CLIMATE ADAPTATION STORMWATER STANDARDS

Stormwater is rain or melted snow that falls onto streets and sidewalks and “runs off.” In rural areas, rain soaks into the ground, while in more urbanized areas, stormwater runoff is directed into storm drains along roads to prevent localized flooding. Check out your community’s network of stormwater pipes and tunnels; you may be surprised at how many miles of underground pipes it takes to keep your city or town from flooding every time it rains.

Stormwater is a major source of pollution in rivers and streams because it carries with it all the pollutants that sit on roads and sidewalks: gas or oil that drips from cars onto roads; pet waste; fertilizer; pesticides; trash; even leaves are a form of stormwater pollution. These pollutants are carried by stormwater drains directly into streams and rivers, with no filtering or treatment. Polluted stormwater runoff is the largest source of pollution in the Charles River, causing the growth of invasive aquatic plants, cyanobacteria blooms, fish kills, and other ecological harms.

Leaves are a form of “nutrient pollution”, because they release nutrients like phosphorus and nitrogen as they decompose. When they decompose on the ground, these nutrients are absorbed by plants and other organisms, and they contribute to a healthy forest ecosystem. But when they decompose in lakes, rivers, and streams, it is “too much of a good thing,” and the excess nitrogen and phosphorus feed algae growth. The algae then decompose and use up oxygen that fish and native plants need.

In Massachusetts, stormwater is regulated at the federal level under the Clean Water Act, at the state level through wetlands and water quality regulations, and through local bylaws and ordinances. The Clean Water Act requires cities and towns to control stormwater discharges from municipal separate storm sewer systems (under what is referred to as an “MS4 permit”), and in the Charles River watershed, communities are also subject to additional limits on phosphorus and other nutrient pollution to help restore the river’s health, as determined by the Charles River “total maximum daily loads” (TMDLs). Together, these laws and regulations require communities to maintain stormwater infrastructure, monitor and mitigate sewage contamination, and control runoff from construction sites. They also set requirements for how new development and redevelopment projects manage stormwater.

Under state stormwater standards, projects that disturb one acre or more of land area must include stormwater controls to partially offset their pollution. (The types of disturbances regulated include clearing, grading, and excavating from such projects as road building, construction of residential houses, office buildings, industrial sites, or demolition.) No stormwater controls are required for projects under one acre. This means both large and especially small projects contribute to increased pollution. For cities and towns in the Charles River watershed, subject to nutrient load reductions, this leads to major challenges as development projects contribute to “nutrient debt” that municipalities are ultimately responsible for addressing.

Current stormwater regulations were written with a focus on water quality, not flooding. Over 90% of pollution can be addressed by managing just the first inch of runoff. But managing only the first inch does little to mitigate flooding or provide the increased drainage capacity that will be required by stronger, more frequent storms we are experiencing thanks to climate change. New projects may meet federal, state, and local rules for water quality, but still leave a large gap for communities wanting to protect their people and property from larger and more damaging storms.

That’s why we are recommending The Climate Adaptation Stormwater Standard (CASS): to build on the strong foundation of Massachusetts’ stormwater rules and ensure they address both water quality and flooding, so our infrastructure, homes, and communities are better protected.

Adopting CASS will protect people and property from the more intense and frequent storms that climate change is bringing. Watershed modeling shows that when adopted, CASS can significantly reduce the increased flood risk associated with climate change. Additionally, CASS closes existing loopholes to require stormwater controls on projects not currently regulated. This saves money for cities and towns by appropriately shifting some of the burden of MS4 compliance from municipal retrofits toward private development.

In 2025, CRWA partnered with the University of New Hampshire’s Stormwater Center to review existing municipal bylaws and stormwater regulations within the 35 cities and towns of the Charles River watershed and identify best-in-class standards.

Watertown and Waltham have set a high bar for stormwater management by requiring developers to retain runoff from a 100-year storm, a standard CRWA considers the “gold standard” for resilience. Both communities have strengthened their bylaws and ordinances to ensure new projects are designed for larger, more intense storms and enhanced flood protection. In addition, several other municipalities have adopted innovative approaches that go beyond state minimum requirements, such as implementing tiered permit systems. These systems expand the range of projects subject to stormwater review, ensuring that even smaller developments address runoff and water quality impacts.

A “100-year storm” is a storm that has a 1% chance of happening in any given year. In 2025, in the Charles River watershed, a 100-year storm would drop 8.78 inches of rain within 24 hours. State stormwater regulations require that projects not increase the peak runoff rate during a 100-year storm compared to existing conditions. CASS goes far beyond this to require improvement compared to existing conditions and the elimination of all runoff from the 100-year storm. These stronger standards are needed to protect communities.

A tiered permit system for stormwater regulations is a framework that sets different requirements depending on the size and type of a development project. Instead of using a one-size-fits-all approach, it creates multiple tiers so smaller projects face lighter requirements while larger or higher-impact projects must meet more stringent standards. Many watershed communities, such as Newton, Arlington, Waltham, Somerville, and Brookline, have already implemented these systems. In our urbanized watershed, a tiered approach is important to capture and address stormwater pollution from smaller projects. Tiered systems fairly scale stormwater requirements to project size, expanding permit coverage without overburdening small developments.

Adopting CASS has minimal direct costs to municipalities. The primary expense is staff time for additional permit review and inspection. These expanded oversight costs, estimated at about $2,000 per pound of phosphorus reduced (based on data from Cambridge and Newton), represent a substantial savings compared to the $50,000 to $150,000 per pound of phosphorus removal through municipal retrofits. Under CASS, implementation costs fall on developers and property owners, and when coordinated with construction, are a small fraction of total project costs. Importantly, by preventing flooding and storm damage, CASS can save cities and towns money in the long run by reducing infrastructure repairs and emergency response costs.

No. Municipal leaders in Watertown and Waltham report that implementation of CASS has not curbed development or added an infeasible burden to proposed projects.