Rain Gardens & Tree Wells: Design, Modeling, and Performance in Florida Stormwater Management

Florida Stormwater Management Training Module  ·  Based on FDEP BMPFast Guidance  ·  Part 1 of 2

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1. Rain Gardens & Tree Wells Overview

Source slides: 1, 2, 3, 4, 5  ·  Topic: Definitions, configurations, and regulatory recognition of rain gardens and tree wells as stormwater BMPs

What Are Rain Gardens and Tree Wells?

Rain gardens and tree wells are engineered or naturalistic low-impact development (LID) practices designed to intercept, infiltrate, and treat stormwater runoff close to its source. Both are classified as bioretention systems and share the fundamental principle of routing surface runoff into a vegetated depression where physical, chemical, and biological processes reduce pollutant loads before water either infiltrates into the soil or exits through a controlled outlet.

While the terms are sometimes used interchangeably in common usage, they have distinct physical configurations in the context of Florida stormwater permitting:

• Rain gardens are relatively shallow, broadly vegetated depressions planted with a mix of grasses, sedges, shrubs, and small trees selected for tolerance of both wet and dry conditions. Their vegetated surface area is typically larger, and they are designed to receive runoff from several contributing sources simultaneously.
• Tree wells are compact bioretention cells constructed around individual trees. The tree canopy, root structure, and engineered media work together to capture and treat runoff from small, directly connected catchments such as individual parking stalls, roadway segments, or downspout drainage areas.

Typical Locations and Applications

Both practices are most commonly sited in areas where impervious surface runoff is concentrated and where right-of-way constraints limit the use of larger detention ponds. Typical installation locations include:

• Parking lot islands and medians, where tree wells capture runoff from individual parking stalls and drive lanes
• Roadway curb extensions and swale buffers, where curbside tree wells intercept road drainage before it enters the storm system
• Building perimeters receiving roof runoff redirected from downspouts
• Commercial and mixed-use redevelopment sites with limited open space for traditional wet or dry detention

Retention vs. Detention Function

Rain gardens and tree wells can be designed to function as either retention BMPs or detention BMPs, depending on site soils, water table conditions, and design intent:

Regulatory Recognition Under Florida Law

Rain gardens and tree wells are formally recognized as qualifying Best Management Practices (BMPs) for stormwater treatment in Florida. Their design criteria, performance standards, and calculation methodologies are documented in:

• FDEP Applicant’s Handbook Volume I, Appendix O — the primary regulatory reference establishing accepted BMP configurations, media specifications, and pollutant removal methodologies for the Environmental Resource Permit (ERP) program
• BMPFast software — the FDEP-sanctioned calculation tool used to demonstrate compliance with water quality treatment standards for rain gardens, tree wells, and series BMP configurations

Figure 1 — Typical Rain Garden and Tree Well Configurations. Cross-sectional comparison showing a tree well surrounding a street tree in a parking lot island (left) and a broader rain garden receiving overflow from multiple upstream tree wells (right). Both systems show engineered media profile, optional underdrain, and surface overflow structure.

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2. Treatment Effectiveness with BAM

Source slides: 6, 11  ·  Topic: Role of Bio-sorption Activated Media in achieving nutrient removal standards for redevelopment projects

What Is Bio-sorption Activated Media (BAM)?

Bio-sorption Activated Media (BAM) is a specialized engineered filter media formulated to provide enhanced removal of dissolved nutrients — particularly total nitrogen (TN) and total phosphorus (TP) — from stormwater runoff. Unlike conventional sand or native soil media, BAM incorporates sorbent materials that chemically bind phosphorus and provide favorable conditions for biological nitrogen transformation.

In Florida’s stormwater permitting context, the use of BAM (or equivalent enhanced media) in rain garden and tree well systems is the critical design variable that determines whether a bioretention system can meet the elevated nutrient removal standards required for redevelopment projects. Standard engineered soil media alone — without BAM — does not reliably achieve the removal rates required under current FDEP performance standards.

Pollutant Removal Performance with BAM

When properly designed with BAM, rain garden and tree well systems are capable of achieving the following annual average pollutant removal rates:

Meeting the Redevelopment Performance Standard

Florida’s Environmental Resource Permit program establishes a redevelopment performance standard requiring that projects which intensify existing impervious cover must achieve a minimum of 45% annual average TN removal and 80% annual average TP removal from the water quality treatment volume. This standard is more stringent than the baseline standard applied to new development, reflecting the requirement to demonstrate net improvement over pre-project conditions.

Treatment Volume Basis

The treatment volume credited to a rain garden or tree well system is calculated based on two primary variables:

• Storage depth — the effective depth of runoff that can be stored within the bioretention cell between the invert of the overflow structure and the surface of the media
• Catchment area — the directly connected impervious and pervious area draining to the bioretention cell, as determined by grading and inlet design

The relationship between these variables and the annual runoff capture fraction is computed within BMPFast using rainfall frequency data specific to the project’s Florida rain zone. Larger storage depth relative to catchment area increases the fraction of annual runoff volume treated, directly improving the effective annual pollutant removal rate.

Groundwater Protection Through Sorption Media

A secondary but important benefit of BAM in sites where infiltration occurs is groundwater quality protection. Because BAM chemically binds phosphorus and filters dissolved pollutants before water reaches the water table, bioretention systems using BAM provide a documented treatment barrier between stormwater runoff and the underlying aquifer. This is particularly relevant for sites in designated water supply protection zones, where regulators require demonstration that stormwater management systems will not degrade groundwater quality.

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3. Example Project Description

Source slides: 7  ·  Topic: Site characteristics, development scenario, and stormwater management objectives for the illustrative BMPFast example

Project Site Overview

The example used throughout this module is a redevelopment project involving a 4-acre light industrial site being converted to a high-intensity commercial use. This change in land use triggers the redevelopment performance standard, requiring the project to achieve at least 45% TN and 80% TP removal from the water quality treatment volume.

Proposed BMP Configuration

The stormwater management design proposes a distributed network of 40 individual tree wells installed throughout the site’s parking areas, internal roadways, and building perimeter. These tree wells collectively receive runoff from the 1.4 acres of directly connected impervious surface. The sources contributing to the tree wells include:

• Parking stall runoff routed via curb cuts into tree well islands
• Internal roadway drainage directed to curbside tree wells
• Roof runoff redirected from downspouts to tree wells at the building perimeter

The tree wells are configured to drain in series — overflow from each tree well is routed to a central rain garden located at the lowest point of the site. This series arrangement allows the rain garden to receive pre-treated runoff from the upstream tree wells, improving overall system performance and reducing the sizing requirement for the rain garden.

Water Supply Designation and Groundwater Requirements

The project site is located within a designated water supply protection area. This designation triggers an additional regulatory requirement: the permit applicant must provide groundwater quantity and quality estimates demonstrating that the proposed stormwater infiltration will not adversely affect the underlying aquifer used as a drinking water source.

Figure 2 — Example Site Layout Schematic. Conceptual site plan showing the 4-acre commercial redevelopment with 40 tree wells distributed across parking areas and roadways, and the central rain garden receiving overflow from all upstream tree wells. Arrows indicate the series drainage flow path from tree wells to rain garden to site discharge.

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4. BMPFast Data Entry & Navigation

Source slides: 8, 9, 10, 11  ·  Topic: Step-by-step guidance for entering site, catchment, and BMP data into the BMPFast calculation tool

BMPFast Software Overview

BMPFast is the FDEP-approved spreadsheet-based calculation tool used to evaluate the annual average water quality performance of recognized stormwater BMPs in Florida. For rain gardens and tree wells, BMPFast computes the fraction of annual runoff volume captured by the system and applies pollutant removal efficiencies based on media type to determine annual average TN and TP removal percentages.

The tool is organized into a series of linked worksheets. Navigation between worksheets is accomplished using software-provided navigation buttons rather than clicking individual worksheet tabs, ensuring that dependent calculations are updated in the correct sequence. Users should avoid navigating by tab unless directed to do so by the tool documentation.

Step 1 — Site Data Entry

The first data entry worksheet captures site-level characteristics that apply to all BMPs in the project. Required inputs include:

• Florida rain zone — the FDEP-designated rainfall region (Zones 1 through 3) corresponding to the project location, which determines the rainfall frequency distribution used in all subsequent volume calculations
• Annual rainfall — the long-term average annual rainfall depth in inches for the project location, typically obtained from NOAA records or FDEP guidance for the applicable zone
• Project type — whether the project is new development or redevelopment, which determines which performance standard applies

Step 2 — Catchment Watershed Characteristics

The catchment worksheet defines the drainage area contributing runoff to the BMP. For tree wells and rain gardens, this includes:

• Total catchment area — the total area (in acres or square feet) draining to the bioretention system
• Impervious fraction — the proportion of the catchment that is directly connected impervious surface, which significantly influences runoff volume generation
• Pervious area characteristics — soil type and vegetative cover for any pervious portions of the catchment contributing to runoff

Step 3 — Tree Well Dimensions and Media Specification

The tree well worksheet requires the following inputs for each tree well (or for the representative tree well if all 40 are identical in size and configuration):

• Surface area — the plan-view area of the tree well opening at grade, in square feet
• Media depth — the depth of the engineered media profile, in inches or feet
• Media type — selection of BAM or alternative approved media, which determines the pollutant removal efficiency coefficients applied by the tool
• Sustainable void fraction — the effective porosity of the media, expressed as a decimal fraction, representing the volume of pore space available for water storage; for BAM this value is typically specified by the media manufacturer and confirmed through testing
• Number of tree wells — the total count of identical tree well units, allowing BMPFast to aggregate total storage and treatment capacity

Step 4 — Rain Garden Dimensions and Storage Data

The rain garden worksheet captures the physical dimensions and storage characteristics of the downstream bioretention basin:

• Surface ponding depth — the maximum depth of water that can pond above the media surface before the overflow structure activates, in inches
• Rain garden footprint area — the plan-view surface area of the bioretention cell at the media surface elevation, in square feet
• Media depth and type — same parameters as for the tree wells; BAM specification is required to meet the 80% TP standard
• Underdrain presence — whether the system uses an underdrain (detention) or relies on native soil infiltration (retention), which affects the volume reduction credit calculation

Figure 3 — BMPFast Data Entry Worksheets. Screen captures showing the site data entry worksheet (top), the tree well dimension and media input worksheet (middle), and the rain garden storage input worksheet (bottom), with key input fields highlighted.

Navigating Between Worksheets

BMPFast provides clearly labeled navigation buttons within each worksheet to advance to the next data entry step or to return to a previous worksheet for revision. The recommended workflow sequence is:

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5. Series BMP Calculation & Results

Source slides: 12  ·  Topic: How BMPFast handles series BMP configurations, prevents double-counting, and evaluates combined system performance

Series BMP Hydraulics

In the example project, the 40 tree wells and the downstream rain garden operate as a series BMP system: runoff first enters and is treated by the tree wells, and any overflow from the tree wells is conveyed directly into the rain garden for additional treatment. This configuration is distinct from a parallel arrangement, where each BMP independently treats a separate portion of the catchment runoff.

The series arrangement has important implications for how BMPFast calculates system performance:

• The rain garden’s influent is defined as the overflow from the tree wells — not the original catchment runoff — meaning only the portion of runoff that exceeds tree well storage capacity reaches the rain garden
• Pollutant concentrations entering the rain garden are already reduced by the treatment that occurred in the tree wells, so the rain garden’s removal is applied to a lower starting concentration
• BMPFast’s series calculation methodology explicitly accounts for this, preventing the double-counting of removal credit that would occur if each BMP were evaluated independently against the full catchment load

Individual BMP Performance Results

Before evaluating the combined series performance, it is instructive to examine what each component achieves individually:

BMP Component	Annual Runoff Captured	TP Removal	Meets 80% TP Standard?
40 Tree Wells (alone)	46.7% of annual runoff	<80%	No
Rain Garden (alone)	72% of annual runoff	<80%	No
Tree Wells + Rain Garden (series)	Combined	80.2%	Yes

Combined Series System Performance

When BMPFast evaluates the tree wells and rain garden as a series system, the combined annual average TP removal is calculated as 80.2% — just above the 80% redevelopment performance standard. This result demonstrates that the proposed BMP configuration is compliant with the FDEP redevelopment standard, and the result can be used directly in the ERP permit application.

Why the Rain Garden Alone Is Insufficient

The rain garden alone captures 72% of annual runoff volume, which is a substantial fraction. However, this capture rate does not translate to 80% TP removal for two reasons:

• Volume throughput: The 28% of runoff that bypasses the rain garden as overflow carries untreated pollutant loads that reduce the annual average removal percentage below the 80% threshold
• Concentration effect: High-intensity storms that cause overflow events tend to carry elevated pollutant concentrations, meaning the bypassed fraction has a disproportionate effect on the annual average load calculation

The addition of the upstream tree wells addresses this gap by capturing and treating runoff from smaller, more frequent storm events before they reach the rain garden, effectively pre-treating the most frequent portion of the annual load and reducing the concentration of influent entering the rain garden during larger events.

Figure 4 — BMPFast Series Results Summary. Screenshot of the BMPFast results worksheet showing combined tree well and rain garden performance for the example project, with the 80.2% TP removal result highlighted and the 80% standard compliance indicator displayed.

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Section 6: Groundwater Protection & Savings

Topics: Annual groundwater recharge, discharge concentration estimates, BAM media groundwater protection, and saving project files for permit submissions — Source slides 12, 13, 14

Annual Groundwater Recharge

The BMP Sizing Tool calculates the average annual groundwater recharge produced by a bioretention system in million gallons per year (MGY). This figure is derived from the long-term continuous simulation, accounting for the local precipitation record, the contributing drainage area, land cover, and the infiltration characteristics of the in-situ soils beneath the facility. Recharge volume is a key metric for jurisdictions that credit stormwater infiltration toward water budget restoration or groundwater replenishment goals embedded in MS4 permits and watershed management plans.

Factors that increase the computed recharge estimate include:

• Larger contributing drainage area or higher impervious cover fraction, which generates greater runoff volume
• Higher native soil infiltration rate (e.g., sandy loam or loamy sand soils with elevated saturated hydraulic conductivity)
• Shallower seasonal high groundwater table, which limits ponding duration and keeps the facility draining actively
• Absence of an underdrain, allowing all infiltrated water to route to groundwater rather than being intercepted and piped to surface discharge

When an underdrain is included in the design, the tool partitions outflow between the underdrain pipe and the groundwater pathway. Only the fraction that bypasses the underdrain and continues downward through the native soil is counted toward the recharge total. Designers seeking to maximize recharge credits should evaluate whether the underdrain invert elevation can be raised, or whether the underdrain can be omitted entirely where native soils are adequately permeable.

Average Discharge Concentration to Groundwater

In addition to recharge volume, the tool estimates the average discharge concentration of key constituents in the water that ultimately infiltrates to groundwater. This metric answers a fundamental regulatory question: is the bioretention facility acting as a net protector of groundwater quality, or could it inadvertently serve as a conduit for contaminants to migrate downward into the aquifer?

The average discharge concentration to groundwater is calculated as a flow-weighted mean across all simulated storm events in the period of record. The tool accounts for:

• Inflow concentration — the event mean concentration (EMC) assigned to each pollutant in the runoff entering the facility
• Media treatment performance — the pollutant removal efficiency attributed to the BAM media layer and associated biological and physical processes within the soil profile
• Mixing and dilution — the effect of clean baseflow and inter-event drainage diluting the treated effluent concentration over the simulation period

How BAM Media Protects Groundwater Quality

Biosorption Activated Media (BAM) is specifically engineered to achieve high pollutant removal efficiencies within the media layer before water reaches the native soil and the groundwater table. This multi-mechanism treatment train is central to the groundwater protection story:

Together, these mechanisms mean that by the time treated water exits the bottom of the BAM media layer and begins migrating through the native soil toward the water table, it has already been substantially cleaned. The native soil provides an additional polishing buffer. The BMP Sizing Tool quantifies this combined treatment performance and presents the resulting groundwater discharge concentration so designers and regulators can verify that the facility will not degrade underlying aquifer quality.

Saving Project Files for Permit Submissions

Once a design is finalized and all sizing, water quality, and groundwater outputs have been reviewed, the BMP Sizing Tool allows users to save the complete project as a .BMPT file. This proprietary file format preserves all input parameters, simulation settings, and computed results in a single portable package.

The .BMPT file format serves several important functions in the project delivery and regulatory workflow:

• Permit submission documentation — The saved file provides a verifiable, reproducible record of the design inputs and model outputs submitted to the reviewing agency. Regulators can open the file, confirm all parameters, and re-run the simulation independently to validate results.
• Design iteration and revision — Saving the project at each major design milestone allows designers to return to any prior version, compare alternatives, and track how changes to drainage area, media depth, or surface area affected performance outputs.
• Interdisciplinary coordination — The .BMPT file can be shared among project team members — civil engineers, landscape architects, environmental scientists, and project owners — ensuring all parties are working from the same verified design basis.
• Post-construction verification — If as-built conditions differ from the permitted design (e.g., a change in surface area or media depth during construction), the original .BMPT file can be opened and the affected parameters updated to produce a revised performance report reflecting the actual installed facility.

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