-r 19. Where E «R - the soil loss in t o n s/hectare El. 30 an er o s i v i t y index Too E * Kinetic Energy of rainfall K L S C P *30 * Rainfall intensity of a 30 min u t e storm e r o d i b i l i t y index Slope length slope gradient in per cent c rop p i n g m a n a g e m e n t factor e r o s i o n control factor The equa t i o n has been d e v e l o p e d by soil c o n s e r v a t i o n i s t s purely through o b s e r v a t i o n a l data, regre s s i o n and statistical analy s i s cf / <ts over ma n y years and the equa t i o n has been d e v e l o p e d and m o d i f i e d a c c ording to the area in question, w h e t h e r it be temperate, tropical or s u b - t r o p i c a l. The e q u a t i o n takes into a ccount grain size, bulk d e n s i t y of material, slope length, slope angle, p e r m e a b i l i t y and, mo s t importantly, the rainfall c h a r a cteristics. n trying to a n alyse the e r o s i o n of slimes it b e c a m e n e c e s s a r y to m o d i f y exis t i n g p a r a m e t e r s suitable to the p a r t i c u l a r area and conditions. (9) W i s c h m e i e r and Smith c a r r i e d out the most s i g n i ficant wo r k on rainfall based on 22m plots on a 5 deg r e e slope t hroughout A m e r i c a and d e v e l o p e d a kinetic energy e q u a t i o n b ased on rainfall intensity. KE «1,213 + 0, 8 9 0 l o g lq w h e r e KE * k inetic e n e r g y n ^9 m / m and * rainfall intensity in mm/hr
20. Hudsonobsei 'ed KF, = 29,8 - ^ J for tropical rainfall in Zimbabwe. The Kinetic Energy is based on the summation of intensities of rainfall for d i f f e r e n t storm times. However the reasurenent and re c o r d i n g of the storms s bevond the scope oi this report and it is hoped to show that a p rediction can be mad e by back analysis of the results obtained for the e r o s i t i v i t y index R. K is d e s cribed as the e r o d i b i l i t y index and assumes that d i f ferent soils erode at d i f ferent rates based on the soil grading analysis, p e r m e a b i l i t y and organic content. The no m o g r a p h shown in table 4,8 had been dev e l o p e d to cover d i f f e r i n g types of soils in A m e r i c a and man y locations w orld wid e and F c o n s i d e r e d a valid nomograph for g eneral soil erodibility. LENGTH A N D G R A D E N T The effect of slope length and g r a d i e n t are rep r e s e n t e d by L and S but are often e v a l u a t e d toge t h e r as LS. L is d e f i n e d as the d i s t a n c e of the point of origin of the flow of w a t e r to the point of d e p o s i t i o n and slope g rad i e n t is ex p r e s s e d as a percentage. W i s c h m e i e r and Smith d e fined the factor, LS - ( x )m (0,065 + 0.04 5 S + 0, 0 0 6 5 S 2 ) T T T H w here x is the slope length and m v a r i e s and is d e p e n d e n t on slope a n g l e but is equal to 0,5 for slopes g r e a t e r than 5 per cent. M o r g a n v 1 ^ r e f i n e d this to mor e m a n a g e a b l e terms.
21. Lf (0,136 +,097 S + 0, 0 1 3 9 s 2 ) '. h e equa t i o n has been d e v e l o p e d for angles of up to 25 t h e refore it is with c a u t i o n that this equa t i o n mu s t be useu for angles g r e a t e r than this as no p rev i o u s research has indicated the equat i o n valid above 25. t is intended to show that the e q u a t i o n is valid to an angle of 35 there a f t e r other factors should be taken into account in the prediction. W i s c h m e i e r and Mayer warned that beyond some critical steepness the formula would prob a b l y o v e r p r e d i c t soil loss. C AND P The crop p i n g m a n a g e m e n t factor C and e r osion control factor P represent the soil loss from a spec i f i c cover c o n d i t i o n eg. c r o p s or g r a s s and the soij. loss due to bpecific pr a c t i c e s such as c o n t o u r i n g or furrowing. The factors do not apply to the case of slimes d a m slopes except pe r h a p s for b e n c M n ^ of slimes but the factors have been taken to be for each slimes dam. The literature survey of articles from 1961 onwards tended to r e i terate the views of e a r l i e r articles or were found not to be d i r e c t l y relevant to the pr o b l e m addressed.,t>
3.0 F ELD W<" 3.1 NTRODUCTON The theory behind the d e v e l o p m e n t of E T C O M was that as the flow of w a t e r ove r the s u rface has a shea r i n g effect on the s u rface particles, the shearing effect should be a m e a s u r a b l e item and is d i r e c t l y related to the rate of loss of soil from the slopes. f this shearing effect c ould be s i m ulated and m e a s u r e d then a r e l a t i o n s h i p could be est a b l i s h e d between E T C O M reading and rate of soil loss. 3.2 E T C O M Fig. 3.0 shows a d i a q r a m of the layout of E T C O M and its a p p urtenances. Th e y c o n s i s t of : la) 2500 kpa, A porta b l e pump wi t h p r e s s u r e g a u g e reading to (b) A back p a c k with a c cumulator, and e l e c t r o n i c pump regulator. W a t e r is pum p e d into the a c c u m u l a t o r to a pres s u r e of 2000 kpa. (c) Low and hig h p r e s s u r e g a u g e s on a d j u s t a b l e tripod. (d) Fixed tripod with 0, 8 m m nozzle c l a m p e d 25,4mm above surface.
i 2 3. 1 UJ o <z> o o o Cl o d H UJ ( / ) UJ z X UJ CD < X o cr Ui s U H Z O (/> O X UJ 3 o o g 5 5 < Xo < O ro O Ll
24. The jets had been calib r a t e d before t aken out into the field. Wa t e r ii_ pumped into the acc u m u l a t o r vessel on the backp a c k which is then c o n n e c t e d to the press u r e gaug e s on the tripod by means of rubber tubing. The tripod and b a c k p a c k is then t r a n sported to some position on the slope w h e r e several readings are taken at peak values i.e. the values at w hich the return of the jet is vertical. Readings were taken in and around the control plots, in w i n t e r and summer. For this reason win t e r is de f i n e d as running from A pril to Se p t e m b e r inclusive and and summer from October to M a r c h inclusive. The p e r i o d over which readings we r e taken was from the b e g i n n i n g of April 1984 to the end of M a r c h 1985. A p p e n d i x B shows the results of frequency intervals of 25 kpa versus the number of observations. 3.3 T A K N G E T C 0 M READ N G S (a) Whe n the c o h e s i v e v a l u e of the m a t e r i a l is low, a hole is p e n e t r a t e d w h i c h c h a n g e s the d i r e c t i o n of the return of w ater on the s u rface of the slimes from a sideways refle c t i o n to a near vert i c a l one, thus
25. (b) As the cohes i v e value increases, the surface particles are seen to become d i s l o d g e d w ithout the m a r k e d d r i l l i n g of a hole as in (a) and the d i r ection of the return of water lifts at a more g e n t l e angle than p r e v i o u s l y shown (c) Wh e n the c o h e s i v e v alue is above the m a x i m u m d i s l o d g i n g force of E T C 0 M (400 k P a ), the return of w a t e r hugs the surface of the slimes
3.4 F ELD M E A S U R E M E N T S 3.4.1 T H E PLOTS A series of t*n plots were chosen on seven slimes dams each m e a s u r i n g 9m by 9m (see Fig. 3.1 and 3.2 for location) w i t h i n the J o h a n n e s b u r g / Germistor. area. T h e plots we r e cho s e n for d i f f e r i n g aspect, slope length, and angle and m e t h o d of d e p o s i t i o n (eg. be nched or uniform s l o p e ). m m d i a m e t e r mil d steel pegs, one metre in length were d r i v e n normal to the slope at three m e t r e intervals g i v i n g a 4 x 4 peg plot of 16 pegs in total. 250mm of each peg, a c c u r a t e l y measured, was left proud of the ground. The pos i t i o n i n g of the plot is critical a s s u m i n g an averaging of results, so p lots were p o s i t i o n e d a p p r o x i m a t e l y in the midd l e of the slope length. t is assumed that e r o s i o n is least at the top of the slope and most at the bottom. T h e r e f o r e an a v e raging v a l u e should be o b t a i n e d at the centre of the slope. S i m i l a r l y pegs w e r e not always placed in the s.r.all g u ileys or rills that occur on the slopes, as it is pres u m e d that some e r o s i o n occurs o u t s i d e of these areas. Rai n gaug e s were intially set up on five of the seven slimes dam s but d u e to c o n s i s t e n t vandalism, only two, w i t h the e x c e p t i o n of one m o n t h on dam 4L37, were found to be re c o r d i n g throu g h o u t the w h o l e of the period. The two wer e on dams 3L37 (Meyer and Charlton) and 4 L 3 7, t was felt that due to tie fairly close p r o x i m i t y of the plots that the two gau g e s as well as readings rece i v e d from the M e t r o l o g i c a l O f f i c e at Rand A i r p o r t in G e r m i s t o n w o u l d suffice.
29. The readings for each rain gauge are shown below (with winter readings given for the period April to S e p t e m b e r ). Rainfall mm Data Source Wint e r Summer Tota 1 4L37 47 386 433 3L39 54 397 451 Rand A i r p o r t 53 545 598 X 51 443 494 T able 3.0 Rain G a u g e Readings - April 1984 - M arch 1985 n J a n u a r y the rain gau a g e on 4L37 was v a n d a l i s e d and replaced. An a v erage reading of the Rand Airport, 31.39 and two other nearby stations were taken and this is shown on the g r a p h in Fig. 3.3 for the year. A ppe n d i x 3 shows rainfall s tatistics for the years 1979 to 1984 inclusive as well as the 25 year and 30 year average at J a n Smuts Airport. T h e rainfall for the period recorded at Rand A i r p o r t is 598mm. No long term data exis t s at this station since dita has only been reco r d e d for the past three years. H ence the 30 year a v erage of 726mm and the average for y e a r s 1979 to 1984 of 644mm at J a n Smuts s recorded. The p e r c e n t a g e c o m p a r i s o n of Rand A i r p o r t to J a n Smuts for the 30 year and year a v e r a g e is 82% and 93% respectively. T h e r e f o r e the rainfall for the test p e r i o d a l t h o u g h lower does not appear to be abnormal.
2' 3to, J Ui > C/> Z3 -J o z z u cc < e E E E E E CD CM 00 m m (T> * * m cc CL < a. Ll U J a UJ (ft o cr r- ro a: o a: Ui <i <T> fo -J K>! - t - o * o (V ww Nl HVdNVH
Readings at the pegs w e r e taken throughout the year and again these wer e split into w i n t e r and summer readings. Due to v a n d a l i s m of one site a comp l e t e new plot had to be set up els e w h e r e but on the other plots r elatively few pegs d i s a p p e a r e d (3 in total). An analysis of the results was c arried out. The readings were placed in o b s e r v a t i o n intervals of 5mm and their frequencies plotted as well as a v e raging the results and d e t e r m i n i n g the standard deviation. The readings were als o split into summer and w i n t e r readings. The a v e r a g e is believed to be a true re p r e s e n t a t i o n of the rate of erosion. As p r e v i o u s l y di s c u s s e d erosion rate will be lower ou t s i d e of the small gulleys so the r e p r e s e n t a t i o n of an average is a more realistic view. A bulk and dry d e nsity was d e t e r m i n e d from the samples taken and the rate of e r osion of dry residue c a l c u l a t e d in tons per hectare. 3.4.2 S A M P L N G Samples for grading, h y d r o m e t e r analyses and m o i s t u r e content were obtained. M o i s t u r e content samples from the top 50mm wer e taken at the end of the summer (April 1981 and May 1985). Very little d i f f e r e n c e was e x p e r i e n c e d in the m o i s t u r e cont e n t s over the year. All of w hich w e r e b e l o w 20%. A p p e n d i x A shows the results of the g r ading and h y d r o m e t e r a n a l y s i s for each of the six slimes dams. T a b l e 3.1 b e l o w shows the m o i s t u r e co n t e n t v a r i a n c e t h roughout the year.
3L39 4L34 Balmoral 4L37 4L36 4L24 4L40 April 84-9,8 18,4 5,5 7," 4,6 May 85 14,2 9,8 h 5 o 7,6 7,8 12,9 7,8 Table 3.1 % Mois t u r e C o ntent at the Surface of Slimes 3L39 was not sampled in 1984 and it is a s sumed that there is little d i f f e r e n c e in m o i s t u r e c o ntent throu g h o u t the year. 3.4.3 SHEAR V A N E READ N G S The shear strength of the surface of the slimes was de t e r m i n e d using the 'TORVANE' shown in Fig. 3.11, a m i n i a t u r e shear van e with a vane height of 5mm. A m i n i m u m of six readings were obta i n e d per plot on each si.,c;s dam. Pre - s o a k i n g of the area was ca r r i e d out before the shear v a n e test to el i m i n a t e c a p i l l a r y stresses that w o u l d have shown an u n r e a l i s t i c a l l y high shear strength. The results are shown in table 3.2.. N, E, S and W refer to th aspect of the slope.
Author Dorren Douglas an Name of thesis Erosion Of The Slopes Of Gold-residue Dams On The Transvaal Highveld. 1986 PUBLSHER: University of the Witwatersrand, Johannesburg 2013 LEGAL NOTCES: Copyright Notice: All materials on the University of the Witwatersrand, Johannesburg Library website are protected by South African copyright law and may not be distributed, transmitted, displayed, or otherwise published in any format, without the prior written permission of the copyright owner. Disclaimer and Terms of Use: Provided that you maintain all copyright and other notices contained therein, you may download material (one machine readable copy and one print copy per page) for your personal and/or educational non-commercial use only. The University of the Witwatersrand, Johannesburg, is not responsible for any errors or omissions and excludes any and all liability for any errors in or omissions from the information on the Library website.