Subsurface Drainage and Pump Station for a Saline Delta Block
A 3,000-feddan (1 feddan = 4,200 m²) irrigated area in Beheira, Nile Delta, has a water table rising to 0.8 m below ground and progressive salinisation. You are asked to design a subsurface drainage system with a drainage coefficient of 4 mm/day, discharging through a new pump station to a main drain. The soil is a silty clay loam with hydraulic conductivity 1.0 m/day. The drains are to be laid at 2.0 m depth, with the water table to be held 1.0 m below ground, so the head above drain level at mid-spacing is 1.0 m. The proposed pump station has three duty pumps of 0.25 m³/s each, with no standby, and the outfall drain has a dry-weather level 2.5 m above the pump sump and a wet-season level 4.0 m higher. The irrigation directorate proposes reusing the drainage water for irrigation after blending with fresh canal water; the drainage water has EC 2.5 dS/m and the fresh water 0.5 dS/m.
1. Compute the required pump capacity and assess whether the proposed station is adequate, including the wet-season head condition.
Read the worked answer
Flow. Area = 3,000 × 4,200 = 12.6 × 10⁶ m². A drainage coefficient of 4 mm/day = 0.004 m/day gives 50,400 m³/day, i.e. 2,100 m³/h or 0.583 m³/s.
Proposed capacity. Three duty pumps at 0.25 m³/s give 0.75 m³/s, about 1.29 times the design flow. On paper that is sufficient, but there are three defects. First, with no standby, one pump failure drops capacity to 0.50 m³/s, below the design flow, just when the water table is highest. Second, the pumps must be checked at the wet-season static head of 4.0 m plus friction and losses; a pump selected near 2.5 m will sit far to the right on its curve and deliver less than rated flow at 4.0 m. Third, the drainage coefficient is an average; peak wet-season rates may be 1.5–2 times higher.
Recommendation. Provide four pumps (three duty plus one standby) or two larger duty pumps with one standby, each rated at the wet-season head, with automatic level control, standby generator and a sump capacity of at least 10 minutes' retention.
2. Estimate the drain spacing using Hooghoudt's equation with the stated data, and state the assumptions and the practical adjustment you would make.
Read the worked answer
Equation. For a homogeneous soil with the water table above drain level, q = (4K h² + 8K d h)/L². With K = 1.0 m/day, h = 1.0 m and q = 0.004 m/day, and neglecting the equivalent depth d for a first estimate:
L² = 4 × 1.0 × 1.0²/0.004 = 1000, so L ≈ 31.6 m.
Adjustment. The equivalent depth d and the radial (entrance) resistance around the drain reduce the effective spacing, typically by 10–20 per cent; I would therefore adopt about 25–28 m rather than 31.6 m. Assumptions: steady-state flow, uniform K, drains running full but not under pressure, and no impermeable layer within about 2 m of the drain.
Practical check. At 25–28 m spacing the system needs roughly 3,000 feddan divided by (spacing × drain length per feddan), which drives the cost; before fixing it I would run a field permeability test, since K = 1.0 m/day is an assumption and the design is highly sensitive to it (L varies as the square root of K). I would also check the collector drain gradients so that the laterals can discharge by gravity into the sump.
3. Evaluate the proposal to blend and reuse the drainage water. What salinity, sodicity and downstream issues must be addressed before approval?
Read the worked answer
Salinity. A 1:1 blend gives EC ≈ 1.5 dS/m; cotton, the likely crop, is tolerant, but the relevant figure is the soil saturation extract, not the water EC. Using a leaching fraction of about 0.1–0.15, the root-zone ECe would be roughly 2–3 dS/m, which is acceptable for cotton but not for sensitive vegetables or citrus. Without drainage the salts would accumulate, so reuse and drainage must be designed together.
Sodicity and toxicity. EC alone is insufficient. The sodium adsorption ratio, chloride and boron concentrations must be measured; a high SAR relative to EC causes clay dispersion and loss of infiltration, requiring gypsum or blending to a lower SAR. The drainage water is brackish and may carry residues from fertilisers and pesticides.
Downstream. Taking water out of the main drain reduces dilution flow and concentrates salts for downstream users, and may affect the drain's environmental function. Reuse must be licensed, be seasonal (not in peak leaching periods), and be monitored at the blending point. Approval should be conditional on a salt balance for the irrigated block and on monthly EC and SAR testing.