Introduction to Investigation Methodology
The effective management of inflow and infiltration (I&I) is a critical concern for civil engineers, land developers, and municipalities. Left unchecked, it can stress wastewater systems, increase treatment costs, and lead to flooding and environmental damage. Therefore, implementing a precise, data-driven methodology to identify and quantify I&I is essential.
This blog will explore the systematic approaches involved in determining the root cause of I&I issues and reducing their impact. Specifically, we’ll discuss the use of Failure Modes and Effects Analysis (FMEA), real-world examples of common failure types, and investigation strategies, including flow and rainfall monitoring and field inspections. By the end of this post, readers will gain actionable insights to enhance their I&I investigation methodology and processes.
Failure Modes and Effects Analysis
Failure Modes and Effects Analysis (FMEA) is a structured technique widely relied upon for investigating I&I issues. This methodology systematically evaluates systems and processes to identify potential vulnerabilities, their causes, and effects. It then prioritizes areas requiring inspection and repair based on their impact on the system.
Key review questions within the FMEA process include:
- How are steps in the process defined?
- What types of failure modes are possible?
- Why might these failures occur?
- What are the consequences of each failure?
- What are the root causes of each failure?
Implemented with engineering precision, this analysis enables investigators to focus limited resources on the most impactful failures. For example, in residential areas with separated storm and sanitary drainage systems, an FMEA framework can help pinpoint key engineered stormwater collection mechanisms connected to sanitary systems (inflow) and areas where water infiltrates through defects in sewer infrastructure (infiltration).
By establishing priorities, FMEA ensures targeted inspections that maximize efficiency in mitigating I&I.
Examples of Common Failure Mode Types
When conducting I&I investigations, certain failure modes frequently emerge. These common types illustrate how inflow and infiltration affect wastewater systems.
Common inflow types:
- Residential downspouts
- Sump pumps improperly connected to the sanitary system
- Foundation drains
- Basement walkout floor drains
- Reverse sloped driveway drains
- Surface water entering basement floor drains
- Missing cleanout caps
- Municipal catch basins connected to sanitary systems
- Cross-connections between storm and sanitary systems
- Open manhole pickholes in low-lying areas
Common infiltration types:
- Defects in manhole chimneys and walls
- Sewer pipe material deterioration
- Faulty sewer joints
- Root intrusion into pipelines leading to subsurface water infiltration
Understanding these failure types is essential for identifying priority areas for remediation and ensuring accurate quantification of I&I impacts.
Failure Modes and Effects Analysis – Inflow and Infiltration
| Failure Modes (I&I Type) | Failure Causes | Failure Effects | Inspection Approach |
| Residential Downspout (Inflow) | – Remnants of historical by-law, engineering standards or enforcement limitations – Lack of a cost-effective outlet to storm system – Improvement of property aesthetics – Incompetence | – Can quickly consume local sewer capacity – Cumulative contributions can inundate trunk sewers, pumping stations and treatment plants | – Drainage Inventory – Fog and Dye Testing |
| Basement Sump Pump (Inflow) | – Remnants of historical by-law, engineering standards or enforcement limitations | – Cumulative contributions can inundate trunk sewers, pumping stations and treatment plants | – Internal Plumbing Inspection – Questionnaire |
| Foundation Drain (Inflow) | – Remnants of historical by-law, engineering standards or enforcement limitations – Lack of a cost-effective outlet to storm system | – Cumulative contributions can inundate local sewers, trunk sewers, pumping stations and treatment plants | – Injection Dye Testing – Wet Weather Lateral CCTV – Plumbing Inspection |
| Basement Walkout Floor Drain (Inflow) | – Lack of a cost-effective outlet to storm system | – Effects relatively minor in isolation | – Fog and Dye Testing – Plumbing Inspection |
| Reverse Sloped Driveway Drain (Inflow) | – Lack of a cost-effective outlet to storm system | – Can quickly consume local sewer capacity | – Fog and Dye Testing |
| Surface Water Flooding Entering Basement Floor Drain (Inflow) | – Property located with risk of surface flooding | – Can quickly consume local sewer capacity | – Overland flow assessment |
| Missing Cleanout Cap (Inflow) | – No prescribed maintenance mandate | – Effects relatively minor in isolation | – Drainage Inventory – Fog and Dye Testing |
| Municipal Catch Basin (Inflow) | – Human Error | – Can quickly consume local sewer capacity | – Fog and Dye Testing |
| Municipal Cross-Connection Between Storm and Sanitary Systems (Inflow) | – Emergency interim solution that becomes permanent – Lack of a cost-effective alternative to relieve either the storm or sanitary at risk of surcharging | – Can quickly consume local sewer capacity | – Fog and Dye Testing |
| Open Manhole Pickhole (Inflow) – Normal Operation Conditions | – Unforeseen risk of surface flooding during sewer design | – Can quickly consume local sewer capacity | – Drainage Inventory |
| Open Manhole Pickhole (Inflow) – Low Lying Operation Conditions | – Misunderstanding of consequences | – Can quickly consume local sewer capacity | – Drainage Inventory |
| Manhole Chimney & Wall Defects (Infiltration) | – Inappropriate materials selection and/or installation | – Cumulative contributions can inundate trunk sewers, pumping stations and treatment plants | – Dry Weather Visual Inspections – Wet Weather Visual Inspections |
| Sewer Material Deterioration (Infiltration) | – Inappropriate materials selection and/or installation | – Cumulative contributions can inundate trunk sewers, pumping stations and treatment plants | – Dry Weather Visual Inspections – Wet Weather Visual Inspections |
| Sewer Joint Defects (Infiltration) | – Inappropriate materials selection and/or installation | – Cumulative contributions can inundate trunk sewers, pumping stations and treatment plants | – Dry Weather Visual Inspections – Wet Weather Visual Inspections |
| Sewer Root Intrusion- Dripping from Roots (Infiltration) | – Proximity to trees | – Cumulative contributions can inundate trunk sewers, pumping stations and treatment plants | – Dry Weather Visual InspectionsWet Weather Visual Inspections |
Inflow and Infiltration Investigation Strategy
An effective investigation methodology involves performing detailed sewer flow monitoring and rainfall monitoring followed by systematic field inspections. This multi-step strategy provides a comprehensive view of the problem and directs targeted action.
Flow and Rainfall Monitoring
Flow and rainfall monitoring form the foundation of effective I&I detection techniques. By simultaneously tracking precipitation data and sewer system performance, municipalities can determine:
- How much I&I occurs within specific catchments
- Which failure type (inflow vs. infiltration) is dominant
For example, significant inflow might cause immediate spikes in flow levels post-rainfall, whereas infiltration typically leads to gradual increases over time. Numerical models or engineering judgment are often used to analyze flow responses during rain events.
These insights guide the next phase—field inspections—ensuring resources are allocated effectively.
Field Inspections
Following monitoring, field inspections are conducted to locate specific sources of inflow or infiltration. Inspections are grouped by cost-effectiveness and implemented sequentially:
Group 1: Initial inflow and infiltration inspections (drainage inventory, fog testing, dye testing) focus on locating engineered stormwater collection features tied to the sanitary sewer (e.g., connected downspouts). These are low-cost fixes that can make a significant impact.
Group 2: If Group 1 doesn’t identify sufficient I&I sources, detailed fieldwork (wet weather visual assessments, subsurface soil and groundwater evaluations, etc.) is performed to uncover less obvious contributors.
Group 3: Site-specific investigations—such as lot grading surveys or storm sewer exfiltration tests—are deployed when needed to address persistent or complex issues.
This phased approach minimizes overall costs while maximizing impact, making it a best practice for municipalities.
Partner with Experts to Resolve I&I Challenges
Effectively managing I&I requires both expertise and collaboration. Civica Infrastructure Inc. specializes in identifying and mitigating I&I challenges through proven methodologies tailored to municipal and development-specific needs, leveraging wastewater system modelling and flood analysis.
Key Benefits of Working with Civica:
- Access to advanced analysis techniques like FMEA and numerical modelling
- Expert guidance on cost-effective inspection and remediation strategies for inflow and infiltration reduction.
- Proven track record in wastewater management system optimization.
What’s Next?
This post touched on the core aspects of I&I investigation methodologies. If you are eager to gain deeper insights into field investigation strategies, or need expert support for your municipality or project, stay tuned for part two of this series.
To learn how to implement these strategies—or to partner with us—please contact us today.
Learn more at:
Methodology of Sanitary Maintenance Hole Inflow and Infiltration
Role of Flow Monitoring in Inflow and Infiltration (I&I) (Part 1)
Flow Monitoring and Its Role in Inflow and Infiltration (I&I) (Part 2)
Case Study: Inflow and Infiltration Reduction of Pilot Areas (Part 1)
Case Study: Inflow and Infiltration Reduction of Pilot Areas (Part 2)
Inflow and Infiltration and Wet Weather Flow Reduction of Pilot Areas (Part 3)
