--- title: "Methods of stream flow measurement" subject: "Energy Resources" chapter: 5 key: "semester_6/energy_resources:flow-measurement-methods-micro-hydro" source: https://www.bctengineer.com/q/semester_6/energy_resources/flow-measurement-methods-micro-hydro site: BCT Engineering --- # Methods of stream flow measurement Energy Resources · Chapter 5: Micro and small hydro power systems - Asked in: year not given, 6 marks - Video: [How to Measure River Discharge: Step-by-Step Flow Rate Tutorial](https://www.youtube.com/watch?v=NJlTdpXyMKQ) — Nepali Xperiment - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/energy_resources/flow-measurement-methods-micro-hydro - Whole chapter: https://www.bctengineer.com/md/semester_6/energy_resources/chapter-5 ## Question Explain the methods used to measure the flow of a small stream for a micro hydro plant (bucket, float, weir, salt dilution and current meter). What is a flow duration curve and how is it used in selecting the design flow? ## Short answer Bucket: collect whole flow, Q=V/t for small streams. Float: surface speed L/t × 0.85 × area. Weir: V-notch Q=1.4H^2.5 or rectangular with Francis formula. Salt dilution: inject salt, measure downstream conductivity curve. Current meter: velocity-area from point measurements. Flow duration curve: flow vs % time exceeded; design flow chosen at 90–95% exceedance. ## Answer #### Bucket method The whole stream is diverted into a container of known volume and the filling time is measured. $Q = V/t$. It is accurate and used for very small flows (below about 10 L/s). #### Float (velocity-area) method Mark a straight length of channel (say 10–20 m), measure the time $t$ for a float to travel it, and the average cross-sectional area $A$. Surface velocity $v_s = L/t$; mean velocity $v = k\,v_s$ with $k \approx 0.8$–0.85. $Q = A\,v$. It is simple but approximate. #### Weir method A sharp-edged rectangular or V-notch weir is built across the stream and the head $H$ over the crest is measured. For a rectangular weir with end contractions $Q = 1.84\,(L - 0.1nH)H^{3/2}$ and for a 90° V-notch $Q = 1.4H^{5/2}$ (m³/s, metres). Accurate for small and medium streams. #### Salt dilution method A known mass of salt is dissolved and poured into the stream at one point, and the conductivity is recorded downstream where it is fully mixed. The area under the conductivity-time curve gives $Q$. It suits turbulent mountain streams where other methods are difficult. #### Current meter A propeller meter measures velocity at several points (usually at 0.6 of depth) in verticals across the section. $Q = \sum v_i A_i$ (velocity-area method). It gives the best accuracy but needs equipment. #### Flow duration curve (FDC) It is a plot of flow against the percentage of time (or days) the flow is equalled or exceeded, constructed from daily or monthly flow records sorted in descending order. ```text Q |\ | \___ | \____ | \______ +------------------ % time 0 50 100 ``` For micro hydro the design flow is normally taken at the flow available 90–95 % of the time (dry season) so that power is dependable, and the area under the curve gives the energy available. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The bucket method is suitable for... A. Large rivers B. Floods C. Very small flows (below about 10 L/s), Q = V/t D. Flows above 1 m³/s Answer: C. The whole stream is diverted into a container. (Key point: Bucket method) 2. Mean velocity in the float method is... A. Surface velocity times 2 B. Surface velocity times k ≈ 0.8-0.85 C. Equal to surface velocity D. Surface velocity divided by 0.2 Answer: B. Then Q = A v. (Key point: Float method) 3. Salt dilution suits... A. Turbulent mountain streams B. Still ponds C. Wide slow rivers D. Dry channels Answer: A. Conductivity downstream gives Q from area under the curve. (Key point: Salt dilution) 4. The design flow from the flow duration curve is normally taken at... A. The flow exceeded 5% of the time B. The maximum flood flow C. The average of all peaks D. The flow exceeded 90-95% of the time Answer: D. This ensures dependable dry-season power. (Key point: Design flow) ## Related in Energy Resources - [Stream flow from weir and float measurements](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Astream-flow-weir-float-numerical) — Energy Resources, chapter 5 - [Site investigation for micro hydro plant](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Amicro-hydro-site-investigation-head-measurement) — Energy Resources, chapter 5 - [Micro hydro net head, power and energy](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Amicro-hydro-power-output-numerical) — Energy Resources, chapter 5 - [Micro hydro scheme components and classification](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Amicro-hydro-scheme-components-classification) — Energy Resources, chapter 5 ## Related in other subjects - [Load duration curve; diesel plant; ROR flow duration](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aload-duration-diesel-import-fdc) — Hydropower Engineering, chapter 3 - [Stream flow from current meter data](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Acurrent-meter-0-035-0-74n) — Engineering Hydrology, chapter 4, asked 2× - [Stream flow from current meter, V=0.045+0.76N](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Acurrent-meter-0-045-0-76n) — Engineering Hydrology, chapter 4, asked 2× - [Hyetograph and intensity-duration curve, Lumle tipping bucket](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Atipping-bucket-lumle) — Engineering Hydrology, chapter 2