Terracing and Bunding Questions and Answers

Answer: B

The keyed loss is 5.47%.

Take one terrace with vertical interval D. On a 10% slope its horizontal interval is 10D. The riser, at ½ H : 1 V, takes D/2 of that horizontally, leaving a level bench 9.5D wide. The original land, measured along the slope, was \(10D/\cos\theta\) wide, with \(\tan\theta = 0.1\):

\[ \begin{aligned} \cos\theta &= 1/\sqrt{1.01} = 0.99504 \\ \text{loss} &= 1 - \frac{9.5D}{10D/0.99504} \\ &= 1 - 0.9453 = 0.0547 \end{aligned} \]

So about 5.47% of the sloping land is lost to the risers. Measuring the loss on the horizontal projection instead gives exactly 5%, which is not an option. The higher values of 6.25% and 6.78% would need a flatter batter or a steeper hill slope, and 4.68% a steeper batter.

Answer: D

The drop is \(D = \frac{3\,W\,S}{(300 - 2\,S)}\).

Take the batter slope as 150%, a rise of 1.5 for each 1 horizontal, so the riser of height \(D\) takes up a horizontal width of \(D/1.5 = 2D/3\). On the original land slope \(S\)% the horizontal distance between terraces is \(100D/S\), and it equals the terrace width plus the riser width:

\[ \begin{aligned} \frac{100D}{S} &= W + \frac{2D}{3} \\ D\left(\frac{300 - 2S}{3S}\right) &= W \\ D &= \frac{3WS}{300 - 2S} \end{aligned} \]

In general \(D = \frac{WS}{100 - SU}\), where \(U\) is the horizontal run per unit rise of the riser. \(\frac{WS}{100 - S}\) is the case \(U = 1\), a 100% (1:1) batter, and \(\frac{2WS}{200 - S}\) is \(U = 1/2\), a 200% batter. \(\frac{WS}{100}\) ignores the riser entirely, which only holds for a vertical riser.

Answer: A

Such areas should adopt bench terraces sloping inward.

An inward-sloping bench drops towards the hill, so runoff collects at the inner edge of each bench and is led away in a drain along the toe of the riser to a safe outlet. The outer edge and the riser stay dry and protected, which is what is needed in hilly regions of heavy rainfall where much of the rain must leave as surface runoff.

Level benches suit areas of moderate, evenly distributed rainfall on permeable soils, where water is to be held and absorbed. Outward-sloping benches suit low-rainfall areas, since they would send heavy runoff over the riser and erode it. Narrow width is a matter of land slope and use, not a drainage design.