Navigating the Latest Mass. Stormwater Handbook Updates

Revisions to the Massachusetts Stormwater Handbook are nearing adoption.

The MassDEP’s Stormwater Advisory Committee is finalizing proposed updates to the Stormwater Handbook to better align with the federal EPA MS4 Permit, and to promote Environmentally Sensitive Site Design (ESSD), Low Impact Development (LID), and resilience against increased flooding, storm damage, and runoff pollution.

Implementing these changes will enhance environmental protection but will also impact land development projects across the state. This includes increased planning efforts throughout a project’s lifecycle and a likely expansion of the stormwater management system, leading to higher overall costs for planning, design, and construction.

As we understand at this time, MassDEP plans to implement the new regulations by the end of 2024. Projects subject to the Wetlands Protection Act or local regulations in municipalities that have adopted the new standards (e.g., local wetlands ordinances and stormwater bylaws) must comply with the latest regulations.

Here is more detail on what Bohler’s New England Team knows about the changes as of this article’s published date, and the potential impacts it will have on development teams throughout Massachusetts.

Increased Rainfall

Projects will be required to utilize NOAA 14 PLUS events for stormwater analysis, which substantially increases rainfall depths across all storms. For example, a 100-year storm event in Southborough, MA will increase from 8.1 inches to 10.3 inches.

This will require larger Stormwater Control Measures (formerly known as Best Management Practices) to mitigate flows in post-development conditions.

Green Infrastructure

Environmentally Sensitive Site Design (ESSD) and Low Impact Development (LID) techniques, or an Alternatives Analysis to document the impracticability of using such techniques, will be required for all projects.

See Related: Benefits of Low impact development Design

This requirement could result in additional at-grade stormwater management or water quality devices, such as rain gardens or bioswales. However, a strategic design that uses these techniques and associated MassDEP credits could help to reduce the size Stormwater Control Measures, minimize land disturbance, and mitigate costs.

Increased Stormwater Recharge

The majority of land development projects will be required to infiltrate one inch of stormwater, increasing the size of infiltrating Stormwater Control Measures.

Increased Treatment Requirements

New developments will be required to remove 90% of Total Suspended Solids (TSS) and 60% of Total Phosphorus (TP). Redevelopments will be required to remove 80% of TSS and 50% of TP.

To help reduce the impact on developable area, infiltration system sizes, and other additional infrastructure, our teams are prioritizing green infrastructure techniques to tackle this requirement. However, this change could still necessitate filtering devices or off-site mitigation, both of which may require additional real estate for stormwater management.

Increased Setbacks

Projects will be constrained by expanded setback requirements for the location of infiltration systems, limiting site layout options despite the increase in regulatory metrics. For example, systems may not be placed less than 100 feet from a slope greater than 5%.

This will require upfront master planning and conceptual site grading to identify adequate space for stormwater infrastructure and maximize site yield.

Subsurface Explorations

MassDEP is proposing a significant expansion in requirements for in situ testing during the stormwater entitlement process.

Depending on the site and existing soil information, this may require upfront, site-wide geotechnical explorations, which is an expansion of the current requirement to test only within infiltration system footprints. For example, a one-acre site could require four test locations to verify on-site soils. Developers should anticipate additional early project costs to comply with this requirement.

Strategizing Stormwater Moving Forward

At Bohler, we recommend starting with a conceptual master planning phase to determine the best approach for each project. Our strategic planning and design efforts help minimize the impact of new regulations while providing a thorough assessment of anticipated costs and timelines.

This information is fluid and continues to evolve as the state works through this process. Bohler is in frequent communication with state agencies, attorneys, and industry professionals to keep our teams and our clients apprised of the latest information.

If you are concerned that your project may be affected by these changes, please contact our team today.