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Maryland Proposes to Advance Stormwater Regulations in Response to Climate Change

September 27, 2023

Following the passing of Maryland Senate Bill 227 in 2021, the Maryland Department of the Environment (MDE) worked with a stakeholder advisory group and the public on the Advancing Stormwater Resiliency in Maryland (A-StoRM) initiative. A-StoRM looks to update Maryland stormwater regulations to incorporate recent precipitation data more effectively and to address urban flooding events. As concluded by the Maryland Commission on Climate Change, the State is likely to experience “chronic and periodic impacts of climate change” such as increases in extreme precipitation, increases in river and pluvial flooding and a continuing rise of tidal waters (MCCC Annual Report, 2022). These observable changes impact the economy and the quality of air, land, and water resources within the State and the Chesapeake Bay.

Based on the A-StoRM initiative, MDE published for public comment, “Proposed Stormwater Management Regulatory Requirements for New Development and Redevelopment,” dated July 26,2023. As noted, Maryland current stormwater management design requirements are based on the Rainfall Frequency Atlas of the United States (USCD, 1961) which incorporates precipitation data from 1938-1957. The proposed regulatory requirements would change the Maryland Stormwater Design Manual (October 200, Revised May 2009) to rely on updated rainfall data covering a larger period. The more current analysis is provided in the National Oceanic and Atmospheric Administration (NOAA) Atlas 14, Volume 2 Precipitation-Frequency Atlas of the United States, Ohio River Basin and Surrounding States, as updated in 2006 (NOAA, 2006). The Atlas 14 updated rainfall data indicates an increase in the 90% average rainfall for the State from 1.0” to 1.4” and a new 1 yr/24-hr design storm rainfall of 3.0” instead of the previous 2.7”. This will represent a fundamental change in how stormwater best management practices are evaluated to meet design storm and environmental site design requirements.

Other proposed changes to the stormwater regulatory requirements include:

  • Modifying the water quality standard to align with the federally adopted 95% capture rule,
  • Modifying the hydrologic soil group classification for all disturbed soils,
  • Inclusion of requirements to ensure that no flooding or erosion related impacts are created from stormwater runoff to existing downstream properties, and
  • Inclusion of requirements to ensure historically overburdened and underserved communities are adequately protected from unmanaged stormwater runoff.

The proposed changes to the stormwater management standards represent the first phase of improvements to Maryland stormwater policy to address the impacts of climate change. MDE is undertaking additional studies and future regulatory changes to address stormwater quantity requirements are anticipated. The currently published changes are intended to move forward in December of 2023 and to become effective in January of 2027. MDE has provided a transition period for projects that receive Concept Plan approval prior to June 30, 2028, and meet other prescribed milestones, however.

For more information on our Watershed Science and Stormwater capabilities within Maryland, or to better understand how these proposed changes may impact your projects, please contact B&L’s Jason Gillespie.