1. SURFACE IRRIGATION SYSTEMS – DESIGN PRINCIPLES.

2. SURFACE IRRIGATION: Definition and Types
Surface irrigation is the application of water by gravity flow to the surface of the field.
Types of Surface Irrigation
Furrow
Consists of furrows and ridges.
Border
Border strips are strips of land with a downward slope but are as horizontal as possible in cross section.
Basin
Is a horizontal area of land surrounded by earthen bunds and totally flooded during irrigation.
Flooding should be done using a large stream size that advances quickly in order for water to spread rapidly over the basin.

3. Criteria for the Selection of the Surface Irrigation Method
Selection depends mainly on
Soil type,
Crops to be irrigated,
The irrigation depth,
land slope,
Field shape,
Labour availability
Water source.

4. Criteria for the Selection of the Surface Irrigation Method Cont.
Soil type
All three surface irrigation methods prefer heavy soils, which have lower infiltration rates.
A light soil with high infiltration rates favours deep percolation losses at the top of the fields, resulting in low field application efficiency.
Type of crop
Furrow irrigation is particularly suitable for irrigating row crops such as maize and vegetables.
Furrows are also more suitable for shallow-rooted crops.
Borderstrip irrigation can also be used for row crops or for close-growing crops that do not favour water ponding for long durations, such as wheat and alfalfa.
Any crop, whether row or close-growing, that can stand a very wet soil for up to 24 hours is best grown in basins.

5. Criteria for the Selection of the Surface Irrigation Method Cont.
Required depth of irrigation application
If the application depth is small, furrow irrigation is the most appropriate method of irrigation.
Large application depths can be applied most efficiently with basin irrigation.
In general, the gross irrigation depth is much larger than the net irrigation depth for all three surface irrigation methods, due to the lower irrigation efficiencies of surface irrigation compared to pressurized systems.
Of the 3 surface irrigation methods, basin irrigation can have higher irrigation efficiency and use less water for the same crop on the same soil than the other methods as water is confined within bunds .

6. Criteria for the Selection of the Surface Irrigation Method Cont.
Land slope
In general , all surface irrigation methods favour flat land as steep slopes would necessitate excessive land levelling in order to avoid erosion.
Flat land with a slope of 0.1% or less is best suited for basin irrigation (which needs a zero slope) since it requires minimum land levelling.
Borderstrip irrigation may be used on steeper land, even up to 5%, depending upon other limiting factors such as soil type.
One has to be cautious with furrow irrigation on such steep slopes. This is because the flow is confined to a small channel (the furrow), which could result in erosion.
Field shape
for all three types regularly shaped fields are preferable,
Labour availability
Basin irrigation requires less labour than the other two methods and might have to be considered if there is a critical labour shortage.

7. Practical values of maximum furrow lengths in metres depending on soil type, slope, stream size and irrigation depth for small-scale irrigation (Source: FAO, 2002)

8. Basin area in m2 for different stream sizes and soil types (Withers and Vipond, 1974 in FAO, 2002)
Selection of an irrigation method based on soil type and net irrigation depth (Source: Jensen, 1983 in FAO, 2002)

9. Figure 1. Components of a surface irrigation system (Source: Walker and Skogerboe, 1987 in FAO,2002)

10. Surface irrigation design stepsEa
Contour map of design area (total area, ha), topography
Water quantity and quality
Soils
Crops
Type of crop
Irrigation method
AWC
RZD
Peak ETc
MAD or P
Dnet IF
Adjusted Dnet
Adjusted Dgross
Area irrigated per day
System capacity
Dgross
infiltration
basic infiltration rate, contact time, stream sizes, lengths of run
Design layout
Adjusted P IC
topography
Mech.
Size water supply infrastructure

11. Step 1: Collect basic resource data at farm
The data include:
A topographic map showing:
The proposed irrigated area, with contour lines
Farm and field boundaries and water source or sources
Power points, such as electricity lines, in relation to water source and area to be irrigated, roads and other relevant general features such as obstacles
Data on water resources, quantity and quality over time, on water rights and on cost of water where applicable
The climate of the area and its influence on the water requirements of the selected crops
The soil characteristics and their compatibility with the crops and irrigation system proposed
The types of crops intended to be grown and their compatibility with both the climate in the area, the water availability and the soils; current agricultural practices should be identified

12. Step 2.
Involves analysing the farm data in order to determine the following preliminary design parameters:
peak and total irrigation water requirements
infiltration rate of soils to be irrigated (and also contact time, stream size, lengths of run)
maximum net depth of water application per irrigation (and irrigation efficiencies)
irrigation frequency and cycle
gross depth of water application
preliminary system capacity

13. Steps 3-9 3. Selection of surface irrigation method, 4. System layout
5. Land levelling
6. Determination of a water reticulation plan
-continuous supply
-rotational
-on- demand
7. Canal Design
Tertiary canal sizing
Secondary canal sizing
Main canal sizing
Canal cross sections
8. Drainage system channel sizing
9. Identification and design of hydraulic structures

14. Steps 10-12 10. Canal longitudinal profiles -main -secondary -tertiary
11. Production of Bill of quantities
Production of canal bill of quantities
Production of structures bill of quantities
List of other materials (fencing, toilets, etc)
12. Production of system operation instructions

15. SOILS ASSESSMENT: The four phases of surface irrigation
Advance Phase – Begins when water is applied at upstream end and ends when it reaches downstream end of field.
Storage or Ponding Phase – Time elapse between the arrival of water at the tail end and the stopping of the inflow at the top end.
Depletion Phase – This is the time between the stop of the inflow at the head end and the appearance of the first bare soil that was under water.
Recession Phase – This is the time when water starts to disappear at the head end until it eventually recedes from the whole field.
Contact or Intake Opportunity Time – Time in hours or minutes that any particular point in the field is contact with water.

16. Definition sketch showing the surface irrigation phases (Source: Basset et al., 1980 ,in FAO,2002)

17. Contact Time Depth of infiltration varies in relation to contact time.
Contact time can be increased by using flatter slopes, increasing length of run or reducing stream flow or a combination.
Contact time can be decreased by steepening the slope, shortening the length of run or increasing the stream flow.

18. Infiltration
Infiltration or intake rate is the rate at which water enters the soil and is expressed in mm/hr.
It is critical to surface irrigation as it determines the following:
The time the soil should be in contact with water (Contact time).
The rate at which water should be applied to the fields.
Also;
Water infiltrates rapidly when soil is dry, but then later slows down until it reaches a steady state referred to as the basic infiltration rate.
When the basic infiltration rate is reached, the cumulative infiltration curve becomes a straight line and the basic infiltration rate curve becomes a horizontal line. See diagram below.

19. Basic infiltration rate and cumulative infiltration curves (Source: FAO,2002)

20. Typical infiltration rates for different soils(Source: FAO,2002)
The infiltration rates of soils are influenced, among others, by the soil texture.
Heavy soils have low infiltration rates by virtue of their small pore sizes, while light soils have high infiltration rates because of larger pore sizes.
The infiltration rate is a difficult parameter to define accurately, but it has to be determined in order to describe the hydraulics of the surface irrigation event.
When planning a furrow irrigation scheme, one can determine the infiltration rate by two methods:
- the infiltrometer method
- the actual furrow method if furrow irrigation is to be used.

21. Irrigation Depth and Contact time
Basic infiltration rate and cumulative infiltration curves are plotted from data obtained from an infiltration test.
The contact time is interpolated from the cumulative infiltration curve as it takes stock of all the water that has infiltrated the soil.
The net depth of irrigation is divided by the field application efficiency to get the gross irrigation requirement.
With this gross depth of irrigation, one goes along the vertical axis of the cumulative infiltration curve , at the infiltration reading equivalent to Dgross, read off the time on the horizontal scale to get the contact time.

22. Design parameters for the infield works (Step 2)
These are ;
Cropwater requirements
Dnet
irrigation efficiencies.
Dgross
Irrigation frequency
Irrigation cycle
System capacity
The parameters are determined more in the same way as for sprinkler irrigation .
The following surface irrigation scheme will be used to demonstrate the process of using and calculating design parameters: 
Nabusenga irrigation scheme, which is a surface irrigation scheme in Matabeleland North Province in Zimbabwe using a concrete-lined canal system
Table 15 shows the given design parameters for the scheme.

23. Table 15: Design parameters for Nabusenga

24. Example on Dnet and Dgross
Based on the design parameters given in Table 14, what are the net and gross depths of water application for Nabusenga irrigation project?
dnet = 130 mm/m x 0.70 m x 0.50 = 45.5 mm
The gross depths of water application at field and at overall level would be:
dgross = 45.5 /0.50= 91.0 mm at field level
dgross = 45.5 / 0.41 = mm at overall level

25. Example on IF and IC determination
What is the irrigation frequency and what can be the irrigation cycle for Nabusenga scheme?
The irrigation frequency is equal to:
IF = 45.5/6.0 = 7.5 days
The system should be designed to provide 45.5 mm every 7.5 days. For practical purposes, fractions of days are not used for irrigation frequency purposes. Hence, the irrigation frequency in our example should be 7 days, with a corresponding dnet of:
dnet = 7 x 6.0 = 42 mm for an IF of 7 days
The dgros at field and overall level will be 42/0.5 = 84.0mm and 42/0.41 = mm respectively.
The adjusted allowable depletion for the 7 days (instead of 7.5 days) is equal to:
P =7x6.0/(130x0.7)= 0.46 or 46%
Based on the above, the Irrigation Cycle (IC) is fixed at 6 days.

26. System capacity (Q)
Refers to the discharge that has to be abstracted from the headwork during a given period per day and it is used for the design of the headwork and the conveyance system. It is determined by the following equation:
Equation 8
Q = V/T
Where:
Q = Discharge (m3/hr or l/sec)
V = Volume of water to be abstracted per day (m3 or l)
T = Irrigation duration per day (hr or sec)
The volume of water to be abstracted per day is obtained as follows 
Equation 9
V = 10 x A x dgross
V = Volume of water abstracted per day (m3)
A = Area irrigated daily (ha)
dgross = Gross depth of application at overall scheme level (mm)
10 = Conversion factor to convert mm to m3/ha

27. Example on area irrigated per day and System capacity
The area irrigated per day can be calculated as follows:
Equation 10
A = At/IC 
Where:
A = Area irrigated per day (ha)
At = Total area (ha)
IC = Irrigation cycle (days)
Example 6
What should be the system capacity for Nabusenga scheme, considering an irrigation duration of 10 hours per day?
The area irrigated per day is equal to:
A = 15/6 = 2.5 ha
The volume of water to be abstracted per day is equal to:
V = 10 x 2.5 x = m3/day
The system capacity, assuming 10 hours of irrigation per day, will be equal to:
Q = /10= 256 m3/hr or 71.1 l/sec

28. System capacity cont.
If, however, this results in large conveyance dimensions, a night storage reservoir could be introduced so that abstraction from the head works could be continuous (24 hours/day) at peak demand.
In such a case, the conveyance system capacity would be 71.1 x (10/24) = 29.6 l/sec.
A summary of the calculated design parameters for Nabusenga is given in Table 16.

29. Table 16: Summary of the calculated design parameters for Nabusenga surface irrigation scheme

30. Night Storage Reservoir
By incorporating a night storage reservoir in the design of Nabusenga scheme, the system capacity has been reduced to less than half, thus allowing a smaller size conveyance system to be used.
NSDs store water during the time when there is abstraction (usually for 14hrs) from the water source, but with no irrigation taking place.
This gives time for the required volume to accumulate so that it caters for envisaged deficiencies.
They are incorporated in the design of scheme when:
The distance from the water source to field is very long, resulting in long time lag between releasing water from source and receiving it in the field.
The canal cross-section or pipe size from the water source should be kept small in order to save on constructions costs.
The flow in the stream, river or spring discharge is not sufficient to meet the required irrigation flow.
Capacity of NSD is calculated by multiplying a continuous abstraction rate by maximum hours of non-irrigation per day. A factor of 20% is added to cater for evaporation and seepage losses.

31. Layout of a surface irrigation scheme

32. GUIDELINES FOR PRODUCING SURFACE IRRIGATION LAYOUTS
Water Conveyance.
Water is conveyed from source to highest point in field either by gravity or by pumping depending on elevation differences between water source and highest point in field.
Where a conveyance canal is used, it should as much as possible follow contours and can adopt flat grades depending on elevation differences between water source and field. Pipelines can be used where there is enough head difference to overcome frictional losses in pipes or when pumping and usually take the shortest distance to field.
Infield Layout
Layout is determined by the following factors:
Topography.
Farm size and degree of mechanization.
Possible length of furrows, border strips and basins.
Two distinct layouts can be adopted.
On flat lands with slopes of less than 0.4%, field canals run along contours, while secondary canals run at right angles to the contours. Furrows are then constructed perpendicular to the contours.
For lands with a gentle to steep topography, secondary canals are constructed on the top edge of field following the contour but slightly running away from it to maintain a gradient for water flow.
Field canals are then constructed in such a manner that they cross elevation contours at almost right angles.
Canals are often constructed at a grade of 1:500 (concrete lined) or gentler slopes for unlined canals.
Furrows are often pegged around 1:300 but it can deviate either side depending on the desired contact time.

33. Land levelling
Proper land levelling is important for efficient surface irrigation.
It involves moving soil in order to have level fields for basin irrigation or uniform sloping fields for furrow or borderstrip irrigation.
When levelling or grading land, one should avoid large volumes of cut and fill. Besides being expensive, too much soil movement tends to leave shallow topsoil in areas of cut, which is not ideal for crop production.
A detailed topographic survey, preferably grid, is needed to calculate the most economic land levelling requirements.
Based on the spot heights of the grid points and the required gradient of the land, the cut and fill can be calculated. The total volume of cut should preferably exceed the total volume of fill by 10-50% depending on the total volume to be moved and the compressibility of the soil.
The three most widely used methods for calculating the amounts of soil cuts and fills are:
Profile method
Contour method
Plane or centroid method
Use of computers
A number of programmes have been written to calculate the land levelling requirements by computer. One such programme, written by E.C. Olsen of Utah State University, is called LEVEL 4EM.EXE.
It calculates land-grading requirements based on the least squares analysis for both rectangular and irregularly shaped fields.

34. IMPORTANCE OF LAND LEVELLING
To enable efficient irrigation
Removal of surface water
Low areas may receive excess water – water logging
High areas may not receive sufficient water – low yields
Salt, as in borders may accumulate in high areas

35. Figure 92: Part of the completed land levelling map for Nabusenga,

36. Determination of a water reticulation plan
-Continuous water Delivery
Each field canal or pipeline receives its calculated share of the total water supply as an uninterrupted flow.
The share is based on the irrigated area covered by each canal or pipeline.
Water is always available, although it may not always be necessary to use it.
This method is easy and convenient to operate, but has a disadvantage in its tendency to waste water.
The method is rarely used in small irrigation schemes.
-Rotational water delivery
Water is moved from one field canal or pipeline or from a group of field canals or pipelines to the next.
Each user receives a fixed volume of water at defined intervals of time.
This is a quite common method of water delivery.
-Water delivery on demand
The required quantity of water is delivered to the field when requested by the user.
This on-demand method requires complex irrigation infrastructure and organization, especially when it has to be applied to small farmer-operated schemes where the number of irrigators is large and plot holdings are small.

37. Design of canals and pipelines
Design of pipelines
In piped surface irrigation systems, water is transported in closed conduits or pipes in part or all of the distribution system from the headwork up to the field inlet.
The pipes can be all buried, with outlets in the form of hydrants protruding above ground level on field pipes.
Or only the conveyance and supply lines can be buried with field pipes being portable and laid above ground.
In the latter case, the above ground pipes are made of aluminium fitted with adjustable gate openings (see Figure next slide).
Piped systems for surface irrigation, unlike piped systems for sprinkler irrigation, do not require a lot of head at the hydrant outlet.
The head should only be sufficient to push water through the irrigation hose that takes the water from the hydrant to the soil.
In view of the low head requirements for the systems, it is possible to employ gravity flow where there is sufficient head to overcome the frictional losses in pipes.
In situations where the head is not adequate, small power pumps would be used with low operational costs.
Pipes with low-pressure rating are also used for these systems as they operate at reasonably low pressures.
If the water level at the headwork is higher than the water level required at scheme level, the water can be transported through the pipes by gravity.

39. Design of pipelines cont.
If the water level at the headwork is lower than the water level required at scheme level, then the water needs to be pumped through the pipe to arrive at the scheme at the required elevation necessary to be able to irrigate by gravity from the field inlet onwards.
For calculation of friction losses, just as in sprinkler design , manufacturer friction loss charts or equations like the Hazen-Williams can be used.
The Hazen-Williams equation is given below; Equation 21:

41. Design of canals and pipelines
The canal dimensions and longitudinal slope, whether for irrigation or drainage, can be calculated through trial and error with the Manning formula.
This formula is derived from the continuity equation and the equation for unsteady flow.
These equations have been simplified by assuming steady uniform flow in the canal (this assumes long canals with constant cross-section and slope).
The Continuity equation is expressed as:
Equation 12
Q = A x V
Where:
Q = Discharge (m3/sec)
A = Wetted cross-sectional area (m2)
V = Water velocity (m/sec)

42. The Manning Formula The Manning Formula can be expressed as:
Equation 13

43. Figure 23: Flowchart for canal design calculations

44. Figure 24 shows the different canal parameters.
As and P, and thus R in the Manning formula, can be expressed as d, b and X, where:
d = Water depth (m)
b = Bed width (m)
X = Side slope = horizontal divided by vertical
For a trapezoidal canal, As is the sum of a rectangle and two triangles.
The manning’s formula is applicable to uniform flow. For uniform flow, the flow cross section is same at all sections along the channel. Amount of water passing every section is the same – mean velocity is constant.

45. Calculation of the cross-section, perimeter and hydraulic radius of a canal
As (Cross sectional area) is sum of a rectangle and the two identical triangles at the edge and is expressed as:
As = b x d +Xd2 = d (b + Xd)
P (Wetted Perimeter) is sum of bed width plus the two side slopes from canal bottom to top of water level. It is calculated from formula c2 = a2 + b2 and expressed as
P = b + 2(d2+(dx)2)1/2 = b + 2d (1 + X 2) 1/2
Hydraulic radius R is
R = As/P = d (b + Xd)/b + 2d (1 + X2) 1/2
When the computed discharge is equal to the required discharge, the velocity needs to be checked to ensure that it is acceptable.
Velocity increases with an increase in gradient. Recommended maximum gradient is 1:300 or 0.33%. With high velocities, flow becomes super-critical and becomes difficult to siphon. The state of flow is checked using the Froude Number Fr which is given by:
Fr = V/(g x l)1/2
Where:
V = water velocity (m/sec)
g = gravitational acceleration (9.81m/sec2)
l = hydraulic depth of an open canal, defined as the wetted cross-sectional area divided by the width of the free water surface (m)
Fr = 1 for critical flow
Fr greater than 1 for super-critical.
Fr less than 1 for sub-critical flow
It is important to maintain a Froude number of 1 or less so that the flow is at or below the critical level.

46. Factors affecting canal discharge
Canal gradient
A canal with steeper gradient but with the same Cross-section discharges more water than a canal with a smaller gradient.
Canal roughness
Influences the amount of water that passes through a canal. A higher Km (lower n) the higher the ability of the canal to transport water, hence the smaller the required cross-sectional area for a given discharge.
Canal shape
Canals with narrower beds and higher water depths have a smaller wetted perimeter, and thus a higher discharge, than canals with larger beds and lower water depths, for the same cross-sectional area.
This is due to the fact that the hydraulic radius R (= As/P) increases if the wetted perimeter decreases, while keeping the wetted cross-sectional area the same
Side slope
For unlined canals, soil texture determines the side slope.
Maximum water velocities
The higher the velocity , the higher the discharge. For unlined canals, ensure that velocity is non-erosive as well as self-cleaning.
Freeboard
Freeboard (F) is the vertical distance between the top of the canal bank and the water surface at design discharge. It gives safety against canal overtopping because of waves in canals or accidental raising of the water level, which may be a result of closed gates. The safe freeboard can be calculated from the following equation:
F = C x h1/2
Where:
C = 0.8 for discharges of up to 0.5 m3/sec, 1.35 for discharges in excess of 80m3/sec
h = water depth (m)
For lined canals, F ranges from 0.40 m for discharges less than 0.5 m3/sec up to 1.20 m for discharges of 50 m3/sec or more. For very small lined canals, with discharges of less than 0.5 m3/sec, the freeboard depths could be reduced to between m.

47. Canal Shapes
Although the trapezoidal canal shape is very common, other canal shapes, including V-shaped, U-shaped, semicircular shaped and rectangular shaped canals, can also be designed as shown in Figure 25.

48. Km and n values for different types of canal surface (Source: FAO,2002)

49. Typical canal side slopes (Source: FAO,2002)
Maximum water velocity ranges for earthen canals on different types of soil (Source: Peace Corps Information Collection and Exchange, in FAO,2002)
The recommended bed width/water depth (b/d) ratios for earthen trapezoidal canals
The bed width should be wide enough to allow easy cleaning.
A bed width of m is considered to be the minimum, as this still allows the cleaning of the canal with small tools such as a shovel.
Lined trapezoidal canals could have similar b/d ratios as given in opposite table

50. Hydraulic parameters for different canal shapes

52. Seepage losses in earthen canals
Unlined earthen canals are the most common means of conveying irrigation water to irrigated lands.
Farmers prefer them because they can be built cheaply and easily and maintained with farm equipment. Unlined canals are also flexible, as it is easy to change their layout, to increase their capacity or even to eliminate or rebuild them the next season.
However, unlined canals have many disadvantages that make them less desirable compared to lined canals or underground pipes. These are:
They usually lose more water due to seepage, leakage and spillage
Rodents can cause leakage
Frequent cleaning is needed because of weed growth
Earth ditches can erode and meander, creating problems in maintaining straight or proper alignments
Labour costs of maintenance of unlined canals are normally higher than of lined canals and pipelines
They provide an ideal environment for the vector of bilharzia
When designing earthen canals, it is important to ensure that the slope is such that the bed does not erode and that the water flows at a self-cleaning velocity .
Relatively flat lands on soils with a high percentage of silt and clay are the most suitable for canal construction, because of low infiltration rates.
In earthen canals, seepage occurs through the canal bed and sides. In areas where relatively permeable soils are used to construct canals, high seepage can be expected.
The higher the seepage losses in the canals the lower the distribution system (conveyance and field canal) efficiencies, since much less water than that diverted at the head works reaches the fields.

53. Seepage losses for different soil types
Seepage losses in earthen canals cont.
Seepage is difficult to predict. Two simple ways to estimate seepage losses are:
1. Measurement of inflow into and outflow from the canal at selected points. The difference between the inflow and outflow measurements will not only represent seepage losses, but evaporation losses as well.
2. Measurement of the rate of fall of the water level in a canal stretch that has been closed and where the water is ponding.
From these losses the estimated evaporation should be subtracted to get the seepage losses.
Usually, seepage losses are expressed in m3 of water per m2 of the wetted surface area of a canal section (P x L) per day. If a field test cannot be carried out, seepage can be estimated from Table below, which gives average seepage losses for different types of soil.
Seepage could be localized where a portion of highly permeable material has been included in the bank or where compaction has been inadequate during canal construction.
Seepage losses for different soil types

54. Canal lining
Canal lining is generally done in order to reduce seepage losses and thus increase the irrigation efficiencies.
It also substantially reduces drainage problems and canal maintenance as well as water ponding, thus reducing the occurrence of vector-borne diseases.
Also, smooth surface linings reduce frictional losses, thereby increasing the carrying capacity of the canals.
Material used for canal lining are:
Clay
Polyethylene plastic (PE)
Concrete
Sand-cement
Brick
Asbestos cement (AC)
The selection of a lining method depends mainly on the availability of materials, the availability of equipment, the costs and availability of labour for construction.

56. Hydraulic design of canal networks using the chart of Manning formula
The hydraulic design of canal networks for irrigation and drainage requires the following steps (Euroconsult, 1989, in FAO ,2002):
1. Design water surface levels in relation to natural ground slope and required head for irrigation of fields or for drainage to outlet, taking into account head losses for turnouts and other structures.
2. Calculate corresponding hydraulic gradients.
3. Divide network into sections of uniform slope (S) and discharge (Q).
4. Determine required design (maximum) discharge per section.
5. Select roughness coefficient (Km or n)
– side slopes
– preferred minimum velocity and permissible maximum velocity
– bottom width/water depth ratio
6. Calculate hydraulic section dimensions and corresponding velocity, using:
– nomograph series, if available e.g the nomograph presented in Figure in next slide (chart of Manning formula)
– basic equations and calculator
7. Check calculated velocities against preferred and maximum velocity values; if it is too high, reduce hydraulic gradient and corresponding bottom slope.
The gain in head should preferably be used in upstream and downstream canal sections but, if this is impossible, it must be absorbed by drop structures. The chart presented in Figure next slide can be used to determine the optimum canal parameters for trapezoidal canal sections through trial and error.

57. Chart of Manning formula for trapezoidal canal cross-sections

58. Canal section sizes commonly used in Zimbabwe
(by The Department of Irrigation ,DoI)
Dept of Irrigation has adopted a 60o trapezoidal canal with a depth of 0.3m, a freeboard of 0.05m and bed widths of 0.25m, 0.30m, 0.375m and 0.5m depending on gradient and capacity or discharge required.
Only the bed width is varied
The total depth of the canals is 0.35m (water depth + freeboard) is easily reached by construction gangs while placing concrete.
It also provides an adequate siphon head and gives efficient flows within range.
Narrowest bed width used of 0.25m is easy to clean
By varying bed width only and not depth, change from one canal section to another is simplified

59. Table 21:Canal capacities for standard DoI canal sections.

60. Calculations of inputs to the Mannings formula
In the calculation of canal flow, the following parameters can be fixed.
Km or n (canal roughness co-efficient) relates to canal quality as it affects water velocity in canal and is provided from tables. Concrete lined canals have a Km value of 55.
Canal gradient S expressed as a decimal. Most of our field canals are pegged at grade 1:500.
Side slope X .DoI uses 60 degree trapezoidal canal where X is 0.58 (Calculated from tan 600 being equal to I/X );
Water Depth d Most canal sections (for DoI) have a fixed water depth of 30cm plus 5cm freeboard for easy transition from one section to the other.
Bed width b can now be the variable that is used in the iteration to come up with the required Q discharge.

62. Longitudinal canal sections
The best way to present canal design data for construction is to draw a longitudinal profile of the canal route and to tabulate the data needed for construction.
Prepared after layout and presents canal data for construction.
Prepared for main, secondary and tertiary canals.
The horizontal axis of a long section shows the distance along the canal and usually starts from a reference point. The vertical axis shows the elevations.
Longitudinal profiles are drawn from topographical-survey data but where such data is insufficient or lacking, a detailed survey of the proposed alignments should be done.
useful for BOQ preparation
Guidelines for preparation of longitudinal canal sections
Direction – water flow is always from left to right
Horizontal scale: 1:1000 for short canals and 1:5000 for long canals
Vertical scale: 1:20 for small canals and low gradient and 1:100 for larger canals and higher grad.
The profile should show the ground level, the bed level and the water level at design discharge
Structures should be marked by a vertical line at the place of the structure, with the structure name written along the x-axis
Distance is measured in meters from the canal inlet with suitable intervals. for very long canals it can be measured in km. distance to structures or major change of direction is always measured and added to the tabulated data.
Ground levels are taken from survey data
Bed levels and water levels are tabulated at the end of each reach, upstream and downstream of each structures where water level changes
Important points
Field canals should have sufficient command over the whole length. As normal practice, the water depth should be more or less 10-15cm above the levelled ground to maintain a good siphoning head.
Secondary and main canals can be designed in cut at points where there are no off takes. The engineer to ensure that there is adequate command at canal off takes. Ideally, place off takes before a drop.
Main / Conveyance canals do not necessarily need to have a water level above ground level since no water will be abstracted from it. It is preferable to design them in cut as much as possible.
Drops to be incorporated when the canal goes in fill, but maintaining the required command.

64. Design of the drainage system
Good water management of an irrigation scheme not only requires proper water application but also a proper drainage system.
Agricultural drainage can be defined as the removal of excess surface water and/or the lowering of the groundwater table to below the root zone in order to improve plant growth.
The common sources of the excess water that has to be drained are precipitation, over irrigation and the extra water needed for the flushing away of salts from the root zone.
Furthermore, an irrigation scheme should be adequately protected from drainage water coming from adjacent areas. Drainage is needed in order to:
Maintain the soil structure
Maintain aeration of the root-zone, since most agricultural crops require a well aerated root-zone free of saturation by water; a notable exception is rice
Assure accessibility to the fields for cultivation and harvesting purposes
Drain away accumulated salts from the root zone.
A drainage system can be surface, sub-surface or a combination of the two.

65. Factors affecting drainage
Climate
An irrigation scheme in an arid climate requires a different drainage system than one in a humid climate.
An arid climate is characterized by high-intensity, short-duration rainfall and by high evaporation throughout the year.
The main aim of drainage in this case is to dispose of excess surface runoff, resulting from the high-intensity precipitation, and to control the water table so as to prevent the accumulation of salts in the root zone, resulting from high evapotranspiration.
A surface drainage system is most appropriate in this case.
In a humid climate, that is a climate with high rainfall during most of the year, the removal of excess surface and subsurface water originating from rainfall is the principal purpose of drainage. Both surface and subsurface drains are common in humid areas.
Soil type and profile
The rate at which water moves through the soil determines the ease of drainage. Therefore, the physical properties of the soil have to be examined for the design of a subsurface drainage system.
Sandy soils are easier to drain than heavy clay soils.
Capillary rise is the upward movement of water from the water table. It is inversely proportional to the soil pore diameter.
The capillary rise in a clay soil is thus higher than in a sandy soil. In soils with a layered profile drainage problems may arise, when an impermeable clay layer exists near the surface for example.

66. Factors affecting drainage cont.
Water quantity . The quantity of water flowing through the soil can be calculated by means of Darcy’s law:
Equation 67
Q = k x A x i
Q = Flow quantity (m3/sec)
k = Hydraulic conductivity (m/sec)
A = Cross-sectional area of the soil through which the water moves (m2)
i = Hydraulic gradient
The hydraulic conductivity, or the soils’ ability to transmit water, is an important factor in drainage flow.
Irrigation practice
The irrigation practice has a bearing on the amount of water applied to the soil and the rate at which it is removed. For example, poor water management practices result in excess water being applied to the soil, just as heavy mechanical traffic results in a soil with poor drainage properties due to compaction.

67. Surface drainage
Ponding or excess surface water can be discharged through an open surface drain system.
Drains of less than 0.50 m deep can be V shaped.
In order to prevent erosion of the banks, field drains often have flat side slopes, which in turn allow the passage of equipment.
The side slopes could be 1:3 or flatter. Larger field drains and most higher orders drains usually have a trapezoidal cross-sections.
The water level in the drain at design capacity should ideally allow free drainage of water from the fields.
Drains must be designed to remove the total volume of runoff within a certain period. If, for example, 12 mm of water (= 120 m3/ha) is to be drained in 24 hours, the design steady drainage flow of approximately 1.4 l/sec per ha (= (120 x 103)/(24 x 60 x 60)) should be employed in the design of the drain.

68. Cross-sections of Surface Drains

69. Surface Drainage Cont.
If rainfall data are available, the design drainage flow, also called the drainage coefficient, can be calculated more precisely for a particular area.
The following method is usually followed for flat lands. The starting point is a rainfall-duration curve, an example of which is shown in Figure 96.
This curve is made up of data that are generally available from meteorological stations.
The curve connects, for a certain frequency or return period, the rainfall with the period of successive days in which that rain is falling.
Often a return period of 5 years is assumed in the calculation. It describes the rainfall which falls in X successive days as being exceeded once every 5 years.
For design purposes involving agricultural surface drainage systems X is often chosen to be 5 days.
Thus from Figure 96(Source FAO, 2002) it follows that the rainfall falling in 5 days is 85 mm.
This equals a drainage flow (coefficient) of 1.97 l/sec per ha (= (85 x 10 x 103)/(5 x 24 x 60 x 60)).

70. Drainage Design cont.
The design discharge can be calculated, using the following equation:
Equation 69
Q = q x A/1 000
Where:
Q = Design discharge (m3/sec); q = Drainage flow (coefficient) (l/sec per ha)
A = Drainage area (ha)
It appears from practice that a drain designed for a 5 days rainfall is, in general, also suited to cope with the discharge from a short duration storm.

71. Drainage Design cont.
The above scenario however, is not necessarily true in small irrigation schemes, especially on sloping lands (with slopes exceeding 0.5%), which may cover an area that could entirely be affected by an intense short duration rainfall.
The design discharge could then be calculated with empirical formulas, two of the most common ones being:
The rational formula
The curve number method
The rational formula is the easier of the two and generally gives satisfactory results.
It is also widely used and will be the one explained below.

72. Drainage Design cont. The formula reads: Equation 70
Where:
Q = Design discharge (m3/sec)
C = Runoff coefficient
I = Mean rainfall intensity over a period equal to the time of concentration (mm/hr)
A = Drainage area (ha)
The time of concentration is defined as the time interval between the beginning of the rain and the moment when the whole area above the point of the outlet contributes to the runoff.
The time of concentration can be estimated the following formula: Equation 71;

73. Table 39: Values for runoff coefficient C in Equation 70
The runoff coefficient represents the ratio of runoff volume to rainfall volume. Its value is directly dependent on the infiltration characteristics of the soil and on the retention characteristics of the land. The values are presented in Table 39.
Example 39
An irrigation scheme of 100 ha with sandy loam soils and a general slope of less than 5% has a main drain of 2.5 km long with a difference in elevation of 10 m. What is the time of concentration?

74. Drainage Design cont.
The rainfall intensity can be obtained from a rainfall duration curve, such as shown in Figure 96. For short duration rainfall, it is necessary to make a detailed rainfall duration curve for the first few hours of the rainfall.
Example 40
In Example 39, the 68 minutes rainfall with a return period of 5 years is estimated at 8.5 mm. What is the design discharge of the drain?
The mean hourly rainfall intensity is (60/68) x 8.5 = 7.5 mm/hour.
The runoff coefficient for sandy loam arable land with a slope of less than 5% is 0.30 (Table 39).
Thus the design discharge for the scheme is:
⇒ Q = m3/sec or 6.25 litres/sec per ha
Once the design discharge has been calculated, the dimensions of the drains can be determined using the Manning Formula (Equation 13).
It should be noted that higher order canal design should not only depend on the design discharge, but also on the need to collect water from all lower order drains.
Therefore, the outlets of the minor drains should preferably be above the design water level of the collecting channel.

75. Hydraulic structures
Hydraulic structures are installed in open canal irrigation networks to serve various purposes:
Control and measure discharge
Control water levels for command requirements
Dissipate unwanted energy
Deliver the right volume of water to meet crop water requirements
Incorporate recycled tail water, if available
The most common structures are:
a. Head works for river water off take
b. Night storage reservoirs
c. Head regulators
d. Cross regulators
e. Drop structures
f. Tail-end structures
g. Canal outlets
h. Discharge measurement structures
i. Crossings, like bridges, culverts, inverted siphons
Depending on the size and complexity of the irrigation scheme, some or all of the above-mentioned structures could be incorporated in the design.

76. Bill of Quantities
During the design stage, detailed technical drawings have to be made.
These drawings are needed during the implementation stage and for the calculation of the bill of quantities and costs.
An implementation programme or time schedule should be prepared as well, to give an estimate of the labour and equipment requirements.

77. Producing bill of quantity for surface schemes.
Bill of quantities are produced so that the project can be costed. Concrete requirements for a canal are calculated from its cross section as detailed below:
0.15
lip
0.05
side
bottom
600

78. BOQs Cont. The area of concrete is calculated as follows:
2 sides Sin 60 = ( ) / L
L = 0.40 / = 0.46m
Area of two sides = 2 x 0.05 x 0.46m = 0.046m2
Length of the bottom part is 0.35 including the side (about 0.015m at each side). The length of bottom therefore becomes 0.32m, giving a concrete area of 0.016m2 through 0.32m x 0.05m.
Area of lip. Length is 0.15m but including part of the slanting side. Length of lip used in the calc. is 0.135m (0.15 – 0.015) giving area of m2 for 2 lips.
Volume of concrete for 1m of canal becomes = 0.075m3
The concrete volume per meter length of canal is then multiplied by the canal lengths.
Contingency factor of up to 20% should be added for loses during concrete mixing, transportation and construction.
In canals we use volume batching and a good concrete mix is 1 : 2 : 3 or sometimes 1 : 3 : 3.
Measuring by volume is based on loose volume and a 50kg bag of cement is 40 litres. The yield of the mix is 60% of the loose volume of cement, sand and stone.

79. BOQs Cont.
Using the 1:2:3 mix, a bag of cement (40l) requires 80l of sand and 120l of stone giving a volume of 240l and a concrete yield of 144l.
A cubic metre of concrete therefore requires the following:
Cement : 1000/144 = 6.94 = 7 bags of 50kg each.
Sand : 7 x 40 x 2 = 560l or 0.56m3
Stone : 7 x 40 x 3 = 840l or 0.84m3
A bill of quantity for surface is in most instances composed of the following:
Materials; cement, sand, stone, canal gates, bricks, reinforcement steel etc for canals, hydraulic structures, drains, roads, toilets etc.
A BOQ is prepared in sections e.g each separate sections for canals, hydraulic structures , drains, ancillary structures like toilets, roads etc.
Transport: for bulk transportation of materials to site.
Labour for canal construction
Equipment for use at site that include concrete mixer, tractor and trailer, water bowser, dozers, motorized graders etc.
A bill of quantities for canals is constructed for various canal size sections independently and the key elements to include are as tabulated below:

80. BOQs Cont. Item Description Qnty Unit Unit Cost Total Cost
250mm canal section 1375m i
Cement
bag Ii
Sand m3
Iii
Stone Iv
Labour skilled
Person-day V
Labour unskilled Vi
Equipment
lump
Vii
Transport
viii
Subtotal

81. An example of elements included in the preparation of bill of quantity for an off-take structure.
Item
Description
Qnty
Unit
Unit Cost
Total Cost
Canal off-takes 12 I
Steel bar 10mm m Ii
Cement
bag
Iii
Sand m3 Iv
Stone V
Bricks no Vi
Canal gates
Vii
Labour skilled
Person-day
viii
Labour unskilled Ix
Equipment
lump X
Transport Xi
Subtotal

82. BOQs Cont.
After having done detailed bill of quantities for all items on project, it is important to have a summarized bill of quantity with total requirements for the various items that can now be used for procurement purposes.
After this ,the System Operation Instructions should then be prepared
Below is an Example of a Surface Scheme BOQ: It is for Nabusenga scheme, downstream of the night storage reservoir