Stormwater Resources


Environmental Site Design in the Critical Area

Prepared by LeeAnne Chandler and Nick Kelly, Critical Area Commission
Jan 10, 2013


... or information about the Critical Area Program or questions relating to State oversight of local programs, e-mail us or call 410-260-3460

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Critical Area Commission
1804 West Street
Annapolis, MD 21401


Stormwater Management and Improving Water Quality in the Critical Area

Bio retention image One of the goals of Maryland’s Critical Area Program is to minimize adverse impacts on water quality that result from pollutants that are discharged from structures and drainageways or that have run off from surrounding lands. To accomplish this goal, the Critical Area regulations include standards for managing and treating stormwater. Improving water quality by removing pollutants is particularly important in heavily developed areas cities and towns, where there are lots of buildings and impervious surfaces such as roads, parking lots, and sidewalks.

In order to effectively implement the Critical Area Program, most jurisdictions use three land use classifications that are defined in the regulations. All land, except federal land, is designated as one of the following classifications, which, in most jurisdictions function as overlay zones:

  1. Resource Conservation Areas (RCA)
  2. Limited Development Areas (LDA), and
  3. Intensely Developed Areas (IDA).

Most moderately and heavily developed areas are classified as Intensely Developed Areas (IDAs). Urban areas such as Annapolis and Baltimore City include significant land area designated IDA; however, smaller municipalities such as Leonardtown, Chestertown, and Princess Anne also include IDA lands. Generally, IDAs are areas that are developed with residential, commercial, industrial, and institutional land uses and where relatively little natural habitat occurred. IDAs are also considered the preferred locations for future growth through redevelopment and/or new development.

In order to meet the water quality goals of the Critical Area Program, the regulations require that any development or redevelopment within the IDA include practices to reduce water quality impacts associated with stormwater runoff. The regulations further specify that these practices must be capable of reducing stormwater pollutant loads from a development site to a level below that generated by the same site prior to development. These regulations have been in place since the effective date of the Critical Area Program in 1985 and pre-date current stormwater management regulations, implemented by the Maryland Department of the Environment (MDE). These regulations and the guidance manual used to implement them are commonly referred to as “The 10% Rule,” referring to the required minimum reduction in stormwater pollutants.

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The responsibility of implementing the Critical Area regulations is delegated to each local government. Therefore, each jurisdiction must ensure that stormwater pollutants are reduced below pre-development levels for development projects located within the IDA. In order to provide a consistent approach to compliance with the pollutant reduction requirements, the Critical Area Commission developed guidance, which was updated in 2003, entitled “Maryland Chesapeake and Atlantic Coastal Bays Critical Area 10% Rule Guidance,” that includes a methodology for determining a pollutant removal requirement and for quantifying the pollutants removed by a variety of stormwater best management practices. This guidance referenced and incorporated information included in the Maryland Department of the Environment 2000 Maryland Stormwater Design Manual, Vol. I & II.

Click here to view the Guidance Information for Critical Area Decision Makers

Environmental Site Design and Improving Water Quality in the Critical Area

Since that time, stormwater management has evolved dramatically in Maryland, both in terms of the overall strategies used to treat stormwater and the most effective types of best management practices (BMPs). In 2000, the Maryland Department of the Environment (MDE) promulgated regulations and a stormwater design manual to address stormwater management and treatment on development sites throughout the State. In 2009, MDE revised the Code of Maryland Regulations (COMAR) 26.17.02 and the 2000 Maryland Stormwater Design Manual (MDE Design Manual), Vol. I & II to require the use of environmental site design (ESD) practices.

In conjunction with the updates to MDE’s stormwater requirements, the Critical Area Commission is currently developing environmental site design criteria for the Critical Area to replace the Commission’s current stormwater guidance manual and Appendix D.4 of the MDE Design Manual (2000). The use of environmental site design in the in the Critical Area will integrate compliance with both the Critical Area pollutant removal requirements and MDE’s environmental site design regulations with the intent to streamline the stormwater management review process and optimize water quality benefits.

Environmental Site Design to the Maximum Extent Practicable – Background

All Maryland counties and their incorporated municipalities are responsible for reviewing and approving stormwater management plans for new development and redevelopment according to the methods specified in the MDE Design Manual. The basic goal of Maryland’s stormwater program is to implement ESD practices to the maximum extent practicable (MEP) to replicate runoff characteristics similar to “woods in good condition.” This is accomplished when stormwater management measures, including ESD practices, are used to provide treatment for the 1-year rainfall event which is 2.7 inches of rainfall.

The MEP standard is implemented through a comprehensive plan review process that requires plans to be submitted at various stages of design. This allows local reviewers to negotiate options for ESD in the earliest phases of project design. The Design Manual requires that no less than 1-inch of rainfall must be treated using ESD. However, this is not a minimum standard, and the full 2.7 inches of rainfall target must be addressed, using nonstructural and structural stormwater practices when necessary. In all cases, after ESD to the MEP is achieved, runoff conditions replicating woods in good condition for the 1-year storm will be achieved. This is the minimum standard for all development projects in Maryland and includes the Critical Area. Because there is no single design approach to treating stormwater on a given site, sound environmental analysis and engineering judgment is essential to developing strategies that use ESD to the MEP.

The Critical Area and Environmental Site Design to the Maximum Extent Practicable

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When integrating ESD to the MEP and the Critical Area standards for stormwater, it is not always necessary to implement additional best management practices to satisfy the requirements. Rather, in the Critical Area, additional analysis of existing and proposed conditions is done to demonstrate compliance in terms of pollutants removed from stormwater. This pollutant reduction targets the Critical Area Program goal of minimizing impacts to water quality from runoff from surrounding lands. The objective of Critical Area ESD is to ensure that runoff from new development projects does not result in an additional nutrient load to the Chesapeake Bay and that pollutant loads in runoff from redevelopment projects is reduced from original levels. The required pollutant reduction is achieved through the use of optimally functioning resource-based site design and treatment practices.

MDE’s design standards use impervious area to be treated or managed as their “currency” or unit of measure while the Critical Area design standards analyze the reduction of phosphorus,a common pollutant found in stormwater. In order to integrate compliance with both Critical Area and MDE standards, the requirements for both programs have been combined in a single spreadsheet compliance tool. In addition, this integrated methodology reflects improvements in the science of stormwater treatment and the engineering of treatment strategies and practices.

Draft Stormwater Guidance Manual

Draft Manual ESD Feb 2013

Commission staff, working with the Chesapeake Stormwater Network, has developed a draft stormwater guidance manual, entitled Environmental Site Design for the Maryland Critical Area. The manual provides an overview of stormwater treatment in the Critical Area and how ESD standards have been integrated into the design of stormwater management plans. It also includes step-by-step instructions for using the Stormwater Spreadsheet Tool, guidance for optimizing pollutant removal in the Critical Area, and Frequently Asked Questions (FAQs) about compliance with the pollutant reduction requirement in the Critical Area.

Draft ESD Spreadsheet

Commission staff has developed a Critical Area Stormwater Spreadsheet Tool to allow designers, engineers, and local plan reviewers to evaluate compliance with the new ESD regulations and pollutant removal performance standard. Version 4.0 of the spreadsheet, released in January 2014, enables the user to track 10% phosphorus reductions and ESD volume reductions for new development and redevelopment projects, both inside and outside the Critical Area. The spreadsheet provides, for the first time, a unified basis for addressing both the MDE and Critical Area regulations in a single tool. This tool should help streamline project review and reduce the need for duplicate submittals.

Please note that this spreadsheet is currently in DRAFT form. Furthermore, while the draft spreadsheet currently calculates 10% phosphorus requirements, Commission staff is testing and refining the spreadsheet to evaluate a potential change in phosphorus reduction calculations to match MDE’s update to stormwater management requirements. Commission staff will provide updates if and when any such changes are proposed.

Commission staff is seeking feedback on its accuracy and ease of use of the spreadsheet. We are requesting designers, engineers, and plan reviewers to test this spreadsheet over the coming months and to send feedback to Nick Kelly (

Additional information on how to apply the ESD guidelines within the Critical Area, including a step-by-step guide on how to use the new spreadsheet, can be found within the Critical Area ESD webinar, hosted by the Chesapeake Stormwater Network.