ADS FAQs – Inflow and Infiltration
This rule-of-thumb works as follows: If you have a monitor basin with typical residential and commercial development you can expect to see dry weather flows on the order of 5 gallons per day per linear foot of sewer. If you notice flow rates that are dramatically different, you should look for an explanation. Are the flow monitor data correct? Is the basin size information correct? Is the basin boundary properly delineated? This rule-of-thumb serves as a useful gut check.
An I/I study needs sufficient data to characterize both dry weather and wet weather performance at each flow monitor location, and in the United States and Canada, it is often the number of storm events that drive the length of the study period. Statistically, the more storm events the better, and practically, we look for 6 to 12 storm events where a meaningful wet weather response is observed in the sewers. For this reason, a minimum 90-day study period is often needed.
Gravity sewers typically need to have flow depths less than full pipe for sensors to be properly installed, maintained, or removed. However, once installed they can measure flow depth and velocity in both non-surcharged and surcharged conditions.
Once suitable locations are identified and flow monitors have been properly installed, routine data review is used to make sure that flow monitors are working correctly. At ADS, we use both automated reviews and human reviews for this purpose. The urban sewer environment is quite dynamic and can be quite hostile at times. As such, we remain on alert for changing hydraulic conditions and/or problems with monitoring equipment that may arise. Problems that are found quickly can often be resolved quickly, and that is our goal. Periodic field confirmations are also a routine practice and ensure that the measurements made by the monitors are reasonable.
Years ago, there was a significant difference, as permanent flow monitors were connected to a telephone line and temporary monitors required a manual cable connection to retrieve data. With mobile communication technology available today, there is much less of a distinction between a permanent and temporary flow monitor. Whether permanent or temporary, you want to find a location with suitable hydraulic conditions that is easy to access, easy to maintain, and has minimal safety concerns.
The nature of the hydraulic conditions at a given location are the primary driver of the technology selected with the purpose of the data being a secondary concern. With that said, what kind of hydraulic conditions are often of most concern during infiltration and inflow studies? Wet weather events where surcharge conditions occur are often of most concern, and this will require measuring flows during transitions into and out of surcharge conditions. For these scenarios, we recommend a suite of flow depth and velocity sensors, where the sensors are installed inside a pipe as opposed to flow depth and velocity sensors installed in the manhole over the manhole channel.
Empirical data suggests that the ideal basin size is around 10,000 LF of sewer, with a goal of maximizing granularity to minimize follow-on condition assessments such as manhole inspection, smoke testing, and closed-circuit television (CCTV) inspection. Smaller basins sizes are generally better than larger basin sizes, but it is important to remember that whatever the basin size, it is preferable to keep basin size as consistent as possible across the study area for better apples-to-apples comparisons.
Assuming that locations with suitable hydraulic conditions are used and that the flow monitors have been properly installed, diligence is required to assure data quality over the full monitoring period. Routine data review is one key aspect. You must keep track of the “pulse” of the data. At ADS, we use both automated reviews and human reviews for this purpose. The urban sewer environment is quite dynamic and can be quite hostile at times. As such, we remain on alert for changing hydraulic conditions and/or problems with monitoring equipment that may arise. Problems that are found quickly can often be resolved quickly, and that is our goal. Periodic field confirmations are also a routine practice and ensure that the measurements made by the monitors are reasonable.
If a consistent level of bias is detected in the data, it can be adjusted accordingly during data review and approval. We also can correct individual outliers and estimate their correct values from surrounding data and observed hydraulic patterns. Outliers can adversely affect engineering metrics and calculations, and they are reviewed and corrected whenever possible. We often refer to these processes as Data Review, Data Editing, and Data Approval.
Product design takes this into account, and materials that are used should be compatible with these environments.
Nighttime flow isolations can be effective in the extreme upper reaches of the sewer system. However, the most useful nighttime flow isolations are the ones where no flow is observed, as you can automatically rule out these stretches of sewer from further evaluation and not have to worry about the accuracy of the weir measurement. As you go a little further downstream, micro-monitoring then becomes an option, but results from micro-monitoring are perhaps best used in a qualitative rather than a quantitative manner.
Yes. ADS flow monitoring services, including data approval in PRISM, are governed by written procedures that are part of our quality assurance and quality control program which is certified under ISO 9001: 2015 standards.
We call this situation a flow imbalance, and this is indeed a problem. This first thing to do is evaluate the data from each flow monitor. Were both monitors installed where you think they are installed? Are both flow monitors configured with the correct pipe shape and pipe height? Is the silt value correct? These are some of the obvious things to check. You also need to evaluate the site hydraulic conditions. Are hydraulic conditions optimal at one location and not the other? If so, be more suspicious about the location with less-than-ideal conditions. If you find that the monitor data appears in good shape, recheck the mapping and verify that the correct flow schematic is used. If that checks out, you may have to do some field work to further validate the actual connectivity and determine if the maps are accurate or not. Sometimes, a simple error is found, and the problem is easily resolved. Other times, the answer is not as obvious and may require cooperation from the utility, the consulting engineer, and the flow service provider to resolve.
The National Oceanic and Atmospheric Administration (NOAA) and USGS both maintain networks of rain gauges across the United States, and data from them are available for public use. Our recommendation is that you can use them to supplement your own rainfall monitor network when they are available but do your homework and use some caution. Also, never rely on them as your sole source of rainfall data. Before you consider using one, first Investigate data availability. We have seen some of these rain gauges where data is intermittent and not always available for the time periods needed. Second, investigate the sample rate reported by the rain gauge. Some locations only report daily totals or hourly totals. A sample rate of 5-minutes is preferred for most I/I studies and related hydraulic modeling applications.
Think of the minimum Rainfall Threshold as a default value to automatically identify rainfall events, not as an ideal value. As a rule-of-thumb, we often do not evaluate a rainfall event for I/I if the rainfall total is less than 0.5 inches. However, there is not a particular rainfall total that is universally accepted as a significant wet weather event in the context of I/I. Rather, it is the response of the sewer system to a given rainfall amount that determines if a rainfall event is significant or not. This amount can vary from one sewer system to another and can even vary within different parts of the same sewer system.
The answer depends on the type of sewer. For storm sewers, specified rainfall return frequencies are used to provide various levels of service and are often specified in local storm sewer design requirements. For combined sewers, specified rainfall return frequencies are often used as well and are derived from combined sewer overflow (CSO) discharge permit requirements. For sanitary sewers, there are no specified requirements. This topic has been discussed on-and-off for years within the industry and between municipalities and regulators, but no consensus has emerged. However, it makes sense to establish a level of service for sanitary sewer systems and use that as a basis to benchmark performance over time and plan system improvements with the desired level of service in mind.
A double mass analysis is one method to compare cumulative rainfall from various rain gauges where consistency between rain gauges is deemed an indicator of proper operation. Keep in mind that a double-mass analysis is generally intended for use evaluating data over long time periods, not individual rainfall events. For example, you could look at cumulative rainfall over a rolling 12-month period. Based on past literature reviews, rain gauges are considered to pass the double-mass analysis if they are within ±20% of reference or within ±20% of each other.
Most I/I studies look to quantify rainfall entering a sewer system as I/I. As a result, frozen precipitation events that occur during an I/I study are often ignored. Keep in mind that the most intrusive effects of I/I on a sewer system result from rainfall, not snowfall. Some municipalities in colder climates have an interest in this, but the analysis is not straight-forward and can be problematic.
The primary, independent recommendation for rain gauge density in the United States is from the American Society of Civil Engineers (ASCE) and the Water Environment Federation (WEF). They recommend a rain gauge density of one rain gauge for every 5 to 10 square miles. There are some finer points to consider with rain gauge density when it comes to topography and perhaps the design storm of interest, but Step 1 is having a rain gauge network with a density that meets ASCE and WEF guidelines.
When determining sewer basin and sub-basin areas for an I/I analysis, do you typically use the entire acreage of the sub-basin, or do you subtract out undeveloped areas, native growth areas, critical areas, etc. for a better comparison of I/I rates between sub-basins.
The ADS ECHO can help you judge the timing of I/I by observing the quickness of the flow response and the rate at which the system recovers. ECHOs can also be used to indicate if the system becomes surcharged. However, if the sewer moves into backwater or is restricted in any way the resulting depth increase will be incorrectly interpreted as an I/I response. Also, converting flow depth data to flow rate data using the Manning Equation in the invert channel in a manhole is not reliable above 1/3 or 1/2 pipe depth (spillover to manhole bench). Because of these two issues the data from level monitors are not generally reliable for quantifying RDII or used as devices to subtract upstream flow from downstream flow.
Yes. ADS offers SLiiCER™ – a web-based infiltration and inflow analysis application. We have a version that has been available for many years and have recently released a new Cloud-based version that integrates with PRISM.