The design process in six steps
- Need: what must the structure do? (carry 0.5 m³/s, hold 20 000 litres, take a 5-tonne tractor)
- Site: slope, soil, water, space, local materials, rules.
- Choose: type and shape (lined or earth channel, round or rectangular tank).
- Size: use formulas to find dimensions.
- Safety margin: add freeboard, cover, or a factor of safety. Real life is never exactly as the sum.
- Draw and cost: make a plan and section with a scale, list the quantities, and find the cost.
If the cost is too high or a check fails, go back and change the choice. Design is a loop, not a straight line.
Sizing a channel: Q = A × v
Discharge Q is the volume of water passing a point each second, in m³/s. Area A is the cross-section of the water (b × d for a rectangle) in m². Velocity v is the speed of the water in m/s.
Q = A × v. Think of a long sausage of water: the cross-section is its area and v is how far it slides each second.
Worked: need Q = 0.5 m³/s and v = 0.5 m/s. Then A = Q ÷ v = 1 m². With width 1 m the depth must be 1 m. Or width 2 m and depth 0.5 m. Both give the same area. We choose the one that is cheaper and fits the site.
The speed v is not free. If it is too high the water scours (eats) the channel. If too low, silt settles and weeds grow. So designers pick a safe v for the lining. Here we take v as given.
Freeboard and safety margins
Freeboard is the extra wall height above the normal water level. It stops water spilling when the flow rises, when wind makes waves or when weeds slow the water. Wall height = d + freeboard.
Other margins: concrete cover over bars, a factor of safety (design for a load bigger than the real one), and a gap for expansion. A margin is cheap. A failure is not.
Reading and making a drawing
A drawing is the language between the designer and the builder. A plan is a view from above. A section is a slice through the structure, like the end of a cut loaf. We draw to a scale, such as 1:50, which means 1 unit on paper is 50 units in real life. So 40 mm on paper is 40 × 50 = 2000 mm = 2 m.
A good drawing has a title, the scale, all dimensions with units, the material and levels (heights above a fixed point). Keep lines clear and numbers big.
Quantity and cost
To cost a lined channel, find the volume of lining: (the length around the channel section) × thickness × length of channel. For our channel the section is a floor plus two walls: b + 2 × wall height.
Cost = volume × price per m³. Add workers, tools and a small extra for waste. Compare two designs and take the cheaper one that still passes every check.
Next, check the design: does it carry the need? Is it safe? Can people maintain it? Does it harm the land or the neighbours? Only then does construction begin.
Try it: pour a channel in the sand
Make a small channel in damp sand with a ruler: width 5 cm and depth 3 cm, so A = 15 cm². Pour 150 ml of water from a mug in 10 seconds along it. Now make it twice as deep and pour at the same speed: does the water stay inside? Predict first, then check. Try making a freeboard wall with the sand.
Key formulas and definitions
- Q = A × v (m³/s = m² × m/s)
- A = b × d (rectangular channel)
- Wall height = d + freeboard
- Lining volume = (b + 2 × wall height) × thickness × length
- Cost = volume × price per m³
- Scale 1:n → real length = drawn length × n
Worked examples
1. A field needs Q = 0.6 m³/s. The water speed is 0.5 m/s. Find the cross-section area.
A = Q ÷ v = 0.6 ÷ 0.5 = 1.2 m².
2. A rectangular channel is 1.2 m wide and 1 m deep. Water speed is 0.5 m/s. Find Q.
A = 1.2 × 1 = 1.2 m². Q = 1.2 × 0.5 = 0.6 m³/s.
3. The water depth is 0.8 m and the freeboard is 0.3 m. How high is the wall?
Wall = 0.8 + 0.3 = 1.1 m.
4. A channel is 2 m wide in real life. On a drawing at scale 1:50 how long is that on paper?
2 m = 2000 mm. 2000 ÷ 50 = 40 mm.
5. A channel needs Q = 0.9 m³/s at v = 0.6 m/s and is 1.5 m wide. Find the depth of water.
A = 0.9 ÷ 0.6 = 1.5 m². d = A ÷ b = 1.5 ÷ 1.5 = 1.0 m.
6. A lined channel has b = 1 m, d = 1 m, freeboard 0.3 m, lining 0.1 m thick and is 20 m long. Find the lining volume and the cost at 6000 per m³.
Wall height = 1.3 m. Length around = 1 + 2 × 1.3 = 3.6 m. Volume = 3.6 × 0.1 × 20 = 7.2 m³. Cost = 7.2 × 6000 = 43 200.
7. Two designs give the same area 1 m²: (A) b = 1, d = 1; (B) b = 2, d = 0.5. Freeboard 0.3 m, thickness 0.1 m, length 10 m. Which lining is cheaper?
A: wall 1.3, around = 1 + 2.6 = 3.6 m, volume = 3.6 m³. B: wall 0.8, around = 2 + 1.6 = 3.6 m, volume = 3.6 m³. They are equal here, so choose by site, flow speed and ease of digging.
Common mistakes
- Mixing units: using litres per second in Q = A × v with m² and m/s. Convert 500 L/s to 0.5 m³/s first.
- Forgetting freeboard, so the channel overflows in a flood or a windy day.
- Calculating the lining only for the water depth and not for the full wall height.
- Reading a scale backwards: at 1:50 the drawing is smaller, so real length = drawn length × 50.