Chapter 5

Soil erosion control Measures

1. CHAPTER FIVE Soil Erosion Control Measures Mengistu Zantet (MSc .) Lecturer @ Hydraulic and Water Resources Engineering department Mizan Tepi university   Email:   [email protected] P.O.Box :  260 Tepi , Ethiopia 11-Jul-21 1 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department

2. Outline 5.1.BiologicalMeasures/Agronomic practices 5.1.1 . Mulching 5.1.2. Crop management practices 5.1.3. Soil Management practices 5.2. Mechanical/Engineering Measures 5.2.1.Terraces 5.2.2. Bunds 5.2.3. Vegetated waterways 5.2.4. Gully control measures 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 2

3. 5.1 Biological Measures/Agronomic practices C onservation measures the preference is always given to this methods due to: It is less expensive Reduce the rain drop impact, increase infiltration rate, reduce runoff volume and decrease the velocity of runoff and wind. It is easier to fit them into existing farming system. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 3

4. Cont.… Agronomical measures are referred to the practices of growing vegetation on mild sloppy lands to cover them and to control soil and water losses. The role of agronomical measures in achieving soil and water conservation has immense importance, much more than engineering measures 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 4

5. The practices included under agronomic measures are Contour cultivation Strip cropping Tillage practices Mixed cropping/inter planting 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 5

6. Contour Cultivation: is tillage and planting of crops across the land slope along the contour lines rather than up and down hill or parallel to field boundaries. Contour cultivation in humid and moist sub humid regions is mainly to reduce soil erosion. It is used in semiarid and drier portions of sub humid regions primarily to increase soil moisture by reducing runoff losses 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 6

7. Contour Cultivation: 11-Jul-21 7

8. Strip Cropping it is the practice of growing alternate strips of different crops in the field across the land slope. The strips are so arranged that the strip crops should always be separated by strips of close-growing and erosion resistance crops. Strip cropping is more intensive practice for conserving the rainwater than contouring, but it does not involve greater effect on soil erosion 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 8

9. Strip Cropping 11-Jul-21 9

10. Tillage practices 11-Jul-21 10

11. Mixed cropping/inter planting 11-Jul-21 11

12. The four types of strip cropping are : Contour strip cropping Field strip cropping Buffer strip cropping Wind strip cropping 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 12

13. 5.2. Mechanical/Engineering Measures Mechanical or engineering measures for protection of soil and water loss are all the methods which involve earth moving They are constructed by manipulating the surface topography. The agronomic measures combined with good soil management practices provide better influence on the detachment and transportation of soil particles in the process of soil erosion 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 13

14. The mechanical measures are not much preferred than agronomic measures because Many mechanical works are costly to install and require regular maintenance. Structures like, terrace and bunds create problems for agricultural operation. At shallow soil depth, the terrace construction exposes the bed rock or less fertile sub-soil and therefore results in low crop yield. There is a risk by severe storm with return periods of 20 years or more, of which terraces can be failed. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 14

15. The principles of water erosion control measures are the same wherever serious water erosion occurs. These principles are: Reduce rain drop impact on the soil Reduce runoff volume and velocity Increase the soil’s resistance to erosion 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 15

16. Prevention and Control Measures for Water Erosion 1) Sheet and Splash Erosion prevented by maintaining plant cover (preventing splash erosion) and maximizing infiltration of ponded water through the maintenance of soil structure and organic matter 2) Rill Erosion : Reducing Flow Velocity (settle suspended particles): Flow velocity can be reduced by either reducing the flow volume or roughening the soil surface. Hardening Soil Surface (prevent detachment) 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 16

17. 3) Tunnel Erosion Tunnel erosion is particularly difficult and expensive to control and not always successful. Combinations of mechanical, chemical and vegetative measures are usually required to control or prevent tunnel erosion 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 17

18. Permanent structures are built for long-term erosion control and are established for a long-term use. permanent measures include terraces, drop structures, spillways, culverts, gabions, ripraps, and ditches temporary measures include contour bunds, sand bags, silt fences, surface mats, and log barriers 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 18

19. The choice of mechanical measures depends on the severity of erosion, soil type, topography, and climate 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 19

20. 5.2.1. Terraces Terracing is an engineering soil and water conservation practice used to control soil and water loss in sloping areas. Terracing involves construction of embankments, ridges, or channels or land leveling in steps across the land slope. In terrace systems the effective length of land slope and slope steepness are reduced to large extent 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 20

21. Terraces 11-Jul-21 21

22. Types of Terraces Graded/diversion terraces : These terraces are constructed to intercept the overland flow and channel it across the slope to a suitable outlet, i.e. grassed water way etc. built at a slight down slope grade from contour. Retention terraces: These terraces are used where conservation of surface water by storing it on hill side is required. They are also termed as level terraces. The permeable soil with the land slope less than 4.5% are suitable for retention type terraces. In areas of low rainfall they are constructed for the purpose of water conservation. They may also be constructed where rainfall is high and the infiltration rate is also high allowing all rainfall to enter to the soil. Bench terraces : They are platform like constructions along the contours of the sloping land. They are generally constructed on lands 6 to 33% slope. In this terrace system, the hilly land is modified in the form of several steps, which intercept the flowing water over the soil surface. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 22

23. Graded/diversion terraces 11-Jul-21 23

24. Retention terraces 11-Jul-21 24

25. Bench terraces 11-Jul-21 25

26. Terrace Design The determination of a terrace components depend up on: The soil characteristic of the area Topography of the area Climate of the area Type of terrace Agricultural practices (tillage practices) 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 26

27. Designing a terrace involves Proper selection of the terrace type Determination of proper spacing (considering the farmable cross-section) Determination of terrace cross-section Terrace spacing : The spacing of terraces is expressed as the vertical distance between the channels of successive terraces 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 27

28. The factors which affect terrace spacing are climate , soil , topography , type of terrace and the tillage practice. But the important ones are climate, topography and the soil type 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 28

29. estimate the terrace spacing. U.S Soil Conservation Service devised a formula for estimating the best vertical interval VI = XS +Y …………………………………………………5.1 Where X = rainfall factor, dimensionless S = average land slope, % Y = Soil infiltration and vegetation cover factor, dimensionless VI = vertical interval, m Y = 0.3, 0.6, 0.9, or 1.2 with the low value for highly erodible soils with no surface residues and the high value for erosion resistant soils with conservation tillage and good crop cover. X = ranges from 0.12 to 0.24 with low value for highly erosive rainfall and high value for less erosive rainfall. Horizontal interval ……………………………5.2 Where S = average land slope, % The spacing computed by the above formula can be modified as much as 25% to allow for soil, climate and tillage conditions. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 29

30. Terrace Length Size and shape of the field, outlet possibilities, rate of runoff and channel size are factors that influence the terrace length. The length of a terrace should be decided so as to avoid the erosive velocity and large cross-section of the channel 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 30

31. Critical Slope Length The slope length of a field at which the overland flow becomes erosive is called critical slope length. Provided the effective slope length below the critical slope length, serious erosion will not take place 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 31

32. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 32

33. The computation of terrace spacing can be accomplished using the following steps. Determine the maximum depth of the productive top soil Find out maximum admissible depth of cut for the land slope of the field and crop to be grown based on the existence of maximum depth of productive soil range After determining the depth of cut, find out the width of terrace using the equation Inward sloping ranges from 2 to 10 % Outward slope ranges from 2 to 8.5% 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 33

34. The size of the shoulder bund: It depends upon the type of the bench terrace . For the terrace sloping inward the size of the shoulder bund is kept nominal (minimum possible) while for sloping outward and level top terraces the shoulder bund comprises larger section for holding runoff. The terrace section and soil characteristics affect the size (angle of repose of soil and permeability). On most soils for outward sloping terraces , Top width of shoulder bund = 30 cm Height of shoulder bund = 45 cm Bottom width =120 cm 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 34

35. 5.2.2. Bunds It is a type of engineering soil and water conservation measure which helps to control soil erosion and retain rainfall water from runoff. They are simple soil embankment structures constructed across land slope by less soil movement than for bench terracing 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 35

36. Bunds 11-Jul-21 36

37. Cont.. The practice is, generally, suitable for lands having 2-10% slope ranges. However , it can also be practiced on areas that have slope greater than 10% but with closer spacing, which may require high cost of construction and on area that have lesser slope than 2% with wider spacing. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 37

38. Classification of bunding systems contour bunding graded bunding 1) Contour bunding: it is construction of bunds that pass through equal elevation. The method can be adapted on all types of soils but not for deep black clay soils. The practice is suitable for areas, which receive annual rainfall less than 600mm. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 38

39. Cont.. It is not technically feasible on land slopes greater than 6% Contour bunding system is sub divided in to the following sub-groups a) Narrow based contour bunds , creates obstruction for crossing farm implements b) Broad based contour bunds 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 39

40. contour bounding 11-Jul-21 40

41. 2) Graded bunding In this bunding system, some grade is provided to the channel behind the bund (0.2 to 0.3%). Graded bunding is used in areas that have average annual rainfall greater than 700mm. However, it can also be used on areas of lesser average annual rainfall if the soil is of heavy texture (clayey). The functions of graded bunds are: to reduce soil erosion To dispose surplus rain water safely to a suitable outlet, the system may require grassed waterway. Graded bunding is not recommended on land slopes less than 2% or greater than 8% 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 41

42. Graded bunding 11-Jul-21 42

43. Types of graded bunds a) Narrow based graded bunds - provides obstruction to crossing farm implements b ) Broad based graded bunds - Does not provide obstruction to crossing of farm implements; the entire area can be put under cultivation. Therefore, the original cross -section of the bunds does not remain unchanged, resulting in the requirement of frequent maintenance. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 43

44. Some of considerations for bund construction: Economy: Rainfall characteristics and soil type: Land submergence: Seepage rate: 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 44

45. Design specifications for contour bunds 1 ) Spacing: In contour bunding, the bund needs to check the surface runoff at the point where its flow attains erosive velocity, and should meet the requirements of agricultural operation. Vertical interval between two bunds (VI): Different empirical formulae have been suggested to estimate the vertical interval between two bunds. a) For areas of heavier rainfall Where VI = vertical interval in (cm) ; S = land slope in percent b) For areas of low rain fall . 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 45

46. Cont.… But the above equations were developed by considering only land slope & rainfall amount; other factors such as infiltration rate, surface cover, etc. were not considered. Incorporating the effects of the remaining factors, COX developed more reasonable relation which is expressed as: VI = 0.3 (XS+Y) Where VI = Vertical interval in (m), S = Land slope in percent X = rainfall factor, Y = infiltration rate and crop cover factor   11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 46

47. Con… 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 47

48. 2) Size of the bund It is determined by the height, top width, bottom width, and side slope of the bund a) Height of the bund It is determined on the basis of the amount of water to be intercepted. The determination procedure can be illustrated as follows: 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 48

49. Cont.. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 49

50. b) Side slope 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 50

51. 3) Earth work: The earth work of the bunding system includes the sum of earth works of the main bund, side bunds and lateral bunds formed in the field. For calculation, the sum of earth works of side and lateral bunds is considered to be 30% of the earth work of the main bund. The earth work of any bund is determined by multiplying cross-sectional area and the total length of the bund. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 51

52. 4) Area lost due to contour bunding It is calculated by multiplying the length of the contour bund by its base width. Cross-Section of Graded Bunds The cross-section of a graded bund should have sufficient carrying capacity and the velocity of flow in the channel must not cause scouring of the channel bed. On the basis of this point the cross-section of graded bund can be determined by using manning equation. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 52

53. 5.2.3. Vegetated/grassed waterways Runoff that may be concentrated by the natural topography or by graded bunds, terraces, or other human works must flow in a controlled manner that will not result in gully formation. This can be achieved by practicing either of the following activities. Reducing the peak flow rates of runoff by full utilization of field protection practices earlier, or Providing a stable channel that can handle the peak flow 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 53

54. Vegetated/grassed waterways 11-Jul-21 54

55. Design of Vegetated Waterways : The design parameter estimation techniques are described below . Waterway capacity Velocity of runoff flow Shape of Vegetated Waterway Cross-section Vegetation type The gradient of waterway Roughness coefficient 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 55

56. The general procedure that has to be followed in designing a vegetated waterway is the following : Step 1: Determine the peak runoff rate from the area Q = CIA Step 2 : Find out the value of permissible flow velocity for vegetated waterway based on soil type and type of vegetation Step 3: compute the cross-sectional area of the waterway to handle the peak runoff rate and permissible flow velocity Q = VA; A = Q/V Keep in view that the runoff rate increases towards the outlet 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 56

57. Cont.. Step 4: After determining the cross-sectional area of the waterway, compute its various dimensions (depth, bottom width, side slope) to suit the area of cross-section obtained. Step 5: Calculate the hydraulic radius and decide the value of manning roughness coefficient R = A/P Step 6: compute the grade (s) of the waterway using manning formula Step 7: Check the elevation of the outlet computed using the grade and the elevation of the field outlet. They should coincide. The grade computed can be rounded or otherwise. Step 8: Using the rounded value of the waterway grade compute the flow velocity at a section. It has to be lower or equal to the permissible velocity. Otherwise, repeat the procedure 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 57

58. 5.2.4. Gully control measures The causes that activate gully formation are: Making the land surface to be without vegetation by over grazing and other biotic pressure, clearing for cultivation, firing, deforestation Improper construction of water channels, roads, rail lines, cattle trails, etc. adoption of faulty tillage practice Not smoothening of rills, small channels or depressions present on the ground surface . 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 58

59. Gully Development Gullies are developed by the following processes, which may be activated either singly or in combination Scouring of the bottom Gully head erosion Sliding or mass movement of soils from the gully banks, due to seepage, freezing & thawing 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 59

60.   Stages of gully development Stage 1: initiation stage Channel erosion & deepening of the gully takes place. Stage 2: development stage: Width & depth are enlarged due to runoff from up stream. The gully depth reaches up to c – horizon Stage 3: healing stage vegetation's start to grow in the channel There will not be appreciable erosion. Stage 4: final stage (stabilization) The gully has been fully stabilized. There will be no further development of gully unless the healing process is disturbed. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 60

61. Classification of Gully: Classification on the basis of shape a ) U -Shape gullies b) V – Shape gullies Classification based on the state of the gully a ) Small gullies b ) Medium gullies c ) Large gullies 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 61

62.   Gully Control : Principles of gully control Determine the cause of gully and take counter measure as early as possible Restore the original hydraulic balance or create new condition. That is, either the flood has to be reduced to its original volume or a new channel has to be provided to accommodate the increased flood. Therefore, for controlling gully erosion, the following activities are very important: A) Improving the drainage area of the gully. B) Stabilization of gully I) Stabilization of the gully head II) Stabilization of the gully side and bed 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 62

63. Structures for gully control: Temporary structures Wire bolsters Netting dams Brushwood dams Log dams   Brick weirs Permanent Structures: Silt trap dams Regulating dams Gully-head dams Gabions e) Drops & chutes 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 63

64. Silt trap dams 11-Jul-21 64

65. Regulating dams 11-Jul-21 65

66. Gabions 11-Jul-21 66

67. Design of Permanent Structures The design of permanent gully control structures is completed under the following three design steps: 1) Hydrologic design E stimation of design runoff rate and flood volume 2) Hydraulic design and determining the dimension of different components of the structure, on the basis of expected maximum runoff rate , 3) Structural design : determination of strength and stability of different parts of the structure 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 67

68. 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 68

69. the end 11-Jul-21 Soil and Water Conservation Engineering [email protected] . lecturer@ Hydraulic and water resources Engineering Department 69