--- title: "Hydrograph and flow duration curve" subject: "Power Plant Design" chapter: 3 key: "semester_8/power_plant_design:hydrograph-flow-duration" source: https://www.bctengineer.com/q/semester_8/power_plant_design/hydrograph-flow-duration site: BCT Engineering --- # Hydrograph and flow duration curve Power Plant Design · Chapter 3: Hydro Power Plant Design - Asked in: 2078 Kartik, 6 marks - Video: [HYDROGRAPH,MASSCURVE AND FLOW DURATION CURVE](https://www.youtube.com/watch?v=Xjls-BiLd10) — Asw Education - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/hydrograph-flow-duration - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-3 ## Question What are hydrograph and flow duration curve? Discuss its applicability. ## Short answer A hydrograph plots discharge against calendar time, showing seasonal sequence; a flow duration curve plots discharge against percentage of time exceeded, losing time order. The hydrograph is used for seasonal energy, flood design, storage and maintenance; the FDC sets design flow, capacity, firm power, unit number, riparian release and site comparison. ## Answer #### Hydrograph A **hydrograph** is a graph of river discharge plotted against **time in calendar order** (hours, days or months). It shows when high and low flows occur, e.g. the monsoon peak in July–August and the dry-season low in March–April for Nepalese rivers. ```text Q (m3/s) | __ | / \ | / \ | / \__ | _____/ \______ +---------------------------> Month Jan Mar May Jul Sep Nov ``` #### Flow duration curve (FDC) An **FDC** is a graph of discharge against the **percentage of time it is equalled or exceeded**. It is made by sorting the same flow data in descending order and plotting each value against its exceedance $P = \dfrac{m}{n+1}\times 100\%$. Time order is lost, but the duration of each flow is shown. ```text Q (m3/s) |\ | \ | \___ | \______ | \__________ +--------------------------> % time exceeded 0 45 65 95 100 ``` #### Applicability of the hydrograph 1. **Seasonal energy:** monthly flows give month-wise energy, needed for wet/dry season tariff and PPA. 2. **Flood design:** peak flood hydrographs size spillways, cofferdams and diversion works. 3. **Storage design:** the mass curve (cumulative hydrograph) gives the reservoir or pondage volume needed to meet demand. 4. **Construction and maintenance planning:** plan works and unit outages in low-flow months. 5. **Sediment planning:** high-flow months carry most sediment, guiding desander operation. #### Applicability of the FDC 1. **Design discharge:** chosen at a set exceedance, e.g. $Q_{40}$–$Q_{45}$ for ROR in Nepal. 2. **Installed capacity and annual energy:** area under the FDC up to the design flow gives energy; plant factor is found from it. 3. **Firm power:** from $Q_{90}$–$Q_{95}$. 4. **Number and size of units:** shows how often the plant runs at part load. 5. **Riparian release:** from the low-flow tail. 6. **Comparison of sites:** a flat FDC means steady flow, ideal for ROR; a steep FDC means storage is needed. | Point | Hydrograph | FDC | |---|---|---| | x-axis | Time (calendar) | % time exceeded | | Shows sequence | Yes | No | | Main use | Seasonal energy, floods, storage | Design flow, capacity, firm power | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a hydrograph plot? A. Discharge against % time exceeded B. Head against discharge C. Rainfall against area D. Discharge against time in calendar order Answer: D. A hydrograph keeps time sequence; the FDC does not. (Key point: Hydrograph) 2. How is an FDC constructed? A. Sort flows in descending order and plot against exceedance B. Plot flows in time order C. Sum flows monthly D. Average flows yearly Answer: A. FDC loses time order but shows durations. (Key point: FDC) 3. Which use belongs to the hydrograph? A. Firm power from Q95 B. Flood design and storage (mass curve) design C. Choosing Q45 design flow D. Riparian release Answer: B. Time sequence is needed for floods and storage; the FDC gives design flow and firm power. (Key point: Hydrograph use) 4. What does a steep FDC indicate? A. Steady flow ideal for ROR B. No monsoon C. Storage is needed D. Flat river Answer: C. A flat FDC suits ROR, a steep one needs storage. (Key point: FDC shape) ## Close variants set in other exams - [Flow duration curve and its applicability](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aflow-duration-curve) — Power Plant Design, chapter 3, asked 3× ## Related in Power Plant Design - [Flow duration curve and unit sizing](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Afdc-unit-sizing) — Power Plant Design, chapter 3 ## Related in other subjects - [Flow duration curve and planning parameters](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Afdc-planning-parameters) — Hydro Power, chapter 3 - [Flow duration curve and power at mean flow](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-capacity-fdc-flow-table-79) — Hydropower Engineering, chapter 3 - [Flow duration curve; firm and secondary energy](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afdc-firm-secondary-energy-69) — Hydropower Engineering, chapter 3 - [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