What does agricultural civil engineering do?
It builds and looks after things that make farming easier and safer:
- Canals and pipes bring irrigation water.
- Drains remove extra water so roots do not rot.
- Farm roads let people, tractors and carts reach fields.
- Ponds and tanks store water for dry months.
- Land levelling and terraces save soil on slopes.
Good works raise yield, save water and protect soil.
Planning a project
- Aim: what must be solved, and for how many people or hectares?
- Survey: measure distances, heights and slope. Look at the soil and the water source.
- Choose the route/site: a gentle slope keeps water speed low so the soil is not cut. Avoid taking good farm land.
- Find the need: water needed = area × need per hectare.
- Cost, time and people: list them and ask the farmers who will use it.
Designing a canal
A small canal has a trapezoid cross-section. Bottom width is b, water depth is h, and sides slope 1 : 1.
Area A = (b + h) × h, and flow Q = A × v (v is water speed).
The bank is built a little higher than the water. This extra height is the freeboard (here 10 cm). It keeps waves and extra rain from spilling over.
Design rule: Q of the canal must be at least the need of the field. If it is bigger, you wasted money. If smaller, crops go thirsty.
Other structures and safe design
Every structure is designed in the same way: find the load or flow, choose the size, then add a safety margin. A road needs a firm base and a sloped top so rain runs off. A pond needs strong banks. A drain needs a slope so water keeps moving.
Try it
In the 3D: in the last step set the field to 30 ha and the width to 40 cm. Reduce the bank height to the smallest value that keeps the blue bar at or above the orange bar.
At home: pour water down a long board tilted a little, then a lot. Which tilt moves sand faster? This is why we avoid steep canals.
Key formulas and definitions
- Water need (L/s) = area (ha) × 2 (in this lesson)
- Trapezoid canal area A = (b + h) × h (sides 1:1)
- Flow Q = A × v (m³/s; ×1000 for L/s)
- Water depth h = bank height − freeboard
- Key terms: survey, route, discharge, cross-section, freeboard
Worked examples
1. A field is 15 ha. Each ha needs 2 L/s. Find the water need.
15 × 2 = 30 L/s.
2. A canal has b = 0.4 m, bank height 0.4 m and freeboard 0.1 m. Speed is 0.3 m/s. Find Q.
h = 0.4 − 0.1 = 0.3 m. A = (0.4 + 0.3) × 0.3 = 0.21 m². Q = 0.21 × 0.3 = 0.063 m³/s = 63 L/s.
3. How many hectares can that canal water at 2 L/s per ha?
63 ÷ 2 = 31.5, so about 31 ha.
Common mistakes
- Choosing the shortest route even when it is very steep. Fast water cuts the soil.
- Forgetting the freeboard and using the whole bank height as water depth.
- Mixing units: Q in m³/s must be multiplied by 1000 to get L/s.
- Designing a canal without checking what the field really needs.