Structural BMP Performance in 2NFORM

This article explains how 2NFORM models structural BMP performance—how design, condition, and maintenance affect runoff and pollutant reductions—and how results support compliance.

In this article:

• Types of Structural BMPs Included

• What Data Goes In

• How Performance Is Estimated

• How This Affects Modeled Reductions

• Why It’s Useful 

Types of Structural BMPs Included

Structural BMPs—such as bioretention cells, infiltration basins, and wet ponds—are long-term investments to reduce runoff and remove pollutants. 2NFORM uses TELR to reflect both design capacity and changing condition over time, turning your inventory into dynamic performance accounting for maintenance planning, funding, and compliance. 

2NFORM accounts for a wide range of stormwater control measures, including decentralized BMP types such as:

• Bioretention or rain gardens
• Infiltration basins and trenches
• Permeable pavement systems
• Green roofs

And larger regional treatment systems or centralized BMP types such as:

• Dry basins
• Treatment vaults
• Wet ponds
• Infiltration basins

Explore the comprehensive types of 2NFORM structural BMPs in more detail.

What Data Goes In

Structural BMP performance is estimated using the a collection of inputs:

• Design specifications (e.g., drainage area, BMP type, treatment capacity and other details relevant to the specific BMP type).
• Installation or construction completion dates (assumed to begin in optimal condition).
• Condition assessments entered through your team's field inspections.
• Maintenance or remediation activities as documented by your team (e.g., media replacement, vegetation restoration, sediment removal). 

How Performance Is Estimated

TELR performs hydrologic accounting and applies nationally derived treatment effectiveness, modified by most recent condition. Centralized systems receive more detailed routing/treatment representation.

• Performance decline curves. Effectiveness naturally decreases over time (e.g., clogging, vegetation stress, media exhaustion).

• Condition-based adjustments. Field assessments directly update modeled performance ensuring your inspection data improves accuracy.

• Maintenance resets.  Remediation activities restore BMPs to higher performance levels, extending their useful life and impact.

How This Affects Modeled Reductions

By incorporating condition and maintenance, 2NFORM:

• Provides realistic current performance instead of assuming BMPs always operate at design capacity.
• Highlights the difference between baseline, current, and potential scenarios, showing how maintenance or planned BMPs shift pollutant reductions.
• Tracks performance over time using reliable defensible methods. 
  
  

Why It’s Useful

• Asset management. Connect inspections and maintenance directly to modeled water-quality benefits.

• Budgeting. Quantify ROI of implementing and maintaining effective BMPs.

• Planning. Makes estimating the added benefit of future stormwater BMP investments easy for immediate inclusion into any funding requests. 2NFORM potential scenarios can be the foundation of required watershed management plan water quality modelling requirements. 

Compliance Note

Structural BMPs are often central to TMDL implementation. With 2NFORM, you can:

• Show each BMP’s contribution to reducing pollutant loads relative to baseline.

• Demonstrate how your program portfolio moves the MS4 toward its TMDL objectives.

• Provide regulators a transparent accounting system that links condition and maintenance directly to compliance outcomes.

If you’d like to talk methods with our science team, contact Support and we’ll connect you.