How to Design an I/I Study for a Collection System
Designing an infiltration and inflow (I/I) study comes down to five decisions: how to divide the system into meterable sub-basins, how many flow monitors and rain gauges to deploy, how long to monitor, what conditions to capture, and how to separate rainfall-derived flow from base sanitary flow. A defensible study typically places a flow monitor at each metershed outlet, uses local rain gauges, captures a dry-weather baseline and multiple qualifying storms, and relies on true area-velocity flow data.
Five-Step Study Framework
Design the study around the decisions the data must support
Metershed boundaries, instrument placement, rainfall coverage, monitoring duration, and RDII analysis should work as one integrated design.
Define metersheds
Divide the system into hydraulically measurable sub-basins.
Set monitor count
Place monitors where basin contributions can be separated.
Cover rainfall
Use local gauges that represent monitored basins.
Capture conditions
Monitor long enough for baseline and storm events.
Analyze RDII
Separate slow infiltration from rapid inflow response.
Divide the System into Metersheds
An I/I study identifies where the problem is only when flow can be isolated by area. Break the collection system into metersheds—sub-basins whose flow can be captured by a downstream monitor. Draw boundaries at natural hydraulic divides such as trunk-line confluences, lift-station basins, and sub-trunk outlets using GIS, sewer atlases, and available record drawings. Smaller, more homogeneous metersheds improve attribution but require more monitoring locations.
Decide How Many Flow Monitors Are Needed
There is no universal meter count. The number is driven by the sub-basins that must be isolated and the precision required for rehabilitation decisions. A common starting point is one monitor per metershed outlet, with additional monitors where upstream and downstream contributions must be separated. When budgets are limited, prioritize older pipe, high-groundwater areas, known wet-weather surcharge locations, and basins with previous overflow history.
Place Rain Gauges Locally
Rainfall varies over short distances, especially during convective storms. Regional and airport gauges may not represent the rainfall that actually reached a monitored basin. Use local, study-specific rain gauges distributed across the project area. A planning density of roughly one gauge per 3–5 square miles is common, with additional gauges where topography or storm patterns create strong spatial variability.
Set the Duration and Target Conditions
Run the study long enough to capture both a representative dry-weather baseline and multiple qualifying wet-weather events. Several weeks of typical diurnal patterns help establish base sanitary flow and groundwater infiltration. RDII analysis commonly benefits from three to five storms spanning different depths and intensities. In many programs, this means 60–90 days timed to the wet season, although the final duration should reflect climate, regulatory requirements, and project objectives.
Separate Infiltration from Inflow
With measured flow and local rainfall, decompose each metershed hydrograph. Infiltration generally appears as a slower, persistent increase in baseline and minimum-night flow associated with groundwater conditions. Inflow generally appears as a rapid flow response closely following rainfall. Quantifying rainfall-derived inflow and infiltration by basin turns raw monitoring data into rehabilitation priorities and model inputs.
Why True Flow Data Matters
I/I decisions depend on measured volume, not depth alone. Wet weather can create backwater and surcharge—the same conditions in which a depth-only conversion becomes unreliable. Use area-velocity measurement that records both depth and velocity. See Level vs. Flow Monitoring and the FlexFlow IQ product page.
Common Mistakes That Weaken an I/I Study
- Using too few monitors to isolate the basins that must be prioritized.
- Relying on regional rainfall records instead of local rain gauges.
- Ending the study after a single storm or monitoring only during dry months.
- Using level-only data to estimate wet-weather volume under surcharge or backwater.
- Selecting sites with turbulence, deposition, unstable hydraulics, or poor access without accounting for those conditions.
Frequently Asked Questions
I/I Study Design FAQ
How many flow meters does an I/I study need?
There is no fixed number; it is driven by how many sub-basins you need to isolate. The rule of thumb is one monitor per metershed outlet, plus monitors wherever you need to separate upstream from downstream contributions. Prioritize suspected high-I/I basins when budget is limited.
How long should an I/I study run?
Long enough to capture a representative dry-weather baseline and multiple qualifying storms, commonly 60–90 days timed to a wet season. RDII analysis needs three to five rain events of varying intensity to be defensible.
What rainfall data does an I/I study need?
Local, study-specific rain gauges co-located with the monitored basins, often one per 3–5 square miles. Regional or airport gauges miss the spatial rainfall variability that drives RDII.
How do you separate infiltration from inflow?
Infiltration shows as elevated baseline and minimum night flows that track groundwater; inflow shows as sharp, short spikes coincident with rainfall. RDII analysis decomposes each metershed's hydrograph against the local rainfall record.
Can I use level-only data for an I/I study?
No. I/I quantification requires defensible volumes, and wet-weather surcharge and backwater break the depth-to-flow relationship. Use area-velocity flow measurement.
Plan an I/I Monitoring Program
US3 can support metershed planning, site selection, flow monitoring, rainfall monitoring, and RDII analysis.
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