MODULE AND CODE: IRRIGATION ENGINEERING ENAG3EI DURATION: 3 HOURS TOTAL MARKS: 160 Internal Examiner : Dr A Senzanje External Examiner : Mr M Zartmann NOTES: STUDENTS ARE REQUIRED IN THEIR OWN INTERESTS, TO WRITE LEGIBLY This paper consists of 9 pages of questions and 16 pages of formulae and data. Please see that you have them all. Answer all questions. Calculators may be used. Complete, detach and attach Fig Q5 (Pg 8) and Fig Q7b (Pg 9) to your answer book. Question 1: PIPE HYDRAULICS [15 marks] a) Define a pipe period and briefly explain how it is applied in the design and operation of valves in pipe networks. (6) b) An aluminium lateral line is 240 m long and has sprinklers spaced 12 m apart. Each sprinkler discharges 1.08 m 3 /hr at a pressure of 300 kn/m 2. Applying SABI irrigation design norms regarding allowable head losses due to friction along the lateral, determine the required practical pipe diameter (mm) for the lateral. (5) c) A 90 mm diameter upvc pipe carries water at a flow rate of 40 m 3 /hr and the water temperature is 20⁰C. Determine the state of flow of water in the pipe. (4) Question 2: OPEN CHANNEL HYDRAULICS [45 marks] a) Briefly explain, with diagrams where possible, what would happen to the absolute water level in an open channel for the following flow conditions when a small smooth step is introduced on the bed of the channel: i) flow upstream of the smooth step is sub-critical (3) ii) flow upstream of the smooth step is super-critical (3) b) In each case, state a single hydraulics principle or concept associated with the following individuals and then give its relevance or application to irrigation engineering: 1
i) BA Bakhmeteff (2) ii) JB Belanger (2) c) An open channel of trapezoidal section, 2.5 m wide at the base (b) and having sides inclined at z:1 = 1:1 to the horizontal, has a bed slope of 1:500. It is found that when the rate of flow (Q) is 1.24 m 3 /s the depth of water (y) in the channel is 350 mm. Assuming the validity of Manning s formula, calculate the rate of flow (m 3 /s) when the depth of flow is 500 mm. (6) d) A long concrete channel of trapezoidal section with sides that slope at 60⁰ to the horizontal is to carry 3 m 3 /s of water. If the bed slope is 1:1800 and Manning s roughness coefficient (n) for the concrete lining is 0.015, determine the optimum dimensions of the channel. (8) e) Water flows at 5.4 m 3 /s under a wide sluice gate into a rectangular prismatic channel 3.5 m wide. A hydraulic jump is formed just downstream of a section where the depth is 380 mm. For this situation, determine: i) the flow depth downstream of the hydraulic jump (m), (5) ii) the type of jump that is formed, (2) iii) the power dissipated in the jump (kw), and (5) iv) whether the flow involves a flow regime transition. (2) f) A V-notch weir is a classic example of the application of vertical contraction of flow in open channels for flow control and measurement. The generalised V-notch weir discharge equation is given as: Q =CH m Where Q = discharge (m 3 /s), H = head of water above the weir (m), C = proportionality constant, and m = weir exponent that indicates the sensitivity of flow to variations in H. The data in Table Q2f were obtained during the calibration of a V-notch weir in a laboratory. Using the 2-point method (or any other suitable approach) determine: i) the parameters C and m in the V-notch discharge equation, and (4) ii) from (i) above and the equation, the discharge when the head of water above the weir is 0.5 m. Is this answer valid? Explain. (3) 2
Table Q2f Head (H) versus discharge (Q) data for calibrating a V-notch weir H (m) 0.105 0.109 0.139 0.164 0.173 0.181 0.189 0.194 0.200 Q (m 3 /s) 0.0030 0.0031 0.0058 0.0086 0.0099 0.0113 0.0123 0.0130 0.0139 Question 3: GENERAL DESIGN [15 marks] a) Drip and micro irrigation systems seem to be quite popular with grape farmers in the Western Cape part of South Africa. Briefly, and in point form, outline why this is the case. (7) b) More and more these days irrigation designers are being asked to design irrigation systems that are multiple use (MUS) rather than single use (SUS). Briefly discuss how you would go about designing an irrigation system for MUS, and in your answer, give examples of situations and irrigation hardware applicable to Southern Africa that you would design differently. (8) Question 4: IRRIGATION PLANNING & SCHEDULING [15 marks] a) Imagine that you have a level, 400 x 400 square orchard with trees spaced 4 m x 5m. The orchard s transpiration rate is 5 mm/day. Quantitatively discuss how climate, crop, soil and spatial information are used in the irrigation planning and design process. (8) b) Table Q4b below gives physical properties for common agricultural soils. Table Q4b: Physical properties of common agricultural soils. Soil Texture Infiltration Rate (mm/hr) Porosity (%) Field Capacity (%) Crop Water (%) Sand 50 38 15 7 Sandy loam 25 43 21 9 Loam 13 47 31 14 Clay loam 8 49 36 18 Clay 0.5 53 44 21 Extractable At full development, a sugar cane crop is measured in an unrestricted soil profile to have an effective root zone of 1.5 m. The maximum equivalent crop evapotranspiration at the midpoint of the growing season is 9 mm/day. Assuming that each irrigation replenishes the soil profile up to field capacity, and using the information in Table Q4b: 3
SCHOOL OF BIORESOURCES ENGINEERING & ENVIRONMENTAL HYDROLOGY i) For a sandy loam soil, how many days are allowed between irrigations if 40% depletion of available water is permitted? (3) ii) For a sandy loam soil, how long would the stand time (T set ) be if irrigation was effected using a 3.5 mm nozzle sprinkler operating at 300 kpa discharging 0.81 m 3 /hr on a 12 m x 15 m layout? (4) Question 5: SPRINKLER DESIGN [15 marks] a) Design a quick-coupling hand move sprinkler irrigation system for the conditions given below. Your design should also include a calculation of the system total dynamic head (TDH) and power required to energise the system. To allow for flexibility in operation, the main line of the system should be placed in the middle of the field in the South to North direction. The land is flat and measures 183 m (L) by 202 m (W) (see Figure Q5 attached on Pg 8) The crop to be grown is tomatoes with an effective rooting depth of 1.0 m The peak consumptive water use rate (ET c ) is 7 mm/day The management allowable depletion (MAD or α) is taken as 40% The soil is sufficiently deep and has a water holding capacity of 122 mm/m Available sprinklers are the 3.5 mm single nozzle type operating at 300 kpa (30 m) pressure, each discharging 0.82 m 3 /hr for an application rate of 6.07 mm/hr on a 9 m x 15 m layout From SABI design norms, take the system efficiency as 75% Ignore the effect of wind on the design Assume the pump is located on the South edge of the field (see Figure Q5) Hours of operation should not exceed 12 hours per day The key design criteria are that the total allowable pressure drop along a lateral should not exceed 20% of the operating pressure of the sprinklers. Similarly, the total head loss along the main line should not exceed 20% of the sprinkler operating pressure. Clearly state any relevant professional assumptions you may make during the design. (15) Question 6: MICRO IRRIGATION DESIGN [15 marks] a) Briefly discuss conditions under which the Martinez drip design approach may not be particularly applicable. (6) b) Sometimes irrigation systems are designed based on generalised guidelines. Table Q6b gives general estimates of drip system equipment requirements for different crop types. 4.
Table Q6b: General estimates of drip system equipment requirements (adapted from Hanks & Keller, 1972) Type of Crop Row Spacing (m) Plants per Hectare Emitters per Hectare Lateral length (m/ha) Ordinary orchards 6 250 500 1500 1900 Dwarf orchards & 3.7 1000 2000 3040 vineyards Wide-spaced row 1.5 15000 7500 6840 crops Greenhouses & close-spaced row crops 1 25000 10000 10640 A typical orchard is to be developed on a field measuring 253 m by 439 m. The orchard will be irrigated using a drip system laid out so that each tree is served by four emitters. The following design conditions are based on peak period requirements at full tree maturity: Operating pressure at the emitter =10 m (1 bar), Peak period crop water requirements = 5 mm/day, Emission (distribution) uniformity = 92%, and Operating hours per day =18 hrs For the above conditions determine (or estimate) the following design parameters: i) the number of emitters required, (3) ii) the required application rate (mm/hr), and (3) iii) length of laterals required (m). (3) Question 7: SURFACE IRRIGATION [15 marks] a) Briefly explain the following terms as applied to land levelling for surface (flood) irrigation design and construction: i) centroid of a field, and (3) ii) formation levels of field points. (3) 5
b) The topographic map of a watercourse (canal) command area is shown in Figure Q7b on Pg 9. The watercourse is to take off from the distributary canal at the indicated point (A). For this irrigation design project, you are required to: i) plan the alignment (and clearly indicate this on the topographic map) of the watercourse to command the maximum possible area (4) ii) estimate the actual area (ha) that would be commanded by your proposed watercourse alignment. Clearly indicate the area that would be commanded on the topographic map (5) Question 8: PUMPS [15 marks] a) Define cavitation parameter, state its range of values, and briefly outline its relevance to irrigation pump installation and operation. (7) b) Table 8Qb lists specific speeds of various pumps available on the market. Table 8Qb: Specific speeds for various pumps available on the market Pump Specific Speed, Ns Type of Pump Indicative Efficiency (metric) (%) A 10 Radial flow 80 B 100 Mixed flow 90 C 40 Francis impeller 93 D 200 Axial flow 80 E 20 Radial flow 82 From the above information, select and justify your choice, of the most appropriate pump for the following pumping requirements: i) rural water supplies, (3) ii) everyday pressurised irrigation, and (3) iii) dewatering a mine (2) 6
Question 9: DESIGN NORMS & STANDARDS [10 marks] a) State the indicative SABI design norms for the following irrigation system design situations: i) system application efficiencies for movable quick-coupling sprinkler systems, (2) ii) recommended irrigation hours per week for a centre pivot irrigation system, and (2) iii) foot valve open area compared to suction pipe diameter. (2) b) American Society of Biological and Agricultural Engineers (ASABE) irrigation design norms are touted worldwide as being very good. Briefly explain why in South Africa one should, in the first instance, use the SABI design norms. (4) 7
COMPLETE, DETACH AND HAND IN WITH YOUR ANSWER BOOK STUDENT NAME:. STUDENT NUMBER: Figure Q5: Field layout for a sprinkler system design (Not to scale) 8
COMPLETE, DETACH AND HAND IN WITH YOUR ANSWER BOOK STUDENT NAME:. STUDENT NUMBER: Figure Q7b: Topographic map of a watercourse (canal) command area (NB: contours in meters) 9
DATA SHEETS & SOME FORMULAE NB: All variables are as defined in class and in the Irrigation Design Manual. A. HYDRAULICS 1. Reynolds Number (Re) Re d i V Or lv Re Or VR Re 2. Froude Number (Fr) Fr V gy Or Fr V gd m 3. General Exponential Equation (used with Table 6.6 in the IDM) h f blq d r i p 10
4. Generic Uniform Flow Velocity Equation in Open Channels Manning & Chezy V = C R x S y 5. Free board or Dry board in Open Channels F b C fb y 6. Unit Tractive Force grs 0 7. Critical Conditions in Open Channel Flow y c = 2/3 E c and y c 3 2 q g 8. Hydraulic Jump in Rectangular Open Channel Flow y y 2 1 1/ 2( 1 8F 2 r1 1) (for 4 < Fr < 20) 11
( y2 y1) E 4y y 1 2 3 E E 2 1 2 2 8 F 3/ r1 1 4Fr 2 2 8F 2 F r1 r1 2 1 1 9. Generalities Density of water = 1000 kg/m 3 Dynamic viscosity of water = 9.2 x 10-4 kg/m s (at about 20ºC) Kinematic viscosity of water = 0.01 cm 2 /s (at about 20ºC) Atmospheric pressure = 10.35 m of water (at sea level) Pressure relationships = 1 bar = 10 m head of water 10. Channel sections i) Hydraulically efficient channel Ratio between b & y 2y b OR 2 z 1 z 12
ii) Shallow section Relation between Q & b/y ratio Q (m 3 /s) 0.30 3.0 14.0 28.0 140.0 285.0 b/y 2.0 4.0 6.0 7.5 14.0 18.0 11. Air entrapment and water hammer ( ) 13
B. IRRIGATION ENGINEERING 1) Generalised friction loss equation (Equation 6.19) blq h f d This equation is used in conjunction with Table 6.6. r i p 2) Power required by a pump prime mover Power gqh ~~ or ~~ Power QH 360E p 3) Generalised 2-point method for power function y=kx p log( y p log( X 1 1 / y2 ) / X ) 2 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. dd z ( ) m*10000m 1000 2 / ha *1000l / m 3 1 (1*24*60*60) s 14
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 Area / plant i) N p Pw A w ii) N psew P w 100 S psr iii) S S / N e p p iv) Area of wetted soil = S p x S r x P w v) EU = 100{1 1.27 CV m /(Np) 1/2 }{q min /q avg } vi) H avg /H min = (q avg /q min ) 1/x 6) Flood (surface) irrigation i) Ax24x7xNIR Q 2.4xT xt xe d h s 15
7) Sprinkler irrigation i) t c = RAW/NIR and t s = RAW/NAR ii) q e = GAR x A or GAR = q e /A iii) t h = t s x n and n c = n x t c iv) N e {A x 10000}/{n c x L e x L d } 16
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 17
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