FINDING Stormwater Discharges
Using
Aerial Infrared Thermography
Gregory R. Stockton
AITscan Division
(800) AIT-SCAN
www.AITscan.com
ABSTRACT
Polluted water
contaminating the surface water and drinking water supply has been identified by
the EPA as one of the most serious environmental problems facing the
GENERAL
Typically, liquids flowing into a stream or lake appear warmer than the surface of the larger body of water, particularly during cooler times of the year because of the relative warmth of the ground a short distance below the surface (Figure 1).
Figure 1) Aerial infrared
image of outfall.
Leaks from nearby lines often come to the surface through lateral flows to a stream or lake bed, or to a slope (Figure 2) leading down to the surface of the water. These leak areas and the warm plume of liquid joining and flowing downstream with the cooler water are visible in the thermal IR spectrum. In most parts of the United States, late fall, winter, and early spring are well suited to this type of inspection because of the greater difference in temperature between ground and surface water, as well as the minimal interference from overhanging foliage (Figure 3).
Figure 2) Aerial infrared image of outfall on bank slope.
Figure 3) Photograph of the typical creek landscape, taken in early
summer.
Why IS THIS NEEDED?
Municipalities must design a stormwater management program that:
- Reduces the discharge of pollutants to the "maximum extent possible",
- protects water quality,
- satisfies the appropriate water quality requirements of the Clean Water Act.
Aerial IR thermographic surveys can help municipalities fulfill this obligation. Stormwater collection systems are engineered to efficiently drain selected areas and to discharge the runoff into surface waters. All too often, these systems convey pollutants from illicit connections, degraded sanitary sewers and septic tanks, and other sources. Until now, locating these point sources has been a labor-intensive task that often relied on taking samples blocks or even miles from the actual pollution source. Traditional methods of pollution source detection, including on-the-ground water quality sampling and visual stream surveys, do not provide effective coverage of large areas, so many problems go undetected. Many municipalities have become interested in using alternate, cost-effective means of pollution detection that overcome the limitations of the traditional labor-intensive approaches, while satisfying Clean Water Act requirements. They would like to identify and abate in-stream increases of bacteria, metals, nutrients, pathogens, herbicides and other pollutants from urbanization, along with malfunctioning septic systems, illegal sanitary sewer and storm drain connections and any other illicit discharges. An aerial IR survey provides an efficient and cost-effective way to find these point sources. After ground verification and analysis of the information collected during an aerial IR survey, officials can take action to deal with malfunctioning systems and illicit discharges. Municipalities can also identify areas that contain priority clusters or higher concentrations of pollutants and prepare lists of individual property addresses located within these clusters. This type of project demonstrates to local residents a heightened awareness by public officials of illegal stream connections, septic system failures, and general water quality issues. Aerial IR surveys, ground verification, and remedial follow-up provide measurable environmental results, including enhanced in-stream water quality, recovery of aquatic species, improved collection systems and septic system maintenance as well as increased knowledge of groundwater movements.
Ground v. aerial
Conducting a ground-based visual survey of a stream requires walking the entire length on both sides. Ground-based IR surveys offer few advantages over visual surveys and may cost more. Its major advantage is that inspectors need only test the water outfalls that show heat signatures, which potentially increases the speed of the survey if there are few anomalies. However, there are additional costs involved with a ground-based IR survey. Given the expense of the personnel and equipment needed, and assuming downtime for adverse weather, sick days, and/or injuries, either type of ground-based survey costs more than most municipalities are willing or can afford to pay. In contrast, aerial IR surveying is quick and efficient. Under good conditions, aerial IR thermographers can scan up to hundreds of stream-miles in one night, and produce a complete, accurate report in a timely manner.
Methodology, Equipment AND CREW
IR imagery often consists of grayscale pictures whose varying shades represent differences in temperature and the emissivity of objects in the images. As a general rule, lighter colors designate warmer objects and darker colors indicate cooler ones. All objects in the images are detected at thermal IR wavelengths in the 3,000nm – 5,000nm range (shortwave) or in the 8,000 nm – 14,000nm range (longwave). Lights and other relatively hot objects are evident because of their heat–not light emissions.
Images taken with an IR camera during a flight are often recorded on videotape and/or saved digitally to on-board hardware and later converted to digital image files, which can then be modified in a number of ways to enhance their value to the end user. Professional survey results require methods and equipment that are specifically designed for the task. In applications where a straight-down view, a large area view and/or where long distances must be covered in a limited time, aerial IR is superior to ground-based IR in all respects. The selection of the proper aircraft, camera mount, IR imager, navigational aids, recording medium, workstation computer equipment, pilot and crew are all critical to success.
Both helicopters and light airplanes can be used to perform aerial IR surveys. Spatial resolution and thermal sensitivity are all-important in aerial IR thermography. It is always better to use a large pixel array, although larger lenses will help if some signal strength degradation is acceptable. Using a more powerful lens does reduce the ground resolution element (GRE)—the size of one pixel on the ground for a given distance. But this reduced GRE also reduces the sensor’s field of view, which limits the area covered on the ground. Also, the aircraft’s movements and vibrations, particularly those of a helicopter, may cause image blurring or smearing, which results from an increase in the apparent speed of the sensor’s view across the ground. The GRE and other thermal imager characteristics need to be carefully considered before the aircraft and imager are selected for a particular job. Our extensive research and experience in aerial IR applications have shown, for example, that a handheld, small-format imager held out the open window of a helicopter will not produce professional results.
A well-maintained aircraft and high resolution infrared imager (Figure 4) with the latest digital videotaping and mapping capabilities equipment are components essential to success. Everything in the aircraft must be secured, and wires clearly labeled, placed out of the way, and shielded from electromagnetic interference. Precise navigation is important in any aircraft, but particularly so in nighttime aerial IR operations. To produce the most valuable report possible, one must record the imagery and exact location of all areas surveyed. Because the pilot and thermographer are extremely busy during the flight, one or both could miss an anomaly. Therefore, all imagery and matching Global Positioning System (GPS) information need to be recorded. During post-flight analysis, each frame of the video will receive methodical and detailed scrutiny. For this reason, the thermal imager video output must be routed through a video encoder/decoder (VED) that encodes the videotape with a continuous stream of GPS-derived information (Figure 5).
Figure 4) Large
format infrared imager, fixed-mounted.
Figure
5) Video encoder-decoder (VED) annotation guide.
The image displays the annotation data—date, time, latitude and longitude, etc., on a strip at the bottom of the image. A digital video recorder records the encoded/annotated imagery, while a laptop computer with specialized mobile mapping software is used to guide the aircraft and map the designated flight path.
Aerial IR imaging is not a job for airsick-prone IR equipment operators or pilots who have not received specialized training in such operations. The aircraft must fly over and along the surface drainage system in a manner that allows the accurate imaging of the target creek, stream, river, or lake. This is very difficult flying which requires a highly-skilled pilot. In the cockpit, a moving-map on a computer connected to GPS antennas permits the crew to monitor the flight path and the aircraft’s location with respect to the drainage area, and guides the pilot along specific flight lines to ensure complete coverage. The infrared thermographer/imager operator controls the settings and interrupts the recording during turns outside the study area, which omits extraneous imagery.
POST-FLIGHT Analysis
After the flight, the videotaped imagery is analyzed using a digital VCR, a high-resolution TV monitor, and an integrated computer system with specialized hardware and software. As the tape plays, the GPS-coded signal received and recorded during the flight is decoded by the VED, which re-creates the original GPS signal and sends it to the computer so that its mobile mapping software interprets the recorded signal as a live signal. The mapping software shows the position of the moving airplane superimposed on a topographic map on the computer screen, while the recorded IR imagery of the area below the airplane appears on a second monitor. GPS signals update the airplane’s position once every second throughout the flight and at the same rate during the post-flight analysis.
To find potential sources of pollution during the post-flight analysis, thermographers view the tape in its entirety—pausing and playing it backward and forward at regular speed and in slow-motion as necessary. For each hour of videotape, many hours of analysis will be required to complete the report. After all anomalous sources are found, they are marked on the topographic map (Figure 6), and IR thermographs are digitally captured. Each anomaly is assigned a number that corresponds to a number on a specific image. The maps and digital images are then brought into an image-processing software application and adjusted for such qualities as contrast and brightness before being scaled for final editing.
Figure 5) Example of a
topographical map marked with anomaly locatios.
Example results
Below are the results of a survey conducted by the
Mecklenburg County Water Quality Program in
Aerial IR surveying pinpointed 62 heat anomalies along 27 miles of two creeks:
Q
One anomaly was identified as a failing 15-inch
sewer line. Charlotte Mecklenburg
Utilities replaced the line and the discharge stopped.
Q Another anomaly was an illegal discharge into the storm drain system from a convenience store. The discharge was removed from the storm drain and tied to the sanitary sewer system.
Q
Ten anomalies were identified as dry weather
flows to storm drains with elevated fecal coliform
bacteria levels. Additional follow-up
field investigations were conducted to identify the sources of these problems.
Q Twelve anomalies were no longer found to be flowing during several field investigations. Inspectors carried out additional investigations to check for recurrence of the discharges.
Q
Ten anomalies could not be located on the
ground. Additional follow-up was
performed in an effort to identify them.
Q
Five anomalies proved to be sewer collection
system features with no discharges to surface waters. No further follow- up was required.
Q Another twelve anomalies were identified as being dry weather flows to storm drains but with no negative water quality impacts. No further follow-up was required on these.
Q Eleven anomalies were attributed to groundwater flow. No further follow-up was required.
Conclusions
Municipalities must comply with Clean Water Act and develop,
implement, and enforce a stormwater management
program that has been designed to minimize the amount of pollutants discharged
into local waters. By using specialized equipment and techniques, aerial IR
thermographers can locate pollution point sources so officials can act to
prevent contaminants from entering waterways. Aerial IR surveys will continue
to assist municipalities in making
Author Biography
Gregory R. Stockton is president of Stockton