university of kwazulu-natal examinations: may...

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UNIVERSITY OF KWAZULU-NATAL

EXAMINATIONS: MAY 2008

MODULE AND CODE: IRRIGATION ENGINEERING ENAG4IE DURATION: 2 HOURS TOTAL MARKS: 100 Internal Examiner : Dr A Senzanje External Examiner : Mr M Zartmann

STUDENTS ARE REQUIRED IN THEIR OWN INTERESTS, TO WRITE LEGIBLY _______________________________________________________________________________ NOTES: This paper consists of 6 pages of questions and 3 pages of formulae. Please see that you have them all. Answer all questions. Calculators may be used. Table 1 and Figure 1 must be handed in with your answer booklet Question 1: General Design [15 marks] a) What key factors do you need to consider when selecting an irrigation system under general circumstances? (5) b) According to Keller (1980), how can you best summarize the process of irrigation system design? (2) c) As an irrigation engineer in training, briefly describe the following:

i) irrigation method. (2) ii) irrigation system. (2) iii) irrigation scheme. (2) iv) irrigation project. (2)

[15] Question 2: Irrigation Planning [15 marks] a) You are in the process of planning irrigation development and need to quantify crop and irrigation water requirements. You have collected some data as given in Table 1 attached. (Table 1 must be handed in with your answer sheet)

i) Complete the crop water requirements and irrigation water requirements in Table 1. (5)

ii) At a minimum, how much water should the farmer store in order for them to meet his/her seasonal irrigation needs per ha? (2)

iii) What is the peak design duty (l/s/ha) for the above system that would be used to size irrigation canals? (2)

b) In irrigation planning, differentiate between the deductive method and inductive method for

determining water duties and canal discharges. (3) c) What are the consequences of not following a proper irrigation schedule? (3) [15]

UNIVERSITY OF KWAZULU-NATAL SCHOOL OF BIORESOURCES ENGINEERING & ENVIRONMENTAL HYDROLOGY

MODULE AND CODE: IRRIGATION ENGINEERING MAY 2008 ENAG4IE _______________________________________________________________________________

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Question 3: Sprinkler Design [15 marks] a) A level field plot measuring 96 m x 96 m is designed for irrigation with a solid set system having low discharge sprinklers. A uPVC mainline line runs through the centre of the plot, with aluminum laterals placed on both sides. The whole plot, planted to high value vegetables, is irrigated simultaneously. The selected sprinklers apply (qs) 0.110 m3/hr at a pressure of 2.0 bars (20 m head of water). The laterals are spaced every 6 m on the main line while the sprinklers along them are spaced every 8 m. The height of the stand (riser) pipe is considered negligible. The head losses in the lateral and mainline are to be restricted to below 20% of the operating pressure.

i) what is the length of each lateral? (1) ii) how many sprinklers are carried on each lateral? (1) iii) what is the flow rate into each lateral? (1) iv) size (practical) the lateral to satisfy the stated design criteria. (4) v) size the mainline (practical) to satisfy the given design criteria. (4) vi) What is the available pressure on the last sprinkler on the last lateral in the

solid set? (4) [15]

Question 4: Micro Irrigation [15 marks] a) In the design of drip irrigation systems, define and outline the importance of: i) a hydraulic unit (or sub-unit). (2)

ii) allowable pressure variation (∆Hs) in a hydraulic unit. (2) iii) design emission uniformity (EU). (2)

b) You are designing a drip system for avocado trees growing in a clayey soil in a cool climate with moderate rainfall and thus aim for a percentage wetted area (Pw) of 55%. If the trees are spaced 9 m x 9 m and the area wetted by an emitter is 10 m2, determine the required number of emitters per tree? (2) c) For the problem in (b) above, you further determine that:

• The effective rooting depth is 1.0 m. • The available soil water is 130 mm/m, • Allowable soil water depletion can safely be 25% (i.e., MAD or α) • The net irrigation requirement is 6.5 mm/day. For the drip design process, calculate:

i) net irrigation requirement per tree as a volume (l/tree/day). (2) ii) available soil water per tree from the soil (l/tree). (2) iii) readily available soil water per tree to be replenished by irrigation (l/tree). (1) iv) design frequency (days) at peak demand. (2)



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[15] Question 5: Surface Irrigation [15 marks] a) In surface (flood) irrigation system design, when you are undertaking preliminary canal layouts based on the contour map of the design area, explain where and why you would:

i) locate or place the main canal? (3) ii) locate and orient the distributary or lateral canals? (2)

ii) locate the main drain or outfall? (2) b) The attached figure (Figure 1) is a contour map of an area were you are designing a surface

irrigation system and you are tasked with laying out the canals such that you command as much of the irrigable area as possible.

i) On the contour map, indicate and label the alignment of the supply canal and main canal to command the maximum possible area. (6)

ii) Approximately how much area are you able to command? (2) [15]

Question 6: Pumps [15 marks]

a) i) In pump terminology, differentiate between brake horse power and water horse power. (2) ii) Which one is a more appropriate measure to consider when selecting pumps for irrigation water pumping? (2) b) What is pump cavitation in irrigation pumping units and how do you ensure that it does not occur? (3) c) i) What is the importance of specific speed in pump design and selection? (3)

ii) Typically, what are the head (H) vs. discharge (Q) characteristics of a pump with a low specific speed compared to one with a high specific speed? (3)

iii) In which specific speed category do commonly used irrigation centrifugal pumps belong or fall? (2)

[15]



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Question 7: Design Norms [10 marks] a) Under what conditions would you deviate from following the SABI design norms in your irrigation system design? (2) b) In the following cases, has the designer satisfied or met the SABI design norms? Provide reasons for your answers.

i) An AGPS301 student has used a design emission uniformity of 85% in the design of a micro irrigation system. (2)

ii) An ENAG3EI student has selected a stand pipe height of 1 m in the design of a sprinkler system when the sprinkler discharge is 0.92 m3/hr. (2)

iii) A recently graduated ENAG4IE student has supervised the construction of an irrigation basin with a cross slope of 0.02%. (2) i) A farmer has submitted to DWAF (for purposes of water permit application) a plan

in which their portable sprinkler system will operate for 110 hours per week. (2) [10]



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Please hand in this page Student Name………………………………………. Student No…………………………….. Table 1: Crop and irrigation water requirements for a crop of onions. Month Month 1 Month 2 Month 3 Month 4 Month 5 ETo (mm/day) 5.9 6 6.4 6.1 5.8 Kc (/mnth) 0.65 0.95 1.05 1 0.85 ETc (mm/day) ETc (mm/mnth) P (mm/mnth) 36 40 41 124 138 Pe (mm/mnth) IWRnet (mm/day) IWRnet (mm/mnth) IWRgross (mm/mnth) Design duty (l/s/ha) NB: Effective precipitation can be estimated from the FAO (1986) method as follows: Pe = 0.8P – 25 (for P>75 mm/month) and Pe = 0.6P – 10 (for P<75 mm/month)



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Please hand in this page Student Name……………………………… Student No…………………..

Figure 1: Contour map for canal layout (for Question 5b)



DATA SHEETS & FORMULAE

NB: All variables are as defined in class and in the Irrigation Design Manual. 1) Generalised friction loss equation (Equation 6.19)

ri

p

f dblQh =

This equation is used in conjunction with Table 6.6. 2) Power required by a pump prime mover

ηρgQHPower =

3) Generalised 2-point method for power function y=kXp

)/log()/log(

21

21

XXyy

p =

4) Generalised equation for the calculation of design duty (dd (l/s/ha)) for a daily gross crop water requirement z (mm/day) over 24 hours.

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛=

smlhammzdd

)60*60*24*1(1/1000*/10000*)

1000( 32

Note: This equation needs to be adjusted accordingly if the flow is not for the full 24 hours per day, and similarly if the gross crop water requirements are not given as mm/day but, say, mm/month. 5) Drip irrigation

i) ww

p PA

plantAreaN /=

ii) ⎟⎟⎠

⎞⎜⎜⎝

⎛=

rp

epw SS

WSNP 100

iii) ppe NSS /= iv) Area of wetted soil = Sp x Sr x Pw



Standard Pipe Sizes Available on the Market Aluminium (mm) uPVC (mm) AC (mm) 32 32 75 51 40 100 63 50 125 76 63 150 102 75 175 127 90 200 152 110 225 178 125 260 203 140 300 254 160 350 304 200 400 250 460

525 600 675 750 825

PIETERMARITZBURG

EXAMINATION PAPER : UN 6 FORM

1. Date of Examination :…………30 May 2008……AM/ PM…………AM………

2 Module : ……………ENAG4IE………………………………………………………..

3 Description : ………………IRRIGATION ENGINEERING…………………………

4 Number of Pages : …………6 Pages of questions and 3 pages formulae……...

5 Duration of Exam : …………2 Hours………………

6 Number of candidates writing this paper : ……8………

7 Please indicate the materials that are to be supplied to the candidate

□ Green Answer Book (12 pages) □ Blue Answer Book (4 pages)

□ Graph Paper □ MCQ

□ Any other Requirements : ……………………………............................

……………………………………………………………………………….

8 Examiner(s)

Internal : Name : ………………… Signature :……………….. Tel no : ………

Name : ………………… Signature : ………………. Tel no : ………

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External :

Name : ………………… Signature : ………………

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6 Master Checked Name : …………………………Signature : …………...

PLEASE NOTE : THIS FORM MUST BE ATTACHED TO ALL EXAM PAPERS AND MUST BE SIGNED BY EXAMINERS

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