10 – Guide to Retention Systems
Part 1 of 2 | Stormwater Management Series
Retention Systems, BMP Types, and Efficiency Modeling
1. Retention Systems Overview
Foundational concepts distinguishing retention from detention
Retention vs. Detention: A Fundamental Distinction
The terms retention and detention describe two fundamentally different approaches to managing stormwater on a developed site. Understanding this distinction is essential before evaluating any Best Management Practice (BMP) for water quality or volume control.
Definition — Retention
Retention systems capture stormwater runoff and prevent it from discharging to surface waters from the site. Captured volume is removed through infiltration into the soil, evapotranspiration, or both. There is no surface discharge for the design treatment volume.
Definition — Detention
Detention systems temporarily hold stormwater and then release it — either to surface waters or to an outfall structure — after a controlled delay. Detention provides attenuation of peak flows and some water quality benefit through settling, but it does not eliminate the discharge volume.
No Surface Discharge for the Treatment Volume
The defining operational characteristic of a retention BMP is that the design treatment volume experiences no surface discharge. This means pollutants carried in that volume are not conveyed off-site via surface waters. Instead, they are retained within the BMP footprint and removed through natural processes — primarily infiltration through the soil profile.
Because retention eliminates the discharge pathway entirely for the captured volume, it generally provides superior pollutant load reduction compared to detention-based systems treating the same volume.
Variable Reduction in Runoff Volume
Retention systems do not provide uniform volume reduction across all storm events. The actual volume reduction achieved depends on:
- The size of the storm event relative to the treatment volume capacity of the BMP
- Antecedent soil moisture conditions and available storage in the reservoir or soil profile
- The hydraulic conductivity of the native soil (infiltration rate)
- The design configuration and maintenance state of the BMP
- Seasonal variation in evapotranspiration rates
Small, frequent storms — which account for the majority of Florida’s annual rainfall events — are most likely to be fully retained. Large, infrequent events may exceed the available storage capacity, resulting in surface overflow that bypasses the retention function.
Common Retention BMP Categories
The Florida stormwater program recognizes a range of retention BMPs that differ in their physical configuration, the types of land uses they serve, and their infiltration mechanisms. The primary categories include:
Contrasting Detention BMPs
For comparison, detention BMPs include wet detention ponds (the most common BMP in Florida’s regulatory program), dry detention basins, and offline storage vaults. These systems:
- Retain water temporarily but ultimately discharge to surface water bodies
- Provide pollutant reduction primarily through settling and biological uptake within the permanent pool
- Do not eliminate the discharge volume — they modify its timing and peak rate
- Are typically credited with a fixed percentage removal efficiency (e.g., 80% TSS removal for a properly sized wet detention pond)
Key Takeaway
Retention BMPs eliminate the surface discharge pathway for the treatment volume; detention BMPs delay and attenuate it. Both serve legitimate roles in stormwater management, but retention systems offer a fundamentally different — and often higher — level of pollutant load reduction, particularly for the smallest, most frequent storm events that deliver the majority of annual pollutant loads.
2. Common Retention BMP Types
Physical configurations and design variants for each major retention BMP category
Retention Basins
A retention basin (also called a dry retention pond or percolation pond) is a shallow impoundment designed with no outlet structure other than an emergency overflow. Its entire function depends on infiltrating the design treatment volume into the native soil before the next storm event. Key design considerations include:
- Bottom and side slopes must be stabilized with vegetation or erosion-resistant material
- Minimum infiltration rate in the receiving soil must be sufficient to drawdown the treatment volume within the required time window (typically 72 hours)
- Basin must remain dry between design storm events to preserve storage capacity
- Side slopes are typically 4:1 (horizontal:vertical) or flatter for mowing access
Florida Context
Retention basins are well-suited to Florida’s sandy soils, which typically exhibit high hydraulic conductivity. In areas with high seasonal water tables, available storage depth is constrained, and the effective volume may be significantly less than the physical basin volume.
Rain Gardens (Bioretention)
Rain gardens are shallow, planted depressions that capture runoff from adjacent impervious surfaces and allow it to slowly infiltrate through an engineered soil media. They combine the benefits of infiltration, evapotranspiration, and biological uptake of pollutants by plant roots and soil microorganisms.
Typical rain garden components include a surface ponding zone (maximum 6–12 inches), a mulch layer for pollutant adsorption and moisture retention, an engineered planting media (often a sandy loam mix), and optionally an underdrain if native soils have insufficient permeability. Where an underdrain is present, the system transitions from a fully retentive BMP to a partial-retention/partial-detention hybrid.
Roadside Swales with Check Dams or Raised Inlets
A standard vegetated roadside swale functions primarily as a conveyance channel, moving stormwater from the road surface to a downstream outfall. However, with relatively minor design modifications, a swale can be converted into a retention BMP that achieves meaningful volume reduction:
Design Modification 1 — Check Dams
Low rock or concrete weirs placed across the swale channel at regular intervals create a series of small ponding pools. Water backed up behind each check dam has additional time to infiltrate before overtopping and moving downstream. Check dams increase both residence time and infiltration opportunity.
Design Modification 2 — Raised Inlets
By elevating the inlet elevation of the downstream pipe or curb cut, a small permanent or temporary pool is maintained in the swale between storm events. Runoff must fill this pool volume before it can exit the swale, forcing a portion of each event to infiltrate rather than discharge. The raised inlet functions as an in-swale retention volume.
Exfiltration Systems
Exfiltration systems (also called French drains or exfiltration trenches) capture runoff in a subsurface perforated pipe or aggregate-filled trench. Water percolates outward through the aggregate and into the surrounding native soil. These systems:
- Are typically installed along the edge of parking lots or beneath roadways
- Receive runoff from surface inlets that direct flow into the trench
- Depend entirely on the hydraulic conductivity of the native soil for their function
- Are vulnerable to clogging by fine sediments if pre-treatment is not provided
- Require periodic maintenance (jetting) to restore hydraulic function over time
Underground Storage Systems
Underground storage systems use buried chambers, vaults, or large-diameter pipes to capture stormwater runoff in the subsurface. Unlike exfiltration trenches, these systems may be designed with or without connections to the native soil:
- Open-bottom chambers allow the stored volume to infiltrate into the underlying soil — functioning as true retention BMPs
- Sealed chambers hold water temporarily for later reuse (e.g., irrigation harvesting) or slow release through an orifice — functioning as detention BMPs
- Underground systems preserve surface area for other land uses, making them particularly attractive for high-density urban sites where above-grade retention is not feasible
- Maintenance access is more difficult than surface systems and requires scheduled inspections
Permeable Pavement with Reservoir Base Course
Permeable pavement systems allow rainfall to pass through the surface layer and accumulate in a granular reservoir base course below. The stored volume then infiltrates into the native subgrade over time. This BMP type is addressed in greater detail in Section 4 of this module; its key structural characteristic relevant here is the layered system:
- Permeable surface layer — the visible pavement (pervious concrete, porous asphalt, or interlocking pavers)
- Filter course — intermediate aggregate layer providing structural transition and filtering
- Reservoir base course — large-void aggregate (typically #57 or #89 stone) storing the treatment volume
- Native subgrade — receives infiltrating water; hydraulic conductivity governs system performance
Swale Conversion: Loss of Retention Function
A critical design caution concerns the inadvertent conversion of a retention-functioning swale to a purely conveyance channel. This commonly occurs when a drainage improvement project:
- Replaces a vegetated swale with a concrete-lined channel to increase hydraulic capacity
- Lowers the outlet invert elevation, eliminating the ponding depth behind a raised inlet
- Removes existing check dams without replacement
- Installs a longitudinal pipe below the swale that intercepts subsurface flow before it reaches the native soil
Design Caution — Swale Conversion
When a swale that was previously credited as a retention BMP is modified such that it no longer provides infiltration of the treatment volume, the project may trigger a re-evaluation of stormwater treatment credits for the site. The loss of retention function must be compensated by an equivalent retention or treatment credit elsewhere on the site or through an approved offset mechanism.
3. Swale Performance & Design
Hydraulic behavior, treatment performance, design requirements, and research findings for vegetated swales
Swales as Dual-Function BMPs: Conveyance and Treatment
Vegetated swales occupy a unique position among stormwater BMPs because they simultaneously serve two engineering functions: runoff conveyance (moving water from one location to another) and stormwater treatment (reducing pollutant concentrations through infiltration, filtration through vegetation, and settling of particulates).
The treatment function is primarily a result of the slow, shallow flow through dense vegetation — particularly when the swale is operating at low to moderate flow depths where the water surface is below the vegetative canopy. Under these conditions, runoff is forced to travel through the grass mat, which physically filters particles and slows the flow velocity sufficiently to allow settling.
Mechanism Summary
Swale treatment performance is driven by three mechanisms working in combination: (1) infiltration of a portion of the flow volume into the soil profile; (2) filtration of particulates and adsorbed pollutants through the vegetative mat; and (3) sedimentation of suspended solids as flow velocity decreases in the shallow, dense-grass environment.
Pollutant Concentration Reduction at Low Slopes
Research and field monitoring data consistently show that swale performance — measured in terms of pollutant concentration reduction — is most effective when the longitudinal slope of the swale is less than 1%. At slopes below this threshold:
- Flow velocities remain low enough to allow particulate settling throughout the swale length
- Ponding behind natural depressions or slight grade changes increases infiltration opportunity
- Residence time is maximized, allowing more complete contact between runoff and vegetation
- The risk of erosion and grass mat damage from high-velocity flow events is reduced
At slopes greater than 1%, flow velocities increase significantly, reducing residence time and the opportunity for settling and infiltration. Swales on steeper grades primarily function as conveyance channels with limited water quality benefit unless structural modifications (check dams) are installed.
Check Dams and Raised Inlets: Increasing Infiltration Volume
As introduced in Section 2, check dams and raised inlets are the primary structural tools for increasing the infiltration volume within an existing or proposed swale. Their function from a hydraulic standpoint is to create temporary or permanent ponding that:
- Stores a defined volume of runoff in the swale channel between storm events (or during small events)
- Forces that stored volume to infiltrate through the swale bottom and sides before the next storm arrival
- Converts a portion of the conveyance function back into a retention function
- Increases the effective treatment volume credited to the swale system
The volume of retention credit available from a swale with check dams or raised inlets depends on the spacing of the control structures, the swale geometry, and the ponding depth created. Each control structure creates an individual retention cell; the total credit is the sum of all cell volumes that can reliably infiltrate within the required drawdown time.
Design Constraints: Applicant’s Handbook Appendix O
Swale design for retention credit in Florida must comply with the requirements set forth in Appendix O of the Applicant’s Handbook for the applicable Water Management District. These requirements establish minimum design standards that must be met before a swale system can receive retention treatment credit. Key constraints include:
Appendix O Criterion — Slope
Longitudinal slope requirements that govern whether the swale can be credited as a retention system versus a conveyance channel, and whether check dam retrofits are required.
Appendix O Criterion — Drawdown Time
The design treatment volume must drawdown (fully infiltrate) within a specified period — typically 72 hours — to maintain capacity for the next design storm. Swales in low-permeability soils may not meet this criterion without soil amendments or underdrain systems.
Appendix O Criterion — Vegetation
Swales must be vegetated with a dense, low-growing grass cover appropriate for Florida’s climate. Bare areas, eroded sections, or invasive plant colonization can reduce treatment performance and may jeopardize compliance with the retention BMP credit.
Appendix O Criterion — Soil Separation from Seasonal High Water Table
A minimum separation distance between the bottom of the swale and the seasonal high water table (SHWT) must be maintained to ensure the unsaturated zone can accept infiltrating water. Where SHWT is high, the available storage and infiltration capacity are sharply reduced.
1982–83 Grassed Swale Performance Study
One of the foundational research studies supporting the use of grassed swales as retention BMPs in Florida was conducted in 1982–83. This field performance study monitored grassed swales under actual rainfall conditions and documented pollutant removal and volume reduction performance. Key findings from this study include:
- Grassed swales demonstrated measurable volume reduction for small to moderate storm events, with the volume reduction attributable directly to infiltration through the swale bottom
- Concentration reductions for common stormwater pollutants (total suspended solids, nutrients) were observed, with performance linked to the low-slope, vegetated conditions in the study swales
- Performance was highly variable depending on antecedent conditions, storm intensity, and the hydraulic loading rate to the swale
- Findings supported the use of grassed swales as a legitimate retention BMP in Florida’s regulatory framework, provided design criteria limiting slope and ensuring vegetation density were met
- The study provided the empirical basis for the design guidelines codified in Appendix O of the Applicant’s Handbook
Research Note
The 1982–83 study remains a cornerstone reference for swale performance in Florida’s regulatory literature. Its findings — particularly the performance relationship with longitudinal slope and vegetative cover — continue to inform current design standards. Practitioners should consult the original publication for specific numeric performance data when developing compliance documentation for swale-based retention systems.
4. Permeable Pavement Design
Structural system layers, long-term void space performance, design options, and site applicability
System Layer Configuration
A permeable pavement retention system is a multi-layer engineered structure in which each layer serves a distinct hydraulic and structural function. Understanding the role of each layer is essential for correct design, construction quality control, and long-term performance assessment.
Void Space Reduction Over Time
A critical long-term performance consideration for permeable pavement is the progressive reduction in void space within both the surface course and the reservoir base course due to sediment accumulation. Over the operational life of the facility:
- Fine particles carried in runoff are deposited within the void spaces of the aggregate as water infiltrates downward
- Surface clogging of the permeable pavement is the most common failure mode and is typically addressed through vacuum sweeping or pressure washing
- Deeper clogging of the reservoir base course is much more difficult to remediate — in severe cases, it may require excavation and replacement of aggregate
- Areas receiving runoff from adjacent unpaved areas or construction zones are at highest risk for accelerated clogging
Design Parameter — Sustainable Void Space
To account for long-term void space reduction, Florida design guidance estimates sustainable void space at 75% of the dry laboratory void ratio for the reservoir aggregate. This reduction factor is applied when calculating available storage volume for treatment credit. For example, if a #57 stone aggregate has a laboratory void ratio of 40%, the design storage volume should be calculated using an effective void ratio of 30% (0.75 × 40%).
Permeable Pavement Options for Small Areas
Permeable pavement is not limited to large-scale applications such as parking lots and roadways. Multiple product options exist for small-area applications, making the BMP accessible for residential sites, landscaped areas, sidewalks, and utility corridors:
Application on a Parcel Basis to Reduce Runoff
One of the most significant planning-level advantages of permeable pavement is its ability to be applied on a parcel-by-parcel basis as an alternative to conventional impervious surfaces. Rather than requiring a large centralized retention pond, a development can distribute its stormwater retention function across the impervious surfaces of the site itself — particularly parking areas.
This distributed approach offers several advantages:
- Runoff volume reduction occurs at the point of generation, minimizing the need for downstream conveyance capacity
- Land area otherwise required for a centralized retention basin is preserved for other uses
- The BMP footprint is the parking surface itself — no additional land area is consumed
- Particularly effective in redevelopment and infill scenarios where space for new stormwater infrastructure is constrained
- Qualifies for retention treatment credit when properly designed with an infiltrating reservoir base course meeting the applicable drawdown time requirement
5. Retention Efficiency Modeling
Methodology, data inputs, and results of the Harper and Baker (2007) continuous simulation study for FDEP
Background: The Need for Efficiency Estimates
While retention BMPs provide a clear hydraulic benefit — no surface discharge for the treatment volume — quantifying the annual pollutant load reduction achieved by retention requires an understanding of what fraction of the total annual runoff volume is actually retained by the BMP across the full spectrum of storm events that occur in a typical year.
This is not straightforward: a retention BMP sized for the 1-inch design storm will capture 100% of events smaller than that threshold but will allow overflow during larger events. The net annual efficiency depends on the statistical distribution of storm events at the site — how often small events occur versus large ones.
Harper and Baker (2007) Study for FDEP
The definitive study quantifying retention efficiency across Florida’s meteorological regions was conducted by Harper and Baker (2007) for the Florida Department of Environmental Protection (FDEP). This study developed retention efficiency tables that are now incorporated into Florida’s stormwater design guidance and used in regulatory compliance demonstrations.
Study Reference
Harper, H.H., and Baker, D.M. (2007). Evaluation of Current Stormwater Design Criteria within the State of Florida. Prepared for the Florida Department of Environmental Protection, Tallahassee, FL. This study remains the primary source document for retention efficiency values used in Florida regulatory practice.
Continuous Simulation Using SCS Curve Number Methodology
The Harper and Baker study employed a continuous simulation approach rather than a single-event or frequency-based method. Continuous simulation processes actual historical hourly precipitation records through a hydrologic model to generate runoff volumes for every storm event in the period of record, then aggregates the results to determine annual loads and retention fractions.
The runoff generation component used the SCS (Soil Conservation Service) Curve Number methodology, which relates runoff depth to rainfall depth as a function of the land surface’s curve number (CN). The CN is a dimensionless index that reflects land cover, soil type, and antecedent moisture conditions — with higher CN values indicating greater runoff production per unit of rainfall.
Methodology Note — Continuous Simulation
The continuous simulation approach captures the temporal variability of rainfall — the sequence of wet and dry periods, the clustering of events, and the variation in storm size — that is lost in single-event design calculations. This is especially important for retention systems, where performance depends critically on the recovery (drawdown) of available storage between events.
Meteorological Data: 45 Sites, 30–50 Years of Record
The study utilized hourly precipitation data from 45 Florida meteorological sites, with records spanning 30 to 50 years of continuous observation. This large dataset ensures that the simulated results are statistically representative of long-term Florida rainfall patterns rather than any particular wet or dry period.
- Hourly data resolution was required to accurately capture storm event timing and inter-event dry periods relevant to BMP recovery
- Sites were distributed across the state to represent the geographic variation in Florida’s rainfall regime
- 30–50 years of record provides a sufficient sample size to represent rare events while averaging out inter-annual variability driven by ENSO and other climate cycles
Simulation Parameters: DCIA and Curve Number Matrix
To generate efficiency tables applicable across the full range of land development conditions encountered in Florida, the study simulated a comprehensive matrix of site conditions:
Directly Connected Impervious Area (DCIA) is the fraction of a site’s total area that is impervious and hydraulically connected to the drainage system — meaning runoff from these surfaces flows directly to a collection point or outfall without first passing over a pervious area where infiltration could occur. DCIA is the dominant variable controlling runoff volume and pollutant loading from developed sites.
The non-DCIA curve number characterizes the hydrologic behavior of the remaining site area — the portions that are either pervious (landscaping, open space) or impervious but hydraulically disconnected (rooftops draining to lawns, for example). The wide range of CN 30–98 simulated in the study ensures the results are applicable to the full spectrum of Florida land use types, from low-density residential with generous open space to high-density urban commercial.
Five Florida Meteorological Rain Regions
Analysis of the 45-station dataset revealed that Florida’s rainfall regime is not uniform across the state. The study identified five meteorological rain regions with sufficiently distinct characteristics to warrant separate efficiency tables:
- Each region is characterized by differences in annual rainfall totals, storm frequency, storm intensity distributions, and seasonal patterns
- South Florida receives the most intense convective events, while the Panhandle experiences more frequent winter frontal systems
- The five-region framework allows design practitioners to apply the efficiency tables most representative of conditions at their specific project site
- Regional boundaries are mapped in the Harper and Baker (2007) report and in subsequent FDEP guidance documents
Treatment Volume Assumption: 100% Removal
A fundamental modeling assumption in the Harper and Baker study is that for any storm event whose generated runoff volume falls entirely within the available treatment volume of the retention BMP, 100% removal of pollutant mass is assumed. This assumption reflects the retention principle: if no surface discharge occurs, all pollutant mass carried in that volume remains on-site.
Modeling Assumption — Implications
The 100% removal assumption for events within the treatment volume is conservative in some respects (it does not credit partial removal for events that overflow) but appropriately captures the binary nature of the surface discharge pathway. Events that do not overflow contribute 100% of their pollutant load to the retained fraction; events that overflow contribute zero removal credit in the model. The annual efficiency is then the probability-weighted average across all events in the simulation period.
The resulting efficiency tables from Harper and Baker (2007) provide design practitioners with annual load reduction percentages as a function of:
- The selected treatment volume (expressed as inches over the DCIA)
- The site’s DCIA percentage
- The non-DCIA curve number
- The applicable Florida rain region
These tables allow a designer to determine, for a given site configuration and proposed retention BMP sizing, what annual pollutant load reduction credit can be claimed for regulatory compliance purposes — without performing a site-specific continuous simulation.
6. Modeling Assumptions & Results
This section addresses the core modeling assumptions used to derive retention efficiency estimates and summarizes the results produced across Florida’s rain zones. The assumptions govern how volume, nutrient retention, drawdown timing, and curve number inputs interact to determine overall BMP performance.
Volume and Nutrient Retention Relationship
A foundational assumption of the modeling framework is that the volume of stormwater retained equals the mass of total phosphorus (TP) and total nitrogen (TN) retained. This proportionality simplification means that efficiency calculations based on volumetric retention directly translate to pollutant load reductions without requiring separate constituent modeling.
Core Assumption
Volume retained = Mass of TP retained = Mass of TN retained. Volumetric retention efficiency directly equals pollutant removal efficiency for both nitrogen and phosphorus.
Drawdown Timing Assumptions
The model uses a two-stage drawdown schedule to characterize how quickly the retention system empties between storm events:
- 50% drawdown occurs within 24 hours of the storm event
- 100% drawdown (full recovery) occurs within 72 hours
This schedule reflects typical infiltration-based retention system performance and is critical for determining the system’s available capacity before the next rainfall event arrives.
Accelerated Drawdown Scenario
When drawdown is accelerated to 36 hours (full recovery), modeled efficiency improves by up to 3.5%. This sensitivity result demonstrates that drawdown rate is a meaningful design parameter, particularly in high-rainfall zones.
First Flush Assumption
The model assumes no significant first flush effect. The first flush phenomenon — in which the highest pollutant concentrations occur in the initial portion of runoff — is not incorporated into the efficiency calculations. This simplification is consistent with the assumption that pollutant concentration is uniform across the runoff volume.
Effect of DCIA and Non-DCIA Curve Number on Efficiency
Retention system efficiency is sensitive to both directly connected impervious area (DCIA) and the curve number (CN) of non-directly connected impervious area (non-DCIA). The key directional finding is:
Efficiency Trend
Efficiency decreases as DCIA increases and as non-DCIA curve number increases. Higher imperviousness and higher runoff potential in the non-DCIA portion both increase total runoff volume relative to the fixed retention capacity, reducing the fraction of runoff that can be captured.
Table Generation Across Rain Zones
To support design and permitting across Florida’s diverse rainfall climate, the modeling effort produced a comprehensive set of lookup tables:
- 80 tables were generated in total
- Tables span five rain zones across Florida
- Each table cross-references DCIA, non-DCIA CN, and retention depth to yield an efficiency value
- These tables form the backbone of the BMPFast software tool described in Topic 8
7 – Regional Variability in Effectiveness
Retention BMP performance is not uniform across Florida. Rainfall patterns, storm frequency, and regional hydrology create meaningful differences in how well a given retention depth performs depending on geographic location. This section examines those regional differences and their implications for design standards.
Performance Efficiency Varies Across Florida
The same retention depth applied in different parts of the state will achieve different levels of pollutant removal efficiency. Florida’s five rain zones reflect gradients in annual rainfall volume, storm intensity, and inter-event dry periods — all of which influence how often and how fully a retention system recovers between storms.
Geographic Sensitivity
A fixed retention depth produces higher efficiency in drier regions (less frequent large storms, longer recovery periods) and lower efficiency in wetter regions where the system has less time to drain before the next storm arrives.
Design Criterion: 0.5-Inch Runoff vs. 1-Inch Rainfall
A key design comparison emerges from the modeling:
Design Criterion Finding
Designs based on 0.5 inches of runoff outperform designs based on 1 inch of rainfall as a sizing criterion. The runoff-based approach accounts for site-specific impervious fraction and soil conditions, resulting in more precisely sized systems that achieve higher capture efficiency per unit of retention volume.
Pensacola vs. Melbourne: A Regional Contrast
The contrast between Pensacola (Florida Panhandle) and Melbourne (central east coast) illustrates how dramatically regional rainfall differences affect design requirements:
- Pensacola requires a larger retention depth than Melbourne to achieve equivalent pollutant removal performance
- Pensacola’s higher annual rainfall and more intense storm events mean the retention system is taxed more frequently, requiring greater storage volume to maintain effectiveness
- Melbourne’s comparatively lower rainfall intensity allows smaller retention depths to achieve the same removal targets
Rain Zone: Higher annual rainfall
Required depth: Larger retention volume
Design challenge: Frequent storm loading limits recovery time
Rain Zone: Moderate annual rainfall
Required depth: Smaller retention volume
Design advantage: Longer inter-event recovery improves efficiency
Performance Against the 80% and 95% Removal Standards
The modeling results reveal that meeting stringent statewide removal standards is challenging under typical design depths:
- Most scenarios fail to meet the 80% removal standard under common design depths, underscoring the need for careful site-specific sizing
- Larger retention depths are needed statewide to achieve 95% removal — the higher standard applicable to Outstanding Florida Waters and other sensitive receiving bodies
- These findings reinforce that default or minimum sizing provisions are insufficient for high-quality water body protection
Regulatory Implication
Because most scenarios fail the 80% removal threshold at typical depths, practitioners must use the efficiency tables (or BMPFast) to verify that the proposed retention depth achieves the project-specific removal target — not simply rely on a rule-of-thumb sizing criterion.
8. BMPFast Software Application
BMPFast is a software tool developed to operationalize the Harper and Baker efficiency table framework, enabling practitioners to calculate retention BMP performance quickly, accurately, and consistently across Florida’s rain zones without manual table lookups or interpolation by hand.
Theoretical Foundation
BMPFast is built directly on the Harper and Baker tabular efficiency relationships. Rather than replacing or approximating the underlying methodology, the software implements the same lookup table structure produced by the continuous simulation modeling, preserving the rigor of the original analysis while making it accessible to users who do not need to work with raw tables.
Regulatory Reference
BMPFast is referenced in the Applicant’s Handbook Volume 1, Appendix O. This reference provides official guidance on when and how the tool should be used in the permitting context.
Required Inputs
The software requires five inputs to determine retention efficiency for a given site and design scenario:
- Rain zone — identifies which set of the 80 tables applies to the project location
- Annual rainfall — used to select or interpolate between rain-zone-specific table sets
- DCIA (Directly Connected Impervious Area) — expressed as a percentage of the total drainage area
- Non-DCIA curve number (CN) — characterizes the runoff potential of pervious and disconnected impervious portions of the site
- Retention depth — the proposed water quality volume expressed as depth over the drainage area
Interpolation Capability
One of BMPFast’s core technical contributions is its interpolation engine:
- The software performs interpolations within tables — resolving efficiency values for input combinations that fall between discrete table entries
- It also performs interpolations between tables — bridging across rain zone boundaries and annual rainfall values not explicitly tabulated
- This two-dimensional interpolation capability eliminates the approximation errors that arise from manual nearest-neighbor lookups
Practical Benefit
BMPFast reduces calculation time and improves accuracy compared to manual table interpolation. For permit applications requiring efficiency documentation, the software provides a defensible, reproducible calculation trail consistent with the Applicant’s Handbook methodology.
Workflow Summary
Appendix — Key Terms Reference (Part 2 of 2)
Term
Volume = Mass Assumption
The modeling simplification that volume retained equals mass of TP and TN retained, allowing volumetric efficiency to stand in for pollutant removal efficiency.
Term
Drawdown Rate
The rate at which a retention system empties after a storm event. The model assumes 50% drawdown in 24 hours and 100% in 72 hours; accelerated drawdown to 36 hours improves efficiency by up to 3.5%.
Term
First Flush Effect
The phenomenon where the highest pollutant concentrations occur in the initial portion of runoff. Assumed absent (not significant) in the Harper and Baker modeling framework.
Term
DCIA (Directly Connected Impervious Area)
Impervious surfaces whose runoff flows directly to the stormwater system without passing over pervious areas. As DCIA increases, retention BMP efficiency decreases.
Term
Non-DCIA Curve Number (CN)
A runoff parameter for the portion of the drainage area not directly connected to the conveyance system. Higher CN values indicate greater runoff potential and lower retention efficiency.
Term
Rain Zones
Five geographic zones across Florida used to classify rainfall climatology for BMP sizing. Each zone has its own set of efficiency tables; performance differs significantly between zones.
Term
0.5-Inch Runoff Criterion
A retention sizing standard based on capturing 0.5 inches of site-generated runoff. Shown to outperform the simpler 1-inch rainfall criterion in terms of achieved removal efficiency.
Term
80% and 95% Removal Standards
Regulatory thresholds for pollutant (TP, TN) removal efficiency. Most scenarios fail 80% at typical depths; achieving 95% statewide requires substantially larger retention depths.
Term
BMPFast
Software implementing the Harper and Baker tabular efficiency framework. Accepts rain zone, annual rainfall, DCIA, non-DCIA CN, and retention depth as inputs; performs within- and between-table interpolation to output removal efficiency.
Term
Applicant’s Handbook Vol. 1, Appendix O
The official Florida regulatory reference that documents the BMPFast tool and the Harper and Baker methodology for retention BMP efficiency calculations in the permitting process.
Term
Table Interpolation
The mathematical process of estimating efficiency values between discrete table entries. BMPFast performs both within-table and between-table interpolation, improving accuracy over manual lookup methods.