Electromagnetic Current Meters and ADCP Quiz

Introduction to Hydrometry

• Hydrometry is the study of streamflow, a key task for hydrologists
• It has seen significant changes, mainly due to the implementation of electronic instruments in environmental science
• Streamflow measurement techniques can be categorized as instantaneous and continuous

Instantaneous Streamflow Measurement - Velocity-Area Method

• Streamflow, or discharge, is the volume of water flowing per unit of time (measured in m³/s or cumecs)
• It's calculated by multiplying water velocity (m/s) by the cross-sectional area (m²)
• The velocity-area method divides the stream into sections to measure water velocity in each
• Discharge is then calculated by summing the product of velocity and area for each section
• This method provides a more accurate estimation, accounting for velocity and cross-sectional area variations

Making a Streamflow Measurement

• ISO 748:2007 provides precise guidelines for measuring stream discharge in streams, rivers, and open channels.
• Stream reach selection should prioritize straight, uniform areas without obstacles (trees, rocks).
• Stream width and vertical measurements are vital for accurate discharge estimations.
• Rougher streambeds necessitate more verticals to accurately measure the required flow rate for compliance with ISO specifications.
• Stream velocity measurement uses a current meter; technological advances in current meters have improved hydrometry greatly in recent years.

The Velocity-Area Method

• This method combines velocity measurements with cross-sectional area calculations to estimate streamflow
• Accurately representing the entire profile of stream velocity is necessary for precise results
• Velocity measurements are typically taken at depths of 60%, 20% and 80% for single measurements, and deeper rivers respectively
• Measurements should last for at least 30 seconds, taking care to face the current meter directly into the stream flow.
• Wading rods and boat-based or cage-based methods are used, depending on the river conditions and sizes to ensure measurements cover the entire stream profile.

Types of Current Meters

• Mechanical Current Meters: These are propellers that measure stream velocity by counting revolutions over time. They are commonly used for river sizes of different magnitudes.
• Electromagnetic Current Meters: These measure fluid velocity in pipes and open channels based on electromagnetic induction. They offer advantages like avoiding obstruction but can be more expensive, and not as widely used today.
• Ultrasonic Flow Measurement (ADCP): Acoustic Doppler Current Profiling (ADCP) is a method using ultrasonic pulses to measure flow direction and transit time and provides advanced, real-time velocity measurements across a river.

Continuous Streamflow Measurement

• Three techniques for continuous stream flow measurement exist in hydrograph (data) collection: stage-discharge relations, flumes and weirs, and ultrasonic flow gauging
• Two International Standards (ISO 1100-1 and 1100-2) cover continuous stream flow measurement

Stage vs. Discharge Relationship

• River stage, meaning the water level or height, is related to discharge using a rating curve
• These curves are obtained via multiple discharge observations at various river stages
• These relations enable continuous discharge measurement from recorded stage measurements
• This relationship is important for various water management and flood prediction purposes
• Gravel-bed rivers are prone to rating curve changes; updates are often required after major flood events

Measuring Stage Height

• Historically, stage height was measured using floats and counterweight systems in stilling wells
• Modern methods include gas bubblers, pressure transducers and capacitance probes for measuring water depth and pressure at the river bed.
• Electronic recordings and telemetry systems now allow more efficient data collection and management.

Flumes and Weirs

• Flumes and weirs are structures to provide continuous discharge data by regulating velocity and cross-sectional area

• A stilling pool allows for consistent flow or velocity input regardless of river levels

• Structures have a fixed cross sectional area, allowing stage height measurement to calculate discharge

• Rating curves for flumes and weirs are derived via experimentation and hydraulic theory, often based on power equations

• ISO 1438 standard provides theoretical rating curves for various weir types (e.g., a V-notch weir)

• Discharge (Q) calculations use equations based on variables such as gravity, discharge coefficient, V-notch angle, and water depth

• Flume and Weir differences:

  • Weirs force water over a structure creating a waterfall.
  • Flumes direct water through a channel without a significant drop.

• V-notch weirs are ideal for low-flow conditions, and trapezoidal flumes work for higher flows

• Sediment accumulation in stilling ponds is a challenge; trapezoidal flumes and other techniques help mitigate this

Continuous Flow Gauging

• Technologies like Acoustic Doppler Current Profilers (ADCPs) and ultrasonic instruments are used to measure flow continuously
• These methods can measure velocity and sometimes cross-sectional area, particularly in cases without large deviations and spreads during flood conditions.

Physical/Geomorphological Estimation

• The geomorphological approach assumes equilibrium between river channels and flow regimes; this allows for utilizing specific channel measurements (depth-to-width ratio, wetted perimeter, and bankfull discharge) to estimate streamflow over time
• It's particularly valuable in determining mean annual floods, considering small flooding events, alongside stream diameter, wetted perimeter, and average depth
• Manning's equation is used to determine average velocity of a river stretch for specific calculations

Dilution Gauging

• This technique relies on diluting a solute/tracer introduced into a flowing river water

• The concentration of the tracer, in combination with other measurements, facilitates the calculation of stream discharge

• Two primary methods:

  • Gulp Dilution: A known volume of tracer is introduced, and the tracer concentration is measured as it flows downstream.
  • Continuous Injection: A continuous tracer injection is used, and the concentration downstream is monitored to determine discharge

• Mixing distance calculation is crucial in this process. This is often a complex calculation depending on factors such as Chezy's roughness coefficient, average stream width, gravity, and average depth within the river.