Case Studies: Wet Weather Inspections to Find Inflow and Infiltration (I&I) – Part 2

The first part of this webinar delved into the diverse factors contributing to the problems of inflow and infiltration (I&I), inflow and infiltration methods, and onsite inspections, with a focus on field inspections conducted during wet weather conditions. This second part outlines some specific cases of wet weather inspections that Civica conducted.

Case Study 1: York Region I&I Reduction Project

The first test was carried out in York Region. During a minor storm, a local creek displayed heightened flow; however, nearby sanitary maintenance holes remained unaffected by the creek’s influence. In a larger storm, the creek’s waters raised, leading to bank overflow and submergence of the maintenance holes.

dry and wet weather inspection

In another case, a drainage channel that was bone dry during a site visit in August had elevated flows during a larger storm. The water level rose significantly and interacted with the sanitary maintenance holes near the channel. Upon removing the lid of the MH at this site, a substantial portion of the drainage channel’s water was entering the sanitary system through defects in the maintenance hole structure.

To combat standing water on maintenance holes, several municipalities are adopting securely fastened covers and frames, with recommendations to raise the structure to prevent interaction if possible (i.e. off road conditions). 

Case Study 2a: Storm Event

The study was conducted in the City of Toronto. It involved 53 millimeters of rainfall spread across a 17-hour period. The objective of the study was to determine discharge points of drainage features that were identified as potential I&I sources through previous dry weather inspections.

A hydrograph from a downstream flow meter placed at the study area’s outlet showed that under dry conditions, the flow remained relatively steady at one to two liters per second. During the wet weather event, however, the flow multiplied by six or seven times. 

Different addresses were revisited to confirm their ultimate drainage connections. Five initial locations on the list were verified as being linked to the sanitary sewer system. A few catch basins situated at the rear of properties posed unique challenges. Despite traditional dye testing, these catch basins exhibited limited visibility, possibly due to pipe storage or defects connecting to the storm sewer system. This provided assurance regarding the drainage paths of numerous potential sources and confirmed connections to either the sanitary or storm sewer systems.

As part of the dye testing work, CCTV inspections were also conducted concurrently. Among the 26 laterals inspected, only four exhibited low or no flow, 20 displayed moderate flow levels, and two registered high flow rates. 

Case Study 2b: Storm Event 

A follow-up case study was conducted in the City of Toronto to validate connectivity and response times of foundation drains during storms to see where some of the I&I was getting in. The goal was also to evaluate the effective zone of influence on foundation drains and see how far away the water infiltration was from the foundation wall. The project involved a much smaller rain event of only 16 millimeters.

A lateral inspection camera was launched on multiple properties to look at the condition of the laterals and make sure they were not in disrepair. When inspected during wet weather, it is important to inspect if the laterals are in poor condition. Dye testing at the surface was also conducted at the downspouts outlet – the first test occurring right at the foundation wall. The results showed that it took only 10 minutes for the surface water to get into the foundation drain and then into the lateral and sanitary sewer system. 

Another test was then conducted shortly after that with a green dye at about 1.7 meters

from the foundation wall. Within 20 minutes the water reached the sanitary sewer system. This testing proved that the majority of the flow was coming from foundation drains and not defects within the laterals themselves. This data is relevant to know when making recommendations as to how far away downspouts should be discharged from the foundation. Based on the testing, the zone of influence on foundation drains is approximately two meters.

case study storm event

As part of the study, 185 downspouts were surveyed within a small section of the street. It was found that a majority of the downspouts (64%) discharge within two meters of the foundation.

Wet Weather Inspection Challenges

Ultimately, not many companies perform wet weather inspections. The reason for this is that wet weather inspections pose significant challenges, including:

  • Wet weather is hard to anticipate, plan and schedule
  • Disruption to field operations
  • Staff and equipment get very wet
  • Safety precautions (traffic control, visibility, etc.)

Conclusion & Recommendations

Despite the challenges, wet weather inspections are highly effective in finding I&I sources. To conduct accurate testing, dedicated forecasting and planning are key to successful wet weather inspection operations. Simulated wet weather inspections are also alternatives in order not to be fully restricted by weather.

The case studies show that flow monitoring is needed to capture elevated flows to prioritize wet weather work. Wet weather inspections should be added to regular I&I reduction programs and foundation drains/downspout discharge distances need consideration for I&I reduction studies.

Connect with Our Experts to Learn More 

Civica is a leader in water resources products, product integration and consulting services. Civica carries out a variety of field services, such as flow and rainfall monitoring, CCTV services, MH inspections, smoke & dye testing, lot and drainage inventory, rainfall simulation testing, wet-weather investigations, MH flood testing, sewer inspection, and more. Connect with the experts at Civica to learn more about any of these services or to discuss your needs.

Learn More At: 

Wet Weather Inspections to Find Inflow and Infiltration (I&I) – Part 1

Private-Side Inflow and Infiltration Reduction

Methodology of Sanitary Maintenance Hole Inflow and Infiltration

Sanitary System Flow Monitoring & Smoke Testing

Newmarket Inflow And Infiltration Reduction Project

Inflow And Infiltration Reduction – Sanitary Sewer Modelling Study

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  1. […] tuned for the second part of this blog that outlines some specific cases of wet-weather inspections that Civica has […]