--- title: "Gauging site selection and current meter use" subject: "Hydro Power" chapter: 2 key: "semester_6/hydro_power:stream-gauging-site-current-meter" source: https://www.bctengineer.com/q/semester_6/hydro_power/stream-gauging-site-current-meter site: BCT Engineering --- # Gauging site selection and current meter use Hydro Power · Chapter 2: Introduction to Hydrology - Asked in: 2071 Bhadra, 3+3 marks - Video: [#stream gauge station#site selection#Engineering Hydrology#NEA#TU,PU,KU, Pou](https://www.youtube.com/watch?v=_IB7oPDK0AU) — Blissy Being - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/stream-gauging-site-current-meter - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-2 ## Question What factors should be considered in selecting a site for a stream gauging station? Describe briefly the procedure of using a current meter for measuring velocity of stream. ## Short answer Select a straight, stable channel with a well-defined control and no backwater effects. Current meter procedure: divide cross-section, measure depth, lower meter to 0.6d (or 0.2d and 0.8d), count revolutions per second, convert to velocity using V=aN+b, then compute discharge by mid-section method. ## Answer #### Factors for selecting a stream gauging site - **Straight reach**: about 5–10 times the river width upstream and downstream, with parallel flow lines. - **Stable channel**: bed and banks free from scour, deposition and shifting, so the stage–discharge relation stays constant. - **Well-defined control** (rock bar, rapids or a weir) downstream, giving a unique stage–discharge relation. - **No backwater** effect from confluences, dams or tides; no eddies or dead water. - **Flow contained in one channel** at all stages, without overflow at floods. - Adequate depth and velocity for the current meter in the dry season. - Easy **access** in all seasons and safe working (bridge, cableway). - Away from tributaries, so the measured flow represents the site of interest (e.g. near the intake for a hydropower project). #### Measuring velocity with a current meter A **current meter** (cup type or propeller type) has a rotor whose speed of rotation $N$ is proportional to water velocity. Its calibration equation is $V = aN + b$. Procedure: 1. Stretch a tag line across the river and divide the width into 15–25 segments (each carrying ≤ 10% of flow). 2. At each vertical, measure the depth $y$ with a sounding rod or weight. 3. Lower the meter (wading rod for shallow water; cable, weight and cableway or bridge for deep water) to the required depth: - Shallow (< 0.6–1 m): one reading at **0.6y** from the surface. - Deeper: two readings at **0.2y and 0.8y**; mean $V = (V_{0.2} + V_{0.8})/2$. 4. Allow the meter to settle, then count the revolutions over a fixed time (40–100 s). $N$ = revolutions/time. 5. Convert to velocity with the rating equation, e.g. $V = 0.51N + 0.03$. 6. Compute discharge by the mid-section or mean-section method: $Q = \sum b_i y_i V_i$. ```text tag line 0 1 2 3 4 5 ~~~~~~~~~~|~~~|~~~|~~~|~~~|~~~|~~~~ \ o o o o o / o = meter at 0.6y \_______o___o___o___o_________/ or 0.2y & 0.8y ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a desirable feature of a stream gauging site? A. A bend with eddies for slow flow B. A reach just below a confluence C. A shifting sandy bed D. A straight reach with a stable bed and banks Answer: D. A straight, stable reach keeps flow lines parallel and the stage-discharge relation constant. (Key point: Reach and channel stability) 2. Why should the site be free from backwater and have a well-defined control downstream? A. To give a unique stage-discharge relation B. To make the river deeper C. To increase the velocity D. To avoid the need for a current meter Answer: A. Backwater and poor control make one stage correspond to several discharges. (Key point: Control and backwater) 3. A current meter's rotor speed N is related to velocity by what? A. V = N^2 B. V = aN + b from calibration C. V = N/depth D. V = depth x N Answer: B. Rotor speed is proportional to water velocity, expressed by the calibration equation V = aN + b. (Key point: Current meter calibration) 4. Where is the meter placed in a deep vertical, and how is the mean velocity found? A. At 0.6y only; mean velocity is that reading B. At the bed; mean is half the reading C. At 0.2y and 0.8y; mean is the average of the two D. At the surface; mean is twice the reading Answer: C. In deeper water, readings at 0.2y and 0.8y are averaged; in shallow water one reading at 0.6y is used. 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