ANSI/ASCE/EWRI 39-23 Overview: Standardized Stormwater Management Design for Multi-Jurisdictional Projects

For a civil engineering firm designing a large logistics park spanning three different counties, the stormwater management plan is a regulatory minefield. One county requires a 10-year storm event for detention design, another mandates water quality treatment for a 25-year event, and the third has unique groundwater recharge criteria. This is the exact scenario where ANSI/ASCE/EWRI 39-23, Standard Practice for the Design and Operation of Stormwater Infiltration Practices, becomes an indispensable tool. It doesn’t replace local codes but provides a unified, scientifically rigorous national framework that engineers can use to design robust, defensible systems that satisfy the core intent of diverse local regulators, especially on projects that cross jurisdictional lines.

What is ANSI/ASCE/EWRI 39-23 in Practice?

Imagine you are a project manager overseeing the site development for a new corporate campus. Your team includes local civil engineers, an environmental consultant, and a review agency unfamiliar with advanced infiltration techniques. ANSI/ASCE/EWRI 39-23 serves as your common technical playbook. It translates complex hydrological science into a standardized process for designing infiltration systems—such as basins, trenches, and bioretention cells—that are intended to mimic natural hydrology by allowing stormwater to soak into the ground. The standard provides the “why” and “how” behind critical design decisions, enabling you to present a coherent, evidence-based plan to multiple stakeholders and regulatory bodies, ensuring everyone is evaluating the design against the same foundational principles.

Core Application: Solving Multi-Region Compliance and Performance Problems

The primary value of this standard emerges in scenarios where consistency and reliability are paramount but local codes may be vague, outdated, or conflicting.

* Cross-Jurisdictional Project Alignment: For a linear infrastructure project like a highway expansion crossing several municipalities, using ANSI/ASCE/EWRI 39-23 as the baseline design standard ensures a consistent level of performance and safety for infiltration practices along the entire corridor. It prevents a “weakest link” scenario where one section is under-designed simply because local code was less stringent.
Filling Local Code Gaps: Many local ordinances state that infiltration is “preferred” but lack detailed guidance on how* to properly design for it. This standard fills those gaps with comprehensive protocols for site evaluation, soil testing, hydraulic modeling, and construction oversight, protecting the engineer from liability and the client from system failure.
* Ensuring Long-Term Function in High-Risk Contexts: The standard is crucial for projects where infiltration failure could have severe consequences, such as near building foundations, in contaminated soil settings, or in regions with sensitive aquifers. Its rigorous pre-design assessment requirements help identify show-stopping constraints early, avoiding costly redesigns post-permit.

Technical & Safety Highlights Through a Scenario Lens

Consider a design team working on a retail development in a region with both expansive clay soils and a strict mandate to reduce runoff volume. The local code simply requires “infiltration where feasible.”

Pre-Design Site Assessment (The “Feasibility” Check): The standard mandates a systematic field investigation beyond a basic geotechnical report. In our scenario, it would require specific in-situ* infiltration rate testing (e.g., double-ring infiltrometer tests) at the exact proposed practice locations and depths, not just relying on generic soil classifications. This might reveal highly variable percolation rates, steering the design away from a large, single basin to multiple, smaller, distributed practices placed in more favorable soil zones.
* Hydrologic and Hydraulic Design: The standard provides a clear framework for sizing. It guides the engineer to calculate not just the peak flow rate (for detention) but, more critically, the water quality volume and recharge volume that must be infiltrated over a specified drawdown time (e.g., 48-72 hours). For our clay soil site, the standard would emphasize extended dewatering times and require a detailed analysis to ensure the practice fully empties between frequent storm events.
* The Unique “Failure Analysis” Requirement: A standout, scenario-specific clause involves planning for system failure. ANSI/ASCE/EWRI 39-23 requires designers to include an overflow or bypass pathway that safely conveys excess water if the practice clogs or becomes overwhelmed. This is not always explicitly required in local codes but is a critical risk-mitigation feature, especially in ultra-urban or high-value property contexts.

Regulatory Context and Professional Utility

ANSI/ASCE/EWRI 39-23 is an American National Standard developed by the Environmental & Water Resources Institute (EWRI) of ASCE. While not a legally enforceable “code” by itself, it is widely referenced and adopted by:

* State and Federal Agencies: As a basis for their own technical manuals or as an accepted compliance pathway.
* Municipal Review Engineers: As a trusted reference to judge the adequacy of submitted infiltration designs.
* Project Owners and Developers: As a specification to ensure a defined standard of care from their design teams.

Professionals who rely on it include:
* Civil Site Design Engineers: For creating technically sound, permit-ready drawings.
* Environmental Planners and Consultants: For conducting the requisite site suitability studies and navigating the permit process.
* Public Works and Municipal Review Engineers: For consistently evaluating complex designs across multiple submittals.
* Construction Managers: For ensuring the infiltration practice is built exactly as designed, with proper soil handling and quality assurance.

Risks of Non-Compliance and Common Misconceptions

Ignoring the guidance in this standard carries significant project risks:

* Permit Denials or Delays: A design based solely on minimal local code may be rejected as insufficiently detailed or risky by a knowledgeable reviewer.
* Catastrophic System Failure: Improper siting or sizing can lead to chronic ponding, overflow damaging adjacent properties, or even subsurface saturation threatening foundations.
* Costly Litigation: If a failed system causes damage, the design professional may be found negligent for not following the nationally recognized standard of care, which this document helps define.

Scenario-Specific Misconceptions to Avoid:

1. “The Standard is Only for Ideal, Sandy Soils.” A major misconception is that ANSI/ASCE/EWRI 39-23 only applies to easy sites. In reality, its detailed assessment and design procedures are most valuable for challenging sites (e.g., clay soils, shallow bedrock, constrained urban areas) to rigorously determine feasibility and design a system that will work safely.
2. “Following This Standard Guarantees Automatic Local Permit Approval.” This standard provides the technical foundation, but local regulations may impose additional constraints (e.g., stricter setbacks, approved plant lists for bioretention, specific monitoring requirements). The engineer’s role is to integrate the standard’s technical protocols with all applicable local laws.

Real-World Scenario: A Manufacturing Campus Expansion

A global manufacturer was expanding its campus in the Midwestern U.S. to include a new, state-of-the-art production facility. The site was underlain by silty clay loam, and the state’s environmental agency strongly encouraged infiltration for groundwater recharge. The local township code, however, was ambiguous.

The design team used ANSI/ASCE/EWRI 39-23 as their governing design document. They performed the mandated detailed soil infiltration tests across the proposed parking lot area, which confirmed infiltration was feasible but at a slower rate. Using the standard’s methodology, they designed a series of subsurface infiltration trenches beneath the parking lot to treat the water quality volume, with a controlled overflow to a conventional detention basin for peak rate control.

During permit review, the township engineer initially questioned the subsurface approach. The design team was able to present their calculations and testing data, all explicitly aligned with the national standard’s sections on site assessment, sizing, and overflow design. This demonstrated a high standard of care and scientific rigor, leading to permit approval without redesign. The project achieved its sustainability goals, met regulatory intent, and avoided months of delay that could have arisen from a less-defensible design approach. This case underscores how ANSI/ASCE/EWRI 39-23 serves as a critical bridge between innovative sustainable design and defensible engineering practice.