Tamil Nadu PSC — Agricultural Engineering (Code 164, degree standard) · India · Tamil Nadu Public Service Commission
Original practice questions written to the published syllabus · not past papersEstimating erosion and designing the bunds, terraces, waterways, spillways, ponds and dams that control it.
In the exams: GATE AG Section 4 Soil & Water Erosion; ICAR AIEEA-PG Unit VII; ICAR AICE subject 40 Unit 5; NCEES PE erosion; PRC ABE land and water (32%); state PSCs
How our questions for Tamil Nadu PSC spread across the units of Soil & Water Conservation Engineering. Difficulty: 25 easy, 22 medium, 4 hard.
| Contour and graded bunds | 7 | |
| Farm ponds | 6 | |
| Bench and broad-base terraces | 5 | |
| Gully control | 5 | |
| Mechanics of water erosion | 4 | |
| Soil loss estimation: USLE, RUSLE and MUSLE | 4 | |
| Drop, drop-inlet and chute spillways | 4 | |
| Wind erosion control and shelterbelts | 4 | |
| Agronomic and biological conservation measures | 3 | |
| Earthen dams: seepage and stability | 3 | |
| Wind erosion mechanics | 3 | |
| Grassed waterways | 2 | |
| Sediment yield | 1 |
Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.
Other factors being equal, which soil is usually the most erodible by water?
D. A silty, very-fine-sand soil low in organic matter with weak structure
Silt and very fine sand are easily detached and easily carried. Clays resist detachment through cohesion; coarse sands resist transport and absorb water quickly, so they produce little runoff.
Further reading: Wischmeier and Smith (1978), USDA Agriculture Handbook 537
Surface mulch reduces water erosion primarily by:
A. absorbing raindrop impact and slowing overland flow
Mulch in contact with the soil protects it from splash detachment and increases hydraulic roughness, so runoff slows and carries less. It acts through C in the USLE; R is a climatic property and does not change.
Further reading: Schwab et al. / Fangmeier et al., Soil and Water Conservation Engineering
Using VI = 0.3(S/3 + 2), with S in percent, the vertical interval between contour bunds on a 6% slope is:
D. 1.2 m
VI = 0.3 × (6/3 + 2) = 0.3 × 4 = 1.2 m. Forgetting to divide S by 3 gives 2.4; forgetting the 0.3 gives 4.0. Dropping the S/3 term gives 0.3 × 2 = 0.6 m.
Further reading: Suresh, R., Soil and Water Conservation Engineering
Surplus arrangements such as waste weirs are provided in contour bunds to:
B. let runoff in excess of the design storage pass safely without breaching the bund
A bund stores only the design runoff. Waste weirs at low points or ends pass the excess in a controlled way; without them water overtops at the weakest point and breaches the bund.
Further reading: Suresh, R., Soil and Water Conservation Engineering
For level bench terraces with vertical risers and a vertical interval of 1.2 m, the earthwork (cut) per hectare is:
C. 1,500 m³
Length of terrace per ha = 10,000/W; cut per metre = W·VI/8; so cut per ha = 10,000 × VI/8 = 1,250 × VI = 1,500 m³, independent of W. 3,000 m³ double-counts fill; 6,000 m³ uses the whole triangle under the original slope (W·VI/2 per metre). 1,042 m³ divides 1,250 by VI instead of multiplying.
Further reading: Suresh, R., Soil and Water Conservation Engineering
On a 20% slope the soil is deep enough to allow at most 0.5 m of cut at the inner edge of a level bench (vertical risers, balanced cut and fill). The widest bench that can be built without cutting below this depth is:
A. 5.0 m
For a balanced level bench, the cut at the inner edge is VI/2. Cut ≤ 0.5 m gives VI ≤ 1.0 m. W = 100 VI/S = 100 × 1.0/20 = 5.0 m. Treating 0.5 m as the VI gives 2.5 m, an unnecessarily narrow bench. 0.2 m multiplies VI by S/100 instead of dividing.
Further reading: Suresh, R., Soil and Water Conservation Engineering
The usual order of the four stages of gully development is:
C. formation, development, healing, stabilisation
A gully first forms by scour of the topsoil, then enlarges and advances its head (most active stage), then vegetation begins to establish (healing), and finally the gully stabilises with a stable gradient and cover.
Further reading: Suresh, R., Soil and Water Conservation Engineering
In a zoned earthen dam, the central core of clay serves mainly to:
D. reduce seepage through the dam
The impervious core controls seepage; the pervious shells provide weight and stability; riprap protects the upstream face.
Further reading: US Bureau of Reclamation, Design of Small Dams
The mode of transport that usually moves the largest share of soil in wind erosion is:
A. saltation
Saltation (short hops of particles about 0.1–0.5 mm) usually accounts for roughly 50–75% of movement. Suspension carries fine dust far but is usually a smaller share by mass; creep is driven by saltation impacts.
Further reading: Morgan, R.P.C., Soil Erosion and Conservation
Wind strip cropping controls erosion mainly by:
D. reducing the unsheltered distance along the wind with alternate strips of erosion-resistant crops
Strips of resistant crops laid across the wind break the field into short lengths, so soil flow cannot build up by avalanching. In the WEQ this reduces L.
Further reading: Woodruff and Siddoway (1965)
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