For Critical Area IDA stormwater management questions, please contact:

Susan Makhlouf

(410) 260-3476

For other stormwater management inquiries, please call (410) 260-3460


Stormwater Management and Improving Water Quality in the Critical Area

Example of a stormwater situation on the side of a neighborhood street
IDA 10% SWM Manual
IDA 10% SWM Spreadsheet
Innovative SWM Guidance (Coming Soon)

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. The Critical Area regulations include standards for managing and treating stormwater within all three Critical Area designations (Resource Conservation Area (RCA), Limited Development Area (LDA) and Intensely Developed Area (IDA)).


Stormwater Management in the RCA and LDA: Lot Coverage

Stormwater management in the Resource Conservation Area and Limited Development Area is managed through limitations in lot coverage. Generally, lot coverage is limited to 15% of a lot or parcel with some exceptions for lots created before December 1, 1985 and under ½ acre. For Critical Area RCA/LDA stormwater management questions, please contact the Critical Area Planner who coordinates with the local jurisdiction where your property is located.​​

Stormwater Management in the IDA: Critical Area 10% Rule

The “10% Rule”  requires that any development or redevelopment within the Critical Area’s Intensely Developed Area (IDA) include stormwater best management practices to reduce water quality impacts associated with stormwater runoff to a level 10% below that generated by the same site prior to development. In conjunction with the updates to the Maryland Department of the Environment’s (MDE) stormwater requirements, the Critical Area Commission has developed a guidance document and spreadsheet to better align MDE’s stormwater goals with the Commission’s “10% Rule.”  

The Critical Area Stormwater Guidance 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.

​The Critical Area ESD Spreadsheet​ allows designers, engineers, and local plan reviewers to evaluate compliance with the Critical Area “10% Rule” phosphorus reductions and MDE’s ESD volume reductions for new development and redevelopment projects, both inside and outside the Critical Area. ​

Innovative Stormwater Management

Commission staff is developing guidance on installing climate resilient stormwater management practices to help communities impacted by frequent storms and rising sea levels.

Learn about new resources for resilient stormwater management practices by checking out this webinar given by the Chesapeake Stormwater Network.

Click here for more information​ about stormwater practices and climate projection data from the Chesapeake Stormwater Network.