---
title: "Submerged and unsubmerged minor bridge"
subject: "Transportation Engineering I"
chapter: 4
key: "semester_6/transportation_engineering_i:minor-bridge-submerged"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/minor-bridge-submerged
site: BCT Engineering
---

# Submerged and unsubmerged minor bridge

Transportation Engineering I · Chapter 4: Highway Drainage

- Asked in: 2067 Mangsir (old course), 4 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/minor-bridge-submerged
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4

## Question

Write a short note on un-submerged and submerged conditions for minor bridge.

## Short answer

Un-submerged condition has HFL below soffit with freeboard, open-channel flow, and normal scour design. Submerged condition has HFL above soffit causing pressure flow or overtopping; adopted for submersible bridges on low-traffic roads with anchored decks, smooth soffit, no parapet, and heavy bed protection.

## Answer

A **minor bridge** has a total length of about 6 to 60 m (IRC). Its hydraulic behaviour depends on the flood level relative to the bridge deck.

#### Un-submerged (free-surface) condition
The High Flood Level (HFL) stays **below the underside (soffit) of the deck**, with a free board (vertical clearance). As per IRC:5, the clearance increases with the discharge: from about 150 mm for very small discharges, to 600 mm at moderate discharges, and 1.2 to 1.5 m for large discharges, with more allowed for floating debris. The flow under the bridge is open-channel flow with a free surface, and the road is not overtopped. This is the normal design condition.

Design: the afflux (rise in water level upstream due to the contraction), the velocity and the scour depth are computed (scour by Lacey's regime theory), and the foundation is taken below the scour level.

#### Submerged condition
The water level **rises above the deck soffit** or even over the road. The flow then becomes pressure (orifice) flow under the deck, or flow over the deck when it is overtopped. The structure is then subject to hydrostatic and drag forces, buoyancy and uplift, debris impact, and heavy scour. The road may be damaged and traffic is interrupted.

A submerged bridge (a "submersible bridge" or a vented causeway) is accepted only on low-traffic roads where floods are rare and of short duration, as it is much cheaper than a high-level bridge. It is designed with:
- strongly anchored decks (to resist uplift),
- a smooth soffit and rounded edges,
- no parapet (or collapsible railings),
- heavy bed protection and downstream aprons.

```text
 Un-submerged                Submerged
 ____deck____ soffit          ~~~~~~~~~~~ HFL
 ~~~~~ HFL ~~~~ clearance     ____deck____
```

## Check yourself

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

1. What defines the un-submerged condition of a minor bridge?
   A. HFL stays below the deck soffit, with free board, giving open-channel flow
   B. HFL rises above the soffit, giving pressure flow under the deck
   C. Water flows over the road surface
   D. The bridge has no free board at all
   Answer: A. In the un-submerged case the HFL is below the soffit with clearance, so the flow has a free surface and the road is not overtopped. (Key point: Un-submerged condition)

2. When the water level rises above the deck soffit, what flow occurs under the deck?
   A. Free-surface open-channel flow
   B. Pressure (orifice) flow, or flow over the deck if overtopped
   C. Critical flow with a hydraulic jump only
   D. No flow, because the bridge blocks it
   Answer: B. Once the soffit is submerged, flow becomes pressure/orifice flow under the deck, or overflow when the deck is overtopped. (Key point: Flow in submerged condition)

3. Which feature is used in the design of a submersible bridge?
   A. Tall solid parapets to keep the water out
   B. Rough, projecting soffit to slow the flow
   C. Strongly anchored decks to resist uplift, with no parapet or collapsible railings
   D. Light unanchored deck slabs
   Answer: C. Submerged decks face buoyancy and uplift, so they are anchored strongly, kept smooth with rounded edges, and have no solid parapet. (Key point: Design of submersible bridge)

## Related in other subjects

- [Buoyancy and stability of floating bodies](https://www.bctengineer.com/question.md?key=semester_4%2Ffluid_mechanics_mechanical%3Abuoyancy-metacentric-height-stability) — Fluid Mechanics, chapter 2
- [Six-pulse converter: supply voltage and firing range](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Asix-pulse-converter-firing-range) — Utilization of Electrical Energy, chapter 3