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New Florida Stormwater Rule & BMPFast Software: Regulations, BMP Analysis, and Permitting Practice

Presenter: Marty Wanielista  ·  Florida Stormwater Continuing Education Series  ·  Updated for the June 28, 2024 Rule Adoption

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1. Course Introduction & Objectives

Source slides: 1, 2, 22  ·  Presenter overview, course goals, and primary tools

This course is presented by Marty Wanielista, a long-standing researcher and educator in Florida stormwater management. The session is designed for licensed professionals, permit applicants, and agency staff who work with Florida’s stormwater permitting system. The core emphasis is on understanding and applying the regulations and rules that govern stormwater quality treatment, with enough practical grounding to move permit applications forward efficiently and correctly.

Primary Course Objectives

By the end of this course, participants will be able to:

• Identify the regulatory framework governing stormwater quality in Florida, including the 2024 rule changes.
• Apply updated Best Management Practice (BMP) analysis methods to real project scenarios.
• Use the BMPFast software to perform required calculations, generate permit-ready outputs, and document design decisions.
• Obtain stormwater permits in a timely, cost-effective manner by understanding what reviewers expect and how to prepare complete submittals.
• Recognize that rules and regulations change over time and build habits for staying current with updates.

BMPFast as the Central Tool

BMPFast is a Windows-based software application developed to support the analytical requirements of Florida’s stormwater rules. It encodes the approved design equations, performance standards, and rainfall data directly, reducing the risk of calculation errors in permit submittals. Throughout this course, BMPFast will be used to illustrate how design inputs translate into regulatory outputs. Participants will see that the software reflects the current state of the rules — and that when rules change (as they did on June 28, 2024, and will continue to change), software updates follow to keep pace.

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2. Sources of Stormwater Improvements

Source slides: 3, 4  ·  How improvements to stormwater rules and methods are developed and adopted

Florida’s stormwater management program does not operate in a vacuum. The methods, performance standards, and allowable BMPs that appear in the rules are grounded in years of research, field testing, and stakeholder input. Understanding where improvements originate helps practitioners anticipate future changes and participate constructively in the regulatory process.

Research, Testing, and Consultants

Improvements to BMP analysis methods flow from multiple directions:

• University research — peer-reviewed studies on pollutant removal efficiency, event mean concentrations (EMCs), and system hydraulics provide the scientific basis for rule updates.
• Field testing and monitoring — real-world data collected from operating stormwater systems in Florida refines theoretical models and identifies where performance standards should be tightened or relaxed.
• Consulting practitioners — engineers and environmental scientists working on permitted projects identify gaps, inconsistencies, or outdated assumptions in existing rules and report findings through professional channels.
• Environmental stewardship groups — advocacy organizations and watershed councils contribute monitoring data and policy priorities that influence rulemaking agendas.

The New Florida Stormwater Rule (Adopted June 28, 2024)

The most significant recent development is the adoption of the new Florida Stormwater Rule on June 28, 2024. This rule revision incorporates updated EMC values, revised performance standards, new terminology, and expanded design options — all of which are addressed in detail throughout this course. The rule was adopted with a phased implementation timeline, with full implementation required by December 28, 2025. During the intervening period, applicants and reviewers must track which provisions are in effect for a given submittal date.

Applicant’s Handbook (A.H. Vol 1)

The Applicant’s Handbook, Volume 1 (A.H. Vol 1) is the primary reference document that translates rule language into design guidance. It contains the equations, tables, and criteria that BMPFast implements computationally. The 2024 rule update is reflected in a revised A.H. Vol 1, including updated Appendix M with current annual rainfall data. Practitioners should treat the Applicant’s Handbook as a living document — it is updated in concert with rule changes and should be referenced by edition date on every submittal.

Software Security and Foreign IP Laws (FS 288.860)

A less familiar but increasingly important consideration in the use of engineering software tools is the requirement to comply with Florida’s technology security provisions. Florida Statute 288.860 addresses concerns about software developed by or sharing data with entities subject to foreign intellectual property laws that may conflict with state data security requirements. Practitioners using BMPFast and other regulated-use software should be aware that compliance with FS 288.860 may affect which software versions or data-sharing features are permissible on state-permitted projects.

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3. BMP Analysis Improvements Overview

Source slides: 5, 6  ·  Summary of what changed in BMP analysis methodology with the 2024 rule

The 2024 rule update brings a broad set of improvements to the technical analysis of BMPs. This section provides a high-level map of those changes so that practitioners can orient themselves before diving into the specifics in later sections. Each of the topics introduced here is addressed in depth elsewhere in the course.

Updated EMCs and Aggregate EMCs

Event Mean Concentrations (EMCs) — the annual average concentrations of total nitrogen (TN) and total phosphorus (TP) in stormwater runoff from different land use types — have been revised based on the latest Florida monitoring data. The update affects the baseline pollutant loads that systems must be designed to treat. Aggregate EMCs, which blend EMC values across mixed land uses within a project boundary, are now more formally defined and documented in A.H. Vol 1, giving applicants clearer guidance on how to handle complex, multi-use sites.

New Performance Standards and Terminology

The 2024 rule introduces revised performance standards with updated percentage removal targets and net-improvement thresholds. Alongside these changes, new terminology has been formally adopted to reduce ambiguity in permit submittals and reviews. Practitioners accustomed to legacy terms should review the updated definitions in A.H. Vol 1 before preparing submittals under the new rule.

Stormwater Harvesting Calculations

Stormwater harvesting — the capture and beneficial reuse of runoff — is now more formally integrated into the performance standard calculations. The 2024 rule provides explicit equations and crediting methods for harvesting systems, allowing applicants to claim pollutant load reduction credit for water that is captured, treated, and used for irrigation or other purposes rather than discharged to receiving waters. BMPFast has been updated to include these calculations directly.

Systems in Series — Equation 9.5

When multiple BMPs are connected in series — meaning the discharge from one system flows into another — the combined effectiveness is calculated using Equation 9.5 from the Applicant’s Handbook. This equation prevents double-counting of pollutant removal while properly crediting the sequential treatment provided by each system. The 2024 update clarifies how Equation 9.5 is applied when one or more systems in a series have been modified or are operating under different performance tiers.

Pre/Post Volume Calculations for Flood Resiliency

The 2024 rule formalizes the use of pre- and post-development volume calculations as a component of flood resiliency analysis. Rather than relying solely on peak discharge comparisons, volume-based calculations better capture the cumulative flood risk contribution of development. These calculations are now integrated with the stormwater quality analysis in BMPFast, allowing applicants to address both quantity and quality requirements in a single workflow.

Cost Data and TMDL/BMAP Program Impacts

The rule update also incorporates updated cost data for BMPs, which is relevant for projects in areas subject to Total Maximum Daily Load (TMDL) allocations and Basin Management Action Plans (BMAPs). Where a project site falls within a TMDL/BMAP watershed, cost-effectiveness analysis may influence which BMP options are required or encouraged. BMPFast includes cost estimation tools that can be used to support these analyses and document the economic basis for BMP selection.

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4. Performance Standards & Annual Rainfall

Source slides: 7, 8, 9, 10  ·  Regulatory targets, dual-standard structure, and updated rainfall inputs

Florida’s stormwater quality rules require that permitted systems demonstrate compliance with specific pollutant removal performance standards. Understanding the structure of these standards — and the rainfall data used to calculate compliance — is foundational to every permit submittal.

Seven State-Directed Performance Standards

The 2024 rule establishes seven state-directed performance standards that apply to stormwater management systems depending on project type, location, and land use. These standards are differentiated by pollutant (TN vs. TP), project category (new development vs. redevelopment), and whether the site falls within a TMDL/BMAP-regulated watershed. Practitioners must identify which standard(s) apply to their project before beginning any BMP sizing calculations.

Dual Standards: Specific Percentage Removal vs. Net Improvement

A key structural feature of Florida’s performance standards is the dual-standard framework. Depending on the project category and receiving water designation, a project may be required to meet either:

• A specific percentage removal target — e.g., a defined minimum percent reduction in TN or TP mass load from the pre-treatment runoff, or
• A net improvement standard — demonstrating that post-development pollutant loads to receiving waters are lower than pre-development loads, regardless of the percentage change.

In some cases, both standards apply simultaneously, and the more stringent result governs. BMPFast calculates compliance with both standards in parallel, flagging which standard controls for a given design scenario.

45% Minimum TN Removal for Redevelopment

For redevelopment projects — sites where existing impervious cover or structures are being modified or replaced — the 2024 rule establishes a minimum of 45% total nitrogen (TN) removal as a floor requirement. This threshold applies even when the net improvement standard would otherwise be satisfied by a smaller reduction. The 45% minimum reflects policy recognition that redevelopment projects in Florida’s urbanized watersheds must deliver meaningful water quality improvements to offset cumulative loading impacts.

Updated Annual Rainfall — A.H. Vol 1 Appendix M (2024)

Annual rainfall depth is a critical input to stormwater load calculations. The pollutant mass exported from a site each year is the product of runoff volume (which depends on rainfall) and pollutant concentration (EMC). If rainfall inputs are incorrect, calculated loads and required BMP performance will be wrong — leading to either over-designed or non-compliant systems.

The 2024 update of A.H. Vol 1 includes a revised Appendix M with updated annual rainfall values for stations across Florida. These values represent a recalculation based on extended rainfall records through recent years, and they differ in some locations from the rainfall values published in the 2000-era version of the handbook. Practitioners who have calibrated their workflows to older rainfall values must update their inputs when working under the 2024 rule.

2024 vs. 2000 Rainfall Data: What Changed

The comparison between 2024 and 2000 rainfall datasets reveals regional variation. Some areas of Florida show higher updated annual rainfall values — reflecting wetter recent decades in those regions — while others show modest decreases. The direction and magnitude of change matters for permitting because:

• Higher rainfall → higher calculated pollutant loads → larger BMP sizing requirements to meet the same percentage removal target.
• Lower rainfall → smaller calculated loads → potentially smaller BMP footprints for the same performance standard.
• Projects permitted under old rainfall data that are now being redesigned or modified must recalculate using current Appendix M values.

Site-Specific Rainfall Evaluation

The 2024 rule explicitly allows — and in some circumstances requires — a site-specific rainfall evaluation. When a project site is located in an area where the nearest Appendix M station is not representative of local conditions (due to geographic isolation, orographic effects, or proximity to coastal rain shadows), the applicant may use a site-specific annual rainfall value derived from a qualified rain gauge record of sufficient length. The analysis must be documented in the permit submittal and is subject to agency review. BMPFast includes a field for entering site-specific rainfall values when Appendix M values are not applicable.

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5. EMCs and Wet Detention Options

Source slides: 11, 12  ·  Event mean concentrations, wet detention design parameters, and advanced credit options

Event Mean Concentrations and wet detention pond design are among the most frequently encountered topics in Florida stormwater permitting. This section explains how EMCs are defined and applied, the key design parameters for wet detention systems, and the expanded credit options introduced or clarified in the 2024 rule.

EMCs Defined: Annual Average TN and TP Concentrations

An Event Mean Concentration (EMC) is the flow-weighted average concentration of a pollutant in stormwater runoff from a specific land use type, measured across multiple storm events over an annual period. EMCs represent the “typical” pollutant loading rate for that land use and are expressed in milligrams per liter (mg/L). In Florida’s stormwater rule framework:

• EMCs are established for both total nitrogen (TN) and total phosphorus (TP).
• They are the primary tool for calculating annual pollutant mass loads from a site: Load (kg/yr) = EMC (mg/L) × Annual Runoff Volume (L/yr) × 10⁻⁶.
• Accurate EMC assignment is therefore critical — an incorrect EMC directly propagates into load calculations and BMP sizing requirements.

Hard-Wired vs. User-Defined EMC Values

BMPFast and A.H. Vol 1 distinguish between two categories of EMC values:

• Hard-wired EMCs — default values for standard land use categories (e.g., residential, commercial, industrial, transportation) that are codified in the rule and embedded in BMPFast. These values are updated with each rule revision and reflect statewide monitoring data. Applicants using hard-wired values do not need to provide site-specific monitoring data to justify the EMC input.
• User-defined EMCs — values derived from site-specific monitoring data or approved literature sources that the applicant believes better represent actual conditions on their site. Using a user-defined EMC requires documented justification and is subject to agency review. User-defined values may be lower than the hard-wired defaults (reducing calculated loads and potentially reducing BMP sizing requirements) only if supported by adequate data.

Maximum Allowable Retention Time (ART) — 200 Days for Wet Detention

Wet detention ponds remove pollutants through a combination of settling, biological uptake, and chemical transformation. The efficiency of these processes depends in part on how long water remains in the pond — the Allowable Retention Time (ART). The 2024 rule establishes a maximum ART of 200 days for wet detention systems. This cap prevents designers from claiming unrealistically high removal efficiencies by modeling excessively long retention times that would not be sustained under actual storm conditions. ART is calculated as the ratio of permanent pool volume to the average annual inflow volume.

Littoral Zone and Managed Aquatic Plants (MAPs) Credit

Wet detention ponds with established littoral zones — the shallow, vegetated perimeter area of a pond — receive credit for enhanced nutrient removal through plant uptake and associated biogeochemical processes. The 2024 rule expands the treatment of littoral zone credit and introduces formal provisions for Managed Aquatic Plants (MAPs). MAPs are floating or emergent aquatic vegetation that is actively managed (harvested on a defined schedule) to remove accumulated nutrients from the system. Credit for MAPs is conditioned on a documented management plan specifying harvest frequency, disposal method, and nutrient accounting. BMPFast includes a MAPs credit calculation module that produces the required documentation.

Anoxic Depth Calculations for TP Removal

Phosphorus removal in wet detention ponds is closely tied to sediment chemistry in the pond bottom zone. Under aerobic (oxygenated) conditions, iron-bound phosphorus remains sorbed to sediment particles. Under anoxic (low-oxygen) conditions, however, iron reduction releases phosphorus back into the water column — a process called internal loading that can significantly impair pond performance. The 2024 rule includes updated guidance on calculating the effective anoxic depth in a wet detention pond’s permanent pool, and BMPFast uses this calculation to adjust TP removal efficiency estimates for ponds where anoxic conditions are likely. Designers can reduce internal loading risk by ensuring adequate mixing, limiting organic sediment accumulation, and maintaining appropriate pond geometry.

Discovery Options: Pond Bottom Analysis

For existing wet detention ponds being evaluated as part of a retrofit or redevelopment project, the 2024 rule provides discovery options — analytical pathways for characterizing actual pond performance rather than relying entirely on modeled defaults. Pond bottom analysis is one such option: sediment cores collected from the pond bottom can be analyzed for phosphorus content, organic matter, and redox indicators to characterize internal loading potential and inform the anoxic depth calculation. This site-specific data can then be entered into BMPFast as user-defined inputs, potentially yielding more accurate (and favorable) performance estimates than the default assumptions.

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