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. Discharge data is often calculated using water level sensors or Doppler velocity meters connected to a remote data logging & telemetry system.
Measuring the changes in water level over time is only one component to developing a stage discharge relationship or stream rating. In order to obtain a good estimate of water flow, point discharge measurements should be taken periodically to establish and maintain a good rating curve. Each discharge measurement is correlated to the water level at the time the measurement was taken. With this information, discharge data can be interpolated and applied to the full range of water level measurements.
Point discharge measurements in wadeable streams can be obtained using a handheld velocity meter mounted to a wading rod. The SonTek FlowTracker is one such instrument commonly used by government organizations such as the U S Geological Survey. In non-wadeable stream environments, a more sophisticated stream discharge measurement system is often required. Towable Doppler velocity systems mounted on either a boat or Trimaran are commonly used to measure discharge in rivers and larger streams.
Selecting a Sensor
A pressure sensor 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 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. The pressure sensor is the most common type of stream gauge sensor because it is relatively cost-effective to install and maintain.
Bubbler sensors 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 shore-mounted air compressor to a surface water orifice in the stream. The orifice is fixed below the minimum expected water level and diffuses the air bubble into the water. Damage from debris and floods is minimal with bubblers, as the bubble tubing and orifice are relatively inexpensive. Bubbler systems offer high accuracy and do not drift over time.
Radar sensors mount above the water and measure the time of travel for a radar pulse to reflect and return to the sensor. Two antennas transmit pulses toward the water surface through a Teflon face plate. The transmitting antenna transmits short radar pulses, and the receiving antenna receives the pulses reflected from the water surface. The time delay from transmission to receipt is proportional to the distance between sensor and water surface. The non-contact solution is a solid choice when a bridge or above-water structure is available.
Acoustic Doppler current meters use sonar for precise water velocity flow measurement in larger streams and rivers. Doppler current meters are mounted in a fixed spot, typically on a riverbank, bridge abutment, or other vertical structure. Doppler current meters can provide significantly more information than water level sensors because they directly measure both water level and horizontal water velocity. They are also widely used in streams and rivers that may experience bi-directional water flow, such as an estuary.
Data Logging and Telemetry
At the heart of the stream gauge station is a NexSens iSIC or SDL data logger, which is typically located on shore. The data logger can be pole mounted or housed in a secondary building or enclosure. Options are available for solar or AC power. When configured with telemetry, data can be transmitted in real-time from the stream gauge site to a project computer, eliminating the need for periodic site visits to upload data.
Two factors help determine the best telemetry method - site conditions and distance to the project 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 is the best choice. If the project site is remotely located relative to the project computer, cellular telemetry becomes the best option. Options are also available for landline phone or Ethernet telemetry if the site has access to a landline or Ethernet hub.
Selecting a Location
The location of a stream gauge station should be chosen based on a variety of general criteria including, stream morphology, safety concerns, accessibility, and location of pre-established gauge stations. The stream should be free flowing and unrestricted by obstructions upstream or downstream that could cause flow diversion or flow backup. A smooth streambed profile with relatively consistent depth across its width is optimal.
When possible, it is oftentimes preferable to install a flume or a weir in the stream. Flumes and weirs are structures of known dimension that force all water through an opening. A weir is a dam built across an open channel with an opening or notch for water to pass through. A flume is a specially-shaped open flow channel. It typically has a converging section where the flow is constricted, a throat section water level is measured, and a diverging section. NexSens iChart Software includes built-in weir and flume calculations that allow the user to automatically output discharge data.
In larger streams and rivers, side-looking Doppler velocity meters that take into account both water velocity and level have been proven to be a more accurate method of calculating discharge. These meters are mounted in a fixed spot, typically on a riverbank, bridge abutment, or other vertical structure. A side-looking current meter is usually installed in the middle of the water column, allowing it to achieve the maximum possible measurement range without interference from the surface or bottom.
Managing Data
Water level data is logged at a user-defined interval (minimum 1 minute). The user also sets the interval at which data is transmitted via cellular telemetry. A common interval for stream gauge stations is to log water level data every 10 minutes and transmit every 30 minutes. Water level data can be transmitted via landline phone, cellular, radio, satellite, or Ethernet telemetry to a remote computer.
iChart is a user-friendly Windows-based software package that serves as the centralized interface and database for all incoming data. A single Software license can manage a virtually unlimited number of remote stream gauge sites. The software allows users to generate customized reports with water level data from all systems in an iChart database. Reports can be converted to PDF, exported to Microsoft Excel, sent to interested parties via e-mail, uploaded to a web server, and more.
NexSens WQData PRO is an optional web-based data management solution for stream gauge stations and other environmental data. The modular datacenter interface can be easily customized to include project-specific themes and information. WQData PRO offers a unique Active Data Sourcing tool that allows users to query and include water level data from USGS gaging stations, National Weather Service sites, and other publicly available datacenters.
Rental Equipment and On-Site Services
Stream Gauge Stations are most commonly purchased and installed for long-term or permanent applications; however, there are commonly short-term level measurement and flood alert applications that make it cost-prohibitive. NexSens Technology offers both standalone and real-time cellular telemetry Stream Gauge Stations with weekly and monthly rental rates to meet project requirements.
To ensure that customers can begin using their Stream Gauge Stations as quickly and efficiently as possible, our application engineers are available for training and technical support, both by phone and in person. Our company offers both extensive field experience and a wide array of deployment hardware to facilitate seamless implementation of the project. Contact our application engineering team for your next stream discharge monitoring project.
