Hydraulic sizing for subsurface drainage projects
The Drainage Calculator sizes HDPE drainage pipes based on the design flow rate, terrain slope, and soil characteristics, using Manning’s equation for conduit flow. The result guides:
For project validation, Techduto’s engineering team reviews the calculation memorandum.
Manning’s equation crosses three variables at once: the higher the design flow, the larger the diameter needed; the steeper the slope, the smaller the diameter for the same flow; and the pipe material enters through the roughness coefficient.
Comes from the contribution area multiplied by the design rainfall — the more intensive the crop, the higher the rainfall considered: 9 mm/day for grains and forage, 12 mm/day for commercial crops, up to 20 mm/day for high-value areas.
At the same flow, steeper terrain drains faster and allows a smaller pipe. Manning’s coefficient (inner wall roughness) changes by material — that’s where the difference between smooth PEAD, corrugated and concrete comes in.
Counter-intuitive, but it’s about inner wall geometry, not material strength. Smooth double-wall PEAD pipe has n = 0.010; perforated corrugated drainage pipe runs around n = 0.016; concrete sits between 0.013 and 0.014. At the same slope and diameter, the lower the coefficient, the higher the flow the pipe carries — smooth wins over both.
Pre-sizing, not a signed design: the result gives an order-of-magnitude diameter and spacing to bring into the design meeting. The final calculation memorandum is the responsibility of a licensed engineer.
Common questions about the drainage calculator
It depends on the pipe wall. Double-wall PEAD pipe with a smooth interior uses n = 0.010; perforated corrugated drainage pipe runs around n = 0.016. For comparison, concrete sits between 0.013 and 0.014 — meaning that, at the same slope and diameter, smooth-wall PEAD carries a higher flow than concrete. The calculator applies the coefficient for the product line you select.
It works as a pre-sizing estimate — to get you into the design meeting with an order of magnitude for diameter and spacing. It does not replace a signed calculation memorandum: that’s the responsibility of a licensed engineer, accounting for soil survey data, local design rainfall, and the applicable standard for your case. If you’d like, Techduto’s engineering team will review your memorandum at no cost.
Drain spacing mainly depends on the soil’s hydraulic conductivity, installation depth, and expected recharge rate — clay-heavy soils (lower conductivity) need closer lines; sandy soils allow wider spacing. The calculator uses the soil and area data you enter to estimate that distance.
Our engineers can validate the calculations and specify the ideal solution.
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