The Broad Meadow Brook Wildlife Sanctuary (Figure 1) is the largest urban wildlife sanctuary in New England. The 435-acre natural area, in the southeast corner of Worcester, Massachusetts, is embedded in an urbanized landscape. The sanctuary is a partnership between the City of Worcester, National Grid, and the Massachusetts Audubon Society (Mass Audubon), which manages the sanctuary’s trail network and visitor center. Broad Meadow Brook Wildlife Sanctuary includes five miles of trails through the area’s mature forest and wetlands in a valley along the Broad Meadow Brook.
The sanctuary does have its challenges, however, including flooding in nearby low-lying neighborhoods and populations of invasive plant species. The brook is also isolated from part of its floodplain by an earthen embankment that has contained a sewer force main for many decades. When the City of Worcester decommissioned that force main as part of ongoing improvements to their wastewater treatment system, Mass Audubon, along with their project partners, the city and the Massachusetts Division of Ecological Restoration (MA DER), seized the opportunity to initiate a holistic restoration program for the sanctuary.
Figure 1: The Broad Meadow Brook Wildlife Sanctuary with its isolated eastern floodplain (right), looking north towards its urbanized watershed (top). Courtesy of City of Worcester.
In 2023, Weston & Sampson joined a team of consultants to assist Mass Audubon and the stakeholder team in that effort. Specifically, Weston & Sampson was tasked with developing a hydrologic model of the brook’s watershed to understand runoff patterns and identify design flows. The effort also included a hydraulic model of the brook and its floodplain to support development of, and evaluate the effectiveness of, several stream restoration design alternatives. Building from that work, we were later tasked with evaluating green and gray infrastructure solutions and watershed-wide strategies that might reduce urban runoff to the sanctuary under current and future climate scenarios, while also improving water quality and providing other important co-benefits.
Watershed Model Development
To better understand present and future runoff patterns and evaluate the potential flood reduction benefits of various green and gray stormwater solutions, we expanded and improved the hydrologic model of the Broad Meadow Brook watershed. The model had been initially developed to identify design flows for the planned stream restoration project within the sanctuary. The watershed model was developed using PCSWMM, a software package that combines the EPA’s SWMM methodology for estimating runoff rates from pervious and impervious surfaces containing piped stormwater systems with a 2D mesh for evaluating surface flooding and flowpaths. This model was developed with a combination of publicly available datasets like soil type and LiDAR, with GIS of existing stormwater infrastructure provided by the city, along with field investigations to spot check critical dimensions and elevations and fill in remaining data gaps.
To ensure its continued reliability and usefulness, the model was calibrated with water level data collected using automated monitoring equipment in a city manhole in one of the low-lying neighborhoods adjacent to the sanctuary. This data was supplemented with continuous water level and streamflow data recorded by MA DER in the downstream Broad Meadow Brook channel. Multiple model input parameters were iteratively modified until the simulated model flow depths and runoff rates reasonably matched historical observations.
Existing Conditions Results
We then used the calibrated model to simulate flooding conditions under present and future climate scenarios to better understand flooding impacts, their likely causes, and how those impacts might worsen in the future. We also sought to identify a baseline against which we could compare the effectiveness of mitigation strategies and individual green and/or gray solutions.
We used the model to evaluate flooding in terms of peak discharge and total runoff volume arriving in the sanctuary, as well as in terms of the extents, depths, and volume of flooding within the watershed’s neighborhoods and streets (Figure 2). Flooding extents ranged from half an acre to over 12 acres during simulated 24-hour design storms ranging from the 2- to the 100-year events. Shorter (i.e., 6-hour) duration events with greater peak rainfall intensity produced simulated flooding from 15 to 20 acres, highlighting the vulnerability of the watershed and its stormwater infrastructure to high intensity events, more so than high volume events. That pattern is expected to continue under a 2070 climate scenario, with flooding extents ranging from two acres to about 25 acres during 24-hour events and more than 30 acres during the shorter, higher intensity events.
Figure 2: Example of model output comparing present (left) and future (right) flooding during a 10-year, 24-hour storm event.
Watershed Related Flood Mitigation Benefits
Based on these no-action findings, the project team then evaluated the anticipated flood reduction benefits of three different watershed-wide mitigation strategies. Those strategies, selected in consultation with the city in the context of their separate, ongoing drainage master planning effort, include:
- Implement the equivalent of one inch of storage for all impervious surfaces within the watershed, consistent with Massachusetts Stormwater Water Quality Standards.
- Reduce impervious surface cover throughout the watershed by 10%. The current average percent impervious within the subcatchments draining to the project area is on the order of 50-55%.
- A combination of 1 and 2.
As expected, the combination (#3) of increasing impervious runoff storage using techniques such as subsurface chambers, green streets, and rain barrels, along with reducing the amount of impervious surface cover, provided the largest benefits. However, model results indicate that due to soil conditions that may limit infiltration, increased storage (#1) was significantly more effective than impervious cover reductions (#2) on their own in this watershed. We also found that the largest reduction in peak and total runoff for all three strategies is experienced during the short-duration, high-intensity events to which this watershed appears vulnerable.
Gray Infrastructure Opportunities
We also evaluated five gray infrastructure options that generally fell into two categories. The first category consisted of capturing runoff from subcatchments within the watershed and re-directing small to moderate event runoff with in-manhole diversion weirs away from neighborhoods prone to nuisance flooding, discharging to other existing outfalls. The second type of gray infrastructure option consisted of the construction of additional relief drains in those neighborhoods. While model simulations did confirm modest reductions in flooding in those neighborhoods, some of these gray infrastructure options actually increased the peak discharge rate to the sanctuary. None of the options addressed increased upland flooding under climate change scenarios or provided a meaningful benefit to water quality or other co-benefits.
Flood Mitigation Benefits from Green Infrastructure Improvements
In addition to high-level, watershed-wide flood mitigation strategies, we assessed the watershed for opportunities to implement green infrastructure with a focus on areas prone to flooding. Through desktop screening, 16 sites were identified as being good opportunities to implement green infrastructure due to available space, land ownership, and proximity to flooding. Within those sites, the team outlined concepts for 42 green infrastructure opportunities (Figure 3) of four main types: subsurface infiltration chambers, bioretention basins, porous pavement, and depaving.
Figure 3: Map of the 42 green infrastructure opportunities identified within the watershed, with the three highest priority clusters (see Prioritization in Table 1) circled in red.
For each of the 42 concepts, estimates for reductions in impervious cover and increases in stormwater runoff capture and storage were made based on the type of green infrastructure incorporated and its approximate footprint. We ran the model to reflect these proposed conditions across a range of design storms to understand their capacity to reduce total runoff from the watershed and potential to reduce flooding impacts, both individually and all together.
The model showed that these green infrastructure opportunities have varied capacity to reduce runoff from their immediate drainage areas, ranging from less than 1% to more than 75%, indicating that some solutions are more beneficial than others. When all 42 green infrastructure opportunities are simulated together, their ability to reduce peak runoff rates to Broad Meadow Brook are modest, approximately 2.7% and 2.0% during the 2- and 10-year storm events respectively. Reductions in total runoff are more significant, approximately 7.4% and 4.3%, respectively, during the same events.
To help Worcester understand which opportunities would likely be most beneficial, both flood reduction benefits and environmental/societal co-benefits were factored in to develop a prioritized list of solutions. The 42 green infrastructure opportunities were ranked by their individual percent runoff reduction within their immediate drainage area. We also assessed each location and opportunity for the presence of priority environmental justice populations, as well as the ability of the proposed green infrastructure to reduce urban heat island effect, improve walkability, and provide ecological benefits to the surrounding area.
This resulted in a prioritized list of green infrastructure opportunities based on both flood reduction benefits and environmental/societal co-benefits. Where there are clusters of projects that ranked highly, the city may be able to implement multiple projects simultaneously, reducing construction mobilization costs and optimizing flood reduction benefits in a given location.
Table 1: Green infrastructure prioritization matrix indicating the scoring methodology to sort various opportunities within the watershed
Summary
The Broad Meadow Brook watershed model represents a powerful tool that can be used to understand present and future flood risk in this or any other watershed and the potential efficacy of green and gray stormwater solutions. Our model results demonstrate that the watershed experiences significant flooding in a range of design storms, but is particularly vulnerable to short duration, high intensity events. The results also demonstrate the watershed’s vulnerability to climate change-driven increases in rainfall magnitude and intensity, primarily due to overwhelmed stormwater infrastructure.
The strength of green infrastructure techniques lies in their co-benefits and in their ability to reduce or eliminate nuisance flooding on a local scale. Ultimately, a combination of gray and green solutions will be required to comprehensively address flooding here. When looking at green infrastructure solutions to address the issue, strategies that increase runoff capture and storage from impervious surfaces were shown to be the most effective. The green infrastructure prioritization process resulted in a list of the top 10 green infrastructure concepts across the Broad Meadow Brook Watershed that the city can now refer to as they move forward in their work to reduce flooding and increase climate resilience.
Alex Simpson is a Senior Project Engineer and Doris Jenkins, PE, LEED GA, is a Project Engineer with Weston & Sampson in their Reading, Massachusetts office. Kylie Tardif is a Water Resources Engineer in their Rocky Hill, Connecticut office.
Published in Stormwater Solutions, November 2024.