Sewer Monitoring Series · Water Environment Federation · 2025
Hydraulic Model Calibration Using Real-Time Sewer Flow Monitoring
Avoiding a $42M Capital Error Through Data-Driven Engineering
“When the model said we were at capacity, the only defensible answer was to measure it.”
Sanitary sewer flow monitoring measures flow rates and rainfall within a wastewater collection system to characterize Rainfall-Derived Infiltration and Inflow (RDII). When hydraulic models are calibrated with field-measured RDII data instead of generic estimates, capital improvement programs become more accurate, better targeted, and significantly less expensive.
Abstract
This case study documents a comprehensive flow monitoring program for a mid-size municipality whose hydraulic model predicted imminent capacity failure requiring $42 million in trunk sewer replacement. A six-month monitoring program demonstrated that the model was overestimating peak flows by 65% because RDII parameters had been derived from generic assumptions instead of field measurements.
After recalibration with measured RTK parameters, the recommended capital program shifted from broad trunk replacement to a $10 million targeted rehabilitation plan. The core lesson is simple: when capacity decisions depend on a model, the model needs defensible field data.
Background
Municipal collection system engineers often make capital improvement decisions based on model predictions, population projections, assumed diurnal patterns, and regional RDII averages. Those assumptions are useful for screening, but they can trigger major capital recommendations before the system has been measured under real operating conditions.
The whitepaper describes a system with more than 1,100 miles of sanitary sewer main, recurring SSO concerns, wet-weather plant loading, older gravity mains, and limited permanent metering. In that context, measured flow and rainfall data became the difference between assumption-based spending and data-driven prioritization.
Methodology
The monitoring network was designed around specific calibration questions: how much RDII entered each sewershed, when peak surcharge conditions occurred, and whether the dominant sources were public defects, private property connections, or operational constraints.
Meters were deployed at trunk interceptors, sub-sewershed outlets, and focused investigation areas. Rain gauges were paired with the network so RDII response could be compared against actual rainfall intensity, timing, and distribution.
Results
The monitoring data showed that the original model overstated peak flows by 65%. Field-measured RDII decomposition changed the engineering interpretation: a large share of the problem came from fast inflow sources that pipe replacement alone would not address.
The recalibrated model indicated that targeted private property I/I abatement, CIPP lining in priority reaches, manhole rehabilitation, confirmed point repairs, and selected pump station upgrades could maintain hydraulic compliance at far lower cost.
Engineering Significance
Model calibration is engineering due diligence. A hydraulic model is only as useful as the RDII parameters behind it, and field-calibrated RTK parameters turn a screening model into a defensible decision tool.
RDII decomposition also determines the solution type. Fast inflow, delayed inflow, and groundwater infiltration require different corrective actions, so measuring the response protects agencies from spending money on the wrong fix.
Conclusion & Next Steps
Municipal engineers should use sewer flow monitoring when capacity, RDII, SSO risk, or model calibration decisions require measured evidence. The downloadable PDF includes the full formatted whitepaper for offline review and sharing.