Group 7 - Measuring Channel Flow 1 PDF

GROUP-7 MEASURING CHANNEL FLOW MISAGAL, JOHN EDWARD MONTES, PATRICIA MARIE NICOR, JOHN RONVER PALAMING, JAIRAH NICOLE Introduction Hydrometry is the study of streamflow, a fundamental task for hydrologists. It has undergone significant changes in recent years due to the integration of electron...

GROUP-7 MEASURING CHANNEL FLOW MISAGAL, JOHN EDWARD MONTES, PATRICIA MARIE NICOR, JOHN RONVER PALAMING, JAIRAH NICOLE Introduction Hydrometry is the study of streamflow, a fundamental task for hydrologists. It has undergone significant changes in recent years due to the integration of electronic instrumentation into environmental science. Streamflow measurement can be divided into instantaneous and continuous techniques.  INSTANTANEOUS STREAMFLOW MEASUREMENT VELOCITY–AREA METHOD Streamflow or discharge represents the volume of water flowing per unit of time, measured in cubic meters per second (m³/s or cumecs). It is calculated as the product of water velocity (m/s) and the cross- sectional area (m²). Therefore: In the velocity–area method, the stream is divided into smaller sections, and the velocity of water in each section is measured. The discharge is then calculated by summing the product of velocity and area for each section: This method ensures a more accurate estimation of streamflow by accounting for variations in velocity and cross-sectional area across the stream. Where: Q is the streamflow or discharge (m3 /s) v is the velocity measured in each trapezoidal crosssectional area, and a is the area of the trapezoid Making a streamflow measurement The international standard ISO 748:2007 outlines precise guidelines for measuring discharge in streams, rivers, and open channels, ensuring the highest standards. While this book provides broader guidelines, adherence to key principles is crucial. The first step is selecting an appropriate stream reach, which should be straight, uniform, and free of obstacles like overhanging trees or large rocks. Areas with reverse flow or backwaters must be avoided. A tape measure or "tag line" is then used to measure the stream's width and establish verticals for cross-sectional measurements. The number of verticals depends on the width and smoothness of the streambed, with rougher beds requiring more verticals for accuracy. For streams wider than 5 meters, a trial discharge measurement may be necessary to ensure compliance with ISO guidelines (verticals representing 5–10% of the total flow). Stream velocity is measured using a current meter, and advancements in current meter technology over the past 20 years have significantly improved hydrometry.  Making a streamflow measurement The velocity–area method measures streamflow by combining velocity measurements with cross-sectional area calculations. Accurate results depend on assuming velocity measurements are representative of the entire profile. A mechanical current meter typically measures velocity at 60% depth for single measurements or at 20% and 80% depths for deeper rivers. Measurements are taken for at least 30 seconds, and the current meter must face directly into the stream flow. When wading, hydrologists use wading rods, which help measure depth and calculate 60% depth conveniently, especially in cold conditions. In larger rivers or during floods, measurements are taken from boats, bridges, or suspended cages. In flood conditions, heavy, weighted propellers ensure measurements through the full profile, but this can be dangerous due to high velocities and debris. For rough estimates, a float can measure surface velocity over a distance. Surface velocity is adjusted using a factor (0.84–0.9) to approximate average velocity, per ISO 748:2007 standards. However, the method is less reliable in small, turbid streams or those with rough beds, where alternative techniques like dilution gauging may be more suitable. The International Standard provides methods for measuring stream velocity, but it is not possible to take multiple measurements due to water traveling faster along the surface than near the stream bed. The most common depth profile is one or two depths, with the current meter being 60 per cent of the stream depth. In deep rivers, it is recommended to take two measurements. A good wading rod is essential for field hydrologists, as it has graduated markings and a depth adjustment for easy reading. The velocity-area method is a reliable streamflow measurement technique, but its reliability depends on sampling strategy and rough bed conditions, making it less applicable in small, turbid streams. Types of current meter Mechanical current meters Mechanical current meters are propellers used to measure stream velocity. They record the number of revolutions over time as water flows in and around the meter. Different sizes and types are available for different stream sizes and velocities. Accurate measurements are crucial, as large propellers are suitable for large swift rivers and small ones for low flow. Until the 21st century, mechanical current meters were the most commonly used tool for measuring stream velocity. Electromagnetic current meters Electromagentic current meters measure fluid velocity in pipes and open channels. The fundamental theory is electromagentic induction, discovered by Michael Faraday in 1831. In streamflow measurement, the conductor is the body of water flowing past a magenetic field between two magnets. Faraday's earliest attempt was in 1832, but he struggled with accurate voltage measurement. Electromagnetic current meters have advantages over mechanical meters, such as not causing obstructions or debris, but are expensive and have not gained as much popularity as ADCP technology.  Types of current meter Ultrasonic flow measurement Acoustic Doppler Current Profiling (ADCP) is a hydrometry method that measures the difference in transit time of ultrasonic pulses propagating with and against flow direction. It has led to significant advances in hydrometry over the past 20 years, allowing integrated stream velocity measurements across rivers. ADCP instruments can measure in real-time a complete velocity profile for streams containing suspended particles, with precision required for nanoseconds. Various types of ADCP instruments exist, including pole-mounted devices, larger boats, and boat-mounted devices with bottom tracking features. CONTINUOUS STREAMFLOW MEASUREMENT Instantaneous streamflow measurement requires a continuous measurement technique for hydrograph data. Three techniques include stage-discharge relations, flumes and weirs, and ultrasonic flow gauging. Two International Standards cover continuous stream flow measurement (ISO 1100–1 and 1100–2). STAGE VS DISCHARGE RELATIONSHIP River stage refers to water level or height. Multiple discharge measurements can create a relationship between river stage and discharge, known as a rating curve. This allows continuous measurement of river stage, which can be equated to actual discharge. Colour shows the velocity at depths (vertical axis). The red colour in the middle at mid- depth is around 2.5 m/s; the blue and purple in the shallow edges is around 0.78 m/s  STAGE VS DISCHARGE RELATIONSHIP The stage vs discharge relationship relies on frequent discharge measurements and a stable riverbed profile. Changes in the riverbed, such as scouring during floods, sediment deposition, or aquatic plant growth, can alter the relationship, requiring adjustments to the rating curve. Gravel-bed rivers are particularly prone to these changes, often necessitating updates after each flood event. A key challenge is the lack of data during flood events, as these are infrequent and dangerous to measure, leading to higher errors in peak flow estimations. It is important to note that stage height is measured continuously, while discharge is estimated using a rating curve. In some cases, concrete structures like flumes or weirs may be required to maintain stability and improve accuracy. MEASURING STAGE HEIGHT Traditionally, stage height was measured using a float and counterweight system in a stilling well, where a float's movement translated into a stage height reading, initially recorded manually and now electronically. Stilling wells remain the most accurate method, often paired with loggers and telemetry systems, but they are vulnerable to damage during floods. Alternatives include: Gas Bubblers: Measure water depth by the force required to push air bubbles through a tube at the riverbed, with equipment safely located away from floods. Pressure Transducers: Measure water pressure at the riverbed, requiring adjustments for atmospheric pressure unless self-calibrating models are used. Capacitance Probes: Measure water depth by detecting electrical transmission, though water chemistry changes can affect accuracy.  FLUMES AND WEIRS Flumes and weirs are stream gauging structures designed to provide a continuous record of river discharge by controlling stream velocity and cross-sectional area. They slow down (or speed up) the stream using a stilling pond to ensure constant or known velocity, regardless of river level. The stream is then directed through a rigid structure with a fixed cross-sectional area, enabling stage height measurements to determine discharge. Once the velocity and cross-sectional area are controlled, the rating curve for a flume or weir can be derived through experimentation and hydraulic theory. These relationships are typically expressed as power equations, influenced by the shape of the structure's cross-section. The ISO 1438 standard provides theoretical rating curves for different weir types. For a V- notch weir, the discharge (Q) is calculated using the equation: Where Q is discharge(m^3/s), g is the acceleration due to gravity, C is the discharge coefficient, θ is the V-notch angle, and h is the water height. For a 90° V-notch, the discharge coefficient CCC is 0.578, and the equation simplifies to: FLUMES AND WEIRS For rectangular weirs, the discharge equation depends on the width of the exit and the coefficient of discharge. The shape of the cross-sectional area in flumes and weirs is crucial, often designed as a V-shape to match the flow regime of the river and data requirements. The V-shape is sensitive to low flows, making it ideal for studying low-flow hydrology, while also accommodating higher flows without overtopping. The V-notch angle varies based on stream size and required sensitivity. However, a challenge is sediment buildup in the stilling pond, which necessitates regular dredging. To address this, trapezoidal flumes are designed to speed up the stream, preventing sediment accumulation. This approach is suitable for small streams but not for large rivers due to the high power and strain involved.  THE DIFFERENCE BETWEEN FLUMES AND WEIRS Flumes and weirs both measure stream discharge continuously, but they operate differently. A weir forces water over a structure, creating a waterfall effect, while a flume allows water to pass through without a drop at the end.  CONTINUOUS FLOW GAUGING In continuous flow gauging, technologies like the ADCP (Acoustic Doppler Current Profiler) or ultrasonic instruments can measure flow. ADCPs, mounted on a fixed structure, measure stream velocity and sometimes cross-sectional area. However, measuring cross-sectional area is more challenging in rivers that spread widely during floods. Ultrasonic flow gauging is especially useful in channels with restricted areas, such as those affected by weed growth, where velocity is impacted by a narrower flow path. ESTIMATING STREAMFLOW Over the past 30 years, significant efforts in hydrological research have focused on developing numerical models to simulate streamflow. These models have evolved to simulate all processes in the hydrological cycle, allowing for a broader range of estimations beyond just streamflow. Despite this, streamflow remains a key focus, often considered the primary output of these models. This section specifically focuses on methods for directly estimating streamflow, as opposed to using simulations. PHYSICAL OR GEOMORPHOLOGICAL ESTIMATION TECHNIQUES The geomorphological approach to river systems is based on the concept that river channels maintain equilibrium with their flow regimes. This means that specific channel measurements, such as the depth-to-width ratio, wetted perimeter, and bankfull discharge height, can be utilized to estimate streamflows over time, particularly for assessing mean annual floods. Key parameters in this estimation include stream diameter, wetted perimeter, and average depth, especially during small flooding events. To estimate the average velocity of a river stretch, the kinematic wave equation known as Manning's equation is employed, expressed as: where: V is the velocity (m/s), k is a constant based on measurement units R is the hydraulic radius (m) s is the slope (m/m), and n is the Manning roughness coefficient. The hydraulic radius itself is calculated by dividing the cross-sectional area of the river by the wetted perimeter. PHYSICAL OR GEOMORPHOLOGICAL ESTIMATION TECHNIQUES In very wide river channels, the mean depth can be used as an approximation for hydraulic calculations, as noted by Goudie et al. (1994). The Manning roughness coefficient, which is crucial for estimating flow resistance in open channels, is derived from the channel's characteristics, including vegetation and bed materials. This estimation process is similar to that of Chezy’s roughness coefficient.Tables listing Manning roughness coefficients are available in various references, including works by Richards (1982), Chow et al. (1988), and Goudie et al. (1994). These coefficients are essential for accurately calculating flow rates and understanding the dynamics of river systems.  DILUTION GAUGING Dilution gauging works on the principle that the more water there is in a river the more it will dilute a solute added into the river. There is a well established relationship between the amount of the tracer found naturally in the stream (Co ), the concentration of tracer put into the stream (Ct ), the concentration of tracer measured downstream after mixing (Cd), and the stream discharge (Q). The choice of tracer depends on its detectability and non-harmfulness to aquatic life, with common tracers including solutions like table salt (NaCl), which can be detected using conductivity meters. There are two primary methods for conducting dilution gauging: Gulp Dilution Gauging - puts a known volume of tracer into the river and measures the concentration of the ‘slug’ of tracer as it passes by the measurement point. With the equation: Where: Q is the unknown streamflow, C is the concentration of tracer either in the slug (t), downstream (d), or background in the stream (0); ∆t is the time interval. The denominator of this equation is the sum of measured concentrations of tracer downstream. Continuous injection method - uses a continuous injection of tracer into the river and measures the concentration downstream. With the equation: Where: q is the flow rate of the injected tracer  DILUTION GAUGING The most difficult part of dilution gauging is calculating the distance downstream between where the tracer is injected and the river concentration measuring point (the mixing distance). This can be estimated using Equation: Where: L = mixing distance (m) Cz = Chezy’s roughness coefficient w = average stream width (m) g = gravity constant (≈9.8 m/s2 ) d = average depth of flow (m) THANK YOU!