-year storm is possible through the use of CMAC at 08 Pond. Anticipated CMAC costs in early 2020 totaled $98,000. Following the retrofit completion in May, 2022 the City will have invested $105,000 to facilitate the installation of this innovative CMAC system. Data from a set of storms totaling 1.63 inches of rainfall in seven days at 08 Pond is shown in Figure 2. Englesby Brook 08 Pond is the second CMAC system in the City of Burlington. The first was the enhancement of an underground detention vault at the Greater Burlington YMCA in 2020. During an analysis period of 3/1/2020 through 11/1/2020, this project retained 70.2% of the stormwater volume during critical wet weather periods, compared to only 16.8% stormwater capture simulated for an identical passive detention system. This presentation will discuss how Burlington is working towards stormwater impaired waters flow restoration and parallel water quality requirements, the background for the Englesby Brook 08 Pond project and highlight aspects of the design, construction, and software configuration. Performance results will be shared for both the 08 Pond and YMCA CMAC projects.