--- title: "Khosla's method for barrage floor design" subject: "Irrigation and Drainage Engineering" chapter: 5 key: "semester_6/irrigation_and_drainage_engineering:khosla-barrage-floor-design-method" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/khosla-barrage-floor-design-method site: BCT Engineering --- # Khosla's method for barrage floor design Irrigation and Drainage Engineering · Chapter 5: Diversion Headworks - Asked in: 2073 Magh, 6+6 marks - Video: [Khosla's Theory of Design of Weirs | Irrigation - GATE](https://www.youtube.com/watch?v=ooZ7Lv3WRLc) — Ekeeda GATE & ESE - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/khosla-barrage-floor-design-method - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-5 ## Question Explain the design method to find the suitable size, length and thickness of floor of barrage using Khosla's seepage theory. Also draw the typical section of barrage showing the different components. ## Short answer Design steps: determine data, scour depth via Lacey, maximum static head, floor length from exit gradient formula, pressures at key points with mutual interference, floor thickness, and slope corrections, then floor thickness t = (4/3)h/(G-1), provide aprons and check. Typical section includes gates, crest, upstream floor, glacis, downstream jump floor, upstream/downstream cutoffs, intermediate piles, aprons with filters, and wing walls. ## Answer #### Khosla's method for the design of a barrage floor Khosla's theory gives the pressure and exit gradient from the flow of seepage water below the floor. The floor is designed in the following steps. **Step 1: Data.** Maximum flood discharge, HFL, pond level (PL), d/s tail water level, bed level, silt factor $f$, safe exit gradient $G_E$ (1/5 to 1/7 for sand), specific gravity of floor material $G = 2.4$. **Step 2: Scour depth.** Lacey's $R = 1.35\,(q^2/f)^{1/3}$, with $q$ = discharge per metre width. Use it for the cutoff depths: u/s cutoff $\ge 1.0R$-$1.5R$ below HFL and d/s cutoff $\ge 1.5R$-$2R$ below the d/s HFL. **Step 3: Maximum static head** $H$ = PL $-$ d/s bed (or floor) level with no flow (usually nil tail water); and under flow with HFL. **Step 4: Exit gradient and length of floor.** $$ G_E = \frac{H}{d}\cdot\frac{1}{\pi\sqrt{\lambda}},\qquad \lambda = \frac{1+\sqrt{1+\alpha^2}}{2},\ \alpha = \frac{b}{d} $$ Taking $d$ as the d/s cutoff depth, find $\alpha$ (hence floor length $b$) such that $G_E \le$ safe value. Provide the total floor length with some margin. **Step 5: Pressures at key points.** Compute $\phi_E,\phi_D,\phi_C$ for each pile line: - u/s pile: $\phi_{E_1} = 100$, $\phi_{D_1} = 100-\phi_D$, $\phi_{C_1} = 100-\phi_E$. - d/s pile: $\phi_{E_2} = \phi_E$, $\phi_{D_2}=\phi_D$, $\phi_{C_2}=0$. Apply corrections: (a) mutual interference of piles $C = 19\sqrt{D/b'}\,(d+D)/b$; (b) floor thickness; (c) slope of the glacis. **Step 6: Floor thickness.** Uplift head at a point $h$ (from corrected $\phi\times H$). Required thickness $$ t = \frac{4}{3}\cdot\frac{h}{G-1} \quad(\text{33\% extra for safety}) $$ The thickness is greatest near the toe of the glacis and is decreased in steps (min 0.5-0.6 m). **Step 7: Floor details.** The d/s floor length is fixed by the hydraulic jump, and the thickness of the u/s floor is provided with nominal values. Provide the u/s and d/s aprons (blocks with inverted filter) of length 1.5-2 times the scour depth, with launching apron. **Step 8: Check** pressure line (HGL), exit gradient, and thickness for both conditions (no flow and flow). #### Typical section of a barrage ```text Pond level ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ gates (piers) HFL ~~~~~ u/s floor | crest |\ glacis | jump floor | d/s apron ==========|__________| \______|============|== blocks u/s block apron | | filter, launching u/s pile (cutoff) intermediate pile d/s pile (cutoff) ``` Components: gates and piers, crest, u/s floor, glacis, horizontal d/s floor (jump), u/s and d/s cutoffs, intermediate piles, aprons with filters and launching aprons, u/s and d/s wing walls. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which relation gives the scour depth R? A. R = 4.75 sqrt(Q) B. R = 1.35 (q^2/f)^(1/3) C. R = H/d D. R = 19 sqrt(D/b') Answer: B. Lacey's scour R = 1.35 (q^2/f)^(1/3), q per metre width. (Key point: Scour depth) 2. How is floor length determined? A. From glacis slope B. From scour depth only C. From pond level D. Take d as d/s cutoff depth and find alpha so that G_E is within the safe value Answer: D. With d fixed, choose alpha = b/d so that exit gradient is within safe value. (Key point: Floor length) 3. What is the thickness formula? A. t = h/(G-1) B. t = (4/3) h/(G-1), with 33% extra for safety C. t = h x G D. t = 2h Answer: B. t = (4/3) h/(G-1), where h is uplift head and G = 2.4. (Key point: Floor thickness) 4. Which corrections apply to key point pressures? A. Mutual interference, floor thickness and slope B. Temperature C. Seepage velocity D. Silt factor Answer: A. Pressures are corrected for mutual interference, floor thickness and glacis slope. 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