Sewer gradient from sight-rail levels: The reduced levels (R.L.) of the string at consecutive sight rails A and B are 203.575 m and 203.475 m, respectively. If the difference in their R.D.s (chainage spacing) is 10 m along the alignment, determine the gradient of the sewer line (state direction as upward or downward relative to A → B).

Introduction / Context:
Setting sewer gradients accurately in the field is crucial to ensure adequate self-cleansing velocity while preventing excessive scouring. Sight rails (and a traveller/boning rod) are used to transfer design levels along the alignment. This problem checks your understanding of how level differences translate to gradient over a known chainage interval.

Given Data / Assumptions:

• R.L. at sight rail A = 203.575 m.
• R.L. at sight rail B = 203.475 m.
• Horizontal distance (R.D. difference) A → B = 10 m.
• Rail strings are set to represent design line; fall or rise between rails equals design gradient over the spacing.

Concept / Approach:
Gradient is defined as fall (or rise) per unit horizontal distance. If downstream is lower, the line has a downward gradient in the direction of flow. Numerical gradient is reported as 1 in N where N = horizontal distance / vertical fall (for downward gradients).

Step-by-Step Solution:
Compute fall from A to B: fall = 203.575 - 203.475 = 0.100 m.Horizontal distance = 10 m.Gradient N = distance / fall = 10 / 0.100 = 100.Since B is lower than A, gradient is downward from A to B.

Verification / Alternative check:
Express as percentage: fall% = (0.100 / 10) * 100 = 1%. A 1% slope equals 1 in 100, confirming the calculation.

Why Other Options Are Wrong:
1 in 100/500/503 upward: levels are lower at B, so the line is not upward.1 in 50 downward: would require a 0.20 m fall over 10 m, which is not the case.

Common Pitfalls:

• Reversing sign when interpreting which point is higher.
• Confusing gradient notation (1 in N) with percentage slope.
• Using pipe invert instead of string level without accounting for traveller height; here we directly compare string R.L.s.

Final Answer:
1 in 100 downward