Canal bank seepage: What saturation gradient (saturation line slope) is generally adopted for ordinary loam soil after a canal has been running for some time?

Introduction / Context:

When an irrigation canal runs for some time, seepage causes a wetted or saturated zone to develop within the earthen banks. The upper boundary of this zone is commonly called the saturation line (or phreatic line). Designers use an empirical saturation gradient to anticipate how far upward and landward the saturation will extend in typical soils, which influences stability, erosion control, and the placement of berms or toe drains.

Given Data / Assumptions:

• Ordinary loam soil for canal banks and embankments.
• Canal has been in operation long enough for a representative saturation profile to establish.
• Saturation gradient is expressed as a horizontal:vertical slope ratio for the phreatic line within the bank.

Concept / Approach:

Empirical practice in canal engineering adopts a typical saturation gradient for ordinary loam soils of about 4:1 (horizontal:vertical). This provides a practical estimate of how the phreatic line will rise into the bank, guiding checks on stability against sloughing, location of inspection berms ('dowla'), and design of pitching or turfing to protect against erosion.

Step-by-Step Solution:

Verification / Alternative check:

Field observations and guidance in canal manuals corroborate that for loam soils, a 4:1 saturation line offers a reasonable design estimate; clayey soils may show flatter phreatic lines, whereas coarser materials may differ, so designers can refine based on site investigations.

Why Other Options Are Wrong:

• 2 : 1 or 3 : 1: Too steep for ordinary loam; tend to underestimate the horizontal spread of saturation.
• 5 : 1 or 6 : 1: Flatter than commonly adopted for loam; may overestimate spread, affecting economy.

Common Pitfalls:

• Using the same saturation gradient for all soil types without adjustment based on investigations.
• Ignoring long-term operation; the phreatic line can shift seasonally with canal levels.

Final Answer:

4 : 1