Retention BMP Design & BMPFast Workflow

Stormwater BMP Series  |  Prepared for Florida stormwater permitting practitioners  |  Source material: Applicant’s Handbook, Appendix O; Harper (2007)

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1. Retention BMP Overview

Source slides: 1, 2, 3  ·  Foundational concepts for retention-based stormwater control

What Is a Retention BMP?

Retention basins and related practices represent the first category of BMPs widely adopted in Florida for stormwater water quality control. Rather than treating and releasing runoff, retention systems eliminate the discharge pathway entirely — making them among the most effective tools available for nutrient and pollutant load reduction. Water held within a retention system is ultimately disposed of through one or more of the following mechanisms: infiltration through the basin floor and walls into underlying soils, evaporation from the open water surface, or uptake by vegetation.

Where Retention BMPs Apply

Retention systems are suitable for large catchments or clearly defined watershed subdivisions where site conditions support infiltration. The design approach described in this module applies to the full retention basin as a stand-alone practice, but the same underlying analytical framework extends to a family of related BMPs that share the core mechanism of holding stormwater in place.

Key Characteristics of the Retention Family

• Retention prevents discharge of stormwater to surface waters — the defining characteristic that separates it from detention-based treatment.
• Water is held in the basin and ultimately disposed of via infiltration or evapotranspiration, with no engineered discharge outlet to a receiving water body.
• Retention was the first BMP category widely used in Florida for stormwater water quality control, predating most other engineered practices.
• The practice is most applicable to large catchments or clearly bounded watershed subdivisions where a single basin can intercept the full contributing area.
• Related BMPs — permeable pavement, rain gardens, vegetated swales, and exfiltration trenches — share the same fundamental mechanism and use the same performance methodology described in this module.

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2. Treatment Depth and Volume

Source slides: 4, 6  ·  The normalizing metric that links storage volume to pollutant removal performance

The Treatment Depth Formula

Treatment Depth is calculated by dividing the total retention volume by the total contributing catchment area. The result is most commonly expressed in inches, providing a dimensionally consistent metric that can be applied uniformly regardless of site size or land use.

Why Treatment Depth Matters

Treatment Depth serves as the normalizing variable that allows performance tables and curves developed from long-term rainfall simulations to be applied to any site. By converting volume into a depth uniformly distributed over the catchment, the metric accounts implicitly for site area and makes comparison across different projects straightforward. A site with a large basin but also a large catchment may have the same TD — and therefore the same expected annual removal — as a smaller site with a proportionally smaller basin.

• TD = Treatment Volume ÷ Catchment Area, with units of inches over the catchment area (not depth within the basin).
• Common reporting units in Florida practice are inches over the catchment area, consistent with how rainfall depths are reported.
• TD directly links the design retention volume to the annual runoff volume removal efficiency used for pollutant load calculations.
• Worked example: A 12-acre site with a retention volume of 0.33 acre-feet yields a Treatment Depth of 0.33 inch — calculated as (0.33 ac-ft ÷ 12 ac) × 12 in/ft = 0.33 in.
• Because TD normalizes for area, it simplifies comparison across development projects with different sizes, CN values, and soil types, while still capturing the essential water balance relationship.

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3. Retention Effectiveness and Capture

Source slides: 5, 6  ·  How annual pollutant removal efficiency is determined from tables and automated tools

Source of the Removal Tables

The performance data for retention BMPs in Florida are drawn from long-term continuous rainfall-runoff simulations published by Harper (2007) and incorporated into Appendix O of the Applicant’s Handbook. These simulations used decades of rainfall records to model how different combinations of treatment depth, curve number (CN), and directly connected impervious area (DCIA) affect the annual fraction of runoff volume that can be captured and retained before the basin fills and overflows.

Simulation Baseline and Recovery Time

The standard Appendix O tables are based on a 72-hour recovery time — the assumed time required for the retention basin to drain (by infiltration or evaporation) back to its empty condition following a storm event, making storage available again for the next event. This 72-hour baseline is conservative and appropriate for many Florida soils with moderate infiltration rates. Sites with faster drainage — such as those underlain by Hydrologic Group A soils — may qualify for recovery time adjustments that increase the tabulated effectiveness values (covered in Section 4).

Interpolation Requirements

The Appendix O tables are organized by discrete CN values and DCIA percentages. Most real projects will not fall exactly on a tabulated row and column, requiring interpolation between adjacent table entries to obtain the correct effectiveness value. This two-dimensional interpolation (simultaneously varying CN and DCIA) is straightforward in concept but error-prone when performed manually, particularly when dealing with fractional values far from table breakpoints.

• Effectiveness is the fraction of annual runoff volume retained — for a fully mixed pollutant traveling with runoff, this fraction equals the annual mass removal efficiency.
• Removal tables are from Harper (2007), published in Applicant’s Handbook Appendix O, and are the regulatory standard for Florida retention BMP permitting.
• Tables are derived from 72-hour recovery time simulations using long-term historical rainfall records representative of Florida rain zones.
• Interpolation is required when the project’s CN and DCIA values do not fall exactly on table breakpoints — both values must be interpolated simultaneously.
• BMPFast automates the interpolation process, selecting the correct table entries and performing two-dimensional interpolation internally, reducing calculation errors and saving time.

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4. Recovery Time Adjustments

Source slides: 8, 9  ·  Crediting faster basin drainage with improved annual removal performance

The 72-Hour Baseline

All standard Appendix O retention tables assume a 72-hour recovery time. This means the simulation credits the basin with restoring full storage capacity once every 72 hours after a storm fills it. Any site that can demonstrate a faster measured or design infiltration rate — resulting in a shorter actual recovery time — is eligible for an upward adjustment to the tabulated effectiveness value.

Adjustment Magnitudes by Recovery Class

Polynomial Curve Application

Recovery time adjustments are not applied as a simple lookup — they are computed using polynomial best-fit curves derived from simulation results for each Florida rain zone. The curves relate recovery time (in hours) to the percentage-point increase in annual effectiveness. BMPFast applies these curves automatically during calculation, selecting the correct polynomial coefficients for the rain zone applicable to the project location and computing the adjusted effectiveness without requiring manual curve reading.

BMPs Eligible for Recovery Time Credit

• Retention basins — the primary BMP covered by this module; recovery time derived from measured in-situ infiltration rate and basin geometry.
• Rain gardens (bioretention) — recovery driven by engineered media conductivity and underdrain configuration.
• Permeable pavement — recovery determined by subbase permeability and pavement surface condition over time.
• Exfiltration trenches — recovery based on trench geometry, aggregate void ratio, and native soil permeability at the trench walls and floor.
• Vegetated swales — recovery estimated from swale cross-section, longitudinal slope, and soil infiltration characteristics.

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5. Example Project Setup

Source slides: 10, 11, 12  ·  A worked reference scenario used throughout the remaining module sections

Site Description

The example project is a 12-acre residential subdivision located south of Orlando, Florida, placing it within the applicable Florida rain zone for central peninsular Florida. The site is being developed from undeveloped land to a conventional residential subdivision with streets, rooftops, driveways, and landscaped lots. A single retention basin is proposed to serve the entire subdivision catchment.

Post-Development Hydrologic Parameters

BMP Configuration and Performance Standard

• The proposed BMP is a retention basin with a 36-hour recovery time, supported by the Hydrologic Group A native soils beneath the basin floor — qualifying the project for a recovery time adjustment above the 72-hour baseline.
• The storage range evaluated in the design spans from 0.33 to 2.5 acre-feet, corresponding to treatment depths of approximately 0.33 to 2.5 inches over the 12-acre catchment.
• The site discharges to an Outstanding Florida Water (OFW), triggering the most stringent performance standard under Florida ERP rules.
• OFW performance requirements: 80% annual TN (Total Nitrogen) removal and 90% annual TP (Total Phosphorus) removal from post-development loads.
• Because the retention system removes pollutants by capturing runoff volume, the required BMP effectiveness (annual fraction of runoff retained) must meet or exceed the mass removal targets — making the OFW standard the binding design constraint.

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Topic 6: BMPFast Navigation and Input

Source slides: 11–14 · BMPFast data entry workflow and BMP selection

Sequential Data Entry Workflow

BMPFast guides the user through a structured, sequential data entry process. Site information is entered first, followed by catchment-level parameters, and finally treatment configuration details. This step-by-step approach minimises the risk of omitting required inputs and ensures the model has a complete dataset before performing any calculations.

Automated Loading Calculations

Once site and catchment data have been entered, BMPFast automatically calculates three key annual output values:

• Annual runoff volume — derived from catchment area, impervious fraction, and local rainfall data
• Total Nitrogen (TN) loading — computed from event mean concentrations applied to the runoff volume
• Total Phosphorus (TP) loading — computed using the same approach with nutrient-specific concentration values

These three outputs become the baseline against which BMP performance is subsequently measured. All figures are expressed on an annual basis, consistent with permit and watershed planning requirements.

BMP Treatment Options Worksheet

The BMP treatment options worksheet presents 13 recognised BMP types within a single scrollable interface. Each BMP entry includes its associated removal efficiency parameters and applicable design constraints. Users select the BMP most appropriate for the catchment conditions being modelled.

Handling Multiple BMPs per Catchment

BMPFast is designed around a one-BMP-per-catchment data model. Where a project requires more than one BMP to treat drainage from the same contributing area, the practitioner must create separate catchment entries for each device. Each entry then carries its own BMP configuration, allowing the software to account for the cumulative load reduction across the full treatment train without conflating performance values.

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Topic 7: Discovery — Plot and Get Depth Tools

Source slides: 15–16 · Removal efficiency curves and treatment depth optimisation

The Plot Button: Visualising Removal vs. Depth

The Plot button generates a graphical curve showing how pollutant removal efficiency (expressed as a percentage) changes as treatment depth increases. The curve is specific to the BMP type and catchment parameters already entered, making it a direct design aid rather than a generic reference chart.

Reading the curve allows the designer to:

• Identify the treatment depth at which a target removal percentage is first achieved
• Observe the rate at which marginal gains in removal diminish as depth increases beyond the target
• Make an informed decision about whether additional storage volume is cost-effective given the incremental improvement on offer

Figure 1 — BMPFast Plot output. Removal efficiency (%) versus treatment depth (acre-feet) curve generated by the Plot tool for the example catchment. The curve flattens noticeably beyond approximately 1.0 ac-ft, indicating diminishing returns at higher storage volumes.

The Get Depth Button: Calculating Volume for a Target

Where the designer already knows the required removal percentage — for example, a permit condition specifying a minimum nutrient reduction — the Get Depth button reverses the calculation. The user inputs the target removal percentage and BMPFast returns the treatment volume (in acre-feet) needed to achieve it.

Interpreting Marginal Gains Beyond the Target

The plot curve extends to 2.5 acre-feet, allowing the designer to see what additional removal is achievable with larger storage volumes. The results for the example catchment demonstrate a pattern typical of retention-based BMPs: substantial gains up to approximately 1 ac-ft, followed by a progressively flatter curve where each additional unit of volume contributes a smaller increment of removal. This information supports value-engineering discussions where project budgets or site constraints limit available footprint.

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Topic 8: Groundwater Protection with Sorption Media

Source slides: 17–19 · Managing nutrient discharge to groundwater through sorption media specification

The Groundwater Loading Problem

Retention infiltration BMPs — designed to capture and percolate stormwater into the underlying soil — are effective at reducing surface discharge volumes. However, they introduce a secondary concern: nutrients dissolved in the retained water pass directly into groundwater as the water infiltrates. Without additional treatment, TN and TP concentrations in the recharge water may exceed groundwater quality standards or contribute to nutrient loading of receiving aquifers.

Sorption Media as a Treatment Solution

BMPFast incorporates a groundwater protection module that allows the user to specify a sorption media layer placed in the basin bottom. Stormwater percolating through this layer is treated for nutrients before it reaches the native soil and enters the groundwater table. The media layer is sized within the software based on the anticipated recharge volume and the required effluent quality.

CTS24 Media Selection

The sorption media selected for the example in the course is CTS24, a proprietary engineered media composed of:

• Clay — provides cation exchange capacity for ammonium and metal adsorption
• Tire crumb — improves hydraulic conductivity and provides additional sorption surface area
• Sand — structural matrix component maintaining media permeability over time

CTS24 is one of several media options available within BMPFast, each with independently characterised treatment rates and service life parameters derived from laboratory and field testing.

Treatment Performance: With and Without Media

The performance benefit of the sorption media layer is quantified by comparing post-treatment TN concentrations in the recharge water under two scenarios:

The CTS24 media reduces TN in the recharge water by approximately 75% — from 1.77 mg/L to 0.44 mg/L — demonstrating that sorption media can meaningfully protect groundwater quality even where large recharge volumes are involved. The 3.77 million gallons per year recharge estimate is used subsequently to size the media volume and calculate service life.

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Topic 9: Filter Area and Media Service Life

Source slides: 20–22 · Sizing the sorption media filter and verifying long-term performance

Minimum Filter Area Calculation

The sorption media filter layer must be large enough to accept the treatment volume within the time constraints imposed by the basin drawdown requirement. BMPFast calculates a minimum filter area of 587.25 square feet for the 1-inch treatment depth scenario.

This figure is governed by the drainage criterion: half of the treatment volume must drain through the media within one day. This one-day drainage requirement prevents the basin from becoming a standing-water nuisance and ensures adequate recovery capacity before the next design storm. The calculation takes the form:

Media Volume Selection

With the minimum filter area established, the media volume is determined by specifying a media layer depth. For the example catchment, a media volume of 5,400 cubic feet is selected, corresponding to a 2-foot media depth across the filter area. The 2-foot depth is a commonly applied standard for CTS24 and comparable sorption media products, balancing treatment capacity against construction cost and basin geometry constraints.

Service Life Verification

The service life calculation confirms how long the sorption media will continue to meet its nutrient removal target before replacement or regeneration is required. BMPFast computes this by dividing the total media sorption capacity (a function of media volume and the specific sorption capacity of the selected media type) by the annual mass of nutrients loaded onto the media from the recharge volume.

For the example, the calculated service life of 10.5 years meets the project’s 10-year design target. This outcome confirms that the selected media volume is adequate without being excessively conservative. The 0.5-year margin above the target provides a modest buffer against variability in annual recharge volumes or nutrient concentrations.

Sensitivity of Service Life to Sizing Decisions

Service life scales linearly with media volume when all other parameters are held constant. A designer who needs to extend service life to 15 or 20 years — perhaps to align with a facility’s major maintenance cycle — can do so by increasing the media volume proportionally. Conversely, where space is constrained, a shorter service life may be acceptable provided O&M provisions for earlier media replacement are incorporated into the maintenance agreement. BMPFast allows rapid iteration between media volume and service life to find the optimal balance for a given project context.

Parameter	Value	Basis
Treatment depth target	1 inch	Design storm / permit requirement
Treatment volume	~0.993 ac-ft	Get Depth tool output at 90% removal
Drainage criterion	50% of volume in 1 day	Drawdown requirement
Safety factor on media rate	2×	Long-term clogging allowance
Minimum filter area	587.25 SF	Calculated by BMPFast
Media depth selected	2 ft	Standard for CTS24 installations
Total media volume	5,400 CF	Area × depth
Annual recharge volume	3.77 MG/yr	BMPFast groundwater module
Calculated service life	10.5 years	Media capacity ÷ annual loading
Design service life target	10 years	Project requirement — met ✓

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Appendix: Quick Reference Cards

Condensed reference summaries for field use, permit documentation, and design review

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