---
title: "Design steps of side drain"
subject: "Transportation Engineering I"
chapter: 4
key: "semester_6/transportation_engineering_i:side-drain-design"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/side-drain-design
site: BCT Engineering
---

# Design steps of side drain

Transportation Engineering I · Chapter 4: Highway Drainage

- Asked in: 2079 Jestha, 6 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/side-drain-design
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4

## Question

Write down the steps of design of side drain in road.

## Short answer

Design steps are: collect catchment data, find time of concentration and rainfall intensity, compute discharge by Q=CIA/360, choose drain shape and slope, size using Manning's equation, check non-silting and non-scouring velocities, add free board, and fix discharge points. The bed slope is at least 0.5%.

## Answer

A side drain carries the runoff from the carriageway, shoulder and nearby catchment along the road. The design steps are:

1. **Collect data**: catchment area ($A$), slope, soil/ground cover, rainfall intensity for the chosen return period (5 to 10 years for side drains), and the longitudinal gradient of the road.
2. **Find the time of concentration** $t_c$ and read the rainfall intensity $I$ for that duration.
3. **Compute the discharge** with the rational formula:
$$
Q=\frac{C\,I\,A}{360}\ \ (\text{m}^3/\text{s})
$$
with $I$ in mm/h and $A$ in hectares, and $C$ the runoff coefficient.
4. **Select the shape and material**: trapezoidal (side slope 1:1 to 1.5:1 in earth), rectangular (masonry or concrete, in hills) or V-shaped (small flows), and fix the bed slope, equal to road gradient but not less than 0.5%.
5. **Size the drain** by Manning's equation:
$$
V=\frac{1}{n}R^{2/3}S^{1/2},\qquad Q=A\,V
$$
Assume dimensions, find $A$, $P$, $R=A/P$, and revise until $Q_{capacity}\ge Q_{design}$.
6. **Check the velocity** is more than the non-silting velocity (about 0.6 to 0.75 m/s) and less than the non-scouring limit of the soil/lining.
7. **Add a free board** (about 0.15 to 0.3 m) and provide lining or check dams where velocity is too high.
8. **Fix the discharge points** (mitre drains, cross drains) so that the length of drain does not exceed the capacity, and design the outfall protection.

## Check yourself

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

1. Discharge is computed by:
   A. Q = A/V
   B. Q = 360 CIA
   C. Q = CIA/360 with I in mm/h, A in hectares
   D. Q = CIA with A in m^2
   Answer: C. The rational formula gives Q in m^3/s. (Key point: Discharge)

2. Bed slope of a side drain is:
   A. Always 10%
   B. Opposite the road gradient
   C. Zero
   D. Equal to road gradient but not less than 0.5%
   Answer: D. A minimum slope ensures flow and avoids silting. (Key point: Bed slope)

3. The velocity is checked to be:
   A. Above 10 m/s
   B. Below non-silting
   C. Equal to scour
   D. Above non-silting and below non-scouring
   Answer: D. Non-silting about 0.6-0.75 m/s. (Key point: Velocity check)

## Related in Transportation Engineering I

- [Design steps of surface drainage](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asurface-drainage-design-steps) — Transportation Engineering I, chapter 4, asked 3×
- [Special considerations in hill road design](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-special-considerations) — Transportation Engineering I, chapter 5, asked 5×
- [Define hill road and design factors](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-define-factors) — Transportation Engineering I, chapter 5, asked 2×
- [Route location in hill road design](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-route-location) — Transportation Engineering I, chapter 5
- [Design steps of superelevation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asuper-design-steps) — Transportation Engineering I, chapter 3, asked 3×

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