Flume

In geography and hydrology, a flume is a narrow artificial or natural channel through which water flows, often constructed to measure, convey, or control the movement of water. The term originates from the Latin flumen, meaning “river” or “stream.” Flumes are widely used in river engineering, irrigation systems, hydrological studies, and hydraulic experiments, and they also occur naturally in steep mountain valleys and gorges where flowing water has eroded a confined passage.

Definition and General Concept

A flume refers to a man-made or natural passageway that confines the flow of water within narrow boundaries, allowing for the observation, redirection, or measurement of discharge. In artificial systems, it is typically made of concrete, wood, metal, or fibreglass and designed to ensure a specific flow regime.
Broadly, flumes serve two main purposes:

1. Engineering Flumes: Built to transport or regulate water.
2. Hydrological Measurement Flumes: Designed to determine the flow rate or discharge of open channels.

Natural flumes, on the other hand, are geomorphic features formed by river erosion in steep or resistant terrain.

Types of Flumes

Flumes can be categorised according to their purpose and structural design.

1. Measuring Flumes (Flow Measurement Devices): These flumes are constructed to measure the rate of flow (discharge) in open channels such as irrigation canals, streams, or wastewater systems. Common types include:
   • Parshall Flume: The most widely used type, featuring a converging section, a throat, and a diverging section. It allows accurate flow measurement with minimal head loss.
   • Venturi Flume: Operates on similar principles to the Parshall flume but designed with smooth transitions to reduce turbulence.
   • Cutthroat Flume: A simplified form without a parallel throat section, useful for small-scale or portable applications.
   • Palmer–Bowlus Flume: Common in closed conduits and sewers, having a curved bottom for partial flow conditions.
2. Conveyance Flumes (Water Transport Structures): Built to carry water across depressions, valleys, or along slopes. They are often elevated or supported on trestles.
   • Historically used in mining operations, especially during the California Gold Rush, to transport water for hydraulic mining.
   • In modern use, employed in irrigation systems, hydropower channels, and industrial water transport.
3. Natural Flumes (Geographical Landforms):
   • Formed by river erosion through narrow rock formations, producing deep, steep-sided channels or gorges.
   • Examples include the Flume Gorge in New Hampshire (USA), a natural granite gorge shaped by glacial meltwater and stream erosion.

Structure and Design

A typical flume consists of three basic components:

• Converging Section: Narrows the flow, increasing velocity.
• Throat Section: Maintains a constant cross-section where flow is measured.
• Diverging Section: Gradually widens to reduce velocity and restore normal flow conditions.

The design of a flume ensures that the water achieves critical flow (transition between subcritical and supercritical states) at the throat, allowing discharge to be calculated from water depth (head) using hydraulic equations.
In flow-measuring flumes, calibration is crucial to establish an empirical relationship between water level and discharge. This relationship is usually expressed as:
Q=KhnQ = K h^nQ=Khn
Where:

• QQQ = discharge (flow rate)
• hhh = head (depth of water at a specified point)
• KKK and nnn = constants determined by flume geometry and calibration

Functions and Applications

Flumes are integral to water management, research, and engineering. Their key functions include:

1. Flow Measurement:
   • Used in irrigation canals, rivers, drainage systems, and wastewater treatment plants to determine discharge accurately.
2. Water Transportation:
   • Convey water over irregular terrain where channels or pipelines are impractical.
   • Historically important in logging, where flumes carried timber downhill using water currents.
3. Hydraulic Research and Education:
   • Employed in laboratory flumes to simulate river flows, sediment transport, erosion, and hydraulic structures on a small scale.
4. Environmental and Hydrological Studies:
   • Used in stream gauging stations and ecological monitoring to measure flow rates for water resource management and flood prediction.
5. Industrial and Agricultural Uses:
   • Found in cooling systems, hydropower plants, and irrigation networks, ensuring efficient water delivery.

Natural Flumes and Geomorphological Significance

Natural flumes occur where rivers or streams cut deep, narrow gorges into bedrock, creating channels that resemble artificial flumes. These are often found in mountainous or glaciated regions.
Characteristics of natural flumes include:

• Steep Sides: Carved into resistant rock such as granite or basalt.
• Swift Flow: Water moves rapidly through the confined space, enhancing erosion.
• Waterfalls and Rapids: Common within the flume due to uneven bedrock erosion.
• Formation Process: Created by glacial meltwater, flash floods, or long-term fluvial erosion.

Example:

• The Flume Gorge in Franconia Notch State Park, New Hampshire, is a natural granite channel approximately 240 metres long and 20 metres deep, formed during the Ice Age.

Advantages of Flumes over Other Flow Measurement Devices

• Low Maintenance: Few moving parts and minimal debris accumulation.
• Accuracy: Provide precise flow measurements across a range of discharges.
• Self-Cleansing: The acceleration of flow through the throat section reduces sediment deposition.
• Versatility: Suitable for both permanent installations and portable field use.
• Minimal Energy Loss: Causes less head loss compared to weirs and other measuring devices.

Limitations

• Cost of Construction: More expensive to install than simpler devices like weirs.
• Sediment Sensitivity: Excessive sediment loads can alter calibration accuracy.
• Flow Range Limitations: Not effective for very low or highly variable flows.
• Installation Requirements: Proper upstream and downstream conditions must be maintained for accuracy.

Examples of Flume Use

• Irrigation Monitoring: The Parshall flume is widely used in canal systems across the United States and Asia.
• Hydrological Research: University laboratories use recirculating flumes to study river dynamics and sediment transport.
• Water Supply Systems: Aqueduct flumes in California and Spain carry water across deep valleys.
• Logging and Mining: Historic flumes in the Sierra Nevada mountains were built to transport logs and water for hydraulic gold mining.