Coastal areas have long been vulnerable to flooding and storm damage—issues that are magnified by climate change and sea level rise. With the increased impacts of flooding, storm surge, more frequent and powerful storms, changing precipitation patterns, groundwater level changes, salt water intrusion, and shifting vegetation hardiness along the coast, stormwater Best Management Practices (BMPs) may need to be refined to ensure long-term effectiveness and to enhance coastal water quality protection.
To help municipalities address these challenges through improved siting, design, construction, and operation and maintenance of coastal stormwater BMPs, the Massachusetts Office of Coastal Zone Management (CZM) and Massachusetts Department of Environmental Protection (MassDEP) funded Assessment of Climate Change Impacts on Stormwater BMPs and Recommended BMP Design Considerations in Coastal Communities. This report was developed by the Horsley Witten Group, Inc., with support from the Woods Hole Group, and includes the following:
- A summary of climate change impacts that are anticipated to affect stormwater BMPs in the Massachusetts coastal zone.
- Design modification recommendations to increase long-term effectiveness of these BMPs.
- Conceptual drawings of proposed design modifications for several existing coastal BMPs, providing specific examples of how some of the report’s recommendations could be implemented.
- Results from 26 field evaluations of coastal stormwater BMPs in Massachusetts to identify potential siting and design factors subject to climate change impacts (Appendix A).
- Projections of future flood elevations to identify flood risk over time for the assessed BMPs (Appendix B).
Vulnerability of Coastal Stormwater BMPs to Climate Change Impacts
Based on projected climate change impacts and observations made through site assessments during the development of this report, coastal stormwater BMPs were found to be vulnerable to:
- Rising sea level and submerged outfalls.
- Rising groundwater and shrinking separation distance between the BMP and the groundwater table.
- Physical impacts of storm surges.
- Chronic wind, sand, and salt exposure.
To address these issues, the report outlines the following recommendations to improve the long-term effectiveness of a proposed stormwater BMP project.
Using a 50-Year Planning Horizon
To ensure that BMPs will operate effectively over the long term, potential climate change impacts should be evaluated during BMP design. A 50-year design life is consistent with the longevity of structural materials. Designers and operators should assume that the BMP will require effective maintenance throughout its design life and plan ahead for changing conditions that could affect the performance of the BMP.
Proper Siting of BMPs
Along with ensuring that the BMP design will be effective over the 50-year planning horizon, the location of the proposed BMP should be evaluated in conjunction with flood projection maps to understand the implications of climate change over the design life of the BMP. In addition, while siting a single BMP in the location where the largest drainage area can be captured—which in coastal areas is often a location closest to the shoreline—is typically the most cost-effective option, BMPs close to the shoreline are at the greatest risk of climate change impacts. To decrease this risk, two or three smaller BMPs sited throughout the drainage area and combined with non-structural practices (e.g., impervious cover reduction) should be considered over the use of a larger, single BMP sited close to the coast.
If the BMP must be sited close to the shoreline due to other constraints, consider the following:
- Site the BMP away from salt marsh edges to minimize disturbance and spread of invasive plants.
- Avoid installing BMPs in areas where they will be exposed to significant storm impacts or sand sources (if clogging is a concern).
- Avoid siting a BMP near high groundwater if the BMP cannot adapt to function with higher groundwater or will be impacted by groundwater intrusion into the system.
Selecting Appropriate BMPs
Select stormwater treatment practices that are appropriate not only for existing site conditions, but also for the conditions expected over the next 50 years. For example, only choose infiltration practices (such as settling tanks and infiltration chambers) for areas where the required depth to groundwater can be sustained in light of expected sea level rise and associated groundwater rise. Also ensure the selected BMP can adapt to wetter conditions. Typically, this approach will prioritize above-ground, vegetated practices over below-ground “gray” infrastructure. For example, a rain garden can convert to a wetland over time as groundwater rises, while an underground infiltration chamber will simply fail when groundwater levels rise too high.
Selecting BMP Construction and Landscape Materials
Choose materials that are appropriate to existing and future site conditions, such as native, salt-tolerant plant species and materials that do not corrode from salt exposure.
Ensuring Redundancy in Design
Redundancy in treatment or storage capacity can help a BMP adapt to impacts of climate change, such as rising groundwater, increased storm surge, and increased rainfall. For example, if large amounts of sand are anticipated with increasing storm surges in a BMP area, the designer can increase the size of the sediment forebay to accommodate sediment overwash and ensure the BMP is functioning between maintenance visits. As the site changes, this redundancy will help to prolong the effective lifespan of the BMP.
Increasing Flexibility in Design
Flexibility in the design of a BMP allows for adaptation to new conditions. Flexibility can include the provision of extra space around the BMP to allow room for future modifications, as well as the use of design modifications that anticipate future conditions. Examples include: 1) incorporating an elbow joint in the underdrain of a bioretention system to ensure the infiltrated water reaches the outlet even with rising groundwater; and 2) designating a reserve area adjacent to the infiltration basin that can expand the BMP’s surface area to maintain treatment capacity in the future.
Choosing “Green” over “Gray”
Practices that are mostly vegetated with minimal underground structures (often referred to as “green infrastructure”) are more likely to successfully adapt to future conditions than substantial, hard, underground structures (known as “gray infrastructure”). While concrete, underground structures cannot expand or move in response to changing conditions, vegetation can shift to more wetland or salt-tolerant communities. This adaptability makes vegetated BMPs more effective over time in areas that are likely to experience increased storm surge or rising groundwater or sea level impacts. However, the ability of vegetated BMPs to evolve and provide a similar level of stormwater treatment depends on the redundancy and the flexibility in the BMP design.
The Even Greater Importance of Maintenance
BMPs subject to climate change impacts will require more attentive maintenance to ensure effective operation. Increased heavy rainfall can cause more frequent clogging of inlets, outlets, and infiltration beds with debris, and increased erosion can clog infiltration systems, requiring debris and sediment removal. As climate change impacts are experienced in the coastal zone, inspection and maintenance of stormwater treatment practices will become more important and require a greater financial investment to ensure long-term effectiveness. Municipalities can help to reduce maintenance needs by implementing targeted nonstructural practices, such as street sweeping following significant overwash events.
Examples of Recommended Conceptual Designs
For demonstrations, including conceptual renderings, of how the design recommendations discussed above can be applied, please see Section 5.0 of the full report.
For More Information
For a more detailed discussion of climate change impacts on stormwater BMPs and recommended BMP design considerations in coastal communities, see the full report. The CZM Coastal Water Quality Program website also provides additional information, strategies, and tools, including information about the Coastal Pollutant Remediation (CPR) Grant Program, which provides funding for design and implementation of BMPs in coastal communities.