Design of SWC.pptx department of soil resource and watershed management

Soil water conservation for soil resource and watershed management department it is used to teach us how run of is remove or eroded soil 1212

1. CHAPTER ONE: RUNOFF DETERMINATION FOR DESIGN OF SWC STRUCTURES 1.1. Characteristics of Surface Runoff Surface runoff (or simply runoff) is the portion of precipitation that makes its way towards the stream channels, lakes or oceans as surface or subsurface fows. Runoff occurs when precipitation rate exceeds infltration rate, and is the most destructive component of rainfall. In the design of SWC structures, the most important factors used are: i) peak runoff rates, ii) runoff volume, and (iii) temporal distribution of runoff rates and volumes. Rainfall is the primary source of water for runoff generation over the land surface. In common course of rainfall occurrence over the land surface, a part it is intercepted by the vegetation , buildings and other objects lying over the land surface; and prevent to reach them on ground surface, called interception. Some part of rainfall is also stored in the surface depressions, referred as depression storage , which in due course of time gets infiltrated or evaporated. When all these losses are satisfied, then excess rainfall moves over the land surface and reaches to the smaller rills, known as overland flow. The overland flow again builds a greater storage over the land surface and draining the same into channels/streams is termed as runoff. Thus, runoff may be defined as that portion of rainfall as well as any other flow, which makes its way towards the river, stream or oceans etc. Since, runoff is through the channel, stream/or rivers etc., therefore, sometimes it is also called as channel flow.

2. Types of Runoff: Based on the time delay between the instance of rainfall and generation of runoff, the runoff may be classified into following three types: a. Surface Runoff: It is that portion of rainfall, which enters the stream immediately after the rainfall. It occurs, when all losses are satisfied and if rain is still continued with the rate greater than the infiltration rate; then excess water makes a head over the ground surface (surface detention), which tends to move from one place to another following land gradient, is known as overland flow. As soon as the overland flow joins the streams, channels or oceans, is termed as surface runoff b. Sub-Surface Runoff: That part of rainfall, which first enters into the soil and moves laterally without joining the water-table to the streams, rivers or oceans, is known as sub-surface runoff or interflow. Sometimes sub-surface runoff is also treated under surface runoff due to reason that it takes very little time to reach the river or channel in comparison to ground water. The sub-surface runoff is usually referred as interflow.

3. c. Base Flow: It is delayed flow , defined as that part of rainfall, which after falling on the ground surface, infiltrates into the soil and meets the water table and flow to the streams, oceans etc. The movement of water in this type of runoff is very slow, that is why it is also referred as delayed runoff. It takes a long time to join the rivers or oceans Sometimes, base flow is also known as groundwater flow. Thus, Total Runoff = Surface runoff (including sub-surface runoff) + Base flow 1.2. Factors Affecting Runoff: The most significant factors affecting runoff are Catchment factors and Rainfall factors. These factor are described one by one as follow: A) Catchment factors or Cook’s Method The method consists of summing numbers each of which represents the extent to which runoff from the catchment will influence a particular characteristic.

4. The effect of four features is considered in Cook’s method, which are: (i) the relief (ii) soil infiltration (iii) vegetal cover, and (iv) soil surface storage Catchment factors of watershed consist of both the watershed as well as channel characteristics . Runoff is influenced by catchment factors such as topography, vegetation, infiltration rates, soil storage capacity and drainage pattern. In addition, the size of the catchment, its shape, orientation, geology and surface culture also affect runoff. The larger a catchment, the more runoff it will generate. Slope steepness is particularly important as soil erosion is more prone on steeper slopes. Surface culture includes the soil tilth, whether there is vegetative cover or not, and other land management activities, e.g. cultivation that would increase erosion. Different characteristics of watershed and channel which affect the runoff, are listed below:

5. 1. Size of Watershed: Regarding size of watershed, if all other factors including the depth and intensity of rainfall are same, then two watersheds irrespective of their size will produce about the same amount of runoff. However , a large watershed takes longer time for draining the runoff to the outlet, as result the peak flow expressed as depth becomes smaller and vice-versa. 2. Shape of Watershed: The shape of watershed has a great effect on runoff . The watershed shape is generally expressed by the terms “form factor” and “compactness coefficient”. The form factor may be defined as the ratio of average width to the axial length of the watershed, expressed as: Form factor = Average width of the watershed/Axial length of the watershed Axial length (l) of watershed is the distance between outlet and remotest point of the watershed. Average width (B) is obtained by dividing the area (A) with the axial length (l) of the watershed. Thus, form factor =

6. The compactness coefficient ( C f ) of watershed is the ratio of perimeter of watershed to the circumference of a circle, whose area is equal to the area of the watershed, is expressed as: R egarding watershed’s shape , two types of watershed’s shape are very common, in which one is fan shape and other is fern shape. The fan shape watershed tends to produce higher peak rate of runoff very early than the fern shape, due to the fact that in former one all parts of the watershed contribute the runoff to the outlet simultaneously, comparatively in little period of time than the fern shape watershed .

7. 3. Slope of Watershed: The slope of watershed has an important role over runoff; however its effect is complex. It controls the time of overland flow and time of concentration of drainage basin which provide a cumulative effect on resulting peak runoff. For example, in case of a sloppy watershed, the time to reach the flow at outlet is less, because of greater runoff velocity, which results into formation of peak runoff very soon; and vice-versa. 4. Orientation of Watershed: This factor affects the evaporation and transpiration losses from the area by making influence on the amount of heat to be received from the sun. The north or south orientation of watershed affects the time of melting of collected snow. In mountainous watersheds, the parts located on the wind ward side of the mountain receive high intensity rainfall, resulting into more runoff yield, while the parts of watershed lying towards leeward side have reverse effect.

8. 5. Land Use The land use pattern and land management practices used have significant effect on the runoff yield. For example, an area which is under forest cover, where a thick layer of mulch of leaves and grasses etc. has been accumulated forms a little surface runoff due to the fact that more rain water is absorbed by the soil. While in a barren field, where no any cover is available, a reverse effect is obtained. 6. Soil Moisture The magnitude of runoff yield depends on the amount of moisture present in the soil at the time of rainfall. If rain occurs over the soil which has more moisture, then infiltration rate becomes very less, which results into more runoff yield.

9. Similarly, if the rain occurs after a long dry spell, when soil becomes dry, then a huge amount of rain water is absorbed by the soil. In this condition even intense rain may fail to produce appreciable runoff. But on the other hand if the rain occurs in a close succession, as in the rainy season, then runoff yield gets sufficiently increased. 7. Soil Type: In the watershed, surface runoff is greatly influenced by the soil type, as loss of water from the soil is very much dependent on infiltration rate, which varies with the types of soil. 8. Topographic Characteristics: Topographic characteristics include those features of watershed, which create effect on runoff. It is mainly undulating nature of the watershed. Undulate land yields greater runoff than the flat land, because of the reason that runoff water gels additional power to flow due to slope of the surface; and tittle time to infiltrate the water into soil. Regarding channel characteristics to describe their effects on runoff, the channel cross-section, roughness, storage and channel density h ave significant effect on runoff yield.

10. 9. Drainage Density: The drainage density is defined as the ratio of the total channel length in the watershed to the total area of watershed. It is expressed as: D.D. = L/A A watershed having greater D.D. includes formation of peak runoff very shortly to that of the lesser D.D. watershed.

11. B) Rainfall Factors Rainfall factors associated with surface runoff and erosion include; rainfall amounts, storm duration, intensity and distribution, as well as seasonal patterns, Dry areas are more prone to erosion than wet areas because prolonged dry spells destroy vegetation cover, and rain storms tend to be of high intensity and thus erosive. The most significant component of rainfall is its intensity, which is a function of the energy the raindrops impact on the soil. The intensity-duration relationship of rainfall gives an indication of expected runoff.

12. The most significant component of rainfall is its intensity, which is a function of the energy the raindrops impact on the soil. The intensity-duration relationship of rainfall gives an indication of expected runoff. For example: I = a/(t+b) Where: I = Rainfall intensity t = Duration of rainfall (min) a & b are constants For any given duration, the graph or equation will indicate the highest average intensity which is probable for a storm of that duration. This is calculated as: I = kTx/tn Where, T = is the return period in years t = is the duration in minutes k, x, and n are all constants Calculations involving rainfall probability must relate to a chosen return period, e.g. for conservation works on small farms, about 10 years.

13. Time of Concentration (Tc) The storm duration which corresponds with the maximum rate of runoff is known as the time of concentration (Tc). It is assumed that during the time of concentration, all parts of the watershed are contributing simultaneously to the discharge at the outlet. Tc is also described as the longest time for water to travel by overland flow from any point in the catchment to the outlet. It is equivalent to the time it takes water to flow from the furthest corner of the catchment to the outlet. The climatic factors of the watershed affecting the runoff are mainly associated to the characteristics of precipitation, which include:

14. Type of Precipitation: Types of precipitation have great effect on the runoff. A precipitation which occurs in the form of rainfall, gets start immediately to flow in form of surface flow over the land surface, depending on its intensity as well as magnitude, Precipitation which takes place in the form of snow or hails, the flow of water on ground surface does not take place immediately, but after melting of the same. During the time interval between occurrence and their melting, the melted water infiltrates into the soil and results a very little surface runoff generation

15. 2. Rainfall Intensity: The intensity of rainfall has a dominating effect on runoff yield. If rainfall intensity is greater than the infiltration rate of the soil, then the surface runoff takes place very shortly, while in case of low intensity rainfall, there is found a reverse trend of the same. Thus, high intensities rainfall yield higher runoff and vice-versa. 3. Duration of Rainfall: Rainfall duration is directly related to the volume of runoff generation due to the fact that the infiltration rate of the soil goes on decreasing with the duration of rainfall, till it attains a constant rate. As result of this, even a mild intensity rainfall lasting for longer duration may yield a considerable amount of runoff. 4. Rainfall Distribution: Runoff magnitude from a watershed depends very much on the distribution of rainfall. 5. Direction of Prevailing Wind: The direction of prevailing wind affects greatly the runoff flow.

16. A storm moving in the direction of stream slope, produces a higher peak in shorter period of time, than a storm moving in opposite direction. The rainfall distribution for this purpose is expressed by a term “distribution coefficient”, which is defined as the ratio of maximum rainfall at a point to the mean rainfall of the watershed . For a given total rainfall, if all other conditions are the same, then greater the value of distribution coefficient, greater will be the runoff and vice-versa. However, for the same distribution coefficient, the peak runoff would be resulted from the storm falling on the lower part of the basin i.e., near outlet.

17. 1.3. Estimation of surface runoff ( The rational & Curve number methods ) The following are among the methods used in soil and water conservation for estimating the rate or the maximum rate of runoff that could occur from a particular catchment: 1. Rational Method: The Rational method is developed on the assumption that: (i) Rainfall occurs at uniform intensity for a duration equivalent to the time of concentration, and (ii) Rainfall occurs at a uniform intensity over the entire area of the catchment In this method, the peak rate of runoff is given by the equation-

18. The Runoff Coefficient (C) is defined as the ratio of the peak runoff rate to the rainfall intensity. Values of C for different slopes and land use conditions , determined from field observations are given in Table 1. When the catchment has areas with different values of C, the weighted value of C should be calculated for the whole catchment. If C 1 , C 2 , C 3 ,… are the values of the runoff coefficient for areas a 1 , a 2 , a 3 , … the weighted value given by:

19. The value of the intensity of rainfall to be used in Rational formula/method should be calculated for the period equal to the time of concentration of the catchment . The time of concentration of a catchment is defined as the time required for water to flow from the most remote point of the catchment to the outlet. When the duration of rainfall equals the time of concentration, all parts of the catchment will be able to contribute to the discharge at the outlet and as such the discharge will be maximum. Empirical formulae are available for determining the time of concentration. One such formula given by Kirpich (1940) is:-

20. According to Haan et al. (1982) the above relationship tends to correlate poorly with gauged runoff measurements of the time of concentration for very small (less than 5 sq. km.) catchments. This is because small catchments are dominated by overland (shallow) flow conditions rather than having well defined network of channels. To overcome this difficulty they suggest the equation for estimating the time of concentration as:

21. The second term in the above equation accounts for the overland flow. In watersheds, the overland flow phase effectively has a maximum travel time because both laboratory and field studies have shown that overland flow conditions can seldom be maintained for a distance longer than 100 to 150 meters. Thus, the value of this term has a maximum value regardless of watershed size because of the upper limits specified for L . However, as the size of watershed increases the maximum length of channel flow L tends to increase. This means that, as watershed size becomes large, the channel flow portion of the time of concentration equation will dominate the overall value. Thus, the above equation will be valid for all sizes of watersheds.

22. The time of concentration can also be assessed from estimates of the velocities of overland flow and the channel flow. If the overland flow traverses more than one kind of surface, the travel times across them should be added up. Knowing the time of concentration of a catchment, the maximum intensity of rainfall possible for this interval can be calculated. The area A of the watershed can be known from the topographical map of the area. The Rational method outlined above is simple and widely accepted. For a proper application of the method, it is important to understand its limitations. The method assumes that the rainfall intensity is uniform over the entire watershed during the duration of the storm, a condition rarely satisfied.

23. Again, for the runoff process to start, the losses due to depression storage and initial infiltration must be supplied. In addition, there is a retardance to flow caused by effects of surface detention and channel storage. The Rational method does not take these factors into consideration. The value of ‘C’ the runoff coefficient assumed to be dependent only on watershed characteristics. This does not seem to be correct as the value of C was observed to change with season as well as rainfall characteristics. These limitations of the Rational formula were discussed by Jaswant Singh (1964). Example 1: Estimate the peak rate of runoff for a 10 year frequency from a watershed of 25 hectares, having 15 hectares under cultivation (C = 0.5), 5 hectares under forests (C = 0.4) and 5 hectares under grass cover (C = 0.45). There is fall of 5 meters in a distance of 700 meters. The distance from the remotest point in the watershed to the outlet is 700 meters.

24. 2. The Curve Number Method: This method, also known as the Hydrologic Soil Cover Complex Number method , is based on the recharge capacity of the watershed. The recharge capacity is determined by antecedent moisture conditions and by the physical characteristics of the watershed. This is an extension of Cook’s method, which allows for variations in the physical conditions of a catchment and also the land use. Like in Cook’s method, four variables are considered and in each case, a selection has to be made from a list of options. Ten categories of land use or cover are offered (row crops, pasture, woods, fallow, farmstead etc) with a choice of soil conservation practices such as contouring and terracing.

25. The hydrologic condition of the catchment is graded good, fair or poor and a subjective assessment of this factor is designated for the four major hydrologic soil groups described earlier. The method relies on subjective non-measurable assessment. Let I a be the initial quantity of interception, depression storage and infiltration that must be satisfied by any rainfall before runoff can occur. It is assumed that the ratio of the actual runoff Q and the rainfall minus the initial loss P – I a (maximum possible runoff) and the ratio of actual retention to the storage capacity S are related by:

26. Where S is the recharge capacity of the watershed in mm. Curve number for different land use conditions and hydrologic soil groups are given in table below. The values apply to antecedent rainfall condition II which is considered as an average value

27. The soils of group A indicated low runoff potential, high infiltration rate, the soils of group B indicated moderate infiltration rate, moderately well drained to well drained. The soils of group C pointed to moderately fine to moderately rough textures, moderate rate of water transmission and the soils of group D pointed to slow infiltration and possible high runoff.

28. Chapter2: General principles for the design of SWC structures The design of SWC structures considers: S everity and extent of erosion damage or risks , The factors causing erosion , as well as The suitability of land to the identified intervention. SWC control measures are directed at protecting the soil from raindrop impact and hydraulic forces of runoff. The process involves three areas of attention: i) Reduction of raindrop impacts on soil (ii) Reduction of overland flows (iii) Increase infiltration rate, and (iv) Slowing runoff velocities 2.1. Factors considered Soil and water conservation structures are usually made by hand labour or machinery although some terraces develop naturally from vegetative barriers.

29. They are particularly important on steep slopes where annual crops are grown and in marginal rainfall areas where there is a need to conserve rainfall in situ. The selection and design of structure depend on many factors such as: Climate and the need to retain or discharge runoff. Farm size and system (large or small-scale, mechanised or non-mechanised). Cropping pattern (perennial or annual, with or without rotations). slope steepness Soil characteristics (erodibility, texture, drainage, depth, stoniness and risk of mass movement). The availability of an outlet or waterway for safely discharging runoff away from cropland. Labour availability and cost The availability of material e.g. stone The adequacy of existing agronomic or vegetative conservation measures.

30. 2.3 Structures for retention or discharge of runoff Structure can be designed either to retain or discharge runoff. They can also be designed so that part of the runoff is retained but the excess, during heavy storms, is discharged. In the higher rainfall areas (e.g. over 1,250 mm per annum), where crops are rarely short of water, or where there is a risk of water logging at certain times, it is usually to design structures to discharge runoff If there is no suitable outlet such as a natural waterway, artificial waterway or grassed slope are created Discharging water onto a footpath, road or existing gully would aggravate soil erosion. On large-scale farms it is usually possible to set land for waterways. In densely settled area this is much more difficult.

31. In the drier areas (e.g. less than 750 mm per annum) it is usually desirable to keep rainwater in situ and to prevent runoff. Other factors that must be considered in reaching a decision, besides the availability of a discharge area or waterway, include the soil type, soil depth, land slope and the risk, if any, of retaining water in situ. Soils in higher rainfall areas that are prone to water logging because they are shallow or because of the clay content, such as the grey soil (planosols) or black cotton soils (vertisols) in other areas, normally require structures that will drain water. Areas prone to landslides become unstable if they are very wet, and conservation structures should be designed to drain the water away.

32. When there is a need to discharge water but no suitable space for a waterway, there are two options. One is to change the land use to a permanent crop or fodder grass that does not require conservation structure. The other is to use contour barriers designed to conserve all the runoff. 2.4. Size of conservation structure The design of any structure to retain or discharge runoff should be based on a reasonable estimate of the volume of runoff (m3) to be retained or the peak rate of runoff (m3/s) to be discharged. A retention structure can rarely be made big enough to capture all runoff during exceptionally wet period, unless the catchment area is very small. One alternative with retention structures is to incorporate a spillway to take the overflow

33. Similarly the design of a structure to discharge runoff can rarely be based on the heaviest storm possible. Usually it is based on the heaviest storm that can be expected in a given period (e.g. 10 years) with the knowledge that a heavier storm, of a magnitude that occurs once in twenty, fifty or a hundred years, could take place (the frequency in years with which a storm of a given amount is likely to occur is known as the return period). 2.5. Risks The risk of damage due to an exceptional storm should be considered when designing structures. If the risk cannot be eliminated, it must be minimised by ensuring that the structures are stable when they are made and carefully maintained afterwards. Failure to pay attention to this point can lead to damage during heavy storms and greater erosion than erosion if the structures had not been installed in the first place. Where there are a series of structures on a hills slope there is a risk if a structure is breached near the top, then those downhill would also get damaged.

34. 2.6. Limitations The planning and construction of SWC structures on smallholder farms can be complicated by the small sizes of plots on given slope. This is because farm boundaries are not necessarily aligned to the contour or following a natural feature such as a crest line or drainage line. Thus, it is difficult to get appropriate site and space for an artificial waterway. Sometimes, the best site for a waterway may already be occupied by a footpath or a gully. Attempting to plan one farm in isolation from the others is likely to cause failure. A catchment plan is needed but there are social implications which must first be resolved. SWC structures can be expensive to install. In particular, gully control structures can be very expensive. There is also a lot of labour needed to excavate terraces, especially bench terraces. SWC requires some level of engineering design, and thus technical know-hoe can be a limitation. SWC structures function by retaining water in-situ, thus denying runoff to downstream areas. This can be a potential source of conflict which should be addressed.

35. 2.7. Management and maintenance SWC structures require regular maintenance and repairs if they get damaged. Grazing in cultivated lands treated with SWC structures should not be allowed as the animals can damage the structures. Instead, fodder should be cut and taken to animals preferably under cut and carry systems. Replanting vegetative materials and lining out of construction and channels should be done at least every season.

36. CHAPTER THREE: MECHANICAL EROSION CONTROL METHODS S oil and Water Conservation (SWC) refers to activities that maintain or enhance the productive capacity of land in areas affected by or prone to soil erosion. Soil erosion, on the other hand, is the movement of soil from one part of the land to another through the action of wind or water Soil erosion by water is caused by : Raindrop impact surface sealing Crust formation leading to high runoff rate and amount High runoff velocity on long and undulating slopes, and Low soil strength of structurally weak soils with high moisture content due to frequent rains.

37. What are soil and water conservation structures? Soil and water conservation structures include all mechanical or structural measures that control the velocity of surface runoff and thus minimise soil erosion and retain water where it is needed. They usually consist of engineering works involving physical structures, made of earth, stones, masonry, brushwood or other material for the construction of earthworks such as terraces, check dams, and water diversions, which reduce the effects of slope length and angle. SWC structures can be designed to either conserve water or to safely discharge it away. They supplement agronomic or vegetative measures but do not substitute for them. Suitability of SWC structure depends on:Climate and the need to retain or discharge the runoff. Farm sizes. Soil characteristics (texture, drainage, and depth). Availability of an outlet or waterway. Labour availability and cost. The adequacy of existing agronomic or vegetative conservation measures.

38. Benefits of conservation structures: Soil and water conservation bears benefits over a longer time span after construction. However, some benefits such as increased crop yields can be attained within the first year. In general, the benefits of SWC can be summarised as follows:- Increased agricultural productivity (higher yields, fodder for livestock ) Conservation of potentially productive land i.e. SWC supports sustainable agriculture Reduced nutrient loss from the soil, and thus less fertilizer requirements Environmental conservation, by storing more water within the soil profile and thus improved catchment hydrology Soil drainage benefit in areas prone to floods or water logging, SWC benefits irrigation and drinking water supplies, by protecting reservoirs from sedimentation SWC protects infrastructure such as roads from erosion damage, e.g. gullies.

39. 3.1. Bunds Contour Bunds / Level Bund A contour bund is an embankment along the contour, made of soil and/or stones, with a basin at its upper side. Level bunds are walls to retain all runoff between two bunds. Overflow should never occur unless as the result of inappropriate lining of the bunds Eroded soil between two bunds is deposited in the basin behind the lower bund Whenever the basin fills with sediment, the bund must be raised. This way, a Bench Terrace will develop in the course of several years.

40. The bund reduces or stops the velocity of overland flow and consequently soil erosion. The bunds passing through the points of equal elevation (contour line) of the land is called contour bund . The contour bunds and level terraces are synonymous . The contour bunds can be adopted on all types of permeable soils such as alluvial, red, laterite brown soil, shallow and medium black soils . In (i), an obstruction is created for crossing of farm implements and the bund can not be cultivated. In (ii), farm implements can cross the bund and the bund can also be cultivated. But bund sections may be disturbed due to crossing of machinery.

41. Clayey or deep black cotton soils should be strictly avoided as they crack when dry and bund fails during rains . In addition, clay soil has the problem of water logging near the bund, infiltration capacity is low. Types: The contour bund is subdivided into i) Narrow base contour bund, ii) Broad base contour bund.

42. DESIGN AND SPECIFICATIONS LEVEL/COUNTOR BUNDS The vertical interval between two bunds is 1 m for slope gradients of less than 15%. For steeper slopes, the vertical interval must be two-and-a-half times the depth of reworkable soil . The design of contour bund includes determination of spacing, both horizontal and vertical and bund cross section . The bund cross section includes base width, side slope and bund height . The bund height should be sufficient to store the expected runoff from a rainfall of 10 year recurrence interval. Over this, extra depth should be provided for the design depth of water over the weir and the free board.

43. The base width, side slope and top width are decided by the nature of soil. Level bunds are about 50–75 cm high and Have a bottom width of 100–150 cm and a water retention basin on their upper side. Usually, tied ridges, placed in the basin about every 10 m prevent runoff from flowing sideways About every 50 m, a gap can be left open to allow oxen pulling ploughs to cross and reach their land. Spacing of Contour Bunds . As the water flows through a sloping land, it attains erosive velocity. The bund should be spaced to intercept the erosive velocity . Again, the spacing should not be too close to interfere with the farming operations. Different relationships are given:

44. C. E. Ramser’s formula . VI = 0.3 (S/2 + 3) (1) S – land slope (%); a and b are constants where, V. I. = vertical intervals between the bunds, S = land slope ( percent ) The above formula does not take into account soil and rainfall characteristics and its applicability cannot be generalized. When the above formula is used for soils with high infiltration rate and good conservation practices such as contour farming, growing of cover crops etc then 25% extra spacing can be used.

45. On the other hand, in soils of low infiltration capacity and unfavourable conservation measures, the spacing should be reduced by 15%. For high rainfall areas, the interval should be reduced and vice-versa. In fact, a general relationship of the following form may be used and the constants should be evaluated for the specific site. V. I. = S/a + b …… (2) a and b depend on soil and rainfall characteristics. Cox’s Formula: M. P. Cox, a water management specialist of USAID gave the following formula. V.I. = ( XS + Y) x 0.3 The values of X and Y are given in Tables 1 and 2, respectively. Horizontal Spacing in between bunds (H.I) V.I = H*100/S%

46. Table 1. Values of Rainfall Factor, X Rainfall condition Value of X Annual rainfall, cm Scanty 0.8 < 64 Moderate 0.6 64-90 Heavy 0.4 > 90

47. Table 2. Values of the Infiltration and Crop Cover Factor, Y Intake rate Crop cover during critical period Value of Y Below average < 3 cm/h Low coverage 1.0 Average or above > 3 cm/h Good coverage 2.0 One of the above factor is favourable and the other is unfavourable 1.5

48. The level bund in front of the slope follows a horizontal line. The basins behind the bund are separated by tied ridges about every ten metres. The newly constructed embankment still needs more revegetation. For this gentle slope, a 1 m vertical interval was used because the slope gradient is less than 15%. In the background, parallel bunds which allow cattle to cross the land during ploughing are set up with some alternating gaps between them .

49. Apply level bunds in arid & semi-arid area i.e. rainfall < 600mm . Yield increase by 25% has been reported due to construction of level contour bund in dry area Berm is constructed in the lower side of the channel to protect the lower ridge The main criterion for determining spacing of bunds is to intercept the runoff water before it attains erosive velocity. This will depends on many factors such as: Slope Rainfall Soil Cropping pattern Conservation practices adopted, etc…

50. Types of Bund: 1. Contour bund . The bunds constructed exactly on contour or with permissible deviation from the contour are known as contour bund. 2. Side bunds . These bunds are formed at the extreme ends of the contour bunds, running along the slope of the land. 3. Lateral bunds . They are constructed between two side bunds along the slope for preventing the concentration of water at one side and also to break the length of the contour bund into convenient segments. 4. Supplemental bunds . The bunds are constructed between two contour bunds to limit the horizontal spacing between the contour bunds. 5. Marginal bund (Peripheral bund). They are formed at the margin points of the watershed, road, river etc to define their boundary.

51. Earthwork Computation and Area Lost for Contour Bunding   The earthwork for bunding includes main contour bund, side and lateral bunds. Generally all the bunds have the same cross section. The length of the side and lateral bunds may be assumed to be 30% of the length of the contour bund. Let L = length of contour bund, m. The horizontal interval (H.I.) is given by   H.I. = (V.I. /S) x 100 Number of contour bunds (N) per hectare of land is given by:   N = 10 4 / [L x H.I.] = 10 4 x S / (L x V.I. x 100) = 100 S / (L x V.I.) Length of contour bund per hectare of land = N x L = 100 S / V.I. Adding 30% length for side and lateral bunds, total length of bund per ha,   L = 130 S / V.I  Area of cross section of bund A = (b + T) x h /2 where b = bottom width, T = top width, and h = height of bund. Therefore, volume of earthwork per ha of bunding,   V = (130 S / VI) x (b + T) x h /2 Area lost per ha due to bunding = total length of bund x base width of bund = (130 S / VI) x b   Percentage area lost due to bunding = (130 Sb / VI) x (100/10000) = 1.3 Sb/VI  

52. Problem1: Calculate the volume of earthwork for a 100 ha catchment which has a land slope of 3% and the percent area lost due to contour bunding. VI = 1.3 m, b = 2.25 m, top width = 0.45 m, height of bund = 0.90 m. Solution . Volume of earthwork for 100 ha is given by V = (130 S / VI) x (b + T) x (h /2) x 100 = (130 x 3/1.3) x (2.25 + 0.45) x 0.9 x 100/2 = 36450 m 3 . Percentage area lost due to bunding = 1.3 Sb/VI = 1.3 x 3 x 2.25 /1.3 = 6.75%.

53. Problem 2. Calculate the V.I. of contour bunds on a 4.5% land slope. The rainfall is moderate with average infiltration rate and good coverage of the land with vegetation. Solution: a) Using Ramser’s formula: V.I. = 0.3 (S/3 + 2) = 0.3 (4.5/3) + 2) = 1.05 m. Due to moderate rainfall, average infiltration rate and good ground coverage, 25% extra spacing can be provided. Therefore, actual V.I. = 1.05 x 1.25 = 1.31 m. b) Using Cox’s formula: V.I. = (XS + Y) x 0.3 X = 0.6, Y = 2.0, V.I. = (0.6 x 4.5 + 2) x 0.3 = 1.41 m.

54. b) Level Fanya Juu A level Fanya Juu (‘throw uphill’ in Swahili language) is an embankment along the contour, made of soil and/or stones, with a basin at its lower side. The Fanya Juu reduces or stops the velocity of overland flow and consequently soil erosion. In contrast with the Level Bund , the soil in a Fanya Juu is moved upslope for construction. The water retention basin is at the lower side of the wall. Tied ridges about every 10 meters are also used here to prevent runoff from flowing sideways.

55. About every 50 m, a gap can be left open to allow oxen pulling ploughs to cross and reach their land. About every 50 m, a gap can be left open to allow oxen pulling ploughs to cross and reach their land.

56. c. Graded bund A graded bund is defined similar to a Level Bund , with the only difference being that it is slightly graded sideways, with a gradient of up to 1%, towards a waterway or river. Are constructed in high rainfall areas. Such a gradient is used to drain surplus runoff if the retention of the bund is not sufficient. Tied ridges with top heights lower than the bund height serve to retard such flow and to provide small basins for water storage. The main functions Graded bunds retain normal amounts of runoff in their basins, but they can drain excess runoff during heavy storms which would cause overflow down slope and cause destruction on level bunds. Most of the soil eroded between two bunds is deposited, while some will be drained sideways during heavy storms and lost from the land.

57. The graded bund in the foreground enters a natural drainage channel which has been protected with a checkdam just below the entry point of the graded bund. The basin behind the bund still has small tied ridges to prevent runoff from flowing too fast and creating erosion behind the bund. Earth bunds are stabilized with revegetation and their outlets reinforced with stones.

58. Tied- ridge in every 10m with top heights less than the bund height which serve to retard sideway flow & to provide small basins for water storages. No gaps can be provided for ploughing oxen to cross (as for level-bund) because the graded bunds serve as drainage line which cannot be interrupted. It is more effective in wet areas as well as in moist areas with clay soils. Whenever possible, use and improve traditional waterways in the area where you intend to apply graded bunds.

59. Make the waterways one year before the graded structures to stabilize them before use. If the bunds are long, the basins behind them must be increased towards the waterway, as more and more runoff will have to pass during storms. The size of the ditch can be 25 cm deep by 50 cm wide at the beginning of the bund, but 50 cm deep by 100 cm wide after about 100–150 m when the bund reaches the river or the waterway.

60. Problem 3. Design a graded bund in sandy loam soil for the following conditions: Length of the bund = 400 m, average slope of land = 2.5%, VI = 1.5 m, grade of bund: for first 100 m = 0.1%, for next 150 m = 0.15%, and for last 150 m = 0.18%. Rainfall intensity for the time of concentration and for the recurrence interval = 16.5 cm/h, runoff coefficient = 0.3. Solution. VI = 1.5 m, S = 2.5%. Horizontal spacing of bund = 100 x 1.5 / 2.5 = 60 m. Area of catchment formed by 2 bunds = 60 x 400 = 2400 m 2 = 2.4 ha.

61. d) Graded Fanya J uu D efined like a level Fanya juu with the only difference i.e. slightly graded sideways towards a waterway, with a maximum gradient of up to 1%. The main function s: Graded Fanya juu retain small amounts of runoff above their wall & they drain excess runoff of heavy storms through the ditch below which would cause overflow and down slope destruction on level (Fanya juu) structures. Some of the soil eroded between two Fanya juu deposited above the wall, some is deposited in the ditch, while the rest is drained sideways.

62. Tied ridges behind the embankment provide small basins for water storage and guide the water over the bund into the ditch below, from where it is drained sideways. SPECIFICATIONS Caution is needed when applying graded Fanya Juu because they require careful design, supervision and maintenance, although conservation is effective. The vertical interval between two graded Fanya Juus is 1 m for slope gradients of less than 15%. For steeper slopes, the vertical interval is two-and-a-half times the depth of reworkable soil. Gradients of 1% are lined out as shown below apply stone-faced bunds whenever possible to make them strong for overflow.

63. The graded Fanya Juu in this drawing enters a natural drainage channel, where a checkdam has been constructed just below the inlet to prevent erosion. The drainage ditch of the Fanya Juu is also reinforced with stone. Small tied ridges are barely visible behind the embankment of the Fanya Juu. They help to prevent sideways flow of water above the embankment. Instead, excess runoff will flow over the wall and enter the ditch. Revegetation is absolutely necessary on the wall to make it strong.

64. MARKING CONTOUR LINES WITH THE LINE LEVEL Contour lines are horizontal lines across the slope joining points at the same elevation. Contour lines are used to line out conservation measures which have to be level. MATERIALS The following items are needed: Waterlevel; Thin plastic rope, 11 m long; Two wooden poles, 2 m long, marked every 10 cm; Meter band or meter stick; Short poles for marking the ground PREPARATION Fix the thin rope with each end to one wooden pole so that exactly 10 m of rope is between the poles. Check length regularly. Mark the middle of the rope at 5 m with a knot. Hang the small Waterlevel in the middle of the rope. Three to four people are needed to survey a level line and to mark it on the ground.

65. MARKING CONTOUR LINES Proceed across the slope as shown in the drawing below. Survey 10 m at a time; in difficult topography only 5 m (half the rope).

66. MEASURING SLOPE GRADIENTS WITH THE LINE LEVEL Follow the steps given below and use the formula to calculate the slope percentage. Take care that you use the correct units (1 meter = 100 centimeters).

67. MEASURING VERTICAL INTERVALS WITH THE LINE LEVEL A vertical interval between two points is the difference in elevation between them. Vertical intervals are used along the slope to mark the spacing between two conservation measures. Vertical intervals of structures on slopes steeper than 15% are calculated on the basis of the depth of soil observed on the slope. MATERIALS The following items are needed: Waterlevel, Thin plastic rope, 11 m long, meter band or meter stick; 2 wooden poles, 2 m long, marked every 10 cm; Small poles for marking on the ground. ASSESSING THE CORRECT VERTICAL INTERVAL On slopes with gradients of less than 15% , the vertical interval is 1 meter.

68. On slopes with gradients of more than 15%, the vertical interval is two-and-a-half times the soil depth. Examples: Slope (%) Depth of soil Vertical interval , m (cm) 5 More than 50 cm 1 m 10 More than 50 cm 1 m 18 60 cm (0.6 m) 1.5 m 25 80 cm (0.8 m) 2.00 m 35 50 cm (0.5 m) 1.25m 45 25 cm (0.25 m) 0.62 m

69. MARKING GRADED LINES WITH THE LINE LEVEL Graded lines are lines across the slope, which have a very small lateral gradient. They are used to line out conservation measures which are graded and have a ditch to drain excess water. For lining out 1% graded measures, the line level also uses a difference of 1% over a length of 10 m. This means the rope has to be fixed on the poles at two levels with a difference of 10 cm, as shown below:

70. For lining out 2% graded measures, fix one end of the rope at 1.2 m (= 120 cm) on the pole, and one end at 1 m (= 100 cm) to give a total difference of 20 cm over a length of 10 m. For 0.5% graded measures, fix rope with a difference of 5 cm. MARKING 1% GRADED LINES ON THE GROUND Always start lining out at waterway or river and proceed slightly upslope (1%). Always use the pole with the rope fixed higher up, nearer to the waterway, and the pole with the rope fixed at 1 m, farther away, as shown below:

71. Step-by-step guide to drawing contour lines Drawing contour lines requires a systematic approach to ensure accuracy and precision. Follow these steps to effectively create contour lines for your map and then develop a cross section: Analyze the topographic map: Begin by thoroughly examining the topographic map you will be working with. Familiarize yourself with the legend, which provides information about: The map’s symbols, Scale, and Contour interval. Note any prominent landforms, such as hills, valleys, or ridges, which will play a key role in generating the contour lines. Identify the contour interval: Determine the contour interval shown on the map. This value represents the vertical distance between each contour line and helps you understand the relative changes in elevation across the landscape. For example, if the contour interval is 10 meters, each contour line represents a 10-meter change in elevation.

72. 1. Find Known Elevation Points: Look for specific points on the map with known elevations, such as ridges or valleys, that are marked with elevation labels. 2. Use these points as a reference for drawing contour lines. 3, Connect these known points, you can create the framework for the contour lines. 3. Determine Intermediate Contour Lines: Once you have identified the known elevation points, it is time to draw the intermediate contour lines. 4. Connect the known points and gradually trace the elevation changes in the terrain. Use a pencil to lightly sketch these lines, making sure they follow a smooth and continuous path. 5. Maintain a consistent contour spacing throughout the map. 6. Add Additional Contour Lines: In addition to the intermediate contour lines, you may need to add additional lines to accurately represent the topography. 7. These lines can help show significant changes in slope, such as cliffs, steep slopes, or terraces. 8. Adjust the contour spacing if necessary to effectively capture these distinctive features.

73. Clear overfall weir A weir is a small dam built across a river to control the upstream water level. Weirs have been used for ages to control the flow of water in streams, rivers, and other water bodies. Unlike large dams which create reservoirs, the goal of building a weir across a river isn’t to create storage, but only to gain some control over the water level. It allows water to flow over its top, often called its crest. The flow over a weir depends on factors like: the length of the weir, the height of the water level above the crest, and the weir’s geometry. Weirs are commonly used to regulate water flow in streams, rivers, and other water bodies, and they play a crucial role in managing water resources and mitigating flooding impacts Formula for flow over a weir Q = CLH 3/2 C = discharge coefficient L = Length of the weir H = Height over the crest level

74. Types of Weirs: Weirs are classified according to: 1. Types of Weirs based on Shape of the Opening Rectangular weir Triangular weir Trapezoidal weir 2. Types of Weirs based on Shape of the Crest Sharp-crested weir Broad- crested weir Narrow-crested weir 3. Types of weirs based on Effect of the sides on the emerging nappe Weir with end contraction (contracted weir) Weir without end contraction (suppressed weir) Classification Based on Shape of Opening Rectangular Trapezoidal Triangular weir 1. Rectangular weir: It is a standard shape of weir. The top edge of weir may be sharp crested or narrow crested. It is generally suitable for larger flowing channels.

75. Flow over rectangular weir: To find the discharge over rectangular weir, consider an elementary horizontal strip of water thickness dh and length L at a depth h from the water surface. Area of strip = L x dh

76. 3.2. TERRACES a) Introduction A terrace is a broad term to include all types of structures made across a slope for the purpose of soil and water conservation. However, there are many definitions by various authors as to what constitutes a terrace. One simple definition describes a terrace as “a more or less change in slope profile with a reduction in gradient of the planted zone”. Thus, reducing slope steepness and/or length is also referred to as terracing . A more elaborate definition describes a terrace as “a unit consisting of a relatively steeply faced structure across the slope (r eferred to as a riser, bank, dyke, ridge, wall or embankment ), that supports above it a relatively flat terrace bed (which may be either flat, or sloping backwards or forwards and may slope laterally)”.

77. b) Functions of terraces Thus, the functions of a terrace include: On sloping lands, terracing reduces overland flow rates thereby reducing soil erosion. Terraces increase the infiltration of water and thus conserve moisture. They help to retain nutrients of the land thus boosting soil fertility –less fertilizer use Terraces can be used to drain away excess floods safely They reshape the slope profile to make other agronomic activities, e.g. mechanised agriculture possible Although terraces are normally to be used on cultivated lands, they can be on grasslands Terraces are of value on practically all soils except those that are too stony, sandy or shallow to permit practical and economic construction and maintenance. In drier areas the construction of terraces even in gentle sloping lands would be essential to enable water retention. The grass grown on terrace banks is used as animal feed Terraces increase the overall productivity of agricultural lands

78. c) Types of Terraces There are different types of terraces based on the design and shape of the channel and the ridge. Regardless of type, all terraces fall into two categories: (i) Graded terraces and (ii) Contour terraces. i) Graded terraces A graded terrace is one made with a gentle slope in the longitudinal direction. Such gradients range about 0.25% to 1%. Graded terraces are designed for high rainfall areas, and are meant to dispose surplus runoff gently off the arable land at non-erosive velocities to a place where it can be safely discharged such as an artificial waterway. Since water is allowed to leave, graded terraces may have lower embankments and/or channels. Graded terraces are open ended and designed where a suitable waterway can be located. ii) Contour terraces A contour terrace, also called level terrace or absorption terrace, is one which is constructed on the contour, meaning it flat and without a slope along its length. It is made to holding the water on the terrace bed so that it infiltrates into the soil. Contour terraces do not require an outlet and are therefore closed at both ends. The terraces are made with higher embankments and bigger channels in order to retain all the water falling on the bed. Contour terraces are suited to dry areas for water conservation.

79. d) Types of terraces T erraces can be directly constructed or developed over the long term through natural sedimentation. Some of the more common terracing technologies used by smallholder farmers include Contour bunds Fanya juu terraces Bench terraces Stone lines Trash lines and vegetative barriers. Terraces are grouped into four main categories: (i) Channel terraces (ii) Progressive terracing (iii) Excavated bench terraces, and (iv) Intermittent terracing The types of terraces under each of these categories are further described here below.

80. Channel terraces: This is a broad category of terrace types involving earth bunding, whereby a channel is created on the upslope while the embankment/bund is on the down slope. Most channel terraces are made for discharge of excessive runoff, and are thus graded. They are suited to high rainfall areas, and are preferred for gentle slopes. Progressive terracing : Progressive terracing involves of construction of barriers at intervals across the slope without the physical re-shaping of the land. The barriers block runoff flows to facilitate bench formation through natural processes of intra-terrace erosion and sedimentation. Such structures or bunds are created using earthen bunds, stones, crop residues, grass strip or vegetative buffers. The idea is to end up with bench terraces developed over time without having to move large volume of soil. Thus, progressive terraces function by slowing down runoff to allow sediments eroded from the intra-terrace area to be deposited uphill of the barrier.

81. Design of terraces Terraces are usually designed to re-shape the slope profile, by reducing the slope steepness and length. This requires the calculation of the most opportune terraces spacing. The right terrace spacing should enable effective functioning as in reducing soil erosion to a minimum. If the spacing is too wide, soil erosion continues to occur, and if too close, the construction is unnecessarily expensive. Identifying convienent terrace location is among the design aspects of terraces Criteria for good terrace location Terracing is achieved when long slopes are broken with earthworks installed across the steepest slope to intercept the surface runoff. Terrace earthworks consist mainly of two parts: (i) an excavated channel, and (ii) a bank or ridge on the one side ( either uphill or downhill) formed with the soil from the excavation. Sometimes the soil is spread over the land to create a level or nearly level steeped terrace as in bench terracing.

82. The planning of a terrace system should be coordinated with the water management system for the entire farm, giving adequate consideration for proper land use. Wherever possible, adjacent farms having fields in the same drainage area may consider joint terracing system. The following factors are considered for selecting terrace location: Farm orientation in relation to agricultural operations, cropping patterns and ease of farming operations Identify location of terraces where there would be minimum maintenance requirement Reasonable investment cost Adequate control of erosion Availability of outlet for discharging excess flows, especially in the case of graded terraces Better alignment of terrace can usually be obtained by placing the terrace ridge just above eroded areas such as gullies and abrupt change in slope Location of roads, fences and other infrastructure in relation to the terraces Top terrace should be laid out first, starting from the outlet end Top terrace should be properly located so that it will not overtop and cause failure of the other terraces below. Terraces are more effective when used in combination with other practices, such as proper agronomic and vegetative practices . Contouring and strip cropping are the best techniques to be used with terracing

83. Rules in terrace construction : Build the outlet two years before terrace construction. Start building from the outlet and top of the watershed Build terrace when the soil is neither too wet nor too dry Compact the soil for every 15cm of fill The top terrace should be constructed with great care regarding its stability & design specification because the safety of the lower terraces depends up on it. Terraces should be checked after exceptionally high rainfall to maintain any broken parts and any other interferences. NB: The terrace length is dependent upon the size and shape of the field, outlet possibilities, runoff rate, infiltration rate and channel capacity.

84. Determining terrace spacing The capacity of channels in terraces can be calculated using Manning’s Equation For simple terraces such as fanya juu terraces, the general formula for terrace spacing is determined as follows: VI = (% slope+2)*0.3 4 Where VI = Vertical interval between terraces in meters The vertical interval can be converted to the horizontal interval using the formula HI = VI x 100 % slope Where:HI =horizontal interval between consecutive terraces in metres

85. 3.3. Waterways Waterways are natural or constructed & shaped to required dimensions and vegetated or stone paved for safe disposal of runoff from field, diversion ditches or other graded structures. Unless it is possible to discharge the water from CODs and terraces into natural watercourses or in to non-erodible areas, construction of artificial waterways leading to rivers or to non-erodible areas, will be necessary. Its dimension must provide sufficient capacity to contain the peak runoff from a storm with 10 years return period. There are three types of waterways used in soil conservation;  

86. Types of waterways a ) Storm water drain Is a ditch which intercepts runoff from non-arable land to keep it off the arable area and discharge it to the grassed water way. Graded at 1 to 2% (0.4%) across the slope. Length 250m (sand) and 400m for clay soil b) Terrace channels Length the same as storm water drain Run across the slope with gradient 1 to 2% Size of the channel is dependent on the catchment area and velocity of flow. Minimum values recommended for terrace channel cross-section are: 2m width, 0.5m depth.   c) Lined/grassed waterways Used as outlet for storm water drains & terrace channels Located in natural depressions Run down slope at grade of the sloping surface They drain into river system or other outlet.

87. Functions waterways O utlets for diversions and terraces, ii) O utlets for farm ponds, iii) O utlets for emergency spillways, iv) D ispose off water collected by road ditches or discharge through culverts, and v) C arry runoff from natural drains and prevent formation of gullies. Design of Grassed Waterways The waterway is designed for an expected runoff from a rainfall of 10 year recurrence interval . The design includes the shape, grade, design velocity and cross section of the channel.

88. Shape of waterway The shape depends upon the field condition and type of construction equipment. Generally, parabolic, triangular and trapezoidal shapes are used. Natural waterways have shapes very close to parabolic. Trapezoidal channels after a long use gradually approximate the shape of parabola. When a V-ditcher is used for construction, triangular shaped waterways are constructed. V-ditcher in combination with a buck scraper can construct a trapezoidal waterway.

89. Channel Grade and Velocity . The topography of the land largely influences the channel grade. The channel grade influences the flow velocity. If the land slope is very high, channels should not run down the general slope to produce erosive velocity. The maximum slope should not exceed 1% and preferably it should be within 0.5 %. The slope should be checked for maximum non-erosive velocity under various conditions by using Manning’s formula. The following values of non-erosive velocity should be used ( Table 4 ). Table 4 . Non-Erosive Velocities in Grassed Waterways S No Cover condition Permissible velocity, m/s 1 Sparse cover 0.9 2 Vegetation to be established by sodding 0.9 to 1.2 3 Dense, vigorous sod established quickly 1.2 to 1.5 4 Well established sod of excellent quality 1.5 to 1.8 5 Well established quality and conditions under which flow can not take place at lower velocity 1.8 to 2.5

90. Where waterways are not lined by vegetation, the following values of critical velocity can also be used ( Table-5 ). The critical flow velocity is defined as that velocity of flow, at which neither silting nor scouring takes place. Table 5: Critical velocity where waterways are not lined with vegetation: S No Nature of soil Critical velocity, m/s 1 Earth 0.3 to 0.6 2 Ordinary laterite (morrum) 0.6 to 0.9 3 Hard laterite 1.2 to 1.5 4 Boulders 1.5 to 1.8 5 Soft rock 1.8 to 2.4 6 Hard rock > 3.0

91. Cross-Section of Waterway. The shape can be Parabolic Triangular or Trapezoidal For different shapes, the cross-section A, wetted perimeter P and top width T can be obtained by using the following formula. a) Parabolic : A = 2 Td /3 P = T + 8 d 2 / 3T b ) Triangular : A = z d 2 P = 2d  (1 + z 2 ) T = 2dz c ) Trapezoidal : A = bd + zd 2 P = b + 2d  (1 + z 2 ) , T = b + 2dz

92. Design Steps of vegetated grassed waterway The following can be one of the procedures.   Step-1. Determine the peak runoff rate generated from the area and to be drained through the waterway . The area to be drained can be obtained from the contour map . The peak runoff rate is estimated using the rational runoff formula . Step-2. Fix the permissible velocity of flow (V) for the type of vegetated grass waterway . Step-3 . Compute the cross-section of the waterway, A = Q / V. It should be kept in mind that the catchment area and the peak runoff increase towards the outlet. Therefore, the cross-sectional area should increase downwards.

93. Step-4. Determine the different dimensions of the channel to get the cross-sectional area computed above . Step-5. Calculate the hydraulic radius R from the known values of A and P . R = A/P Step-6. Compute the slope S of the waterway using Manning’s formula. S = V 2 n 2 / R 4/3 Step-7. If the available condition permits the slope, it is alright. Otherwise modify the slope as less as possible. Recalculate the velocity and discharge with the changed slope. If they are satisfied, design is final. Otherwise with new dimensions of the cross-section the waterway has to be redesigned.

94. Problem 4.9 . Design a trapezoidal shaped grass waterway to carry a peak discharge of 4 m 3 /s along a slope of 0.5%. Side slope = 0.5%, side slope 2:1 and Manning’s n = 0.048   Solution. We assume a bottom width b = 2m, A = bd + zd 2 = 2d + 2d 2 , where d = depth of channel. P = b + 2d  (1 + z 2 ) = 2 + 2d  (1 + 2 2 ) = 2 + 2  5 .d From Manning’s formula, Q = A R 2/3 S 1/2 /n or, Q n/  S = A R 2/3 R = A/P = (2d + 2d 2 ) / (2 + 2  5 .d) = = (d + d 2 ) / (1 +  5 .d) Therefore, (4 x 0.048) /  0.005 = (2d + 2d 2 ) [(d + d 2 ) / (1 +  5 .d)] 2/3 Or, 2.71529 = (2d + 2d 2 ) [(d + d 2 ) / (1 +  5 .d)] 2/3

95. By trial and error, d = 1.0 m (approx). Then R = (1 + 1 2 ) /(1 +  5 x 1) = 0.618034 m. Velocity of flow, V = (1/0.048) (0.618034) 2/3 (0.005) 1/2 = 1.07 m/s. For well established vegetation, this is non-erosive velocity. A = 2 (1 + 1 2 ) = 4 m 2 . Q = AV = 4.28 m 3 /s. The section meets the discharge requirement. A freeboard of 15 cm should be provided. Finally the dimensions are: b = 2 m, height = 1.15 m, side slope = 2:1 Problem 4.10. Determine the dimensions of trapezoidal shaped grassed waterway to carry a peak runoff of 4 m 3 /s. The slope is 0.3% and permissible flow velocity is 1.0 m/s.   Solution. Let the bottom width b = 2 m. depth d = 1 m. side slope = 2:1. Area of cross section A = bd + zd 2 = 2 x1 + 2 x 1 2 = 4 m 2 . Wetted perimeter, P = b + zd  (1 + z 2 ) = 2 + 2x 1  (1 + 2 2 ) = 6.47 m.

96. R = A/P = 4 / 6.47 = 0.618 m. V = R 2/3 S 1/2 /n , take n = 0.045 Therefore, V = (0.618) 2/3 (0.003) 1/2 /0.045 = 0.883 m/s. Velocity is safe. Discharge, Q = AV = 3.533 m 3 /s. Capacity is less. Select another size. Let b= 2.5 m. A = 2.5 x 1 + 2 x 1 = 4.5 m 2 . P = b + zd  (1 + z 2 ) = 2.5 + 2 x 1  (1 + 2 2 ) = 6.972 m. R = A/P = 4.5/ 6.972 = 0.645 m. V = (0.645) 2/3 (0.003) 1/2 /0.045 = 0.909 m/s. Velocity is safe. Q = AV = 4.09 m 3 /s. The carrying capacity is sufficient. Therefore, final dimensions are: b = 2.5 m, d = 1.15 m including freeboard, side slope = 2:1.

97. 3.4. DIVERSION DITCHES A diversion ditch ( also known as cutoff drain or storm-water drain ) is an open channel, made across the slope, with the ridge on the downhill side Diversion ditches are usually constructed above cropped land to protect cultivated land from excessive flooding coming in from home compounds, roads, gullies and other surfaces. It is the first line of defiance and is vital to the protection of the entire farm since all the structures lower down are designed on the assumption that all the runoff from outside the arable land is well controlled. If the diversion ditch fails the runoff ensuing would certainly breach the lower conservation works. Diversion ditches called variously storm water drain, storm water chanel, diversion terrace, cutoff ditch, cutoff drain, diversion ditch, or other combinations of these terms. However, the major distinguishing feature is whether the structure is meant to drain away excess flows – in which case it is called a “drain” or for water conservation – then the term “ditch” applies.

98. 3.4.1. Functions of diversion ditches The primary purpose of a diversion ditch is to protect intercept surface runoff which flows down from higher ground and convey the water safely to an outlet such as a waterway. A diversion ditch protects other SWC structures downhill behind it from erosion damage In dry land areas, diversion ditches can be used for water harvesting by directing water coming from the upper catchments into farmlands. In places where there is relatively high rainfall, and in areas with Vertisols, diversion ditches are used as drainage ways to let out water from the farmland to the natural or artificial waterway. Diversion structures may also be used to control gully erosion, and dissipating excess flows from roads, home compounds and hilly areas. They are used for water conservation or as water retention structures, and tree crops grow inside the ditch, especially in dry areas. Diversion ditches can pond runoff which can be used to raise the soil moisture of adjacent farmlands through seepag

99. 3.4.2. Design of diversion ditches The design of diversion ditches should be based on the outlet conditions, topography, land use and soil type. As diversion ditches are made to protect cultivated land, they should be designed to carry the peak runoff from the contributing catchment during the worst that can be expected in a 10 year period. The location of a diversion ditch is usually determined after checking the outlet conditions, topography, land use, soil type and length of slope. To achieve effective protection of farmland, the diversion ditch or cutoff drain, should be constructed between uncultivated and cultivated land. a) Allowable Velocity The dimensions of a diversion ditch are determined using the manning equations with permissible velocities shown below. Q = (AR 2/3 S 1/2 )/n Where, Q = flow rate, m3/s A = area of flow, m2 V = mean velocity, m/s R = hydraulic radius, m, (R= A/P; where P is the wetted perimeter) S = bed slope, m/m n = Manning roughness coefficient = 0.03 for drains

100. The channel shape and cross-sectional area are related. But since most diversion ditches are designed with a trapezoidal cross-sectional area, then the values of A and P are calculated as follows: A = bd + Zd 2 ; and P = b + 2d (1+Z 2 ) 1/2 Where, Z = side slope of a trapezoidal channel. Z should be less than the angle of repose of the saturated material. In any case Z should not be steeper than 1.5: 1 (Horizontal: Vertical). b = bottom width of the drain, d = depth of water

101. b) Channel shape and dimensions The channel cross-section can be made: Rectangular Parabolic Semi-circular or Trapezoidal Manully excavated ditches are usually rectangular or trapezoidal, while those made using machinery can be trapezoidal, parabolic or semi-circular. The trapezoidal shape is preferred because it accords larger capacities than either rectangular or parabolic shapes. The channel width and depth must be appropriate to the cross-sectional area calculated For an average smallholder farm, the channel dimensions range about 0.6 to 1.4 m bottom width, with a top width ranging 1.2 to 2.8 m.

102. The depth of diversion ditches can be about 0.3-0.7m. On larger mechanised farms, the side slope of the channel should be 4:1, but in small-scale farming a steeper gradient of up to 1:1 is recommended to save land. Channel dimensions depend on the gradient of the ditch. For gentle slopes, wider, shallow channels are used while on steep slopes, narrow, deep channels are more appropriate. In any given situation there are usually several possible dimensions which are related to the bed gradient as Generally, the channel cross-section should have a wide shallow cross section to minimise the risk of erosion and allow easy crossing by people and livestock.

103. Table 2.1: Allowable discharge rates for various channel dimensions and gradients of diversion ditches Ditch dimensions Gradient of ditch Bottom width Depth Top width Cross-sectional area 0.1 0.2 0.3 m m m m 2 m3/s m3/s m3/s 0.6 0.3 1.2 0.27 0.14 0.20 0.31 0.8 0.4 1.6 0.48 0.30 0.42 0.67 1.0 0.5 2.0 0.75 0.54 0.77 1.21 1.2 0.6 2.4 1.08 0.88 1.25 1.25 1.4 0.7 2.8 1.47 1.34 1.88 1.88 Note: These values assume earth lining and 1:1 side slopes. Source Thomas, 1997

104. c) Channel gradient The channel gradient of a diversion ditch depends on the topography of the area and vary between 0.1% and 1% with 0.5% as the optimum. For light sub-soils, (sandy silt and sandy loam) channel gradient is 0.1%-0.2% and for heavy sub-soils (clay and clay loam) it is 0.4%-0.5%. For small discharges about 1% gradient can be used. In order to minimise the risks of sedimentation and over-topping, the diversion ditch should not be long. d) Channel Length Generally, the length of a diversion ditch is commensurate with the width of the farm. But for very small farms it may be necessary to excavate one continuous channel across farm boundaries with one outlet, rather than several small ditches with respective waterways.

105. This has social implications as neighbouring farmers should all be agreeable. To minimise the risks of sedimentation and overtopping, a diversion ditch should not be too long, preferably not exceeding 250 m for areas with highly erodible soil. On more stable soil, e.g. red clay loams, the length should not be more 500m . In exceptional cases, a longer ditch could be made if it is the only way to reach a safe outlet or waterway. e) Free Board A Free board is an additional capacity added to the design depth of channel. The design is based on the peak discharge, which will occur at the outlet of the ditch. For earthen channels, extra depth equivalent to a 10% increase in the design depth is used. A freeboard is necessary to take care of unexpectedly high flows, and to act as a safety net in case of sedimentation of the channel or settlement of the embankment.

106. f) Channel lining and stability A grass lined channel is more stable than an earth lined channel. Grass resists scouring and slows down the flow of water and encourages deposition. If it grows tall it can block the channel. Broad shallow channels are usually lined with creeping grasses whereas narrow and deep channels are more likely to be earth lined. In areas of small-scale farming, the channels are usually narrow and dug into the subsoil so that condition are not suitable for a grass lining and the design should be based on the assumption that the lining is of earth.

107. g) Layout and construction Diversion ditches can be laid out with a line level or quickset level. The layout should start from the outlet end and stakes placed along the proposed diversion. The design and layout should conform to the principles of catchment planning and the farmers should be involved at all stages. The diversion ditch should be constructed before the land downhill is terraced. Construction should start from the outlet end. The time taken to excavate a given volume of earth for a diversion ditch depends on: the type of soil soil moisture content and size of cross-sectional area of the ditch It is more difficult and expensive to dig clayey soil than sandy soil. Generally, it would take an active person about 8 hours of hard work to excavate 3 to 4 m3 of soil. The embankment is usually compacted and stabilised with grass planting. The excavated soil from the ditch is placed on the downhill side to form the embankment. The embankment helps to further increase the capacity of the diversion ditch. A berm of about 15-30 cm (strip or ledge) is left between the embankment and the channel in order to prevent sliding of the soil. A good grass cover is then established on the embankment

108. C HAPTER FOUR: EROSION CONTROL IN TORRENTS AND GULLIES 4.1. Open channel flow Open channels are natural or manmade conveyance structures that normally have an open top , and they include rivers, streams and estuaries. When the surface of flow is open to atmosphere , in other terms when there is only atmospheric pressure on the surface, the flow is named as open channel flow. The governing force for the open channel flow is the gravitational force component along the channel slope. Water flow in rivers and streams are obvious examples of open channel flow Other occurrences of open channel flow are flow in irrigation canals , sewer systems that flow partially full, storm drains , and street gutters. In most applications,the liquid is water and the air above the flow is usually at rest and at standard atmospheric pressure Open-channel flow can occur also in conduits with a closed top, such as pipes and culverts, provided that the conduit is flowing partially full.

109. The Froude Number When the flow is dominated by the gravity, then the type of flow can be identified by a dimensionless number, known as Froude Number given by the following formula: Fr = V/√gd Where, V is the mean velocity of flow D is the hydraulic depth (= A/T) A is the cross-sectional area, T is the top width and, g is the acceleration due to gravity (9.81 m/s 2 ) Depending on the effect of gravity relative to inertia , the flow may be subcritical, critical or supercritical When, Fr < 1 the flow is subcritical Fr = 1 the flow is critical Fr > 1 the flow is supercritical

110. 4.2. Classification of Open Channel Flows A channel in which the cross-sectional shape and size and also the bottom slope are constant is termed as a prismatic channel. Most of the man made (artificial) channels are prismatic channels over long stretches. The rectangle, trapezoid, triangle and circle are some of the commonly used shapes in made channels. All natural channels generally have varying cross-sections and consequently are non-prismatic. a) Steady and Unsteady Open Channel Flow: If the flow depth or discharge at a cross-section of an open channel flow is not changing with time, then the flow is steady flow , otherwise it is called as unsteady flow . Flood flows in rivers and rapidly varying surges in canals are some examples of unsteady flows . Unsteady flows are considerably more difficult to analyze than steady flows.

111. b) Uniform and Non-Uniform Open Channel Flow: If the flow depth along the channel is not changing at every cross-section for a taken time, then the flow is uniform flow. If the flow depth changes at every cross-section along the flow direction for a taken time, then it is non-uniform flow. A prismatic channel carrying a certain discharge with a constant velocity is an e xample of uniform flow. c) Uniform Steady Flow: The flow depth does not change with time at every cross section and at the same time is constant along the flow direction. The depth of flow will be constant along the channel length and hence the free surface will be parallel to the bed.

112. d) Non-Uniform Steady Flows: The water depth changes along the channel crosssections but does not change with time at each every cross section A typical example of this kind of flow is the backwater water surface profile at the upstream of a dam.

113. Most Economical Channel Section: A channel section is the shape of the cross-section of a waterway. The most economical channel section is the one that has the least wetted perimeter for a given cross-sectional area. This minimizes the friction losses and maximizes the flow rate. The most economical section of a rectangular channel is one which has hydraulic radius equal to half the depth of flow. The hydraulic radius is the ratio of the cross-sectional area to the wetted perimeter. Specific energy and Critical Flow Depth Specific energy refers to the energy per unit weight of fluid in an open channel flow , comprising the sum of the kinetic and potential energies . It is a critical parameter in hydraulic engineering, influencing the flow behavior and indicating the balance between the channel slope, velocity, and water surface It represents the total head above the channel bottom and is instrumental for the application of the Bernoulli equation. Understanding specific energy is critical for evaluating open channel flow scenarios that involve various depths, velocities, and channel

114. I n an open channel flow, the specific energy, denoted as E, is comprised of two components: the flow depth and the velocity head . Mathematically, it can be expressed as: Where: E = specific energy [m] y = flow depth [m] V = flow velocity [m/s] g = gravitational acceleration constant [9.81 m/s2] For a rectangular channel with uniform velocity distribution and width B, the above equation can be expressed in terms of the unit discharge (q) as:  

115. Where: q = unit discharge [m2/s] The unit discharge (q) is the volumetric flow rate per unit width of the channel, as shown in the following equation: q = Q/B Where: Q = volumetric flow rate [m3/s] B = width of the channel [m]  

116. The Specific Energy Diagram is essential in determining the critical depth (yc) of flow in open channels. The critical depth is the point at which critical flow occurs and specific energy is at a minimum for a given flow rate, as shown in the diagram below. This point separates supercritical flow from subcritical flow. In a specific energy diagram, the flow is considered subcritical on the upper portion of the critical point, where the flow depth is greater than the critical depth. In this regime, the flow is described as tranquil and slow-moving.

117. Reynolds Number The Reynolds number is named after the British physicist Osborne Reynolds. He discovered this while observing different fluid flow characteristics like flow of a liquid through a pipe and motion of an airplane wing through the air. He also observed that the type of flow can transition from laminar to turbulent quite suddenly. Reynolds number is a dimensionless quantity that is used to determine the type of flow pattern as laminar or turbulent while flowing through a pipe. Reynolds number is defined by the ratio of inertial forces to that of viscous forces. It is given by the following relation: Reynolds Number = Inertial force/ Viscous force Re = ρVD/ μ Where, Re is the Reynolds number ρ is the density of the fluid V is the velocity of flow D is the pipe diameter μ is the viscosity of the fluid Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluidwith large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion.

118. If the Reynolds number calculated is high (greater than 2000), then the flow through the pipe is said to be turbulent. If Reynolds number is low (less than 2000), the flow is said to be laminar. Laminar flow is the type of flow in which the fluid travels smoothly in regular paths. Conversely, turbulent flow isn’t smooth and follows an irregular math with lots of mixing. Reynolds Number Example Problems Calculate Reynolds number, if a fluid having viscosity of 0.4 Ns/m2 and relative density of 900 kg/m3 through a pipe of 20 mm with a velocity of 2.5 m. Solution 1 – Given that, Viscosity of fluid μ

119. From the above answer, we observe that the Reynolds number value is less than 2000. Therefore, the flow of liquid is laminar.

120. Hydraulic Jump Hydraulic jump is the jump or standing wave formed when the depth of flow of water changes from supercritical to subcritical state. When the slope of open channel decreases from steep to mild, the depth of flow of water increases toward the critical depth and a flow instability occurs at some point. The flow becomes turbulent until the new normal depth is attained in the downstream. This is called a hydraulic jump. It is required to understand what are different depths of flow, to understand the definition of hydraulic jump. Depth of flow: Depth of flow is the depth at which water flows above the ground level in an open channel. Critical depth: Critical depth of an open channel is the minimum depth of water above ground level at which the velocity of flow is very high and flow takes place with more of turbulence. The velocity of water at this depth is called as critical velocity. Supercritical depth: Super critical depth is the depth of water which is smaller than critical depth and it represents very hard and super-critical situation for basic flows taking place in dams , weirs and many irrigation structures. The velocity of water at this depth is greater than the critical velocity. Flow in this region is called supercritical.

121. Subcritical depth: Subcritical depth is the depth greater than critical depth . The velocity of water at this depth is smaller than the critical velocity. Flow in this region is called subcritical flow. Basic Characteristics of Hydraulic Jump: 1. The jump is unsteady, irregular 2. Based on wind directions and heavy wind blow, it changes its property and can be choppy and undular sometimes. Uses of Hydraulic Jump: The hydraulic jump is necessarily formed to reduce the energy of water while the discharge downfalls a spillway. It becomes necessary to reduce its energy and maintain stable velocities, that phenomenon is called energy dissipation in hydraulic structures. Types of Hydraulic Jumps – Based on Froude’s Number: Basically a hydraulic jump occurs in many types depending on topographical features and bed surface roughness and many other natural interface relations. This hydraulic jump types can be probably expressed based on Froude’s number: 1. Undular Hydraulic Jump – Froude Number (1 to 3):Undular Jump is irregular, not properly formed and there are certain turbulences in water particles.

122. 2. Weak Jump – Froude Number (3 to 6) Weak jump takes place when the velocity of water is very less and the water particles cannot be stable and flows in various ways. Below diagram is flow in Weak Jump

123. 3. Oscillating Hydraulic Jump – Froude Number (6-20) Oscillating jump forms when an oscillating jet enter into super critical state and there the number of particles starts oscillating in clockwise or either anticlockwise direction, forming slighter tides or waves to the top surface . Also the flow is dependent on heavy blow of air in one direction. 4. Steady Hydraulic Jump – Froude Number (20 to 80) In steady jump, the bed surface is quite rough so the particles start to tend in one direction with heavy velocity and turbulence, frictional losses are more in this type of jump.

124. 5. Strong Hydraulic Jump – Froude Number (greater than 80) Strong jump is a perfect jump formed when frictional losses are more, air pressure division is equal and velocity is very high that losses take place. The water changes its state from super critical to subcritical in very shorter length when compared to all other types of hydraulic jumps, so this jump is highly preferred in dam structures.

125. 4.3. Torrents A torrent is a sudden and violent rush of water, typically caused by heavy rainfall or a dam or levee breach. Torrents are characterized by their high speed and volume of water, which can cause significant damage to anything in their path. Unlike floods, which can last for days or weeks, torrents are typically short-lived but can be extremely destructive. It can be caused by heavy rainfall, snowmelt, or a sudden release of water from a dam. 4.3.1 Torrent Control Measures Are the mitigation measures that are employed to reduce risk from torrents Risk Likelihood Reduction Risk Consequences Reduction Risk Avoidance Risk Acceptance Risk Transfer, Sharing, or Spreading The mitigation measures are categorized as structural and non-structural. Though these terms are almost universally used to differentiate between the various options available to disaster managers, much disagreement exists concerning the actual delineation of what makes structural or non-structural risk control measures

126. The structural mitigation is defined as a risk reduction method performed through the construction or altering of physical environment by using engineered solutions. Non-structural mitigation is defined as a measure that reduces risk through the modification of human behavior or natural processes without requiring the application of engineered solutions. It must be noted that, while there are several mitigation measures that will clearly fit into one category or the other regardless of the definition of the terms, there are also many that could go either way, and may appear as one form in this text and another form elsewhere. a) Structural measure Structural mitigation measures are those that involve or dictate a necessity for some kind of construction, engineering, or other mechanical changes or improvements aimed at reducing hazard risk likelihood or consequence. They often are considered at “man controlling nature” when applied to natural disasters. Structural measures are generally expensive and include a full range of regulation, compliance, enforcement, inspection, maintenance, and renewal issues. Though, each hazard a unique set of structural mitigation measures that may be applied to its risk, these measures can be grouped across some general categories.

127. The general structural mitigation groups to be described are: Building codes Resistance measures Hazard resistant construction is clearly an effective way to reduce vulnerability to select hazards. Construction and regulatory measures Relocation Occasionally, the most sensible way to protect a structure or a people from a hazard is to relocate it or them away from the hazard. Structural modification Construction of community shelters The lives of community residents can be protected from a disaster’s consequences through the construction of shelters designed to withstand a certain type or range of hazard consequences. Construction of barrier, deflection, or retention systems Detection systems Physical modification Treatment systems Redundancy in life safety infrastructure

128. b) Non-structural Mitigation Non-structural mitigation, as defined previously, generally involves a reduction in the likelihood or consequence of risk through modifications in human behavior or natural processes, without requiring the use of engineered structures. Non-structural mitigation techniques are often considered mechanisms where man adapts to nature.” They tend to be less costly and fairly easy for communities with few financial or technological resources to implement structural measures The following section describes several of the various categories into which non_structural mitigation measures may be grouped: Regulatory measures Regulatory measures limit hazard risk by legally dictating human actions. examples: Land use management (Zoning). This legally imposed restriction on how land may be used. Open space preservation (green spaces). This practice attempts to limit the settlement or activities of people in areas that are known to be at high risk for one more hazards.

129. Community awareness and education program The public is most able to protect themselves from the effects of a hazard if they are first informed that the hazard exists, and then educated about what they can do to limit their risk. Public education programs are considered both mitigation and preparedness measures. An informed public that applied appropriate measures to reduce their risk before a disaster occurs has performed mitigation. However, a public that has been trained in response activities has participated in a preparedness activity. Often termed “risk communication,” projects designed to educate the public may include one or more of the following: i) Awareness of the hazard risk ii) Behavior iii) Pre-disaster risk reduction behavior iv) Pre-disaster preparedness behavior v) Post-disaster response behavior vi) Post-disaster recovery behavior Warning systems inform the public that hazard risk which has reached a threshold required certain protective actions.

130. 4.4. Gully E rosion and Control Measures 4.4.1. Gully Erosion It is advanced form of rill erosion in which the size of rills is enlarged and cannot be smoothened by tillage operations. As the volume of concentrated water attains more velocity on slopes, it enlarges rills in to gullies. Here rills become so deep that ordinary tillage tools cannot smooth the ground out. Gully Development Stages: The gully development is recognized in 4 stages i) Formation Stage: in this stage, the channel erosion and deepening of the gully takes place. It normally proceeds slowly where the top soil is fairly resistant to erosion.

131. ii) Development Stage: It mainly causes upstream movement of the gully head and further enlargement of the gully in depth and width. The gully cuts to the C-horizon of soil, and the parent materials are removed rapidly as water’ flows. iii) Healing Stage: In this stage vegetation starts growing in the gully. No appreciable erosion takes place. iv) Stabilization Stage: In this stage gully reaches a stable gradient, and attain a stable slope. Classification of Gullies Gullies can be classified based on three factors viz. their size, shape (cross section) and state of gully.

132. 1) Gully classification based on size Classification Specifications of gully Very Small gullies 3m deep, bed width<18m, side slope varies Small gullies 3m deep, bed width>18m, side slope varies Medium gullies 3m-9m deep, bed width>18m, side are uniformly sloping Deep and narrow gullies >9m deep, bed width varies, side slope varies, mostly steep and vertical

133. 2) Based on Shape of Gully U-Shaped: These are formed where the topsoil and subsoil have the uniform resistance against erosion. Because the subsoil is eroded as easily as the topsoil, nearly vertical walls are developed on each side of the gully V-Shaped: it forms where the subsoil has more resistance than top soil against erosion. 3) Based on the Stage Active Gullies: Dimensions enlarge with time. Found in plain areas generally Inactive Gullies: Dimensions constant with time found in rocky areas The rate of gully erosion depends on: The runoff producing characteristic of the watershed, Soil characteristic, Size and shape of the gully and The slope

134. 4.4.2. Gully Control Measures I . Watershed management Retention of water on the watershed through mechanical and vegetative measures is useful in the effective gully control program . It is highly advisable to retain as much water as possible in the path catchment through different retention techniques . Proper management of the runoff water and increasing the vegetative cover of the watershed improves : T he watershed hydrology I mproves the watershed condition I ncreases the watershed condition I ncreases infiltration R educes overland flow, and enhances the gully healing process Microbasins , level terraces, and plantations are some viable techniques to water retention in the gully catchment.

135. II. Diversion of runoff In some situations, it is not possible to conserve the water within the catchments either because there is a high discharge at specific point or because retention of water might lead to crop damage or cause landslide. The runoff must then be diverted from the gully head by use of the diversion ditch. Careful consideration should be given to the disposal area. III. Conveyance of water safely If it is not possible to infiltrate the water in the catchments or divert it away from the gully head; it must be conveyed through the gully without causing any further erosion. This is only possible if the gully head, floor and sides are stabilized

136. IV. Gully stabilization; Stabilization involves the use of appropriate structural and vegetative measures in the head, floor and side of the gully Vegetative measures : The use of vegetative material , in gully control offers an inexpensive and permanent protection . Vegetation will protect the gully floor and banks from scouring. Grass in the gully floor slow down the velocity of the runoff and causes deposition of silt.

137. b) Structural measures To give vegetation an opportunity to establish, runoff control structures may be needed in the gully. Gully control structures are either temporary or permanent . Temporary structures are not watertight and are expected to last just long enough to allow vegetation to establish. Permanent structures are used where the temporary ones cannot work because of poor or unstable soil or because the gully is a natural waterway and runoff cannot be diverted elsewhere. If the stabilization of a gully head appears too costly or difficult, there are two approaches. One is to divert runoff away from the gully head so that it ceases to erode. The other is to place a check dam close enough to the gully head so that it will trap sediment, raise the floor level and submerge the head. Stabilizing gully head: Stabilization of small gully heads of less than 1.5m, where the discharge is not more than 0.1m 3 /s, may be done by reshaping and the use of grass sod or a brushwood carpet .

138. i) Brush layering This is formed by the use of small tree branches that have the leaves still intact . It is particularly beneficial where grass sods cannot work because the flow is to much or the slope is too great. The purpose of brushwood, as with grass sod, is to reduce the waterfall erosion and safety d ischarge the water to the floor of the gully. Construction starts with reshaping of the gully to reduce the slope. ii) Grass sod The grass sod is carefully uprooted so that most of the soil remains attached to the roots. This will ensure that the grass establish quickly. The gully head is reshaped and the sod lay in position to form a protective carpet that protects the soil from further erosion by the flowing water . Stakes can be used to anchor the sod to the ground . Its use is limited to small gullies with low discharge and a head height less than 1 meter .

139. Permanent gully control structures Permanent gully control structures are used in medium to .large gullies with medium to large drainage areas. It protects the gullies from further development and helps in storing water. Permanent structures are used for the following purposes. 1. To control the overfall at the head of a large gully, 2. To drop the discharge from a vegetated waterway into a drainage ditch, 3. To take up the fall at various points, and 4. To provide for discharge through earth fills Three main types of permanent gully control structures generally employed for stabilizing slope are: i) Drop spillway ii) Chute spillway, and

140. iii) Drop inlet or pipe spillway These drop structures are used to specific site conditions. Drop spillways are used along the gully bed to act as control points. But, to convey the water safely from gully head to gully bed, chute spillways are employed. While chute spillway checks scouring of gully head , t he drop spillway saves gully bed from erosion below the crest level of the structures. Drop inlet or pipe spillways are used at-appropriate locations in the gully for storage of water. All these structures may be employed together at different suitable locations The structures have to meet the following functional requirements. i) It should have sui'ficient capacity to pass design discharge. ii) The kinetic energy of the discharge must be dissipated within the confines of the structure in a manner and to a degree that will protect both the structures and down stream channel from damage.

141. Stabilization of the gully beds The possibility of reclaiming the gully should be investigated. The velocity of the water must be reduced to encourage deposition and prevent any planted material from being washed away. In shallow and gently sloping gullies simple checks known as thresholds or scour checks can be used. These can be effective where the discharge and velocity are low. Stabilization of bigger gully heads In bigger gullies with higher discharges, check dams should be constructed.

142. Types of check dams I. Brush wood check dams It is constructed of small wood branches and poles, interwoven together, either by wire, rope or sisal. These are temporary structures, easy to construct, and use cheap readily available materials. The purpose of this is to retain sediment and to allow the runoff filter through it slowly. It is suitable method for small gullies. There are two types of brushwood checkdams. These are: Single row brushwood checkdam Double row brushwood checkdam II . Stone check dam This is preferred where stones are available. Stones are laid in the trench until they are flush with the gully floor . The middle section should be lower than the sides to form a spillway so that water moves through the middle of the structure and away from the sides . Laying stones construct it carefully across the floor of the gully. Space between the large stones should be filled with small ones.

143. III. Loose rock check dam It is economical type of structure for gully control. A trench key up to 0.5 m in depth at the base and another trench should extend in to both abutments penetrating, up to 0.5 m reaching up to the crust level of the check dam. Then the construction of the body of the check dam can be continued up to 1 m height and thickness. IV. Gabions check dam Gabions are boxes of galvanized wire mesh. Such boxes can be filled even within small stones. The mesh will prevent water flow removing the stones. The boxes filled up with stones are heavy enough to resist movement even by large water flows and high stream velocities. Unlike concrete they do not crack. Consequently gabions can be recommended on very steep gully heads with large water flows.