Open-channel instruments – what does a critical depth meter measure? A critical depth meter (a calibrated flume or control that establishes critical flow) is primarily used to measure which quantity in an open channel?

Introduction:
Critical depth meters (such as flumes that force Froude number to unity at a throat) convert a difficult-to-measure discharge into an easily measured depth at a calibrated section. This question asks you to identify the primary hydraulic quantity obtained by using such a device.

Given Data / Assumptions:

• Open-channel flow with a control that creates a critical section (Fr = 1) at a known geometry.
• Free-flow (modular) conditions without submergence at the control for proper calibration.
• Empirical rating relates depth at the control to discharge.

Concept / Approach:

At critical flow, specific energy is minimized, and for a given control geometry the depth at the throat (or at a gauge point) is a unique function of the discharge. Therefore, by measuring depth at that location, one infers the flow rate Q using a calibration curve or formula. Instruments like the Parshall flume and Venturi flume are examples of critical-depth meters used worldwide for flow gauging in irrigation canals and wastewater channels.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturer calibration tables and standard hydraulics texts provide Q–h relationships for flumes; field use confirms accurate discharge measurement over wide ranges when submergence criteria are met.

Why Other Options Are Wrong:

Velocity only or depth only: The device’s purpose is to infer Q from depth under controlled conditions, not to report velocity or depth in isolation.Hydraulic jump detection and slope measurement are different diagnostic tasks.

Common Pitfalls:

Using the meter under submerged conditions without correction factors; submergence invalidates the simple Q–h relation for many flumes.

Final Answer:

rate of discharge (flow rate) in the open channel