Leveraging Public Infrastructure to Identify Sources of Private-side Inflow and Infiltration

Leveraging Public Infrastructure to Identify Sources of Private-side Inflow and Infiltration

Over the past five years, the District of North Vancouver has combatted I&I within the Lynn Valley Sewer Catchment by targeting public infrastructure, but efforts have now turned to private-side infrastructure.

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Newly rehabilitated mains surcharging during off-peak hours. Lift stations running two pumps in the middle of the night. CCTV operators pumping down mainlines to complete inspections.

We all know what private-side inflow and infiltration looks like. The problem is that it is difficult to find. Costs can be prohibitive and associated with reports and data that are impractical in an operations setting.

The District of North Vancouver is committed to I&I reduction. The district is a municipality of 85,935 people situated on the side of the North Shore Mountains in the Lower Mainland area of British Columbia. Winters are wet, and summers are dry. Seventy percent of the district’s sanitary sewer pipes were installed between 1943 and 1970. A number of factors including steep terrain, high volume of annual rainfall, and the age of the system make I&I an important issue for district operations.

One area in particular, the Lynn Valley Sewer Catchment, has received recent attention due to restricted flow capacity at the downstream outlet where heavy winter rainfall events can lead to sanitary sewer overflows. The Lynn Valley Sewer Catchment encompasses an area of approximately 3.1 square miles and services approximately 5,625 private properties. The primary land use within the area is single-family residential. The district owns all infrastructure up to the private property lines. Sanitary laterals within private property are the responsibility of individual property owners.

Over the past five years, the district has combatted I&I within the Lynn Valley Sewer Catchment by targeting public infrastructure. Efforts have been moderately successful but the district has not been able to replicate the flow reduction results observed from a previous project that targeted private-side I&I.

Taking action

In March 2017, the district created a Reduction in Inflow and Infiltration Team. The team consists of a project engineer, foreman and laborer. The project engineer coordinates repair contracts and oversees the program. The foreman provides inspection and contract administration services for contracts executed as part of the program and also completes field investigations to identify sources of I&I. The laborer completes field inspections and assists the foreman with daily tasks.

The district collects an array of sanitary infrastructure data. Included in this data is a 15-year-old sanitary CCTV program, a SCADA system that logs historical information, and a user-friendly GIS mapping system that displays attributes of district infrastructure. Since I&I eventually appears in public infrastructure, the district hoped to use the existing sanitary system and its associated data to identify potential sources of private-side I&I.

CCTV inspection

Inspection data within the Lynn Valley Sewer Catchment has been integrated with the district’s internal GIS mapping system. Since the district began its CCTV program in 2003, approximately 78 percent of the sanitary gravity mains within the Lynn Valley Sewer Catchment have been inspected. Approximately 30 percent of those mains have been inspected multiple times in different years. The majority of inspections were completed in winter months when the ground was saturated and it was raining.

With the aid of the GIS mapping system, the I&I team looked through CCTV reports and videos to identify live connections and junctions.

If multiple CCTV videos were available, videos were cross-referenced to confirm the presence of live flow in each video. Laterals that were not live in each video were removed from the list and assumed to have been active due to regular sanitary operation. In general, the team noticed more recent videos showed increased volumes and flow rates from laterals.

The investigation identified 377 laterals with significant flow into the main. These laterals represent properties with high potential for private-side I&I. Since the district owns all laterals from the main to the property line, it is possible some I&I assumed to be private could originate from the public side of the pipe. The average length of public laterals in the Lynn Valley Sewer Catchment is 31.5 feet. The next steps of the program include mechanisms that allow the district to separate public from private-side I&I.

When the team began the exercise, all wet laterals were flagged regardless of volume and flow rate. The team realized early in the process that this method would identify too many laterals to investigate. The 377 flagged laterals represent those with moderate to significant observed flow. The GIS mapping system allowed the team to easily identify the residential address associated with each lateral for future use and documentation.

Pump cycle data

Four small lift stations are located within the Lynn Valley Sewer Catchment. The team analyzed pump cycles observed between 2 a.m. and 4 a.m. when sanitary flow is typically at a minimum. In an attempt to capture only pump cycles from private-side I&I, the team looked at data from rain events preceded by several days of dry weather.

The flow response from private-side I&I in the district is typically instantaneous where the response from groundwater entering public infrastructure often takes longer to appear in the system. The difference between dry- and wet-weather pump cycles was assumed to be attributed to I&I from private properties.

In the team’s analysis of four subcatchments, two showed strong evidence of private-side I&I. The combined 107 properties within those two subcatchments became a priority for further investigation.

Inspection chambers

Approximately 20 percent of the properties in the Lynn Valley Sewer Catchment have sanitary inspection chambers (clean-outs). The I&I team inspected 495 inspection chambers to identify properties with high potential for private-side I&I.

The team used the district’s GIS mapping system to create simple inspection maps. If team members observed a steady stream of clear water in the lateral at the inspection chamber in wet weather, the property was flagged for further investigation. Flagged laterals include all flow from a steady trickle to a full pipe. Once inspected, the team entered the results back into the GIS mapping system to track the locations of each inspection chamber inspected, cleared, and flagged.

Of the 495 laterals inspected, 372 were cleared and 115 were flagged. Another eight were undetermined because the team was not able to access those laterals.

Next steps

The next step in the program is to further investigate the properties flagged in the three methods described above. If not already present, a sanitary inspection chamber is installed at the property line. Preference is given to properties where significant flow was observed in the mainline CCTV video.

Sanitary inspection chambers allow for easy access to the lateral with a push camera. The push camera can then be used to identify source(s) of I&I. Cross connections are often observed as a Y connection in the private-side lateral. Infiltration is viewed as active gushers, runners, drippers, or seepers from broken pipe segments or open joints. The push camera has the added benefit of video capture, which allows the team to bring what they see in the field into the office for analysis and documentation. Once sources of I&I are confirmed, the team creates a remediation plan.

Leveraging public infrastructure is a cost-effective and efficient method to identify sources of I&I on private property. The district was able to take the findings from the program and turn it into specific repair works.

The success of the program depended heavily on dedicated field staff. Without their expertise and specialized skill set, much of the private-side I&I would not be identified and corrected.



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