--- title: "Factors affecting flood hydrograph, baseflow separation" subject: "Engineering Hydrology" chapter: 5 key: "semester_5/engineering_hydrology:flood-hydrograph-factors-baseflow" source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/flood-hydrograph-factors-baseflow site: BCT Engineering --- # Factors affecting flood hydrograph, baseflow separation Engineering Hydrology · Chapter 5: Hydrograph Analysis - Asked in: 2078 Bhadra, 3+3 marks - Video: [HIE: Module 3 - Hydrograph & its components and Base flow separation](https://www.youtube.com/watch?v=KxMslGG4uAc) — SAVI SVIT 6CV2018 - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/flood-hydrograph-factors-baseflow - Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-5 ## Question Discuss how different factors affect flood hydrograph. What are various methods for baseflow separation? ## Short answer Climatic factors: rainfall intensity, duration, areal distribution, antecedent moisture. Catchment factors: size, shape, slope, land use, vegetation, drainage density, storage. Baseflow separation methods: straight-line, fixed-base length N=0.83A^0.2, variable-slope, master recession curve. ## Answer #### Factors affecting the flood hydrograph **Climatic factors** - **Rainfall intensity and duration:** high intensity gives a higher, sharper peak; long duration gives a larger volume and a broader hydrograph. - **Areal distribution and storm movement:** a storm moving downstream gives a higher peak than one moving upstream. - **Antecedent moisture:** wet soil increases runoff. - **Evaporation, temperature and snow-melt.** **Catchment factors** - **Size:** a larger area gives a longer time base and a later, lower specific peak. - **Shape:** a fan or circular shape gives a high, sharp peak; an elongated shape gives a flatter one. - **Slope:** steep slopes shorten the time to peak and increase the peak. - **Land use, vegetation and soil:** forest and permeable soil reduce the peak; urban and bare areas raise it. - **Drainage density:** a higher density gives a quicker response. - **Storage:** lakes, ponds, depressions and flood plains flatten the peak. **Channel factors:** cross-section, roughness, slope and channel storage. #### Methods of baseflow separation 1. **Straight-line method:** join the start of the rising limb to the end of direct runoff by a straight line. 2. **Fixed-base length method:** the end of direct runoff is taken at $N=0.83A^{0.2}$ days after the peak ($A$ in km$^2$). Extend the pre-storm recession forward to the time of the peak and join it to this end point. 3. **Variable-slope method:** extend the pre-storm recession forward to the peak time and the post-storm recession backward to an inflection point; join the two by a straight line. 4. **Semi-log recession (master recession curve) method.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Higher rainfall intensity on a catchment generally gives: A. A lower, flatter peak B. A higher, sharper peak C. No change in peak D. Later peak with same volume Answer: B. High intensity produces more runoff quickly, raising and sharpening the peak. (Key point: Rainfall effects) 2. Which catchment shape gives a high, sharp peak? A. Elongated B. Fan or circular C. Narrow strip D. Any, shape has no effect Answer: B. In fan/circular basins flows arrive together; elongated basins give flatter hydrographs. (Key point: Catchment shape) 3. Which factor flattens the flood peak? A. Urbanization B. Steep slopes C. Lakes, ponds and flood plain storage D. Higher drainage density Answer: C. Storage attenuates the peak; urban areas, steep slopes and high drainage density raise it. (Key point: Storage and land use) 4. For a 100 km^2 catchment N = 0.83 A^0.2 is about: A. 2.1 days B. 8.3 days C. 0.83 days D. 20 days Answer: A. 100^0.2 = 2.51, so N = 0.83 x 2.51 = 2.1 days. 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