Can I Level with You? Benefits and Limitations of Level-Only Flow Monitoring Technology

This presentation explores the benefits, limitations, and lessons learned from deploying level-only monitoring technologies in wastewater collection systems. Two case studies will be highlighted: one from a small utility and another from a medium-sized utility, each employing different level-only monitoring technologies (the iTracker and the SmartCover) to support wastewater modeling and master planning. The presentation will walk through the process of developing dry- and wet-weather flow estimates for hydraulic modeling to inform master planning and capital improvement plan development. Many utilities opt to develop hydraulic models and perform infrastructure capacity planning without investing in flow monitoring in their collection systems. The costs associated with technology, monitor deployment, and maintenance are the largest barriers to traditional depth-velocity flow monitoring. In contrast, level-only monitoring is significantly more affordable and does not require training or specialized contractors to deploy, retrieve, or relocate the monitors. The reduced cost barrier and ease of deployment, relocation, and retrieval allow utilities that would otherwise forego flow monitoring to develop a deeper understanding of system hydraulics and rainfall dependent inflow and infiltration (RDII), as well as use that understanding to predict and prevent sanitary sewer overflows (SSOs). However, level-only monitoring involves several assumptions, including steady uniform flow, and there is inherent uncertainty in flow estimation from level-only data. Level-only monitoring relies on pipe slope, Manning's n, and manhole topology as inputs for calculating flow using Manning's equation. Any uncertainty in these inputs can lead to significant errors in calculated flow. Traditional depth-velocity flow monitoring methods, by comparison, provide much more reliable flow estimates. Therefore, understanding the benefits, assumptions, and drawbacks of level-only monitoring is critical, along with employing processes to minimize data uncertainty. This presentation will outline the steps two utilities followed-from site selection to data processing-in leveraging level-only monitoring to better understand flows in their collection systems. The two case studies that will be presented are the Brushy Creek MUD Wastewater Master Plan and the Gainesville Sewer Model project. Brushy Creek MUD is a small utility that serves 7,000 connections just outside of Austin, Texas. The utility discharges all wastewater flow to a regional treatment facility. Flow monitoring data is not collected at any point in the collection system, including discharges to the regional treatment facility. Four iTracker loggers were installed on major collector lines to aid in developing dry- and wet-weather flow loadings. The iTracker depth data was converted to flow using Manning's equation, and the flows were balanced using SCADA wet well level data and winter water demand data. The magnitude of the flow response to wet-weather events was analyzed and informed the selection of a 4.0 peaking factor for modeling of peak wet weather flow conditions. The iTracker data proved valuable in developing this master plan by validating flow loadings in the hydraulic model for a system with no historical flow data and a limited budget for model development and calibration. This project is complete. The City of Gainesville Department of Water Resources (DWR) is a medium sized utility just outside of Atlanta, Georgia. Currently, Gainesville DWR maintains 328 miles of gravity sewer, 78 lift stations, and 5 regional lift stations that discharge into two water reclamation facilities (WRFs). Throughout their system, 34 level-only sensors are deployed. Prior to the sewer hydraulic modeling effort, Gainesville DWR deployed these monitors, SmartCovers, in 2024. SmartCover collected depth data and converted that data to flow data using Manning's equations. Flows are to be balanced using the regional lift station runtime data, Flat Creek WRF influent data, and 2024 winter water demand data. While this project effort is still ongoing. It is anticipated to be finalized by April 2025. This project looks to build upon and refine the methodology utilized in the Brushy Creek MUD Wastewater Master Plan. Level-only monitoring can be useful to wastewater utilities large and small for a variety of reasons. Level-only monitoring equipment is a fraction of the cost of a temporary flow metering contract and can be deployed by utility staff or engineering consultants. Level-only monitors can easily be relocated throughout the system to allow for micro-metering or investigation of specific problem areas. While traditional depth-velocity flow monitoring provides best-in-class flow data, it often requires hiring specialized contractors to deploy and costs upwards of $10,000 per flow meter per month. In addition to initial deployment, moving meter locations requires remobilization of the contractor and incurs additional costs. The capital cost of a level-only monitor is around $3,000. Engineering consultants and utilities can purchase these monitors directly and incur little to no additional cost to install and retrieve data. However, level-only monitoring data should never be taken at face value, and requires careful consideration as to the degree of uncertainty in the data and how sewer or manhole conditions may impact depth and subsequent flow calculations. This presentation will help utilities considering level-only meters to obtain valuable data and to understand the shortcomings of the technology. Both depth-velocity and level-only monitoring are subject to real world conditions. Collection systems do not always maintain perfect hydraulic conditions and equipment in the field can fail or experience issues that result in uncalibrated results. Many issues experienced in flow monitoring are often the result of poor site selection or unknown hydraulics at the site. Level-only monitoring sites should be manholes with good hydraulic conditions. These manholes should have benching, 180 degrees between inlet and outlet, and be on gravity mains with consistent slopes. All these features should be investigated on candidate manholes before monitor installation. Additionally, site selection should prioritize locations where field data exists to allow for flow balancing. Any existing flow monitoring at wastewater treatment facilities and SCADA data collection at lift stations should be leveraged. The main takeaway points of this presentation are as follows: - Level-only monitoring is a cost-effective meaning for gathering supplemental flow information within collection systems - Level-only monitoring relies on steady state, uniform flow assumptions. The reliability of which is significantly impacted by site conditions and downstream hydraulics. - All data and associated technology have a purpose and benefit; however, it is our responsibility to understand the limitations and assumptions that these tools rely on. - While level-only monitoring needs to be careful reviewed and utilized in the context of hydraulic model calibrations and master planning efforts, these monitors have a wide-array of other applications in operation and maintenance efforts.