To monitor stream flow and provide real time data that will allow operators to discharge treated water into the stream without adverse effects on water quality.
Provide real time flow monitoring data through an existing SCADA system
Provide a minimum flow warning so the plant can operate within consent conditions
Maintain a secure data record to demonstrate compliance with consent requirements.
Install a system that allows for future development.
Provide both high quality and a low maintenance system.
Present a cost effective solution to meet compliance
As there are few instruments available that can accurately measure stream flow directly, it is standard practice to measure water level (stage), and then calculate flow data (discharge) from this by means of a graphed curve. This rating curve shows the relationship between stage and flow and is calculated from the flow velocity and flow volume at measured levels. More information on this process is contained here
This project included the following components:
Continuous measurement of water level using electronic instruments.
Developing a relationship between water level (stage) and flow from a series of manual stream gaugings. (Developing the rating curve)
Calculate flow in real time using the current water level measurement and the flow relationship (the rating curve)
Electronic data output of water level and flow to the client's SCADA system as well as local LCD display
All of the equipment, apart from bubble tubing and chamber, was installed into an enclosure inside the site control room. The LCD display was recessed into the lid of the enclosure allowing quick reference to the logger data, without entering the enclosure.
Water level is measured via an air tube running from the Ott Bubbler to a bubble chamber mounted in the stream. The water level is calculated very accurately by measuring the pressure required to push an air bubble down the length of the tube. The differential pressure and calculated water level is referenced against a manual staff gauge installed at a surveyed datum in the stream. This datum is recorded for future reference back to the site if any physical changes or washouts occur and all data is referenced back to the staff gauge to maintain accuracy and allow for errors due to instrument drift and site specific factors such as silt build up.
Other sensors could have been used to measure water level however, in this case a bubbler was chosen to ensure maximum accuracy, easy maintenance and equipment longevity. Ott is a leading manufacturer of hydrological instrumentation and because the only equipment in the stream is low cost (tubing and bubble chamber), the cost of replacement due to flooding, crushing or other physical damage is hugely reduced.
The datalogger records the water level, and uses a pre-programmed rating table or equation that calculates flow in real time. Data is stored in the logger and also output in real time to both the LCD display and the site SCADA system.
This system consists of a water level sensor in an open channel, but would work equally well if not better behind a weir or other control structure. It provides timely and robust information, and data that will stand scrutiny from a regulator, or in fact the Environment Court if required. Once the initial flow measurements have been completed, over a period of say, 12 or 24 months, the site requires minimal maintenance (depending on conditions in the waterway) and calibration checks of the derived equation in the logger.