--- title: "RT works for a bridge; launching apron" subject: "Irrigation and Drainage Engineering" chapter: 6 key: "semester_6/irrigation_and_drainage_engineering:river-training-bridge-launching-apron" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/river-training-bridge-launching-apron site: BCT Engineering --- # RT works for a bridge; launching apron Irrigation and Drainage Engineering · Chapter 6: River Training Works - Asked in: 2068 Chaitra (old course), 4+4 marks - Video: [What is River Training Works in Bridge Construction | Floor Apron & Flexible Apron, Stone Pitching](https://www.youtube.com/watch?v=2e3AZxByUkI) — REINFORCE - 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/river-training-bridge-launching-apron - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6 ## Question With neat sketch (plan and section), show the river training works for a bridge. What is a launching apron? How is it designed? ## Short answer River training works for a bridge include guide bunds, approach embankments, pitching, and a launching apron. A launching apron is a layer of loose stone at the toe that slides into the scour hole to prevent undermining. Design uses scour depth R=0.47(Q/f)^(1/3), depth below bed D', apron width 1.5D', and launched thickness 1.5t. ## Answer #### River training works for a bridge (4 marks) At a bridge on an alluvial river, the river is contracted to the bridge waterway, so training works are provided to guide the flow axially and prevent flanking: **guide bunds** on both sides upstream, **approach embankments**, **pitching** of slopes and the river banks, **launching aprons** and sometimes **spurs** upstream. ```text Plan U/S D/S ____________________ | ____ / guide bund \ approach |=| / \ ( curved head )========|=|=( ) \____________________/ embankment|=| \____/ flow ---> waterway W between abutments ``` ```text Section through guide bund __6 m__ HFL+1.5 / \ HFL/_________\ 2:1 / pitching \ _/__ filter ____\_ launching apron ``` #### Launching apron (2 marks) A **launching apron** is a layer of loose stone (or concrete blocks or gabion) laid on the bed at the toe of the pitching. When scour develops at the toe, the stones slide into the scour hole and form a protective layer on the slope, preventing undermining of the bund. #### Design of launching apron (2 marks) 1. Scour depth $R = 0.47(Q/f)^{1/3}$ with $f = 1.76\sqrt{d_{mm}}$. 2. Maximum scour below HFL: $2R$ (nose) or $1.5R$ (shank). Depth below bed $D' = $ scour $-$ flood depth. 3. Launching slope 2H:1V, so the stone volume must cover a slope length $\sqrt5 D' = 2.236D'$. 4. Thickness of launched apron $= 1.5t$ where $t = 0.06Q^{1/3}$ is pitching thickness. 5. Width laid $= 1.5D'$ and thickness laid $= \dfrac{1.5t\times 2.236D'}{1.5D'} = 2.24t$ (about $2.25t$). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which works are provided at a bridge on an alluvial river? A. Weirs B. Canal head regulators C. Only a dam D. Guide bunds, approach embankments, pitching, launching aprons, sometimes spurs Answer: D. They guide flow axially and prevent flanking of the contracted waterway. (Key point: Training works for a bridge) 2. What is a launching apron? A. A bridge deck B. A concrete floor C. A layer of loose stone at the toe of pitching that slides into a scour hole and protects the slope D. A filter below the bed Answer: C. When scour develops, the stone launches down the slope and prevents undermining. (Key point: Launching apron) 3. How is the depth D' of apron found? A. Maximum scour below HFL (2R nose, 1.5R shank) minus flood depth B. R + flood depth C. 2R + flood depth D. Equal to R Answer: A. R = 0.47 (Q/f)^(1/3) with f = 1.76 sqrt(d mm), and D' is measured below bed. (Key point: Apron depth) 4. Why is the apron width 1.5D' and laid thickness about 2.25t? A. Slope is 1:1 B. To reduce cost C. Launching slope 2H:1V needs slope length sqrt5 D' = 2.236D' covered at 1.5t, so laid thickness is 2.24t over 1.5D' D. Because 1.5t = laid thickness Answer: C. Volume equality: T = 1.5t x 2.236D' / 1.5D' = 2.24t. (Key point: Apron width and thickness) ## Related in Irrigation and Drainage Engineering - [Objectives of river training works](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ariver-training-objectives-five) — Irrigation and Drainage Engineering, chapter 6 - [Guide bund with plan, section, nose; 2500 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aguide-bund-q2500-bed755) — Irrigation and Drainage Engineering, chapter 6 - [Launching apron, 7000 m3/s, HFL 114 m](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Alaunching-apron-q7000-hfl114) — Irrigation and Drainage Engineering, chapter 6, asked 3× - [Guide bank and launching apron, 5000 m3/s](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aguide-bank-q5000-hfl254) — Irrigation and Drainage Engineering, chapter 6 - [Thickness of launching apron, 7 m shank](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Alaunching-apron-shank-7m-q8000) — Irrigation and Drainage Engineering, chapter 6 - [Objectives of RT works and spur spacing](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ariver-training-objectives-spur-spacing) — Irrigation and Drainage Engineering, chapter 6 ## Related in other subjects - [Plan and vertical section of panelled door](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Apanelled-door-plan-section) — Building Technology, chapter 6 - [Bridge components sketch; river bank protection](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-sketch-components-river-bank-protection) — Transportation Engineering II, chapter 5 - [Bridge site factors; river bank protection structures](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-river-bank-protection) — Transportation Engineering II, chapter 5 - [Bheri river suspension bridge collapse](https://www.bctengineer.com/question.md?key=semester_8%2Fengineering_professional_practice%3Abheri-suspension-bridge-collapse) — Engineering Professional Practice, chapter 7