--- title: "Designing drainage canals in paddy fields" subject: "Irrigation and Drainage Engineering" chapter: 9 key: "semester_6/irrigation_and_drainage_engineering:drainage-canal-paddy-field-design-steps" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/drainage-canal-paddy-field-design-steps site: BCT Engineering --- # Designing drainage canals in paddy fields Irrigation and Drainage Engineering · Chapter 9: Water Logging and Drainage - Asked in: 2072 Asoj, 10 marks; 2072 Magh, 10 marks (asked 2 times) - Video: [surface drainage design | water balance equation | Irrigation numerical](https://www.youtube.com/watch?v=YYmufytG5M8) — Sudip Dhakal - 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/drainage-canal-paddy-field-design-steps - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-9 ## Question Explain in details the procedures (all steps required to arrive at the design discharge) of designing drainage canals in irrigated paddy fields. ## Short answer First, find design 3-day rainfall for the return period. With initial water depth and crop tolerance (maximum depth and duration limits), determine drainage rate D by trial so that field water levels stay within limits. Then design discharge Q = D × area, and size the drain cross-section using Manning’s equation, ensuring non-silting, non-scouring velocities and freeboard. ## Answer Drainage canals in paddy fields (Terai of Nepal) are designed to remove excess rainwater within the time for which rice can tolerate flooding. The design discharge is obtained from the drainage rate and the area, and the canal is sized by Manning's formula. #### Procedure 1. **Collect rainfall data** and find the maximum 3-day rainfall of each year; carry out a frequency analysis (Gumbel or growth factors) to get the 3-day rainfall for the selected return period (5–10 years for field drains; larger for main drains). Design rainfall $R = $ mean annual maximum × growth factor. 2. **Fix the crop tolerance:** the initial water depth in the field $h_0$ (about 40–50 mm) and the maximum depth and duration the rice can bear, for example 300 mm for 1 day and 200 mm for 3 days (bunded fields). 3. **Fix ET and percolation:** neglect them (safe side) or deduct them if their values are known. 4. **Draw the water-level history:** the rain falls uniformly over 3 days and the drainage $D$ removes water, so the level rises at $(R/3-D)$ per day; after the rain it falls at $D$ per day. $$ h_{peak}=h_0+R-3D, \qquad T_e=\left(3-\frac{h_e-h_0}{R/3-D}\right)+\frac{h_{peak}-h_e}{D} $$ 5. **Find the drainage rate $D$** such that the peak depth and the duration above the limit depths do not exceed the tolerance (solve by trial). Convert: 1 mm/day = 0.1157 l/s/ha. 6. **Design discharge** of the drain: $Q = D \times A$, where $A$ is the area served (ha), with a reduction for large areas if justified. 7. **Hydraulic design:** choose the side slope, bed slope (following the land slope) and Manning's $n$ (0.025–0.035 for earthen drains), then solve $$ Q = \frac{1}{n}AR^{2/3}S^{1/2} $$ for the bed width and depth. Check that the velocity lies between the non-silting limit (about 0.3 m/s) and the non-scouring limit (about 0.6–0.9 m/s for earth). 8. **Add free board** (about 0.15–0.3 m) and fix the water level of the drain at least 0.3 m below the field outlets so that the fields drain freely. 9. **Layout and structures:** field drains, collector drains, main drain and outlet; provide culverts, drops and check structures where necessary. #### Worked outline If the initial depth is 50 mm, the 3-day design rainfall is 400 mm and the tolerance is 250 mm for 2 days, the rate $D$ comes out at about 46.5 mm/day (5.4 l/s/ha), so a 10 ha block needs $Q = 5.4\times10 = 54$ l/s, and the drain section follows from step 7. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the design rainfall for paddy drainage obtained? A. Maximum annual 3-day rainfall with frequency analysis for a 5-10 year return period B. Average annual rainfall divided by 365 C. Maximum 1-hour rainfall D. Monsoon total rainfall Answer: A. Annual maximum 3-day rainfall is analysed by Gumbel or growth factors for the chosen return period. (Key point: Design rainfall) 2. During the 3-day storm, how does field water level change, and what is the peak depth? A. Rises at D per day; h_peak = h0 + R B. Rises at (R/3 - D) per day; h_peak = h0 + R - 3D C. Falls at R/3 per day D. Constant at h0 Answer: B. Rain adds R/3 per day while drainage removes D, so after 3 days the depth is h0 + R - 3D; afterwards it falls at D per day. (Key point: Water-level history) 3. How is the drainage rate converted to design discharge? A. Q = D / A B. 1 mm/day = 1 l/s/ha C. 1 mm/day = 0.1157 l/s/ha and Q = D x A D. Q = D x A in m^3/s with D in mm/day directly Answer: C. D found by trial is converted (1 mm/day = 0.1157 l/s/ha) and multiplied by area served. (Key point: Drainage rate to design discharge) 4. How is the drain section sized? A. Lacey's regime formula only B. Darcy's law C. Orifice flow equation D. Manning's formula Q = (1/n) A R^(2/3) S^(1/2), with velocity between non-silting and non-scouring limits Answer: D. Bed width and depth are solved from Manning's equation with n 0.025-0.035 and velocity checked against limits, plus free board. (Key point: Hydraulic design) ## Related in Irrigation and Drainage Engineering - [Short note: Design of drainage of irrigated land](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adrainage-irrigated-land-design-note) — Irrigation and Drainage Engineering, chapter 9 - [Drainage canal for 5 ha paddy field](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adrainage-canal-paddy-5ha-300mm) — Irrigation and Drainage Engineering, chapter 9, asked 2× - [Necessity and methods of drainage of irrigated land](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adrainage-necessity-methods) — Irrigation and Drainage Engineering, chapter 9, asked 3× - [Design discharge for surface drainage, bunded field](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asurface-drainage-design-discharge-bunded) — Irrigation and Drainage Engineering, chapter 9 - [Short note: Design of hilly irrigation canals](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ahilly-canal-design-note) — Irrigation and Drainage Engineering, chapter 3 - [Short note: Design of lined canals](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Alined-canal-design-note) — Irrigation and Drainage Engineering, chapter 4 ## Related in other subjects - [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× - [Steps in designing a constant delay filter](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Aconstant-delay-design-steps) — Filter Design, chapter 2, asked 2× - [Steps in designing a microprogram control unit](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Amicroprogram-design) — Computer Organization and Architecture, chapter 3 - [Short note: Design procedures of microwave filters](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Afilter-design-procedures-note) — RF and Microwave Engineering, chapter 6