DCIA Definition, Curve Number Methods, and BMPFast Project Modeling

Stormwater BMP Training Series  |  Applicant’s Handbook Vol. 1, 2024  |  Topics: DCIA Standards, CN Infiltration, Composite CN, BMPFast Setup & Results

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1 · DCIA Definition & Standards

Topic 1  ·  Source slides 1, 2, 3  ·  Applicant’s Handbook Vol. 1, 2024

What Qualifies as Non-DCIA?

An impervious surface may be classified as non-DCIA (i.e., “disconnected”) when runoff passes across a pervious strip before reaching a conveyance. The minimum required pervious flow path depends on the Hydrologic Soil Group (HSG) of the receiving soil:

Demonstrating a Narrower Width

The Applicant’s Handbook allows a site-specific demonstration that a pervious strip narrower than the default distance is sufficient to achieve disconnection. This analysis must be conducted using the 3-year / 1-hour design storm and must show that the pervious buffer fully infiltrates or substantially attenuates runoff from the contributing impervious area under those conditions.

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2 · CN Method & Soil Infiltration

Topic 2  ·  Source slides 2, 4, 7, 10  ·  SCS/NRCS Curve Number methodology

The S′ Storage Equation

The SCS/NRCS Curve Number (CN) method quantifies the maximum potential soil storage using the following relationship:

CN Values and Storage Capacity by Soil Type

The table below illustrates how CN values translate into storage depth and runoff behavior across representative soil types and land covers:

Land Cover / Soil Type	HSG	CN Value	S′ Storage (inches)	Notes
Open/Sandy soils, good cover	A	50	10.00	High infiltration; large storage
Moderate-permeability soils	C	78	2.82	Limited storage; more runoff
Impervious surfaces	—	95	0.53	Runoff begins at only 0.1 in. abstraction
Compacted HSG A soil	A (compacted)	~55	~8.18	~10% CN increase due to compaction

Key CN Behavior Notes

• CN = 95 is the standard value applied to impervious surfaces. With such a high CN, runoff generation begins almost immediately — after just 0.1 inch of initial abstraction — meaning virtually all rainfall becomes runoff.
• A CN of 50 (HSG A soil) stores 10 inches of rainfall before any runoff is generated, making HSG A soils highly effective for stormwater disconnection.
• Soil compaction from construction activity can raise CN values by approximately 10 percentage points (e.g., CN 50 → CN 55), meaningfully reducing infiltration capacity and increasing runoff volume.
• For HSG A soils, a 10-foot pervious strip can effectively disconnect approximately 32 feet of contributing impervious width under the design storm criteria.

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3 · Composite CN Calculation

Topic 3  ·  Source slides 7, 9, 10  ·  Volume-based composite CN methodology

Why Disconnection Affects the Composite CN

When impervious area is disconnected from a conveyance system, its runoff is routed across a pervious buffer. In the CN modeling framework, this changes how the impervious area’s runoff is accounted for — it no longer flows directly to the outlet but instead contributes to the effective CN of the pervious drainage area that receives it. This causes the composite CN of the pervious area to increase, reflecting the combined infiltration demand placed on the soil.

Worked Examples: Disconnecting 1.1 Acres

Volume-Based vs. Simple Average: Why It Matters

In the HSG A example above, the correct volume-based composite CN is 83.81. Using a simple area-weighted average of the impervious CN (95) and pervious CN (50) produces 86.5, which appears higher — but this is misleading. The volume-based method properly accounts for the runoff generation dynamics of each cover type; a simple average fails to reflect the actual storm runoff depth and will produce incorrect load estimates.

• The composite CN is sensitive to the fraction of DCIA: as more impervious area is disconnected, the effective CN of the receiving pervious area rises, capturing the increased hydraulic loading on that soil.
• The composite CN is also sensitive to pervious area size: a smaller pervious receiving area must absorb the same disconnected runoff and therefore carries a higher effective CN.
• Regulators and review engineers will reject simple or area-weighted CN averages in ERP submissions — always use the volume-based composite approach.

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4 · BMPFast Example Project Setup

Topic 4  ·  Source slides 5, 6, 11, 12  ·  BMPFast software configuration walkthrough

Site Parameters

The following example site is used throughout this module to demonstrate BMPFast configuration and results interpretation. All areas and CN values should be confirmed against site-specific soils mapping and impervious cover delineation before use in a permit application.

BMP Configuration: Rain Garden

Catchment Topology and BMP Routing

In BMPFast, the catchment topology must be explicitly defined to correctly represent the flow path from each land cover type to the outlet. Key configuration steps include:

• Define the catchment areas separately: DCIA (0.2 ac), disconnected impervious (1.1 ac), and pervious (0.7 ac) must be entered as distinct contributing areas with their respective CN values.
• Set the routing topology: The disconnected impervious area routes to the pervious buffer first. The pervious buffer and DCIA both route toward the outlet (or BMP inlet).
• Assign the BMP to the correct position in series: BMPFast allows multiple BMPs in series within a single catchment. The rain garden should be positioned to receive the combined runoff from the disconnected area before it reaches the surface discharge point.
• Verify the BMP media properties: Bold & Gold media has defined hydraulic conductivity and nutrient removal characteristics built into BMPFast’s library. Confirm the version matches current Florida Department of Environmental Protection approved specifications.

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5 · Results: Disconnecting DCIA Impact

Topic 5  ·  Source slides 13, 14, 15  ·  BMPFast annual runoff and pollutant load outputs

Annual Runoff Volume Reduction

Total Nitrogen Load Reduction

Scenario Comparison Table

Scenario	Annual Runoff (in/yr)	TN Load (kg/yr)	BMAP Target Achievable?
Baseline — all impervious connected (DCIA)	28.24	10.27	Unlikely without BMP
Disconnected — 1.1 ac non-DCIA, no BMP	16.97	6.18	Possible (site-dependent)
Disconnected + Rain Garden (0.15 ac, 2 ft)	1.91	—	Yes, with BMP
Connected (DCIA) + Rain Garden	—	—	Yes, with properly sized BMP

Key Findings and Reporting Metrics

• Disconnecting 1.1 acres of impervious area reduces annual runoff by approximately 40% (from 28.24 to 16.97 in/yr) on HSG A soils — without any engineered BMP.
• Adding the 0.15-acre rain garden with Bold & Gold media reduces annual runoff by a further 89% (to 1.91 in/yr), demonstrating the compounding benefit of structural BMPs after disconnection.
• Total nitrogen load is reduced from 10.27 to 6.18 kg/yr by disconnection alone — a reduction of approximately 40% — which may be sufficient to meet BMAP discharge load targets on some permitted sites.
• Both the connected-with-BMP and disconnected-with-BMP scenarios can meet performance criteria; the optimal approach depends on site constraints and the applicable load allocation in the BMAP.
• BMPFast reporting outputs should include surface discharge mass (kg/yr per pollutant) and surface discharge volume (acre-feet/yr or inches/yr) as the primary metrics for permit submittal.

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Appendix · Quick-Reference Cards

Summary reference cards for field use, design review, and permit submittal preparation.

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