Critical Area Commission for the Chesapeake and Atlantic Coastal Bays

Critical Area 10% Rule Guidance Manual

Within the Critical Area there are three land use classifications:

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

Most jurisdictions use these classifications as overlay zones. Intensely Developed Areas are generally areas that were developed with residential, commercial, industrial, and institutional land uses at the time of the original Critical Area mapping and where relatively little natural habitat occurred. IDAs are also considered the preferred locations for future growth through redevelopment and/or new development.

The Criteria set forth in conjunction with the Critical Area Act require that any development or redevelopment within the IDA be accompanied by practices to reduce water quality impacts associated with stormwater runoff. The Criteria further specify that these practices must be capable of reducing stormwater pollutant loads from a development site to a level at least 10% below the load generated by the same site prior to development. This requirement is commonly referred to as the “10% Rule.”

The responsibility of implementing the Criteria is delegated to each local government. Therefore, each jurisdiction must ensure that the 10% Rule is met for development projects located within the IDA. In order to provide a consistent approach to compliance with the 10% Rule, the Critical Area Commission provides 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, entitled “Maryland Chesapeake and Atlantic Coastal Bays Critical Area 10% Rule Guidance” is the result of revisions to prior publications printed in 1987 and in 1993. The current guidance was reviewed and officially adopted by the Critical Area Commission on December 3, 2003.

The current guidance reflects some significant changes in strategies for addressing stormwater management and treatment in Maryland. The guidance references and incorporates information included in the Maryland Department of the Environment (MDE) 2000 Maryland Stormwater Design Manual, Vol. I & II. The guidance addresses some of the inherent differences between the State’s stormwater management program and the Critical Area 10% Rule, and provides updated information for handling unique development situations.

It is important to note that this guidance information applies to development and redevelopment of properties located within the Critical Area and designated as an Intensely Developed Area (IDA). Some of the information and concepts presented in this document may not be applicable to properties designated as Limited Development Area (LDA) or Resource Conservation Area (RCA).

The Guidance Manual may be downloaded in several ways:

  • See Table Below.




File Size (kb)

Cover Color Cover for Manual 1 54
Title Title Page 1
Contents Acknowledgements, Table of Contents, List of Tables, List of Figures 5 69
Section 1 Introduction to the manual, history  of the "10% Rule" guidance, and summary of each sections. 4 86
Section 2 Discussion of the selection of total Phosphorus as the keystone pollutant. 2 75
Section 3 Summary of methodology for determining "10 Rule" compliance. 2 109
Section 4 Methodology for compliance with the "10% Rule" on individual residential lots. 44 475
Section 5 Compliance with the "10% Rule" on individual residential lots. 6 192
Section 6 Offsets and offset fees for projects where "10 Rule" compliance on the project site is not feasible. 14 159
Section 7 Frequently Asked Questions 16 294
Section 8 References and Additional Resources 4 106
Appendix A Description of urban runoff pollutants 2 127
Appendix B Selection criteria for the selection of phosphorus as a keystone pollutant for purposes of the calculations. 2 102
Appendix C Discussion of the "simple method" for computing pollutant load export. 8 379
Appendix D Technical memo about a revision to the value used for total phosphorus concentration in the calculations. 4 93
Appendix E Descriptions, advantages, limitation and schematic drawings of stormwater Best Management Practices 52 3,130
Appendix F Descriptions of and information about Residential Water Quality Plan Best Management Practices 12 558
Appendix G Technical memo about establishing an offset fee based on the actual cost of stormwater management 4 113