Innovative Parabolic Depth Sensor Allows Agencies to Detect Growing Blockages, Conduct Micro-metering and Measure CSOs

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1 Innovative Parabolic Depth Sensor Allows Agencies to Detect Growing Blockages, Conduct Micro-metering and Measure CSOs Patrick Stevens, PE, ADS Kevin Enfinger, PE, ADS ABSTRACT Dozens of sewer agencies in the United States and Australia have found important uses for an innovative depth sensor that relies on the combination of traditional ultrasonic depth technology with the parabolic microphone seen on the sidelines of football games. People will recall from high school geometry that the parabola has the distinctive characteristic of directing any sound energy approaching it to the mathematical focal point of the parabola. This allows the sideline microphone to hear every time Payton Manning yells OMAHA before a snap. It is possible to make the same sideline parabolic microphones operate as a speaker and transmit focused sound back to Payton Manning s replacement. All it takes to get the device to operate in both modes is to place both a speaker and a microphone at the focal point of the parabola. This is exactly what ADS Environmental Services has done with its Long Range Depth (LRD) sensor and its stand-alone version, ECHO. The result is a technology that is mounted inside the manhole riser section and can measure distance to the water surface as far as 6.1 m (20 feet) below. A depth calibration conducted from the surface will allow the device to measure precise sewer flow depth measurements. KEYWORDS Depth-only measurement, parabolic depth sensor, blockage detection, micrometering. 699

2 BACKGROUND Whisper Dishes The best way to discuss this new technology is to describe the similarity to Whisper Dishes. Whisper Dishes appear in science museums and many outdoor public facilities. Whisper Dishes consist of two large parabolic reflectors that face each other and can be separated across a room, across a street or even farther. Figure 1 is a schematic showing how they are arranged. Figure 1 Principle behind a pair of Whisper Dishes Figure 2 is a close-up view of a Whisper Dish in Dallas, Texas. Two people placing their heads in the circular frame on the opposing dishes can literally hear each other whisper. At the same time ambient sounds are minimized at the listener s position. Figure 2 Example of a Whisper Dish reflector from Dallas, Texas 700

3 The science behind the Whisper Dish is a parabolic curve and its unique relationship with the focal point of the parabolic curve. We have all seen these parabolic reflectors in action on the sidelines of sporting events. Figure 3 shows an audio technician on the sidelines of a football game to listen in on what is being said on the field. Ornithologists use the same technique to record the songs of birds in far-away nests. Satellite TV antennas on the roofs of homes are also parabolic reflectors. The important reflective property of the parabolic curve is that any sound originating at the focal point is focused in a very directional beam. Conversely any sound approaching the companion Whisper Dish is reflected to the same focal point, amplifying it greatly. Figure 3 Physical principle of focal point and parabolic curve. 701

4 The Technology ADS Environmental Services has developed a unique acoustic depth sensor that simultaneously operates as both sides of the Whisper Dishes to accurately measure the distance to a water surface. Figure 4 is a schematic view of the sensor showing 1) the horizontally-mounted and highly-directional transducer, 2) the parabolic reflector/housing, 3) the directional acoustic path of the ultra-sonic sound leaving the sensor. The design refocuses and amplifies the returning ultrasonic echo onto the transducer, allowing it to recognize very weak reflections and calculate distances to the water surface of up to 6.1 m (20 feet). Housing with proprietary reflector also protects the sensor in an air bubble. Not drawn to scale Highly directional transducer to transmit and receive. The sound is very directional and can detect water surface up to 6.1 m (20 feet) below. Figure 4 Operating features of the acoustic Long Range Depth sensor. Figure 5 displays a view of the reflective housing from the underside and it is seen that the large diameter transducer is mounted in the horizontal position at the focal point of the parabolic reflector. The sensor face is vertical and prevents the collection of condensation on the face. The dead zone common to all ultrasonic sensors Figure 5 View of the housing with precision parabolic reflector. 702

5 occurs in the distance between the sensor face and the parabolic reflector, allowing the sensor to measure the distance to a water surface right up to the bottom of the sensor. The Long Range Depth (LRD) sensor shown in Figure 5 is a sensor that connects to an ADS Triton meter as one of several sensor types that it can operate. There is a standalone version of the technology called the ECHO and it is displayed in Figure 6. The LRD and ECHO mount at the top the manhole and but the ECHO contains its own battery and wireless modems. The focused beam will paint a target of around 25 cm (10 in.) at a distance of 3.1 m (10 feet) from the sensor face. Figure 6 Narrow acoustic 'beam' will paint a small target. The shower curtain rod type of mounting allows the sensor to be installed directly above most invert channels. Figure 7 demonstrates the flexibility of installation positions. Figure 7 Flexible mounting system allows sensor to be above invert in various locations. 703

6 The ECHO also is equipped with a pressure sensor that can measure surcharge up to 2.5 meters over the sensor. During surcharge a bubble forms inside the housing and keeps the sensor from being fouled so that it immediately becomes active as when the water level drops below the sensor. Five alarm levels can be set to notify the operator of any level of pipe depth or manhole surcharge elevation, including an overflow above the sensor as shown in Figure 8. Figure 8 Pressure sensor allows the ECHO to detect an overflow from the manhole in which it is installed. 704

7 Blockage Detection The Water and Sewer Department in the City of Murfreesboro Tennessee had implemented a regularly-scheduled cleaning program at several locations that routinely experienced blockages. As a test of the LRD technology, two LRD sensors were installed upstream and downstream of the known blockage location. Figure 9 is an aerial view of the blockage location on the yellow 450 mm (18 inch) sewer flowing from east to west. The LRD sensors were installed in the blue-circled manholes upstream and downstream of the blockage area. Figure 9 Aerial view of yellow 457 mm sewer line that experienced regular blockages. 705

8 Figure 10 is a view of the upstream LRD installation with a distance of 4.8 m to the water surface of the 457 mm sewer. Figure 11 is a view of the downstream installation with a distance of 6.7 m (22 feet) above the water surface. Note that there was a side sewer penetrating almost half the manhole diameter at the downstream site and the sensor was able to obtain an accurate measurement. Figure 10 View of upstream sewer 4.8 m below sensor. Figure 11 View of downstream sewer with water 6.7 meters below sensor. Note side sewer penetrating into manhole. 706

9 On 22 June, ten days after the 10 June installation, the two LRD sensors simultaneously recorded a blockage. Figure 12 displays the two depth hydrographs and both reflect the pattern we would expect. The upstream sensor recorded increasing depths leading to a surcharge on the 23 rd. The pattern recorded by the downstream meter is a relatively constant flow that would be expected downstream of an orifice-type blockage. Although no alarms were set up during this test, the ECHO is able issue alarms for up to 5 user-selected depths. Had alarms been set, one alarm certainly would have been at full pipe and that alarm would have occurred around noon on Saturday, 23 June. But if we study the pattern of the upstream meter we can see the pattern began to take on a different shape shortly after the early-morning hours of Friday, 22 June. ADS Environmental Services Pipe Height: MFLRD-01\mp1\LRDEPTH Downstream sensor flat-lines after blockage begins to develop, while upstream sensor continues to increase. MFLRD-02\mp1\LRDEPTH Depth1 (in) 15 Downstream 6.7 m 10 5 Upstream 4.8 m 0 19 Tue Jul Wed 21 Thu 22 Fri 23 Sat 24 Sun 25 Mon Time 07/22/2016 Figure 12 Depth hydrographs of both upstream and downstream sensors, each identifying the blockage that occurred on Friday 22 June. 707

10 Micro-metering There is a lot of buzz in our industry concerning micro-metering. Depending on who is using the term, it can refer to 1) the principle of looking for Rainfall Dependent Infiltration Inflow (RDII) in small and smaller meter basins, 2) a metering device that can measure shallow flows in small sewersheds (microbasins) or 3) the practice of sequential metering in which a second or third round of metering is conducted upstream of the original meter. The LRD and ECHO sensors are showing promise as a tool to obtain a yes/no indicator of RDII sources in the second or third round of such metering. Figure 13 is an example one of four small and similarly-sized sewersheds that typically would not be metered by conventional flow meter because of the cost. This small sewer shed is two blocks long and contains 335 meters of sewer. Practitioners are looking for a low cost way to achieve a yes/no indicator in branches in a study basin. Figure 13 Example of a small 602 m. sewershed that typically would not be metered. 708

11 Figure 14 displays the depth data collected for the four micro-basins and is apparent that only one of the four small basins warranted additional metering or SSES work. ADS Environmental Services Q06010\mp1\DFINAL Q15010\mp1\DFINAL Q18460\mp1\DFINAL Pipe Height: 0.67 Q18FM070\mp1\DFINAL RG FIRE16\mp1\RAIN Depth only data in these 8 inch pipes leads to same conclusion in this 602 meter basin Rain (in) 0.15 Depth1 (ft) Dec Fri 6 Sat 7 Sun Time Figure 14 Depth data by itself is able to determine which of the four micro basins experiences RDII. Of course, looking for RDII with depth-only data suffers from certain weaknesses. The first problem occurs if the sewers being studied become surcharged during rain events. But in theory this would be known during the first round of metering that depth-only metering would be ruled out. The second weakness appears if the sewers being studied have a wide range of slopes. A flat sewer could exhibit a large depth change during a storm whereas a more-leaky sewer with a steeper slope may exhibit a small change and appear to be good. The issues can be noted and overcome with good field work during the installation of depth sensors. 709

12 CSO Applications The two critical depths that CSO managers need for public notification or to calculate overflow volumes are the depth on the upstream side of the overflow weir and the water level in the receiving water. Figure 15 shows an ECHO sensor mounted in the manhole above the regulator to determine if the elevation is above the weir elevation and then to use a weir equation to calculate overflow volume. Figure 15 ECHO sensor measuring water surface elevation in a CSO regulator. Figure 16 is the LRD sensor measuring elevation of the receiving water outside of the CSO regulator. By comparing the two elevations an operator can determine if an overflow is active. Figure 16 LRD sensor measuring elevation of receiving water body. 710

13 CONCLUSIONS The parabolic Whisper Dish technology employed by the LRD and ECHO sensors is a new and innovative addition to the Collection System Management tool box. It offers the opportunity for managers to detect blockages in such a way that scheduled maintenance visits to hot spots can be replaced by on-demand cleaning conducted only when the data indicate that a blockage is developing. Its low cost combined with wireless communication and multi-year battery life makes it an economical way to monitor known hot spots as well as overflow alarms and measurements to quantify flow at CSO regulators. Managers can also use the sensor to measure tidal or river elevation to help sort out if an overflow occurred. 711