Case Studies for the Identification and Quantification of Sanitary Maintenance Hole Inflow and Infiltration (I&I)

Sanitary sewer capacity can be quickly overwhelmed during large rainfall or snowmelt events. Maintenance holes (MH) —even ones not in low-lying areas—can become submerged and produce an inflow of water to sanitary systems. Inflow and infiltration inspections are designed to assess sanitary sewer capacity analysis, giving you a more comprehensive picture of your wastewater asset management needs.

The case studies below show the effectiveness of dual drainage modelling in helping to account for MH inflow and therefore allow municipalities to implement solutions to reduce inflow and infiltration.

Case Study 1

The first case study involves a 32-hectare subdivision in southern Ontario. Using LiDAR and GPS data in tandem with air photography, storm inlet, and maintenance hole characteristics, the study built a dual draining model. Five maintenance holes (10% of MHs in study area) were identified as being prone to submergence during the model runs. This analysis and resulting inflows into the sanitary sewers occur when the storm inlets operate at 100% efficiency. In other words, since storm inlets are designed to remove storm water from the surface, the greater the storm inlet efficiency, the less surface water that should be available for sanitary MH inflows.

Some municipalities recognize that partial or complete blockage of storm inlets can occur on a regular basis and incorporate storm inlet blockage factors into their design criteria. The storm inlet blockages normally occur due to debris transported and deposited at inlet (catch basin) grates. A 50% blockage factor, typically used in some design standards, was also analyzed. Since blocking storm inlets leaves more water at the surface, the impact to sanitary inflows worsens, with higher peak flows and volumes entering the system.

Case Study 2

The second case study had a 100-hectare largely residential study area in the Greater Toronto Area as its subject. Similar data used in the first study was also leveraged here to build a dual drainage model. The results of the modelling exercise found 13 sanitary maintenance holes (7% of analyzed MHs in study area) became submerged during an approximately 25-year design storm.

In this scenario, the total I&I within the case study area resulted in 0.48 L/s/ha produced by these 13 submerged MHs and a total I&I volume of 260.5m3.

Case Study 3

A 45-hectare residential subdivision in the GTA predominately built in the mid-late 1950s was investigated as part of Case Study #3. LiDAR data was not available at the time of the study, so GPS survey data was collected for 141 cross-sections (over 500 data points collected) plus air photography, storm inlet, and MH characteristics, that were used to build an inlet-to-inlet dual-drainage model.

In this case study, the total I&I within the case study area resulted in 0.55 L/s/ha produced by the submerged MHs (24.7 L/s peak flow) during an approximately 25-year storm and also produced > 380 m3 of I&I volume. Over 50% of the MHs were modelled to have at least 50 mm of ponding water, with 18 MHs (18%) that had significant ponding (>100mm).

This type of exercise is useful for identifying and prioritizing the MHs with the largest I&I potential and highest return-on-investment after sealing.

Reasons to Perform Maintenance Hole (MH) Flood Testing

MH flood testing was conducted at all MHs identified in Case Study #1 to understand how much inflow was occurring at the modelled flood depths. MH flood testing was completed and compared to flow rate assumptions using the orifice equation and assuming pickholes as the only inlets. The result was an underestimation of MH inflow by approximately 60-260% for the locations in Case Study #1 if it was assumed that only the pickholes contributed to MH inflow. MHs sunken below the pavement will allow additional rainwater and snow melt to collect and provide increase the depth of submergence. Furthermore, deteriorated pavement around maintenance holes will produce fractures and gaps that the water can seep into and cause damage to maintenance holes. These can be quickly identified as part of routine MH inspection work.

Having such a large disparity between anticipated inflow and infiltration and the modelled amount could lead to severe consequences to wastewater assets if not proactively addressed. Dual drainage modelling and MH inspections provide the insights needed to make the necessary adjustments before issues occur.


The dual-drainage modelling methodology is a robust, repeatable method for identifying previously undetected inflow sources. These inflows add a large proportion of the total I&I during significant storm events into wastewater systems. With the right modelling tool, this approach for identification and quantification of sanitary sewer I&I is practical and can be applied on a system-wide basis.

Literature review shows that this approach is the best-in-class asset management method for MH I&I condition assessment. Municipal programs such as the City of Toronto Basement Flooding Studies and City of Windsor Sewer and Coastal Flood Protection Master Plan have performed similar analyses.

Based on the findings from multiple case studies, municipalities should implement a dual-drainage analysis approach to analyze existing areas for MH I&I and to verify that the storm sewer designs in new areas are actually capturing the five-year peak flows during the five-year storm and are not surcharging to basement levels during the 100-year storms. The same can be applied for any storm drainage design standard, not only for a five-year storm sewer design.

The proposed approach has significant benefits to municipalities intending to reduce sanitary sewer overloading, environmental spills, and basement flooding, particularly during large storms when residual sewer capacity is at a premium.

The approach not only identifies the vulnerable locations through a robust, physically based approach which accounts for the topographic, hydrologic, and hydraulic processes that produce and expose surface flows over and around MHs, but it also quantifies the amount of water that will enter the sanitary sewer due to deteriorated structures. Reductions of inflow can translate to increased capacity for development. Neglecting these abundant sources of I&I will likely not achieve desired I&I reduction targets.

Recommendations by Wastewater Management Experts at Civica

Reducing I&I on private property can be challenging. These challenges include access to private property, defect identification, flow quantification, remedial action planning, and coordination, as well as costs and subsequent liabilities. The findings and recommendations present a new option to municipalities to identify and remediate public sources of I&I. These sources can be identified consistently through a systematic approach and will leverage existing technologies to rationalize fixing I&I in the public right-of-way in a cost-effective manner.

It’s worth noting that even if a MH is not submerged during initial testing that degradation to the pavement over time can lead to these previously well-functioning MHs becoming submerged and potentially contributing towards a wastewater system overload.

The proposed methodology should be used to determine the flow reductions in the sanitary sewer system after the approval of a proposed MH sealing method. Civica has tested fully sealed and gasketed MH products, with >99% inflow reductions observed. 

Have All Your Inflow and Infiltration Inspection Needs Addressed by the Experienced Specialists at Civica

Civica is a leader in municipal wastewater management solutions and water flow monitoring systems. Our expertise spans across sanitary and storm sewer systems as well as natural watershed asset planning and asset management.

We offer consulting on inflow and infiltration inspection services, maintenance hole inspection, sanitary sewer capacity analysis, CCTV sewer inspection, sewage water management, flood analysis, stormwater management consulting, and more. Contact us today for a free consultation.

Learn more at:

Methodology of Sanitary Maintenance Hole Inflow and Infiltration

Basement Flooding Area 9 Inflow And Infiltration Reduction Pilot

Basement Flooding Protection Program Capacity Assessment Study

Pilot Inflow And Infiltration Reduction Project

Shining Hills (Bazil) Inflow & Infiltration Reduction Project

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