--- title: "Design steps of surface drainage" subject: "Transportation Engineering I" chapter: 4 key: "semester_6/transportation_engineering_i:surface-drainage-design-steps" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/surface-drainage-design-steps site: BCT Engineering --- # Design steps of surface drainage Transportation Engineering I · Chapter 4: Highway Drainage - Asked in: 2076 Baisakh; 2075 Bhadra; 2066 Magh (old course) (asked 3 times) - Video: [BASIC TRANSPORTATION ENGINEERING - SURFACE DRAINAGE AND ITS METHODS](https://www.youtube.com/watch?v=_LH-PqYrVLw) — Bhavik Joshi - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/surface-drainage-design-steps - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4 ## Question Write down the design steps of the surface drainage system (design of longitudinal drains of a road to drain off the surface water). ## Short answer Design steps: collect alignment, gradient, catchment, rainfall data; fix return period (5-10 years) and intensity; compute time of concentration; calculate discharge by rational method Q=CIA/360; select drain shape, lining, gradient; design section using Manning's equation with freeboard; check velocity against non-silting and non-scouring limits; provide lining or adjust gradient; fix cross-drain spacing; prepare drawings. ## Answer Surface drainage design of a longitudinal drain (side drain) proceeds as follows (IRC SP:42 / NRS 2070). 1. **Collect data**: road alignment and longitudinal gradient, cross-section, catchment area and its slope, soil and land use, rainfall records, outfall locations. 2. **Fix the design return period** (for example 5 to 10 years for side drains) and find the rainfall intensity $I$ for the time of concentration $t_c$. 3. **Time of concentration**: $t_c=t_{inlet}+t_{flow}$, where $t_{flow}=L/V$ (the length of drain divided by the flow velocity). 4. **Calculate discharge** by the rational method: $$ Q=\frac{C\,I\,A}{360}\ \ (\text{m}^3/\text{s}) $$ where $C$ is the runoff coefficient, $I$ in mm/hour and $A$ in hectares. 5. **Select the shape** (trapezoidal, rectangular, V-shaped or U-shaped) and the material or lining, and the longitudinal gradient (normally the road gradient, but not less than about 0.5%). 6. **Design the section** by Manning's equation: $$ Q=A\,V,\qquad V=\frac{1}{n}R^{2/3}S^{1/2} $$ Assume a section, find $A$, wetted perimeter $P$ and $R=A/P$, and iterate until the capacity equals or exceeds $Q$. Add a free board of about 0.15 m. 7. **Check the velocity**: it must be greater than the non-silting velocity (about 0.6 to 0.75 m/s) and less than the permissible non-scouring velocity for the material (for example about 0.3 to 0.6 m/s for sandy soil, 1.0 to 1.5 m/s for hard clay, about 3 m/s for stone masonry and up to about 6 m/s for concrete lining). 8. **Provide lining or checks** if the velocity is too high, and increase the gradient or reduce the section (to raise the velocity) if it is too low. 9. **Fix the spacing of cross drains and outfalls** (turnouts or mitre drains) so that the drain length does not exceed the capacity. 10. **Prepare drawings and quantities.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is time of concentration calculated? A. t_inlet x L/V B. t_inlet + L/V C. L x V D. V/L Answer: B. tc = inlet time + flow time, where flow time = length of drain / velocity. (Key point: Time of concentration) 2. In Q = C I A / 360, what units apply? A. I in cm/h, A in m^2 B. I in mm/h, A in hectares, Q in m^3/s C. I in mm/min, A in km^2 D. I in m/s, A in hectares Answer: B. With I in mm/hour and A in hectares, Q is in m^3/s. (Key point: Rational formula) 3. How is the drain section designed? A. By Manning's equation, iterating until capacity >= Q B. By choosing any size C. By the rational method alone D. By Darcy's law Answer: A. Assume a section, find A and R, use Manning's V and Q = AV, and revise until capacity meets or exceeds Q. (Key point: Section design) 4. What velocity check is made after sizing? A. Equal to zero B. Between non-silting and non-scouring limits C. Only above scouring limit D. Above 10 m/s Answer: B. Velocity must exceed non-silting (about 0.6-0.75 m/s) and stay below non-scouring for the material. (Key point: Velocity check) ## Related in Transportation Engineering I - [Design steps of side drain](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aside-drain-design) — Transportation Engineering I, chapter 4 - [Surface water drainage in hill roads](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-surface-drainage) — Transportation Engineering I, chapter 5, asked 2× - [Catch water drains in hill roads](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Acatch-water-drain-hill) — Transportation Engineering I, chapter 5, asked 2× - [Surface drainage system](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asurface-drainage-system) — Transportation Engineering I, chapter 4 - [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× - [Collection and disposal of surface water](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asurface-drainage-collection) — Transportation Engineering I, chapter 4, asked 2× ## Related in other subjects - [Surface drainage design, 40 ha Terai field](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asurface-drainage-40ha-terai) — Irrigation and Drainage Engineering, chapter 9 - [Internal/external drainage; water logging; Terai surface drainage](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainternal-external-drainage-water-logging-terai) — Irrigation and Drainage Engineering, chapter 9 - [Passband, transition band, roll-off and design steps](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Apassband-rolloff-design-steps) — Filter Design, chapter 1 - [Water logging remedies and surface drainage in Terai](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Awater-logging-causes-surface-drainage-terai) — Irrigation and Drainage Engineering, chapter 9