OTT RLS Radar Level Sensors Help Optimize South Dakota TMDL Monitoring
By Christel Valentine, Hach Hydromet
Non-contact sensors using accurate pulse radar technology are helping the South Dakota Department of Environment & Natural Resources more efficiently monitor water levels at current and future statewide Water Quality Monitoring sites.
The South Dakota Department of Environment & Natural Resources (DENR), working with the East Dakota Water Development District (EDWDD), continue to implement the state’s Water Quality Monitoring program by monitoring areas of the Big Sioux River basin, the Vermillion River basin, and the Minnesota River basin – all within the eastern part of the state. Total Maximum Daily Loads (TMDLs) can be developed through the use of the information provided by the monitoring results. TMDLs determine the maximum amount of a pollutant that a water body can receive without violating water quality standards. The TMDL process identifies all sources of the pollutant causing the impairment and then determines how much it must be reduced to meet water quality standards.
For the South Dakota DENR, gathering water quality data for TMDL compliance can be complex within the state’s vast landscape. The Big Sioux River basin, which forms the Iowa-South Dakota border, drains an approximate 4,280 square miles in South Dakota. The Vermillion River basin covers an area of 2,652 square miles in southeastern South Dakota, while the Minnesota River basin, found in the northeastern corner of South Dakota, drains an area of 1,572 square miles within the state.
For the East Dakota Water Quality Monitoring Project, new instruments are being placed in the various locations along the three rivers and their selected tributaries in order to maintain a continuous record of water levels in the channels. Discrete level measurements are automatically made and recorded to develop a long-term record of water discharged into the watersheds.
“The EDWDD project is not just for current TMDLs, but in anticipation of future TMDLs as well,” says Sean Kruger, Environmental Program Scientist with the South Dakota DENR. “The sites where we are installing gauges are long term monitoring sites from which we will continue to collect data for years to come. Currently, the most common TMDLs in that area are suspended solids and bacteria.”
“We had been collecting water quality data for several years at many sites where we did not have stage information to go along with it, and this had made TMDL development exceedingly difficult,” says Patrick Snyder, Environmental Senior Scientist with the South Dakota DENR. “We now see a real need for collecting this data for the long term. We want data we routinely collect to be more useful for TMDL development if we need it, and this requires the ongoing collection of water level data.”
The department had deployed a limited number of water level sensing instruments, such as bubblers and shaft encoders, to monitor surface water levels at temporary water quality monitoring sites throughout the state. With the implementation of Water Quality Management (WQM) sites for the East Dakota Water Quality Monitoring Project, however, it was determined to find an alternative method for measuring and recording water levels.
“It had been difficult and expensive to install these type sensors because they have to be submerged in the water and then protected, which typically required encasing them in a conduit and anchoring them to the streambed,” says Barry McLaury, Environmental Program Scientist with DENR. “Then, a length of cable had to be strung from the submerged sensor to a separate enclosure on land that contained the data logger.”
The DENR had a number of these sensors wash away during ice floes and flash floods.
“Nature had a way of causing big problems with some of our units,” says McLaury. “Even though they were installed well, we lost a number of them in bad storms.”
New Radar Sensors Deployed
For the WQM sites serving the East Dakota Water Quality Monitoring Project, the DENR began evaluating new level sensor technologies that could provide more permanence and be easier to install. At a number of these sites the DENR elected to install the OTT Radar Level Sensor (RLS), a new, non-contact radar device with pulse radar technology that is designed specifically for surface water level measurements.
Rather than being submerged, the 4.6 lbs OTT RLS mounts unobtrusively to a bridge, frame, pipeline, or extension arm. The OTT RLS employs two antennas enclosed in a compact housing (8.7” x 6.0” x 7.5”) that transmit pulses down toward the water surface through a Teflon faceplate. The transmitting antenna transmits short radar impulses in the 24 GHz ISM band, which are reflected off the water’s surface and captured by the unit’s receiving antenna. Approximately 16 individual measurements are conducted each second and averaged after 20 seconds to minimize the influence of surface waves.
Initially the department installed four of the sensors on the Big Sioux River near Sioux Falls in the spring of 2009.
“We had our system integrator set up the systems for us, which included integrating each RLS unit with a recorder and power supply,” McLaury says. “A cable runs from a small enclosure (14” x 12” x 6”) to the radar level sensor, allowing both to be mounted discretely side-by-side.” Included in the enclosure are a data logger and a 12 volt battery. The solar panel is mounted adjacent to the enclosure. The DENR specified the data loggers be programmed to record water levels every 15 minutes, and battery voltage is also simultaneously recorded.
The OTT RLS is mounted directly above the water surface, such that the radar beam is perpendicular to the water. “They were very easy to install -- pretty much plug and play,” McLaury says. ”We mounted the radar level sensors on the sides of bridges, and each one took about half a day to install. A key safety advantage is that we weren’t in the water,” he says. “Also, we didn’t need to install a conduit down into the water and anchor all of it to the streambed. As far as the ease of mounting the equipment, they were probably one of the easiest installations we’ve ever worked with.”
“We basically just bolted these two small units on the side of the bridge and we were done,” confirms Kruger. “And, the data we were seeing coming back from them pretty well convinced us to install those additional RLS units.” To date, OTT RLS units have been deployed at 9 TMDL sites serving the East Dakota Water Quality Monitoring project.
DENR personnel drive to the bridges on a scheduled basis, open the enclosure and connect a laptop computer to the data logger and download the data. The entire procedure takes just a couple of minutes. Wireless systems are available so that data can be downloaded remotely, and the system integrator made provisions so that the department could easily add telemetry in the future.
“Compared to some of the other type units we've used in the past, it's very easy to download the data with the RLS,” says McLaury.
“And there was no learning curve with the software,” Kruger adds. “A lot of times it takes a while to figure out how to program measurement devices. But this was the simplest that I've ever dealt with. We had the recorders programmed so that data can be shared easily between us and the East Dakota Water Development District.“
Virtually maintenance free, the OTT RLS does not require any routine maintenance other than visual inspection of the radar's face plate and check for obstructions within the radar beam. Quality control measurements can easily be carried out by comparing the OTT RLS measurement against a measurement read from a staff or wire weight gage.
Besides ease of installation, the non-contact functionality and performance capabilities prompted the DENR to deploy the OTT RLS units. Measurements are not affected by air temperature, wave action, humidity, flooding, floating debris, or contaminated water. Accuracy is ±0.01 ft. for water level measurements, and will not drift over time.
The OTT RLS provides a large measurement range with a small blanking distance and narrow beam width. The sensor connects easily to most data loggers via standardized communication interfaces: SDI-12, 4-20 mA, and RS-485. Because of its extremely low power consumption during measurement and stand-by – only 12 mA @ 12V – it is ideal for remote or solar powered sites.
Critical Tool for Assessments
Section 303(d) of the federal Clean Water Act requires that states develop Total Maximum Daily Loads (TMDLs) for water bodies. TMDLs differ from other pollution management efforts in that all sources of impairment must be accounted for, including contributions from point sources (federally permitted discharge) and nonpoint sources. Moreover, all pollutant loads must be allocated to specific sources. Having accurate, continuous level measurements is critical to these assessments.
“Presently, we have Water Quality Monitoring stations across the state and there are quite a few of them that aren't located near a USGS (U.S. Geological Survey) station. Our long-term plan is to have level sensing units at those sites,” says Kruger. “In the long run, we believe they’ll help us to meet TMDL guidelines more efficiently and cost-effectively.”
About The Author: Christel Valentine is Associate Product Manager II for Hach Hydromet