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:

Guidance Publications

• Bay Smart Guide
• Overview of House Bill 1253 - 2008 Legislation
  (Adobe Acrobat file 220KB)
• A Guide to the Conservation of Forest Interior Dwelling Birds in the Chesapeake Bay Critical Area
  (Adobe Acrobat file 270KB)
• Critical Area 10% Rule Guidance Manual
• Private vs. State Tidal Wetlands
• Forest Mitigation
• Public Walkways
• Living Shorelines
• Backyard Makeover
• Golf Courses in the RCA - Final
• Native Plants for Wildlife Habitat and Conservation Landscaping - Chesapeake Bay Watershed - U.S. Fish & Wildlife Service
• Local Government Assistance Guide - Critical Area Buffer
• Local Government Assistance Guide - Lot Coverage

NEW PUBLICATION!

This new publication was developed to assist homeowners with planting and maintaining shoreline Buffers. It includes lots of information about the importance of the Critical Area Buffer and includes Buffer Management Plans that can be used to satisfy Buffer planting requirements for new construction on waterfront lots.