High-Density Strawberry Tunnel System in Coastal British Columbia

Scenario

A 0.8-hectare protected agriculture operation near Abbotsford, BC, grows day-neutral strawberries under polyethylene high tunnels. Water source is a shallow well (pump max 2.8 bar, 30 m head), with tight elevation control across beds but significant friction sensitivity due to micro-tubing manifolds feeding 16 parallel laterals per tunnel. Constraints include: strict 2.5 m maximum friction loss per lateral (to preserve 10–15 kPa minimum pressure at last emitter), winter freeze risk limiting pipe burial depth (<0.3 m), and requirement for food-grade LDPE laterals compatible with fertigation injectors.

Given Data

• Flow Rate: 0.0041 m³/s (14.8 m³/h — 0.75 L/h emitters @ 0.3 m spacing, double-line configuration)
• Pipe Length: 82 m (maximum lateral length per tunnel, including 5 m manifold offset)
• Pipe Diameter: 32 mm (common LDPE size stocked onsite)
• Roughness Coefficient (C): 120 (conservative value for flexible LDPE with minor internal deposits after first season)

Calculation

Hazen–Williams friction loss:

hf = 10.67 × L × Q1.852 / (C1.852 × d4.8704)

With:

• L = 82 m
• Q = 0.0041 m³/s
• C = 120
• d = 0.032 m

Step-by-step:

1. Q1.852 = 0.00411.852 ≈ 2.61 × 10−5
2. C1.852 = 1201.852 ≈ 3,120
3. d4.8704 = 0.0324.8704 ≈ 1.14 × 10−6
4. Numerator = 10.67 × 82 × 2.61 × 10−5 ≈ 0.0229
5. Denominator = 3,120 × 1.14 × 10−6 ≈ 0.00356
6. hf = 0.0229 / 0.00356 ≈ 6.43 m → exceeds 2.5 m limit

Tool evaluates alternatives: at d = 50 mm, hf = 2.3 m — acceptable. Also confirms C = 120 is appropriate (field turbidity tests showed 12–15 NTU feed water).

Result and Decision

Tool output: friction_loss = 6.43 m, recommended_pipe_diameter = 50.0 mm. The grower upgraded from 32 mm to 50 mm black LDPE laterals (ASTM D3309, 1.25" nominal). This reduced measured pressure drop from 72 kPa (observed in pilot) to 21 kPa — achieving uniformity coefficient >95% across all 16 laterals and enabling consistent weekly fertigation without recalibration.

Lesson

Flexible plastic laterals degrade hydraulic performance faster than rigid pipes — always derate roughness coefficient (C) by 15–25% for LDPE/PEX in long-term fertigated systems, and verify recommendations against actual pressure measurements during commissioning.