PRACTICAL AND DESIGN ASPECTS OF TERRACING (2)-4.pptx

PRACTICAL AND DESIGN ASPECTS OF TERRACING

1. SWAMI VIVEKANAND COLLEGE OF AGRICULTURAL ENGINEERING & RESEARCH STATION, RAIPUR, CHHATTISGARH ( INDIRA GANDHI KRISHI VISHWAVIDHYALAYA, RAIPUR, CHHATTISGARH) Presentation on – Practical and design aspects of Terracing Course title - Soil & water Conservation Engineering Course code – SWE 505 Presented by- Shilpa Indorkar Shivangi Manikpuri Siddhant Shukla Snehil Vishal Kerketta Guided by – Dr. Prafull Kumar Katre sir

2. Terracing A terrace is an earth embankment, constructed across the slope to control runoff and minimize soil erosion. In limited widths of strips, the slope length naturally available for runoff is reduced. The main difference between bunding and terracing is that in bunding the length of slope is reduced while in terracing the length of slope as well as the degree of slope is reduced.

3. Terracing

4. Classification of terraces

5. Classification of terraces They are classified into two types Broad base terrace Bench terrace Broad base terrace adopted where – Main purpose is to remove or retain water from land suitable for cultivation. Bench terrace – Main purpose is to reduce slope. It is adopted when slope range from 6% to 33%.

6. Bench terraces Consist of construction of series of platform along contours. Hill slope cut into step like formation Makes protected platform available for agriculture practices.

7. Classification of bench terraces Bench terrace with level and table top Bench terrace sloping outward Bench terrace sloping inward Bench terrace Puertorician Classification based on purpose Hill type bench terrace – Used for hilly areas Irrigated bench terrace – used in irrigated conditions Orchard bench terrace – Are narrow width terraces, also referred as intermittent or step terraces.

8. Bench terrace sloping outward Used in low rainfall area with permissible soil. Shoulder bunds are essential Suitable for orchards Used to mild slope Slope up to 15%

9. Bench terrace sloping inward Adopted in heavy rainfall areas. Here major portion of rainfall to be drained Also known as hill type terraces. Back slope stabilization Runoff towards hillside

10. Bench terrace with level and table top Suitable for areas of medium rainfall Should have deep and permeable soil Also referred as irrigated bench terraces

11. Layout of terracing If available Topographic maps or high resolution digital elevation models are used to develop an initial plan. Layout should include examination of physical conditions of site, such as- Slope, soil depth, texture, erosion, presence of rocks, wetness, vegetative cover and present land use. It should also include specifications of terraces, sites and types of waterways, sites of roads, and other farm installations needs. Also include human factors such as the farmer’s plans and resources,labour conditions, and tools to be used .

12. Survey procedure Equipment required for survey Dumpy level Measuring tape and rod Soil auger Technique for 1. Level terrace – contour or leveling technique 2. For upland bench terrace – Graded contour technique 3. Setting of base line – Setting up and down base line along respective slope.

13. 4. Use of centre line method - When specification tables are not available, a quick calculation of the VI can be made in the field. Use a level to determine and stake the VI of the terraces along the base line. This should be followed by graded contouring or levelling surveys according to the type of terrace to be built. After staking out all the contours or graded contours, add one line of marked stakes in between them. This line serves as the bottom line of the upper terrace and the top line of the lower one. Continue adding stakes so as to cover the whole area. A top line should be added to the first terrace on the upward slope, and a bottom line to the last terrace on the downward slope. This method is recommended for hand-made terraces where centre-lines should be kept and observed as non-cut and non-fill lines.

14. Centre line method of survey

15. Design Of Bench Terrace Determination of design components of bench terrace mainly depends on the average annual rainfall, soil types, soil depth, and average slope of the land. these information are essential for the design of the same. The design of bench terrace involves the following major parameters for determination. 1. Type of bench terrace 2. Width of bench terrace 3. Spacing of bench terrace 4. Terrace cross section.

16. Step 1 Decide the type of bench terrace to be constructed :- Type of bench terrace to be constructed among all above types, is based on the rainfall and soil condition of the area. Step 2 Decide on the length of the bench :- The length of a terrace is limited by the size and shape of the field, the degree of dissections and the permeability and erodibility of the soil. The linear length of terraces per hectare can be calculated by the following equation:

18. Step 5 Calculate the terrace spacing (Vertical Interval):- Terrace spacing is the vertical distance between two successive bench terraces. It is equal to the double of depth of cut, it depends on the soil depth, bench width and land slope. After the slope and the width are determined, the Vertical Interval (VI) can be calculated by a simple equation. The simple equation using slope and the width of the bench as the main inputs is as follows: Where, VI: Vertical interval in m S: Slope in percent (%) W: Width of bench in m

19. For batter slope1:1 For batter slope ½: 1

20. Step 6 Calculate depth of cut:- Once the width of terrace is decided, the depth of cut can be calculated according to the following formula – Case 1:  When the terrace cuts are vertical                                                                      S is the land slope in percent; D is the depth of cut and W is the width of terrace. Case 2:  When the batter slope is 1:1                                                    

21. Case 3:  When the batter slope is ½: 1 Step 7 Calculate cross section area:- The design of the terrace cross section consists of deciding The batter slope, Dimensions of the shoulder bund, Inward slope of the terrace and the dimensions of the drainage channel in case of terraces sloping inward, and Outward slope in case of terraces sloping outward. 

22. The cross-section can be computed by the following formula: CA: Cross-sectional area of the cut triangle, in W: Width of bench, in m H: Height of riser, in m Fig: Cross section of bench terraces.

23. Steps of construction

24. Construction Equipment • A variety of equipment is available for terrace construction, including bulldozers, scrapers, motor graders, and hydraulic excavators • Smaller equipment, such as moldboard and disk plows, is suitable for slopes of less than about 8%, but the rate of construction is much slower than with heavier machines. • Soil and crop conditions are likely to be most suitable for construction in the spring and fall. Terrace Construction and Maintenance

25. • Settlement of Terrace Ridges • The amount of settlement in a newly constructed terrace ridge depends largely on soil and water conditions, • type of equipment, • construction procedure, and • amount of vegetation or crop residue. • The settlement based on unsettled height will vary from 5% or less for a motor grader to 10 to 20% for a bulldozer. Terrace Construction

26. Proper maintenance is as important as the original construction of the terrace. However, it need not be expensive since normal farming operations will usually suffice. Any breakovers should be repaired as soon as possible. The terrace should be watched more carefully during the first year after construction, and any excessive set- tlement , failures, or cracking repaired. Channels may occasionally need to be cleared of deposited sediment or ridges rebuilt. Terrace Maintenance

27. Tillage Practices In a terraced field, all farming operations should be carried out as nearly parallel to the terrace as possible. The most evident effect of tillage operations after several years is the increase in the base width of the terrace. Reversible plows can be used to increase ridge heights or to redistribute soil that has accumulated in the channel. Terrace Maintenance

28. A l s o c a ll e d c h a nn e l t y p e t e rr a c e Graded along the slope(lognituditional) U s e d i n a r e a h a v i n g h i g h r a i n f a ll Side slope is generally flat(for farming operation) R e m o v a l o f e x c e ss w a t e r C u tt i n g a s h a ll o w c h a nn e l o n u p h i ll s i d e a n d u s e t h i s s o i l t o b u i l t e m b a n k m e n t GRADED TERRACE

29. Reduce slope length. Minimize soil erosion. Intercept the runoff and divert to safe points. Utilize the land left between terrace an d ridge for growing crop or forage. OBJECTIVES

30. In humid region ,where water erosion is a problem and drainage is necessary. 6-8% slope. Area with high degree of mechanization. LOCATION AND CONDITION FOR USE

31. LAND SLOPE SOIL CHARACTERISTICS CROPPING PATTERN SOIL MANAGEMENT PRACTICE CLIMATIC CONDITION D E S I G N - ( g o v er n e d b y )

32. Spacing- V.I.=aS+b (emperical formula) F O R H U M I D R E G IO N - V . I .= S + 4 / 1 F O R N O R M A L S OI L - V . I .= S + 6 / 1 Horizontal interval(H.I.) = (V.I./S) 100 Depth of cut (d)-h+sW/2 Width of side slope- S(f)= W/h S(b)=W/d+sW DESIGN OF GRADED TERRACE

33. Level Terrace Uses Conserve the moisture content control the soil erosion Location Rainfall if low Rainfall intensity is not excessive Land Slope is less that 6% Infiltration and Soil Permeability are enough to absorb water

34. Selection criteria The area should be large The area of poor population density Design specification Terrace length – comparatively much longer than channel type terrace Channel gradient – absolutely level Width – 14-15m Ridge – level to store extra runoff Vertical interval – VI = (S/10) + 2 Horizontal spacing, side slopes, terrace length

35. Construction procedure The main construction component of this type of terrace is embankment. The construction of embankment is done with the help of soil taken from both the sides of ridge. For providing a greater safety, an allowance is given to the embankment height, called settlement allowance. And to create a greater runoff storage area the land immediately above the ridge should be levelled. The terrace end is either kept close or open to the protected area. Construction is performed in the same way as the graded terrace.

36. Protection and Maintenance Special attention is given for protection and maintenance of ridges The overtopping or leakage caused by the trapped water should be checked, immediately. All tillage operations should be done with great care, so that there should not be any damages to terrace edge When cultivating the land between the terraces, then it should be on contour because contour act as an auxiliary measure to harvest the surplus runoff.

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