If you work in the water resources industry, you are bound to come across inflow and infiltration analysis in sewers. Engineers who work with these systems have developed extensive processes and jargon for evaluating inflow and infiltration from flow metering data. This post provides an overview of inflow and infiltration and describes common analysis tools that are used in this field.

Components of Flow

Inflow and infiltration (I/I) are components of flow in sewers that are generated from rainwater and groundwater. Inflow refers to the direct flow of rainwater into sewers from surface runoff.  Infiltration refers to the groundwater flow sources entering the sewer through cracks in the pipes, manhole defects, or other sources. Inflow responds faster to rainfall and infiltration responds slower. This is shown in the figure below of typical inflow and infiltration components developed from modeling tools in H2Ometrics.

Inflow and infiltration figure

Inflow and Infiltration Components

In separate sewer systems, with separate pipes for stormwater and sanitary sewerage, inflow and infiltration are considered extraneous flow sources to be minimized. For this reason, they are often lumped together and abbreviated as “I/I”. It is common for peak flows in a sewer system to reach 5 – 20 times the average flow during a large rain event due to I/I.

For analysis, the I/I flow is separated from the dry wether flow (DWF) in order to develop the rainfall dependent inflow and infiltration (RDII) hydrograph. The RDII hydrograph represents the runoff response from the system due to rainfall. This is useful for developing I/I metrics and performing modeling. The figure below shows the breakdown of these components from the H2Ometrics metrics tools. The RDII flow is separated from the total flow by subtracting an estimate of the dry weather flow during the storm event. An estimate of the dry weather diurnal flow can be developed from dry days in the metering data. H2Ometrics contains modeling tools to automate this process.

DWF and RDII Figure

RDII and DWF Breakdown

I/I Analysis

Once the flow components have been identified and separated, several metrics can be computed. These metrics are useful for comparing metering districts and assessing system performance over time. The most common metrics are described below:

  • Max flow – this is the peak total flow that occurred during the storm.
  • Maximum peaking factor (Max PF) – this is the ratio of the max flow to the average flow for the meter. This metric is unitless, which makes it useful to compare peak flow across different meter districts with varying sizes.
  • RDII Volume – this is the volume of the RDII flow.
  • Capture Coefficient (C%) – this is the percentage of rainwater that is captured by the system. It is computed by dividing the RDII volume by the rain volume. The rain volume is computed from a nearby rain gauge by multiplying the rain depth for the storm times the tributary area of the meter. This metrics is also unitless, which makes it useful to compare capture volumes across meter districts with varying sizes.

Example Spreadsheet

Here is a link to a spreadsheet that contains example formulas to compute the I/I metrics listed above. This spreadsheet is an example of the data that is exported from an H2Ometrics I/I analysis.

H2Ometrics I/I Tools

H2Ometrics has extensive tools for performing I/I analysis. The metrics computed can be customized to suit the needs of the user. H2Ometrics has metrics tables and export tools to send the results to a spreadsheet (example for download provided above) for incorporation into an engineering report. These tools make it possible to perform analyses in minutes that used to take hours or days of work in spreadsheets. Learn more here.