Challenges of the New Subdivision Flow Monitoring to Assess Performance of Inflow and Infiltration (I/I) Prevention

The innovative New Subdivision I/I Prevention Program has yielded great results for multiple municipalities when it comes to infiltration and inflow (I/I) reduction. That said, it doesn’t mean that the new subdivision I/I prevention plan is without its challenges. Let’s take a look at some of those challenges laid out in the flow monitoring plan.

Challenges in New Subdivision Flow Monitoring Program

Wastewater flow monitoring is not a new technique; it has been used in existing neighbourhoods for decades to assess capacity and help in municipal wastewater management and planning.

What is new is that new subdivisions are usually too small to use traditional flow monitoring techniques. What’s more, it is difficult to separate inflow and infiltration from the measured wet-weather flow during subdivision flow monitoring.

Here are some other challenges of the new subdivision I/I prevention plan in more depth.

Catchment Characteristics

Within existing neighborhoods, most flow monitoring programs select an ideal drainage area of 25-75 hectares (ha) or larger to monitor flows. New subdivisions have monitoring catchment areas between 2.5 ha to 25 ha, with the most common size of 10 ha to 15 ha.

The drainage area for a new subdivision is limited to its boundary (such as the number of new homes) and may not drain to a single outlet, requiring a vast amount of monitors to isolate homes built by one developer or another. These small areas producing low sewage flow rates and speaks to the size challenges mentioned earlier.

Low Flows in New Developments

Due to the small catchment area and high construction standards plus additional water conservation technologies employed in these new homes, flows encountered in these projects are usually very low. Area-velocity meters, which are the most commonly used flow monitoring equipment, typically won’t work due to the technological limitations. Area-velocity meters measure flow depth through ultrasonic sensors or pressure transducers and velocity using doppler velocity sensors. Both measurements can be compromised when the flow is low. In the early phases of new subdivisions, the flow observed in the sanitary sewer can be extremely low and the depth of flow are quite often less than 2 cm.

Low flow means that the sensors won’t be fully submerged, which in turn means poor depth measurements. To achieve accuracy, you need primary devices tailored for these conditions, like weirs and flumes that provide higher accuracy and resolution. 

Data Analysis

Adding to the noise in the data, the next set of challenges is refining the methodology to minimize overestimation of the I/I responses and the cumulative error in the analysis. The two I/I key performing indicators are rainfall derived I/I (RDII) and groundwater infiltration (GWI).

The best engineering judgment, which comes with years of monitoring experience, is needed when interpreting such analysis. New construction as part of the program follows stringent building practices, and sewage infrastructure deterioration is expected to be very minimal.

Direct measurement of GWI can be difficult to assess and empirical estimates such as assuming GWI equivalent to 85% of the minimal dry-weather can be applied to quantify GWI contribution. However, it was found that this approach may not be the best representation for GWI present in new subdivisions. A series of midnight dry weather investigations was performed to attempt to trace extraneous flow within a new development with bucket test flow monitoring results and compare to water consumption data. Ultimately, it was found that a very large percentage of the flow measured in the middle of the night appeared to originate from homeowner usage.

Modifications to the way GWI is calculated is currently ongoing in an attempt to characterize infiltration in a more accurate manner.

Real-Time Data Access and Management

With the amount of data coming in, you will need a proper data management platform to make sense of it all. Cloud computing and technological advantages have made it easier to process these large amounts of data by improving accessibility across teams and stakeholders, allowing for scalable storage, and providing scalable computing power. DataCurrent, an online data management, viewing and analysis platform, gives users a holistic view of flow monitoring projects.

The Importance and Benefits of New Subdivision I/I Prevention Program

There are many tangible benefits to I/I reduction produced by the new subdivision I/I prevention plan. The emphasis on inflow and infiltration inspection in advance of construction and other pre-emptive subdivision flow monitoring has proven to be a strong municipal wastewater management strategy. So far, 40 subdivisions have participated and 80% demonstrate a conforming I/I response.

Contact Civica Infrastructure to Get Assistance with I/I Reduction/Prevention

Civica is a leader in municipal wastewater and stormwater management solutions. 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, flow monitoring, capacity assessments, collections systems consulting, and more.

Click here for more information about the New Subdivision I/I Prevention Program. Or contact us today for a free consultation.

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New Subdivision Flow Monitoring to Assess Performance of Inflow and Infiltration (I/I) Prevention