With the implementation of Florida’s updated stormwater rule, design flexibility has increased—particularly through the use of off-site compensatory treatment and Regional Stormwater Management Systems (RSMS). However, these strategies may lead to the discharge of partially treated stormwater runoff to wetlands.

While this approach offers permitting efficiency, it also raises a key technical question:
How do we demonstrate that a wetland has the assimilative capacity to receive this runoff without causing an adverse impact?

📜 Regulatory Background: Rule 62-611, F.A.C.

The good news is wetlands are consumers of nutrients. In fact, wetlands need nutrients to thrive. The issue is determining the total amount of nutrients that can be assimilated by a wetland system while remaining within healthy limits. The answer lies in Chapter 62-611, F.A.C., Florida’s Wetlands Application Rule, which outlines procedures to evaluate wetland assimilative capacity. Originally developed for wastewater treatment wetlands, this rule can also be applied to stormwater discharges—especially in the context of project designs that discharge partially treated runoff to natural wetlands while using off-site compensatory treatment within the same watershed.

🔬 Key Wetland Capacity Criteria

Rule 62-611 sets specific hydraulic and nutrient loading limits for discharges to artificial treatment wetlands and natural (receiving) wetlands. We will use the more stringent limits for natural wetlands in this procedure:

• Hydraulic Loading:
  ≤ 2 inches/week (annual average)
  Detention Time: > 14 days (annual average)
• Nutrient Loading (Assimilative Capacity):
  • Total Nitrogen (TN): < 25 g/m²/year
  • Total Phosphorus (TP): < 3.0 g/m²/year
• Discharge Limits:
  • To Receiving Wetlands: TN = 3 mg/L, TP = 1 mg/L

🚧 How This Applies to Projects

Typical event mean concentrations (EMCs) for project runoff are already below the discharge limits. For example, the EMCs for roadway projects are:

• TN: 1.52 mg/L
• TP: 0.20 mg/L

This means that for many projects, discharges to wetlands may comply with Rule 62-611 without additional nutrient treatment, as long as the wetland has sufficient assimilative and hydraulic capacity.

📌 Why This Matters

By applying the wetland assimilative loading criteria under Rule 62-611, engineers can demonstrate regulatory compliance for wetland discharges from partially treated stormwater systems. This can support more cost-effective stormwater designs, reduce the need for additional BMPs or treatment infrastructure, and allow the use of off-site compensating treatment to achieve full compliance with the new, more stringent performance criteria later this year.

📂 Downloadable Design Example

To help illustrate this approach, we’ve provided a downloadable spreadsheet example that walks through the process of evaluating a wetland’s assimilative and hydraulic capacity for a representative stormwater discharge scenario. Note that other developments that discharge to the same wetland system may add to the cumulative loading, so provisions for calculating this loading are included in the spreadsheet.

📥 Download the Design Example Spreadsheet

🧰 Tips for Stormwater Engineering Consultants:

• Use Rule 62-611 procedures to evaluate wetland assimilative loading capacity.
• Ensure discharge concentrations and loading rates from your project are within allowable thresholds for the receiving wetlands.
• Consider the cumulative loading from other developments within the same basin that may discharge to the same wetlands.
• Larger wetland systems are more likely to assimilate additional nutrient loading without adverse impact.
• Coordinate with the WMD early in design to confirm consistency with the ERP Rules and Applicant’s Handbook.

This website includes details on how the new stormwater rule works, updated forms, applicants handbook Volume IIs, and implementation timeline including when it is effective and who is affected.

Read Here

FDEP, [Updated] Environmental Resource Permit (ERP) Applicant’s Handbook Vol. I §8.3.3(a) 

Read Here

This document focuses on how municipalities, utilities, and related organizations can use advances in technology to implement “smart data infrastructure” for wet weather control—that is, how they can use advanced monitoring data to support wet weather control and decision-making in real time or near real time. 

Read Report

The study results suggest that it is imperative to consider future sea level rise conditions in stormwater design in low-lying coastal areas of Florida and around the world to prevent poor pond performance and increased risk of flooding in the future.

Read Study

An informative narrative of the how stormwater can be effectively reused.

Read Study

This manual is a compilation of information that will be useful to anyone interested in the St. Johns River Water Management District’s (SJRWMD) environmental resource permitting (ERP) program. It was developed as a result of the enactment of section 373.4131 of the Florida Statutes which directed the Department of Environmental Protection (DEP) and the water management districts to adopt statewide ERP rules.

Read Manual

Each Water Management District has adopted an ERP Applicant’s Handbook Volume II (“Volume II”) which contains the District-specific design and performance criteria for stormwater quantity, flood control, stormwater quality and any special basin criteria or other requirements that are applicable within the geographic area of the specific water management district. 

Read Handbook

This Applicant’s Handbook Volume II accompanies Chapter 62-330, Fla. Admin. Code, and the Environmental Resource Permit Applicant’s Handbook Volume I (General and Environmental). Applicant’s Handbook Volume I is applicable to all environmental resource permit applications, and provides background information on the environmental resource permit (ERP) program.

Read Handbook

The Connecticut Department of Environmental Protection (CTDEP) has been actively involved in the operation of Connecticut’s Nitrogen Credit Exchange (NCE) since 2002. During the 2002-2009 period the total value of credits bought and sold was $45.9 million, representing 15.5 million nitrogen credits exchanged.

Read Summary 

Technical Release 55 (TR-55) presents simplified procedures to calculate storm runoff volume, peak rate of discharge, hydrographs, and storage volumes required for floodwater reservoirs. These procedures are applicable in small watersheds, especially urbanizing watersheds, in the United States. First issued by the Soil Conservation Service (SCS) in January 1975, TR-55 incorporates current SCS procedures. This revision includes results of recent research and other changes based on experience with use of the original edition.

Click on the link below to open the document in a new window:

Urban Hydrology for Small Watersheds – TR-55, January 1999