Model Post-Construction Stormwater Ordinance Explained

This model ordinance establishes a comprehensive framework for managing stormwater runoff from new development and redevelopment projects. Its primary purpose is to protect public health, safety, and local water resources by mitigating the impacts of increased impervious cover, which can lead to flooding, stream channel erosion, and nonpoint source pollution. The ordinance is designed to be the principal local regulation governing the design, construction, and long-term maintenance of permanent stormwater management facilities.

The ordinance operates by establishing a set of performance standards that new development and redevelopment must meet, rather than prescribing a single set of structural practices. This performance-based approach provides design flexibility while ensuring that post-construction runoff conditions do not degrade receiving waters. It is intended to work in conjunction with other local development regulations, such as those for Erosion and Sediment Control during construction, zoning, and Floodplain Management.

A key feature of this model is its reliance on a separate, locally adopted Stormwater Design Manual. This allows the ordinance itself to remain stable over time, while the technical criteria, approved practices, and design specifications in the manual can be updated as engineering and scientific understanding evolves. The ordinance mandates the creation of such a manual to provide the specific details needed for implementation.

Key provisions

The model ordinance is structured around a permitting process that requires applicants to submit a stormwater management plan for review and approval before any land disturbance activities can begin. The core components of the ordinance define its applicability, performance criteria, and procedures for administration and enforcement.

Applicability and Exemptions

The ordinance applies to all major subdivision and site plan applications that meet a specified land disturbance threshold. A crucial provision states that it also applies to smaller activities that are part of a larger common plan of development, preventing projects from being segmented to avoid compliance. Certain activities may be exempt, such as additions to existing single-family homes, agricultural and logging activities conducted under an approved conservation plan, and repairs to existing stormwater facilities. The ordinance also establishes special sizing criteria for redevelopment projects, recognizing the unique constraints of infill and previously developed sites.

Performance Standards

Section 5 of the model ordinance outlines the general performance criteria that all applicable sites must address. These standards form the technical core of the regulation and are intended to be detailed further in the jurisdiction’s Stormwater Design Manual. The primary standards include:

Water Quality: Structural stormwater practices must be designed to remove a specified percentage of the average annual post-development total suspended solids (TSS) load. The model suggests a target of 80% TSS removal, which is presumed to be met if practices are sized to treat the water quality volume (WQv) and are designed, built, and maintained according to the design manual.
Channel Protection: To prevent stream degradation from increased runoff volumes and durations, the ordinance requires that channel protection criteria be met. The model discusses several approaches, such as extended detention of the 1-year, 24-hour storm, cautioning against relying solely on controlling the 2-year peak discharge, which has been shown to be ineffective at preventing channel erosion.
Peak Flow Control: Site designs must incorporate practices to control the peak flow rates of stormwater discharge for specified design storms (e.g., 2-year, 10-year, and 100-year events) to prevent downstream flooding.
Groundwater Recharge: The ordinance requires that post-development annual groundwater recharge rates mimic pre-development conditions. This is achieved by promoting infiltration of runoff from impervious surfaces to the maximum extent practical.

Permitting Procedure

No building, grading, or other land development permits may be issued until the requirements of the stormwater ordinance are met. The process begins when an applicant submits a permit application that includes a stormwater management concept plan, a legally recorded maintenance agreement ensuring long-term care of the facilities, and a permit review fee. The designated jurisdictional authority reviews the application for completeness and compliance. The model specifies a timeframe for the authority to approve or disapprove the plan. If a plan is disapproved, the applicant may revise and resubmit it. Once the plan and maintenance agreement are approved, the necessary land disturbance permits can be issued.

Waivers and Mitigation

The ordinance includes provisions for waiving stormwater management requirements in whole or in part under specific conditions. A waiver may be granted if an applicant demonstrates that meeting the requirements is not feasible due to site constraints, if alternative requirements have been established in an approved watershed plan, or if runoff will be managed by an approved off-site facility. When a waiver is granted, the applicant must provide acceptable mitigation. This can include monetary contributions (fee-in-lieu) to a dedicated fund for public stormwater projects, the dedication of land for conservation or buffer creation, or the construction of a stormwater retrofit on a previously developed property.

Maintenance and Enforcement

Long-term functionality of stormwater practices is secured through a legally binding maintenance agreement that must be executed before a permit is issued. This agreement acts as a property deed restriction and outlines the property owner’s responsibility for perpetual inspection and maintenance. While the model defines a “Stop Work Order” as an enforcement tool, it leaves the specifics of inspection schedules, penalties for non-compliance, and other enforcement mechanisms to be detailed by the adopting jurisdiction. Municipalities often supplement these provisions by requiring performance bonds to ensure proper installation and referencing a broader enforcement code for penalties.

Selected provisions, annotated

The ordinance also applies to land development activities that are smaller than the minimum applicability criteria if such activities are part of a larger common plan of development that meets the following applicability criteria…

Model Stormwater Ordinance, Section 1.3

This provision is a critical anti-circumvention clause. It prevents developers from phasing or subdividing a large project into smaller pieces that would individually fall below the regulatory threshold, ensuring that the full stormwater impact of the entire development is managed comprehensively.

Annual groundwater recharge rates shall be maintained, by promoting infiltration through the use of structural and non-structural methods. At a minimum, annual recharge from the post development site shall mimic the annual recharge from pre-development site conditions.

Model Stormwater Ordinance, Section 5(C)

This performance standard addresses an impact of impervious cover that is often overlooked: the reduction of groundwater recharge. By requiring that post-development recharge mimic pre-development hydrology, the ordinance helps maintain baseflow in streams during dry weather and supports local groundwater supplies.

To protect stream channels from degradation, a specific channel protection criteria shall be provided as prescribed in the current stormwater manual… research studies indicate that [2-year peak control] does not protect channels from downstream erosion, and may actually exacerbate erosion…

Model Stormwater Ordinance, Section 5(E)

This provision highlights a shift toward more effective stream protection. It explicitly warns against relying on the traditional 2-year peak discharge control method, guiding practitioners toward more robust criteria like extended detention, which better addresses the erosive power of more frequent, longer-duration storm events common in urbanized watersheds.

What makes it a useful model

This model ordinance serves as a strong foundation for communities seeking to establish or update their post-construction stormwater regulations. Its primary strength lies in its performance-based framework, which focuses on achieving specific hydrologic and water quality outcomes rather than mandating a narrow list of approved technologies. This approach encourages innovative site design and allows engineers to select the most effective and efficient practices for a given site’s constraints and opportunities.

The model’s structure, which separates the legal authority of the ordinance from the technical specifications of a Design Manual, is a highly practical feature. It allows a municipality to adapt to new research, improved designs, and evolving local priorities by simply updating the design manual, a process that is typically faster and less cumbersome than amending the ordinance itself. This ensures the program remains current with best practices in stormwater management.

Furthermore, the inclusion of modern performance standards for groundwater recharge and stream channel protection makes it a forward-looking document. It also provides built-in flexibility through its waiver and fee-in-lieu provisions, allowing regulators to handle challenging redevelopment sites and other constrained projects without compromising the environmental objectives of the ordinance. This combination of robust standards, flexibility, and administrative practicality makes it an excellent starting point for any community in the model ordinances library.

Adaptation checklist for municipalities

Before adopting this model ordinance, a municipality must customize numerous provisions to align with local authority, conditions, and administrative procedures. Key items requiring localization include:

Define Jurisdictional Authority: Replace all instances of “(jurisdictional stormwater authority)” and “(appropriate review agency)” with the specific names of local departments or officials responsible for plan review, permit issuance, and enforcement (e.g., Department of Public Works, Planning Board).
Establish Applicability Thresholds: Set specific numeric values for land disturbance that trigger the ordinance’s requirements (e.g., disturbance of 5,000 square feet or more).
Define Redevelopment Criteria: Establish a clear, quantitative definition for what constitutes a “redevelopment” project to determine when special sizing criteria apply.
Adopt a Stormwater Design Manual: Formally adopt a local or state Stormwater Design Manual by reference. This manual must contain the detailed sizing criteria, design specifications for approved practices, and pollutant removal performance data.
Set Specific Performance Standards: Finalize the numeric performance standards, such as the required percentage of TSS removal (e.g., 80%) and the design storms for channel protection and flood control (e.g., 1-year, 10-year, 100-year).
Establish Application Procedures and Timelines: Specify the exact contents of a permit application, set realistic review timelines (in business days) for staff, and outline the public notice or hearing process if applicable.
Develop a Fee Schedule: Create and adopt a formal fee schedule for permit application review. If offering a fee-in-lieu option, establish a clear methodology for calculating the required monetary contribution.
Incorporate Local Maps: Reference official local maps for critical resources such as jurisdictional wetlands, floodplains, and designated stream corridors where special protections may apply.
Strengthen Enforcement Language: Add specific sections detailing inspection authority and frequency, notice of violation procedures, and a schedule of civil and/or criminal penalties for non-compliance with the ordinance or an approved maintenance agreement.
Ensure Compatibility with Other Codes: Review and cross-reference other local regulations, including zoning, subdivision, Stream Buffers, and Illicit Connections ordinances, to ensure consistency and avoid conflicts.