Overview
The volumetric flow rate of water (discharge) in rivers and streams is commonly estimated from the measured water-surface elevation (stage) using an empirical fit to measurements of stage and concurrent discharge. This is referred to as the stage discharge relationship or stream rating. Stage data is measured with water level sensors connected to a data logging system and is often transmitted by telemetry to a computer.
NexSens has simplified the sensor interface, data collection, telemetry and software associated with setting up and operating stream gauge stations. Specifically designed for environmental monitoring applications, the iSIC data logger offers superior data acquisition performance in extreme conditions. Options for land-line, cellular, radio and satellite telemetry are available. The iSIC data logger is housed in a rugged, NEMA 4X enclosure with a long-life, rechargeable battery.
Data Logging and Telemetry
At the heart of the stream gauge system is a battery operated data logger, which is typically located on shore. It can be pole mounted, or housed in a secondary building or enclosure. Figure 1 shows a pole mounted NexSens iSIC Data Logger near a small Pennsylvania stream. The data logger stores water level data at a specified time interval.
When configured with telemetry, data can be transmitted from stream gauge site to a project computer. Figure 2 shows a NexSens 3100-iSIC Cellular Data Logger housed in a corrosion resistant stainless steel enclosure and mounted at a bridge site.
Two factors help determine the best telemetry method site conditions and distance to a base computer. When the project computer can be located within a few miles (line-of-site) or few hundred feet (non-line-of-site), license free spread spectrum radio telemetry, such as the NexSens 4100-iSIC Data Logger and 4100-Base Radio is the best choice. The distance can be extended with use of repeaters, which for the NexSens system is simply an additional 4100-iSIC. When the project site is remotely located, relative to the project computer, cellular telemetry becomes the best option. The NexSens 3100-iSIC Cellular Data Logger is easily configured for various cellular providers and supports GSM/GPRS, CDMA, iDEN and EDGE technologies. Cellular signal strengths should be confirmed during the site survey and before setting up a cellular data account. When cellular signals are weak a NexSens 2100-iSIC Phone Data Logger or a NexSens 4200-iSIC Phone-Radio Data Logger should be considered.
Batteries and Solar Charging
All NexSens iSIC Data Loggers ship with as standard 12VDC, 8.5 AHr sealed lead acid battery. This battery is designed for typical applications and should work well in most gauge station applications. A NexSens A22, 20-watt solar panel will typically keep the battery above the threshold voltage required for proper operation. Consider an A23, 30-watt panel for extreme northern climates. Larger battery options accommodate non solar systems or projects which require other sensors and water samplers. The NexSens A03 12VDC, 26 AHr battery is designed to fit into the optional stainless steel enclosure. For ultra-high power applications consider NexSens A09 55 12VDC, 55 AHr deep cycle marine battery.
Selecting a Sensor
There are five common types of sensors
Ultrasonic Sensors
Ultrasonic sensors (Figure 3) mount above the water and measure the time of travel for a sound wave to reflect and return to the sensor. These sensors are the least accurate of all the gauge station sensor but offers simplicity in installation when mounting to a bridge or structure above the water. They do not require contact with or submergence in the water. They are unaffected by debris and floods and are less likely to be damaged by a nearby lightning strikes. Accuracy is degraded with air temperature variations, mist and foam on the water.
Pressure Sensors
A pressure sensor (Figure 4) measures the combined pressure exerted on it by the atmosphere and the head of water above it. A vented pressure sensor corrects for changes in barometric pressure. The pressure sensor is the most common type of stage gauge sensor. The sensor is fixed below the minimum expected water level and a cable containing the sensor signals and vent tube runs from the sensor to the data logger.
Bubblers
Bubbler systems (Figure 5) measure the water level by detecting the pressure required to force an air bubble through a submerged tube. The pressure is proportional to the water level. A bubble tube runs from the bubble generator, which is typically mounted on shore, near the data logger to a surface water orifice. The orifice is fixed below the minimum expected water level and diffuses the air bubble into the water. Damage due to debris and floods will be minimal as the bubble tubing and orifice are relatively inexpensive. Bubbler systems offer high accuracy, which can be slightly degraded by sediment build-up.
Shaft Encoders
Shaft encoders (Figure 6) connect to floats which follow changes in water level. As the float position changes, the shaft turns and the encoder tracks the position. The least expensive of all stage gauge sensors, shaft encoders offer an affordable alternative and when properly installed high accuracy and reliability. The installation will require a stilling well, which for some sensors like the OTT Thalimedes can be a simple 4-inch pipe with locking cap. See Figure 8 for a typical installation.
Radar Sensors
Radar sensors (Figure 7) mount above the water and measure the time of travel for a microwave to reflect and return to the sensor. The microwave signal is less influenced by temperature gradients, mist and foam on the water. Accuracy is not as good as the bubbler or pressure transducer but the non contact mounting a 115 ft distance to water specification on the OTT Radar Level Sensor (RLS) make it a solid choice when a bridge or above water structure is available.
Software
NexSens iChart software (Figure 9) is a Windows based program, which simplifies and automates many of the tasks associated with acquiring, processing, analyzing and publishing gauge station data. iChart has an extensive library of predefined device drivers for popular water level sensors. New monitoring devices are quickly configured by selecting from a drop-down list of manufacturers. There is no complicated programming required to set up data loggers and real-time monitoring networks. Simply choose a sensor, select a telemetry option, and your computer is ready to begin retrieving data. Data processing, reporting, exporting and sharing is automated and easy. One button export to Microsoft Excel and Adobe PDF are notable features.
Web Datacenter and Web Data Applets
WQData (Figure 10) is a secure web datacenter providing an online interface for viewing gauge station data. It offers 24/7 instant access to project data using any web browser. Specifically designed for environmental monitoring applications, WQData is easy to setup and maintain. Your project specific web datacenter is automatically generated by iChart software and posted to the NexSens servers. The datacenter can be setup as a secure (password protected) site or an open site for easy access by anyone with the web address.
Included with WQData is the NexSens Web-Data Applet. Simply embed the applet code on your webpage to present website visitors with project data. An 'ALL DATA' button on the applet directs the visitor to the project datacenter where project maps, data, tables, graphs, statistics and other data visualization tools provide a professional and fully featured project website. An integrated project forum allows all team members to communicate and share information.
