Rainfall Simulator
Instruction Manual FEL3 ISSUE 9 September 2011
Table of Contents Disclaimer ................................................................................................................... 1 Copyright and Trademarks ...................................................................................... 1 General Overview ....................................................................................................... 2 Equipment Diagrams................................................................................................... 3 Important Safety Information....................................................................................... 6 Introduction.............................................................................................................. 6 The Control of Substances Hazardous to Health Regulations (1988)..................... 6 Water Borne Hazards .............................................................................................. 7 Electrical Safety....................................................................................................... 7 Description .................................................................................................................. 9 Overview.................................................................................................................. 9 Installation ................................................................................................................. 10 Advisory................................................................................................................. 10 Electrical Supply .................................................................................................... 10 Mains Water Supply .............................................................................................. 10 Assembly ............................................................................................................... 10 Commissioning ...................................................................................................... 11 Operation .................................................................................................................. 13 Operating the Equipment....................................................................................... 13 Equipment Specifications.......................................................................................... 14 Overall Dimensions ............................................................................................... 14 Electromagnetic Compatibility ............................................................................... 14 Equipment Location............................................................................................... 14 Environmental Conditions...................................................................................... 14 Routine Maintenance ................................................................................................ 15 Responsibility ........................................................................................................ 15 General.................................................................................................................. 15 Laboratory Teaching Exercises................................................................................. 16 Index to Exercises ................................................................................................. 16 ii
Table of Contents Exercise A ................................................................................................................. 17 Exercise B ................................................................................................................. 20 Exercise C................................................................................................................. 23 Exercise D................................................................................................................. 26 Exercise E ................................................................................................................. 29 Contact Details for Further Information ..................................................................... 31
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Disclaimer This document and all the information contained within it is proprietary to Armfield Limited. This document must not be used for any purpose other than that for which it is supplied and its contents must not be reproduced, modified, adapted, published, translated or disclosed to any third party, in whole or in part, without the prior written permission of Armfield Limited. Should you have any queries or comments, please contact the Armfield Customer Support helpdesk (Monday to Thursday: 0830 – 1730 and Friday 0830 - 1300 UK time). Contact details are as follows: United Kingdom
International
(0) 1425 478781 (calls charged at local rate)
+44 (0) 1425 478781 (international rates apply)
Email:
[email protected] Fax: +44 (0) 1425 470916
Copyright and Trademarks Copyright © 2011 Armfield Limited. All rights reserved. Any technical documentation made available by Armfield Limited is the copyright work of Armfield Limited and wholly owned by Armfield Limited. Brands and product names mentioned in this manual may be trademarks or registered trademarks of their respective companies and are hereby acknowledged.
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General Overview The use of simulated rainfall in soil erosion studies has many advantages. It can speed up research, it is cost-effective, gives better control over variables and is more dependable. Simulated rainfall may be applied at any selected intensity, for selected duration on known plot management conditions. Crop type and stage of development can be varied as can soil texture and moisture status. Natural rainfall, however, must be accepted as and when it comes; its intensity or duration cannot be controlled. To be useful in soil erosion studies the simulated "rain" produced must closely approach natural rainfall in certain characteristics. Rainfall intensity and uniformity of intensity, drop size and drop size distribution, and impact velocity of rain drops are key parameters. For true erosive compatibility the energy of the simulated storm should closely match that of natural rainfall of similar intensity. The FEL3 Rainfall Simulator is a spinning disc simulator which enables good dropsize distribution and representative kinetic energy to be achieved at a wide range of intensities. The simulator will enable the student of soil erosion to investigate the techniques and parameters required for good rainfall simulation. The FEL3 Simulator can also be used in the laboratory or in the field for a wide range of research from studies of infiltration under sprinkler irrigation to estimating soil loss in high intensity tropical storms. Erodability of soils can be studied in the laboratory and the influence of crop cover on the effect of rainfall can also be investigated.
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Equipment Diagrams
Figure 1: Installation drawing for Rainfall Simulator
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Armfield Instruction Manual
Figure 2: Sectional View of Spray Head Assembly
Figure 3: Control Panel
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Equipment Diagrams
Figure 4: Accessory details
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Important Safety Information Introduction Before proceeding to install, commission or operate the equipment described in this instruction manual we wish to alert you to potential hazards so that they may be avoided. Although designed for safe operation, any laboratory equipment may involve processes or procedures which are potentially hazardous. The major potential hazards associated with this particular equipment are listed below.
INJURY THROUGH MISUSE
INJURY FROM ELECTRIC SHOCK
INJURY FROM HANDLING LARGE OR HEAVY COMPONENTS
INJURY FROM ROTATING COMPONENTS
RISK OF INFECTION DUE TO LACK OF CLEANLINESS
Accidents can be avoided provided that equipment is regularly maintained and staff and students are made aware of potential hazards. A list of general safety rules is included in this manual, to assist staff and students in this regard. The list is not intended to be fully comprehensive but for guidance only. Please refer to the notes overleaf regarding the Control of Substances Hazardous to Health Regulations.
The Control of Substances Hazardous to Health Regulations (1988) The COSHH regulations impose a duty on employers to protect employees and others from substances used at work which may be hazardous to health. The regulations require you to make an assessment of all operations which are liable to expose any person to hazardous solids, liquids, dusts, vapours, gases or microorganisms. You are also required to introduce suitable procedures for handling these substances and keep appropriate records. Since the equipment supplied by Armfield Limited may involve the use of substances which can be hazardous (for example, cleaning fluids used for maintenance or chemicals used for particular demonstrations) it is essential that the laboratory supervisor or some other person in authority is responsible for implementing the COSHH regulations. Part of the above regulations are to ensure that the relevant Health and Safety Data Sheets are available for all hazardous substances used in the laboratory. Any person using a hazardous substance must be informed of the following: Physical data about the substance Any hazard from fire or explosion Any hazard to health Appropriate First Aid treatment
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Important Safety Information Any hazard from reaction with other substances How to clean/dispose of spillage Appropriate protective measures Appropriate storage and handling Although these regulations may not be applicable in your country, it is strongly recommended that a similar approach is adopted for the protection of the students operating the equipment. Local regulations must also be considered.
Water Borne Hazards The equipment described in this instruction manual involves the use of water, which under certain conditions can create a health hazard due to infection by harmful micro-organisms. For example, the microscopic bacterium called Legionella pneumophila will feed on any scale, rust, algae or sludge in water and will breed rapidly if the temperature of water is between 20 and 45°C. Any water containing this bacterium which is sprayed or splashed creating air-borne droplets can produce a form of pneumonia called Legionnaires Disease which is potentially fatal. Legionella is not the only harmful micro-organism which can infect water, but it serves as a useful example of the need for cleanliness. Under the COSHH regulations, the following precautions must be observed:
Any water contained within the product must not be allowed to stagnate, ie. the water must be changed regularly.
Any rust, sludge, scale or algae on which micro-organisms can feed must be removed regularly, i.e. the equipment must be cleaned regularly.
Where practicable the water should be maintained at a temperature below 20°C. If this is not practicable then the water should be disinfected if it is safe and appropriate to do so. Note that other hazards may exist in the handling of biocides used to disinfect the water.
A scheme should be prepared for preventing or controlling the risk incorporating all of the actions listed above.
Further details on preventing infection are contained in the publication “The Control of Legionellosis including Legionnaires Disease” - Health and Safety Series booklet HS (G) 70.
Electrical Safety The equipment described in this Instruction Manual operates from a mains voltage electrical supply. The equipment is designed and manufactured in accordance with appropriate regulations relating to the use of electricity. Similarly, it is assumed that regulations applying to the operation of electrical equipment are observed by the end user. However, it is recommended that the Residual Current Device (RCD) supplied (alternatively called an Earth Leakage Circuit Breaker - ELCB) be fitted to this
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Armfield Instruction Manual equipment. If through misuse or accident the equipment becomes electrically dangerous, an RCD will switch off the electrical supply and reduce the severity of any electric shock received by an operator to a level which, under normal circumstances, will not cause injury to that person. If the electrical supply to the laboratory already incorporates an RCD, then the device supplied with the equipment need not be used. If the electrical supply does not incorporate such protection then the loose RCD supplied by Armfield Ltd should be fitted by a competent electrician either in the supply to the laboratory or in the supply to the individual item of equipment. See the drawing below for full installation instructions. Note: If any doubt exists whether the electrical supply incorporates a device then the RCD supplied should be fitted. At least once each month, check that the RCD is operating correctly by pressing the TEST button. The circuit breaker MUST trip when the button is pressed. Failure to trip means that the operator is not protected and the equipment must be checked and repaired by a competent electrician before it is used. Click on the link below to invoke the drawing: Drawing Number BM20491 Printed Versions of this Instruction Manual Please note, this drawing is appended at the rear of this manual
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Description Where necessary, refer to the drawings in the Equipment Diagrams section.
Overview The equipment consists essentially of two units, the Rainfall Simulator and its service module which stands alongside. The service module comprises a glass fibre tank which is connected to the mains water supply via a ball-cock to maintain the level. Water is pumped from the tank to the rainfall simulator by a centrifugal pump and flexible PVC tube. The service module also carries the electrical control panel for the water pump and for the motor-driven spray head on the simulator. The simulator proper is made up of a metal framework supporting the spray head assembly, which may be either placed directly on the ground for field studies or fitted into a tray for indoor experiments. The framework is complete with spray-containing PVC curtains. Water from the supply unit is controlled by a flow control valve and measured with a flowmeter, both mounted on the simulator framework. Flow is supplied to a vertically orientated nozzle directed downwards and adjustable in height. Two nozzles are supplied for different water flow rates and pressure at the nozzle is indicated on a pressure gauge in the nozzle spray. Water from the nozzle is intercepted by a horizontal rotating disc driven by an electric motor mounted above. The disc is made up of two circular plates each of which has three segmented apertures of 40° in it. If these discs are clamped together with the apertures aligned then an effective aperture of 40° results. The discs may be clamped in position to give apertures ranging from 5° to 40° in 5° steps, the aperture angle being read off a scale. The upper edges of the apertures are raised to stop water falling through the apertures from the top surface of the discs. Water from the nozzle which is intercepted by the disc is thrown off centrifugally into a collector and returned to the supply tank via a plastic tube. The speed of rotation of the disc system is controlled with a motor speed controller on the electrical control panel which also has a speed indicator. Various accessories are supplied for the rainfall experiments. A tilting stand provides a surface which may be inclined at various angles to the horizontal by means of a hinged supporting strut fitting into any one of a number of notches. A 150mm square test plot allows drainage and run-off measurements to be made on soil samples in the laboratory and three pairs of field test plot accessories permit data to be obtained in the field. Six rain gauges and six sample vessels are provided.
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Installation Advisory Before operating the equipment, it must be unpacked, assembled and installed as described in the steps that follow. Safe use of the equipment depends on following the correct installation procedure. Assembly can be completed with a basic tool kit.
Electrical Supply ELECTRICAL SUPPLY FOR VERSION FEL3-A: The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 220/240V, 50Hz. Check that the voltage and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. Connection should be made to the supply cable as follows: GREEN/YELLOW
-
EARTH
BROWN
-
LIVE (HOT)
BLUE
-
NEUTRAL
Fuse Rating
-
10 AMP
ELECTRICAL SUPPLY FOR VERSION FEL3-B: The equipment requires connection to a single phase, fused electrical supply. The standard electrical supply for this equipment is 120V, 690Hz. Check that the voltage and frequency of the electrical supply agree with the label attached to the supply cable on the equipment. Connection should be made to the supply cable as follows: GREEN/YELLOW
-
EARTH
BROWN
-
LIVE (HOT)
BLUE
-
NEUTRAL
Fuse Rating
-
20 AMP
Mains Water Supply a. Connect the tray drain tube to permanent laboratory drain with a suitable length of 50mm bore flexible hose (not supplied). b. Connect the sump tank inlet pipe to a laboratory supply and allow water to enter into the sump tank. The water level in this tank is automatically controlled by the 'ball-cock' operation.
Assembly All numerical references relate to figures 1, 2, 3 and 4 in the Equipment Diagrams. Note: All bolt fasteners are pre-fitted to the required drilled holes of the relative framework. 10
Installation 1. Attach the four adjustable feet to the end frame members. 2. Place tray (6) on a firm surface and ensure that the drain point is located adjacent to a permanent laboratory drain. 3. Fit the spray head assembly (3) to the upper frame member (2) and secure this configuration to the top support of each end frame member (1). (Note: the required assembled position of the flowmeter (5) to ensure the correct location of the end frame members). 4. Bolt the three frame ties (7) to the four upright legs of the assembled framework and fit the flowmeter assembly (5) into the pre-fitted retaining clips. 5. Lift the assembled unit and place this centrally into the tray. 6. Locate the service module and control panel (4) adjacent to the tray and connect the three flexible hoses (17) and (18) as illustrated. Ensure that all hose clips are tight and that the largest diameter hose (52mm I/D) is located into the supporting bracket pre-fitted to the sump tank flange. 7. Assemble the curtain (8) around the four frame legs by supporting the elasticated cord with the four retaining clips. The curtain should be fitted with the two edges placed midway between any two frame legs in order to provide access into the test area and the elasticated cord should be tensioned before fastening the retaining clips so that maximum support for the curtain material is provided. 8. Connect the electrical cable for the spray head geared motor to the socket on the side of the control panel and secure the cable to the adjacent frame using cable ties (19).
Commissioning Establish that the installation is totally complete and proceed as follows: 1. Check that the disc locking knob is secured. 2. Open control panel door and switch door isolator to the ON position. (Two keys are supplied for the door lock - one secured to the framework external to the control panel, the second key taped to the internal base of the control panel). 3. Press the speed controller ON/OFF switch to the ON position and check that the indicator light is operating. 4. Rotate the speed control button and check the operation of the geared motor and speed indicator over the total speed range. 5. Check that the disc rotates freely and that 'fouling' does not occur. Should any 'fouling' be apparent, then the disc position can be re-set by adjustment of the two shaft grub screws. 6. Operate the speed control button so that the disc rotates very slowly and turn the speed control button to zero immediately the disc aperture setting figures are visible.
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Armfield Instruction Manual 7. Rotate the disc locking knob and check that the aperture opening can be adjusted over the full range. Set the aperture to maximum opening of 40° and secure the disc locking knob. 8. Rotate the speed control button over the full speed range to check that the disc aperture adjustment has not caused fouling to occur. Repeat this operation for each aperture setting. 9. Set the speed controller to approximately mid-speed (60 rpm). 10. Close the flow control valve and switch on the pump. 11. Gradually open the flow control valve until water is ejected from the spray nozzle. Note: It is important that when operating the equipment for the first time, there is no undue time delay between operations 10 and 11 as any delay would cause the pump to run in a 'dry' condition which could result in damage to the pump seals. 12. Operate the flow control valve over the full movement and check that the flowmeter and pressure gauge are operating correctly. 13. Check all pipe connections for water-tightness and tighten any joints if necessary. The apparatus is now ready for experimental testing.
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Operation Operating the Equipment See Laboratory Teaching Exercises for details on operating the equipment.
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Equipment Specifications Overall Dimensions Height
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2.65m
Width
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2.30m
Depth
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1.60m
Electromagnetic Compatibility This apparatus is classified as Education and Training Equipment under the Electromagnetic Compatibility (Amendment) Regulations 1994. Use of the apparatus outside the classroom, laboratory or similar such place invalidates conformity with the protection requirements of the Electromagnetic Compatibility Directive (89/336/EEC) and could lead to prosecution.
Equipment Location The equipment requires connection to a single phase fused electrical supply. A 4m length of cable is supplied with the equipment. The equipment requires connection to a cold water supply of 1.5 litres per second at 3.0 bar (absolute).
Environmental Conditions This equipment has been designed for operation in the following environmental conditions. Operation outside of these conditions may result reduced performance, damage to the equipment or hazard to the operator. a. Indoor use; b. Altitude up to 2000 m; c. Temperature 5 °C to 40 °C; d. Maximum relative humidity 80 % for temperatures up to 31 °C, decreasing linearly to 50 % relative humidity at 40 °C; e. Mains supply voltage fluctuations up to ±10 % of the nominal voltage; f.
Transient over-voltages typically present on the MAINS supply; NOTE: The normal level of transient over-voltages is impulse withstand (overvoltage) category II of IEC 60364-4-443;
g. Pollution degree 2. Normally only nonconductive pollution occurs. Temporary conductivity caused by condensation is to be expected. Typical of an office or laboratory environment
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Routine Maintenance Responsibility To preserve the life and efficient operation of the equipment it is important that the equipment is properly maintained. Regular maintenance of the equipment is the responsibility of the end user and must be performed by qualified personnel who understand the operation of the equipment.
General In addition to regular maintenance the following notes should be observed: 1. The equipment should be disconnected from the electrical supply when not in use. 2. Water should be drained from the equipment when it is not in use. 3. The exterior of the equipment should be periodically cleaned. DO NOT use abrasives or solvents. 4. The service module should be periodically cleaned to remove debris and deposits on the walls. DO NOT use abrasives or solvents. The control panel incorporates a fuse to protect the live supply to the speed controller and water pump. The fuse is located inside the control panel below the speed controller on/off switch. If necessary, the fuse should be replaced with a 32mm x 6.3mm (1¼” x ¼”) QUICK BLOW rated at 15 Amps.
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Laboratory Teaching Exercises Index to Exercises Exercise A Exercise B Exercise C Exercise D Exercise E
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Exercise A Objective To assess the variation in intensity and uniformity of simulated rainfall with increasing disc aperture.
Equipment Set Up Position the centre of the test table directly below the nozzle using a plumbline.
Place a grid of suitable containers upon the test table in a configuration similar to one shown above. Containers may be round or rectangular and of any convenient size. Small rain gauges or measuring cylinders are ideal. Number each container and clearly mark its position. Additional Equipment:-Wooden board approx. 0.6m x 0.6m; accurate measuring cylinders 500ml or 1000ml capacity; stop watch.
Theory There is a close association between rainfall intensity and soil erosion - in general, the higher the intensity the greater the erosion. For a given Pressure-Flow-Disc speed combination the intensity of simulated rainfall is controlled by aperture size. Large disc apertures allow more "rain" to strike the test area increasing the intensity of the rainfall. Intensity (I) is usually expressed as a depth of water falling in unit time eg. 100mm/hr, and can be calculated using the equation:
Where Q = Volume of water in each container (ml) A = Area of container (cm2) t = Time (mins) I = Intensity (mm/hr)
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Armfield Instruction Manual The uniformity of distribution of simulated rainfall on the test area is important since lack of uniformity may give unreliable results. Uniformity may vary with pressure, disc speed and aperture size. A measure of uniformity is given by Christiansen's Coefficient (Cu) which is calculated from the following formula: Where:
m = mean of observed depths n = number of observations x = deviation of individual observed depth from the mean.
Initial values of variables to be used Pressure Gauge
= 0.4 bar
Disc Rotation Speed = 100 rev/min Disc Aperture
= 10°
Readings to be taken Cover the collecting vessels with a wooden board and operate the simulator at the selected values until a steady state is attained. Remove the board and start the stop watch simultaneously. Allow rainfall to strike the target for the desired storm duration, eg. 10 mins. Cover the collectors with the wooden board and close down the simulator. The volume of water in each container is then measured with the measuring cylinder. Note the cross-sectional area of the collector. Repeat the experiment for disc apertures of 15°, 20°, 25°, 30°.
Results Construct a table of results similar to that shown below:
Plot a graph of intensity against aperture size:
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Exercise A
Students may wish to map the uniformity distribution and locate areas of greatest intensity and/or uniformity. Students may also wish to assess the effect of pressure and disc speeds on uniformity and intensity.
References Hudson, N.W. Soil Conservation, Ch.4 p.59. Batsford, London (1976). Christiansen, J. E. The uniformity of application of water by sprinkler systems. Agricultural Engineering 22: 89-92 (1941). Morin, J., et al., A rainfall simulator with rotating disc. Trans. A.S.A.E. 10: 1, 74-77, 79. (1967).
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Exercise B Objective To obtain the drop size and distribution of simulated rain and to investigate the effect of pressure and aperture size on the distribution.
Equipment Set Up The simulator is set up for normal laboratory use, with or without the test table in position. Additional Equipment: Sheets of 3mm filter paper (Whatman No 1); methyl blue crystals or B. H. methylene blue; backing board to suit size of paper used.
Theory Natural rainfall consists of drops which range from very small to approximately 6mm diameter. The energy of the rainstorm is the sum of the energies of single drops which is a function of their velocity and impact velocity. To assess the erosivity of rainfall the size and relative number of drops of any size must be known. A common method of assessing drop size distribution is to measure the diameters of marks made on sheets of absorbent paper previously coated with a dye such as methylene blue. These marks are then compared with a calibration curve which is obtained by dropping water drops of known diameter, from a known height, onto similarly prepared filter paper. This calibration should be made for all tests but to facilitate the experiment the curve may be used with 3mm thick filter paper.
The curve is of the general form D = k.dm where D = stain diameter d = drop diameter 20
Exercise B k, m = empirical constants Note: Scales are log-log.
Initial values of variables to be used Pressure Gauge = 0.4 bar Aperture
= 10°
Disc Speed
= 70 rev/min
Readings to be taken With the simulator operating under steady state conditions at selected pressure and disc aperture, sheets of prepared filter paper impregnated with dye are exposed to the rainfall for sufficient time to form clear marks with minimum smudging and minimum overlapping of drops. Some practice will be needed before the best exposure time is found. The number and sizes of the marks left on the filter paper are recorded and compared with the calibration to give the drop size distribution. It is normal to group the drops into diameter classes, eg. 0.75mm to 1.0mm, and to count the total number of drops in each class. Class divisions of 0.5mm or 1.0mm are recommended. The test should be repeated several times for several positions relative to the centre point of the nozzle. The height of sampling should be such as to enable a true sample of drops to be made. The influence of pressure drop size distribution can be investigated at pressures of 0.2, 0.3 and 0.5 bar for similar aperture and speed. For a constant pressure of 0.3 bar assess the drop size distribution for apertures of 5° and 30°.
Results Make a table of results similar to that below:
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Armfield Instruction Manual
Plot results as graphs and compare with those of natural rainfall. What effect does increasing pressure have on drop size distribution?
References Hall, M.J. Critique of methods of simulating rainfall. Water Resources Research. 6 (4); 1104.
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Exercise C Objective To determine the kinetic energy of simulated rainfall at various rainfall intensities.
Equipment Set Up The simulator is set up for normal laboratory use. Facilities to measure intensity as in Test 1 and drop size distribution as in Test 2 are required unless test results are already available.
Theory Erosivity is defined as the potential ability of rain to cause erosion. Kinetic energy and intensity are the major contributing factors to the erosivity of rainfall. Intensity is an easily measured parameter and is available from many weather stations. Many attempts to predict kinetic energy from measurements of intensity have therefore been made. One such equation is: K.E. = 11.87 + 8.73 log I where I = Intensity (mm/hr) However, kinetic energy is best computed for a storm by adding the energies of the individual drops as calculated from:
where m = mass (kg) v = impact velocity (m/s) In these calculations, impact velocity should be used. Where information is limited or when velocities are thought to approach terminal velocity, the values of Gunn and Kinzer (1949) may be used. The graphs overleaf show impact velocities for a similar simulator to the FEL3 for two nozzle sizes, compared with terminal velocities of Gunn and Kinzer.
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Armfield Instruction Manual
Initial values of variables to be used Nozzle 1 HH 12 should be fitted (11/2 H 30 if available) Pressure
= 0.4 bar
Disc Speed = 70 rev/min Aperture
= 20°
Readings to be taken Measure average intensity as in Test 1. Determine drop size distribution as in Test 2.
Results Construct a table of results with headings as below: AVERAGE INTENSITY =
mm/hr
Calculate the kinetic energy for each class and sum to get total energy in joules. 24
Exercise C Energy of rainstorm is usually expressed as J/m2/mm, or the kinetic energy per unit area per mm of rain applied.
Convert the kinetic energy to this form and plot a graph of this against intensity. Compare the values obtained for kinetic energy with those predicted by the equation: E = 11.87 + 8.73 log I where I = Intensity, mm/hr E = Energy J/m2/mm/s
References Gunn, R. and Kinzer, G.D., The terminal velocity of fall for water droplets. Journal of Meteorology. 6; 243-248 (1949). Morin, J. et al., A rainfall simulator with rotating disc. Trans A.S.A.E. 10:1; 74-77, 79 (1967). Wischmeier, W.H. and Smith, D.D., Rainfall energy and its relation to soil loss. Trans. American Geophysical Union 39; 285-291 (1958).
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Exercise D Objective To obtain a relationship between splash erosion and intensity of simulated rainfall.
Equipment Set Up The simulator should be set up for laboratory use with the test table in a horizontal position. Splash cups are placed in several positions relative to the centre of the nozzle spray. Rain gauges or other suitable containers are placed near to each splash cup.
Theory In order to determine the erosivity of rainfall, quantitative measurements of actual erosion, under fixed conditions of soil erodability and rainfall energy, are required. A simple and precise assessment of splash erosion can be obtained using splash cups similar to those used by Ellison (1947). The weight of sand splashed from the cup in a given period can be related to the intensity or the kinetic energy of the rainfall. The splash cups provided are 75mm diameter brass cylinders with gauze soldered into the bottom to allow water to drain through the fine sand which is placed upon a layer of cotton wool in the cylinders. The sand must pass a 60 gauge mesh but be held by a 70 gauge. After levelling the sand flush with the top of the cup, the splash cup is weighed. The splash cups are placed in shallow water until the sand is saturated and exposed to the rainfall. After exposure and oven drying the cups are re-weighed and the sand loss calculated. Sources of error due to edge effects and initial high rates of wash-off from the splash cups make the use of a correction factor essential. The graph on the following page is from Hudson (1965) and combines the edge effect Bisal (1950) correction with a correction for wash-off. Further details of the technique are contained in Kinnell (1974).
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Exercise D
Note: Log-Log scale. For accurate use re-plot on log-log paper using co-ordinates given.
Initial values of variables to be used Pressure
= 0.4 bar
Disc Speed = 70 rev/min Aperture
= 10°
(or other combinations as required)
Results Place the splash cups with carefully prepared sand after weighing, in the desired position on the test table, locate the rain gauges to suit. Cover with a board and when steady state rainfall is attained, uncover for a known time. Re-cover with board and stop simulator. Remove the splash cups and oven dry; record the weights. Remove rain gauges and measure contents with measuring cylinder. Repeat the test for apertures of 15°, 20°, 25° and 30°. Repeat test at pressures of 0.3 bar and 0.6 bar. Make a table of results similar to that below and plot sand against intensity for the three pressures.
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Armfield Instruction Manual
References Bisal, F. Calibration of splash cup for soil erosion studies. Agric. Engineering 31:621 (1950) Ellison W.D. Soil erosion studies - pts I-VII - Agric. Eng. Mich. 28 (1947) Hudson N.W. The influence of rainfall on the mechanics of soil erosion with particular reference to S. Rhodesia. - Unpublished MSc thesis, Univ. Cape Town (1965). Kinnell, P.I.A. Splash erosion - some observations on the splash cup technique - Soil Sci.Soc.Am. 38 (1974).
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Exercise E Objective To measure soil loss due to wash-off from laboratory soil samples under simulated rainfall. To obtain a relationship between soil loss and intensity of rainfall.
Equipment Set Up The simulator is set up for laboratory use and the test table is positioned beneath the spray. Wash-off trays are placed on the table as shown below. Measurement of intensity near the tray as in test 4 is also necessary.
Theory Soil erosion can take place by splash action or by wash-off. Splash erosion is related to the impact energy of raindrops and wash-off is a function of overland flow velocity, slope and slope length. Quantitative measurements of wash-off erosion can be assessed with simulated rain under controlled conditions of rainfall erosivity, slope and soil condition. Soil must be carefully prepared for testing to obtain good results. The method recommended is that of Moldenhauer (1965). Soil of the desired texture is passed through an 8mm sieve and placed in 200gm layers into the tray with no compaction. Light levelling with a wooden board completes the treatment. Soil loss is measured by collecting the run-off from the test plot, carefully pouring off the water after settling, and drying the soil collected in an oven at 105°C for 24 hours before weighing it.
Initial values of variables to be used Pressure
= 0.4 bar
Disc Speed
= 100 rev/min
Aperture
= 10°
Slope of test table = 5% (or other combination to give desired intensity range)
Readings to be taken After preparation of soil trays and positioning as shown, expose to rainfall for a period of 5-10 mins. Collect wash-off, oven dry soil and weigh. 29
Armfield Instruction Manual Repeat the test for aperture angles of 15°, 20°, 25° and 30°. Repeat the test for pressures of 0.3 bar and 0.6 bar.
Results Present the results in a table similar to that below:
Plot a graph of soil loss against intensity for the three pressures.
Students may wish to compare soil loss from plots of different soils for similar intensity and drop size distributions. Comparisons between clay, sandy clay and sand should be made. Students may also wish to investigate the effect of slope of the test table on soil loss for one soil.
References Ellison, W.D. Soil erosion studies, Pts I-VII Agric. Eng. 28: (1947). Moldenhauer, W.C. Procedure for studying soil characteristics using disturbed samples and simulated rainfall. Trans. A.S.A.E. 8 (1) (1965). Morin, J. et al. A rainfall simulator with a rotating disc. Trans A.S.A.E. 10 (1) (1967).
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Contact Details for Further Information Main Office:
Armfield Limited Bridge House West Street Ringwood Hampshire England BH24 1DY Tel: +44 (0)1425 478781 Fax: +44 (0)1425 470916 Email:
[email protected] [email protected] Web: http://www.armfield.co.uk
US Office:
Armfield Inc. 436 West Commodore Blvd (#2) Jackson, NJ 08527 Tel: (732) 928 3332 Fax: (732) 928 3542 Email:
[email protected]
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