Collection System Modelling Utilizing Flow and Rainfall Data (Part 2)

In the previous post, an overview of Civica’s fifth webinar was provided. Part 1 explained how Ontario’s infrastructure growth and climate changes are affecting collection systems and wastewater management. Part 2 will take a deeper look into flow data and the different models used to assess dry-weather flow (DWF) add wet-weather flow (WWF).

Why Use Flow Data for Hydraulic Models?

When using rain and flow data, the model can be calibrated for both dry-weather flow (DWF) and wet-weather flow (WWF) conditions. A calibrated model allows engineers and operators to understand the system’s behaviour, make informed decisions, and improve the overall management and operation of the wastewater collection system.

Accurate and reliable data are necessary to ensure the models’ accuracy and improve our understanding of the behaviour of the collection systems. While it is possible to have one rain event to calibrate the model, typically there needs to be anywhere from three to six rain events of different intensities and different volumes to have reliable results.

What Flow Data Is Required to Calibrate Hydraulic Models?

When flow monitoring data is used to assess the performance of an existing sanitary system, the following information is needed:

• Population data to estimate the per capita flow rate
• Flow monitoring data and rainfall data so that the model can be calibrated for both dry and wet weather.

By using population and flow data, a dry-weather flow generation rate can be established (e.g., 250 L/cap/day). Also, with flow data, the groundwater infiltration (GWI) can be established (e.g., 85% of minimum flow).

Using rain and flow data, the model can be calibrated for wet-weather flow conditions.

Dry-Weather Flow (DWF) Data Analysis

When it comes to dry-weather flow data, the flow monitor performance and data quality should be carefully reviewed for:

• Data consistency: There should be steady dry-weather flow patterns without any major jumps between days or months.
• Mass continuity: If two meters are present at a location, the downstream flow meter should have more volume than the upstream flow meter.
• DWF base flows and seasonal base flows: Dry-weather flow might be higher or lower depending on the groundwater infiltration.
• Weekdays and weekends diurnal pattern: The dry-weather flow patterns should be separated based on weekdays and weekends because the peak flow during the week is different than on weekends.
• Per capita flow rate within reasonable ranges (e.g., 100 to 450 L/c/d): This is a quick way of knowing if the population is underestimated or overestimated or if there are some issues with the flow data.

Wet-Weather Flow (WWF) Data Analysis

When it comes to wet-weather flow data, the flow monitor performance and data quality should be carefully reviewed for:

• Data consistency: There should not be any spikes within the data or gaps or discrepancies between the flows.
• Mass continuity: If there is a flow meter that is downstream of another flow meter, the volume has to be higher than the upstream flow meter.
• No gaps during storm events: If there is a storm event and there are gaps in the flow data, especially if it happens during the peak of an event, the data cannot be used.
• Reasonable time to peak for I/I response: If there is a storm event at midnight and the I&I data occurs at 2:00 a.m., it is important to question why this happened.
• Volumetric runoff coefficients for each rain event: The volumetric runoff coefficient should be less than one. 

Hydraulic Modelling Calibration Criteria in Southern Ontario 

Many municipalities in southern Ontario have adopted the CIWEM/WaPUG for the calibration criteria and targets. Below is an overview of the criteria.

Parameter	General	Critical Locations	Comments
Shape	Good match (NSEC if used >0.5)	Good match (NSEC if used >0.5)	An evaluation technique may be used to compare the shape such as the Nash-Sutcliffe Efficiency Co-efficient (NSEC) method together with a visual check.
Time of peaks and troughs	±0.5 hour	±0.5 hour	The timing of the peaks and troughs should be similar having regard to the duration of the event.
Peak depth (un-surcharged)	±0.1m or ±10% whichever is greater	±0.1m
Peak depth (surcharged)	+0.5m to – 0.1m	±0.1m	Relaxation may be appropriate in deep sewers.
Peak flow	+25% to -15%	±10%
Flow volume	+20% to -10%	±10%	Excluding poor/missing data

Civica’s Collection System Management Services

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

Learn More at: 

Flow Monitoring in Collection System Modelling (Part 1)

Flow and Rainfall Monitoring – Ensuring Data Quality (Part 1)

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

Methodology of Sanitary Maintenance Hole Inflow and Infiltration

Flow and Rainfall Monitoring – Technology, How to Operate & Maintain

City of Toronto’s Sewer Infrastructure and Sewer Capacity Assessment Guidelines