Flow Monitoring and Its Role in Inflow and Infiltration (I&I) Studies (Part 2)

In the previous post, an overview of Civica’s fourth webinar in the Flow Monitoring Webinar Series was provided. Part 1 explained the role rainfall monitoring plays in inflow and infiltration (I&I) studies. Part 2 will take a deeper look into flow analysis, including dry-weather flow (DWF) analysis and wet-weather flow (WWF) analysis.

Flow Analysis

When it comes to flow analysis, there are many things to take into consideration, specifically dry-weather flow (DWF) analysis and wet-weather flow (WWF) analysis.

Dry-Weather Flow (DWF) Analysis

While dry-weather analysis isn’t directly related to I&I, it is important to understand the dry-weather flow so that it can be separated from the wet-weather flow (WWF) when rain events occur.

It is also important to analyze dry-weather flow so that municipalities can get a better understanding of groundwater infiltration rates. Industry standard is to assume that approximately 85% of flow during the minimum nighttime flow is from groundwater infiltration.

Dry-weather flow analysis is typically conducted at various times throughout the year, as seasonality can affect groundwater infiltration rates.

Wet-Weather Flow (WWF) Analysis

Wet-weather flow analysis seeks to understand the flow response in sewers during rain events. Using a flow separation tool, the measured WWF is compared against the predicted dry-weather flow to determine the I&I flow and help illustrate how much rainfall is getting into the wastewater system.

This data also generates various KPIs, such as the I&I Rate (L/s/ha), I&I Volume (m³), CV%, and peaking factors (i.e., the quickness and severity of peak flows).

When municipalities want to understand what is going to happen over time or over various storm events, there is a need to measure numerous events…the more the better. Q vs. i plots can be used to measure flow intensity versus I&I rate, while a R vs. V plot measures rainfall volume versus flow volume.

Thematic Mapping

Thematic mapping is a great way to visualize I&I separation and/or projections across a large area (municipal-wide).

It presents the results in such a way that it makes it easier to understand what’s happening at different catchments and different geographies. This helps to prioritize which areas should be focused on and maps out the problem areas.

Limitation of Wet-Weather Flow Analysis

There are some limitations to wet weather flow analysis, such as:

• Rain Gauge Placement: This refers to how many rain gauges are used. If the rain gauge is 50 kilometers away, or even 10 kilometers away, it’s likely going to be hard to understand the impact of rainfall on wastewater system flow.

• Duration of Monitoring Program: The longer the data set is, the more confidence there will be in the results and in the correlation of those variables. It’s also important to consider seasonality and the time of year the data is being collected.

• Small vs. Large Areas: Smaller is typically better because the larger the catchment area, the harder it can be to understand what is happening at a local level. For instance, there may be a wastewater catchment (sewer shed) that has massive inflow and infiltration and has flooding or has potential for flooding, but it gets muted by the rest of the flow from upstream areas. 

• Bracket Areas: There are a lot of challenges when it comes to subtracting flows and making sure the right data is used to calculate the wet weather flow for a given area.

• Data Quality: The quality of the data is paramount for this type of analysis. Having the right data and good quality data is important to be able to conduct these analyses and have confidence in the results and the recommendations being made.

Planning Actions Based on Flow and Rainfall Analysis Results

To create actions, it’s important to understand the data and the impact of the data, such as whether flooding or backups occurred during the rainfall event.

One of the most common problems is Inflow and Rapid Infiltration. These are quick responses shortly after and during rain events. Some of the tools and techniques used during and after rain events to identify these problems include:

• Flow monitoring
• Lot inspections
• Smoke testing
• Dye testing
• Wet weather CCTV
• Wet weather maintenance hole inspections

The goal of these tools and techniques is to find and disconnect the inflow sources and anything that may be contributing stormwater to the sanitary sewer system.

Infiltration, on the other hand, is the prolonged flow response after rain/melt events. It uses data such as flow volumes at various times of the year. Some of the tools and techniques used during and after rain events include:

• Flow monitoring
• CCTV inspections
• Maintenance hole inspections
• Wet weather inspections

The goal of these tools and techniques is to find and remediate system defects.

Post-Remediation Quantification

There are various methods that can be used once investigations have been conducted and system defects have been found and remediated. Each technique has advantages and disadvantages. These methods include:

• Flow Monitoring (Regression Analysis): A regression analysis of I&I vs. rainfall can be used to evaluate the performance of rehabilitation efforts.

• At-Source Estimates: This includes flow reductions calculated at-source by measuring the estimated contributing area and developing an inflow/infiltration curve. 

• Hydraulic Modelling: Models can be used to quantify the changes in I&I generation rates pre- and post-remediation.

Why Choose Civica?

Civica is a leader in municipal wastewater management solutions and water flow monitoring systems. They 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 Civica today for a free consultation.

Learn more at: 

Role of Flow Monitoring in Inflow and Infiltration (I&I) (Part 1)

Tipping Bucket Rain Gauges and All Season Weighing Precipitation Gauges

Flow and Rainfall Monitoring – Ensuring Data Quality

Flow and Rainfall Monitoring – Ensuring Data Quality (Part 2)

Flow Monitoring and It’s Role in Inflow and Infiltration (I&I) Studies

Flow and Rainfall Monitoring – Available Technology and How to Effectively Operate and Maintain It (Part 1)

Flow and Rainfall Monitoring – Available Technology and How to Effectively Operate and Maintain (Part 2)

Inflow and Infiltration Inspection Case Studies

Methodology of Sanitary Maintenance Hole Inflow and Infiltration