---
title: "Rapids, drop structures, causeways, siphons"
subject: "Transportation Engineering I"
chapter: 4
key: "semester_6/transportation_engineering_i:rapids-drops-causeways"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/rapids-drops-causeways
site: BCT Engineering
---

# Rapids, drop structures, causeways, siphons

Transportation Engineering I · Chapter 4: Highway Drainage

- Asked in: 2064 Poush (old course), 2+2+2+2 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/rapids-drops-causeways
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4

## Question

Describe in brief rapids, drop structures, causeways and inverted siphons. Give sketches wherever possible.

## Short answer

Rapids are long rough-bedded chutes that control velocity on steep slopes. Drop structures dissipate energy via stepped vertical falls into a lined pool. Causeways are low-level crossings at stream bed, allowing flood overtopping, with vented or unvented types. Inverted siphons are pressurized pipes dipping below obstacles, requiring 2–3 m/s flow and silt traps.

## Answer

#### 1. Rapids
A **rapid** is a steep, long, rough-bedded channel (a chute) used to carry a drain down a steep slope over a long distance. Roughness elements (boulders, cross ribs) keep the velocity within the non-scouring limit. It ends in a stilling basin.

```text
 drain \___rough bed___ \
                         \ chute (slope steep)
                          \____stilling pool__
```

#### 2. Drop structures
A **drop structure** (stepped drain or cascade) lets the water fall vertically in one or more steps where the ground is too steep for a normal gradient. The jump forms in a lined pool at the foot, dissipating energy. It protects the drain from scour and erosion.

```text
 ______
       |  step 1
       |____
            |  step 2
            |____ pool
```

#### 3. Causeways
A **causeway** (drift or Irish bridge) is a road crossing built at or just above the stream bed, allowing floods to flow over the road. It may be **unvented** (a ford, with paving only) or **vented** (with pipes or small openings for low flows). It is cheap and used on low-traffic roads across wide, shallow streams whose floods are short and rare, but traffic is blocked during floods. It needs aprons and cut-off walls on both sides.

```text
 road ---\_____ low-level slab _____/--- road
          =vents=  <-- low flow
```

#### 4. Inverted siphon
An **inverted siphon** is a closed pipe or conduit that dips below a road, canal or valley and rises again, flowing full under pressure. It carries drainage water across depressions or under another structure where it cannot pass above. It needs a velocity of about 2 to 3 m/s to prevent silting and a silt trap/air vent.

```text
 inlet \                  / outlet
        \________________/
          (pipe full, pressure flow)
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is a rapid in drainage works?
   A. A closed pipe flowing full under pressure
   B. A steep, long, rough-bedded chute that carries a drain down a steep slope
   C. A vertical fall of water in one step
   D. A road crossing at stream bed level
   Answer: B. A rapid is a steep rough-bedded chute; roughness keeps velocity non-scouring and it ends in a stilling basin. (Key point: Rapid)

2. How does a drop structure protect a drain on steep ground?
   A. Water falls in steps and a jump in the lined pool at the foot dissipates energy
   B. It keeps the water velocity high to prevent silting
   C. It stores the water for irrigation
   D. It carries the water under pressure below the ground
   Answer: A. Water falls vertically in one or more steps, and the hydraulic jump in the lined pool dissipates energy, preventing scour. (Key point: Drop structure)

3. Where is a causeway (drift) suitably used?
   A. High-traffic highways crossing deep gorges
   B. Low-traffic roads across wide, shallow streams with short, rare floods
   C. Only where floods are frequent and long
   D. Where the road must stay open during every flood
   Answer: B. A causeway is cheap and lets floods pass over the road, so it suits low-traffic roads and short, rare floods; traffic stops during floods. (Key point: Causeway use)

4. What characterises an inverted siphon?
   A. An open channel on a steep slope
   B. A pipe flowing full under pressure that dips below an obstacle and rises again
   C. A stepped drain with pools
   D. A paved ford with no vents
   Answer: B. An inverted siphon is a closed conduit flowing full under pressure beneath a road, canal or valley, with velocity about 2 to 3 m/s to prevent silting. (Key point: Inverted siphon)

## Related in Transportation Engineering I

- [Hill road drainage structures](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-drainage-structures) — Transportation Engineering I, chapter 5, asked 4×
- [Cross drainage structures and types](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Across-drain-types) — Transportation Engineering I, chapter 4, asked 3×

## Related in other subjects

- [Drop structures: need and types with sketches](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adrop-structures-types-sketch) — Irrigation and Drainage Engineering, chapter 7, asked 6×
- [Types of cross drainage works with sketches](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acd-works-types-definition-sketches) — Irrigation and Drainage Engineering, chapter 8, asked 9×
- [Importance of ductile detailing, sketches](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aimportance-of-ductile-detailing-sketches) — Design of RCC Structure, chapter 12