MAPA1 HI DROLOGI CAL STUDY ( LIMPOPO ' S RI VER)

EMILIO EUGENIO D'OLIVEIRA MERTENS

JOÃO JOSE MIMOSO LOUREIRO

ABSTRACT

Lack of observations in the flow discharges and runoff, taken at the future location of mapai's dam, have compelled us to the essaying of diversed methodology viewing its obtention.

technic which has conducted us t o most consistant and significant results in conjunction with those observed and calculated for other locations at the catchment area.

It was selected the method of the specifica1 runoff

RESUME

L'abscence des observations relatives aux débits et écoulements measurables au futur lieu du barrage du mapai, nous a forcé d'essayer diverse méthodologie pour en obtenir.

I 1 a ;te choisie la method: de la technique dés débits specifiques que nous a conduit a des resultats tres concor- dants et significatives en conjunction avec ceux observés et calculés pour les autres lieu du bassin versant.

142

1 - CATCHMENT AREA 1.1 - Site, area, relief and hydrography:

The hidrographic basin of the Limpopo River has its major part in the territories of South Africa, Rhodesia and Botswana, its area of 412 O00 km2 being devided in the following manner (Drawing 1):

South African Republic .................... 193 500 km2 Rhodesia .................................. 66 O00 km2 Botswana .................................. 73 O00 km2 Mozambique ................................ 79 500 km2

Rounded off, the catchment area is situated beyween 220 and 260 South and 269 and 350 East, its highest altitude being 2.300 metres near the city of Lydenburg.

In National territory, situated between parallels 210 and 250 South and meridians 310 and 359 East, the basin has to the North, that of the River Save, to the South, that of the River Incomati and to the East, that of the River Govuro, and a coastal strip where a few closed catchment areas are found from which the water-sources accumulate in lakes.

In Mozambique there is no noticeable irregularity, this occurring only in the limiting zone to the south of the Limpopo, in a reduced area with eleva- tions of 400 metres.

In its total length the average height is of 840 metres, its being 977, 964 and 950 metres, respectively in Beitbridge, Mapai and Trigo de Morais.

The average slopes of the course of the water are:

Upper stream ....................................... 2, 50 dlan Central stream ..................................... 1,80 m/km Lower stream ....................................... 0,Og m/km

The Limpopo is one of the most important rivers of South Africa and Mozambique and, as happens with the Incomati River, it is contained in the lower part of the great drainage area, which includes more than half of the Transvaal and a considerable part of South Rhodesia.

./.

143

The Limpopo is a strange river, very changeable and capricious, perhaps its vol5 due to the influence of the dissimilarity of its hydrographical basin;

me of water is extremely variable as in dry weather it is very reduced and during the rainy season, reaches heights of 7 metres which flood large areas of ground in the central and lower courses. The Limpopo River, when it enters our terri- tory, has already a definite bed, where it has three large tributaires: on the right bank, the Elephants River, and on the left bank, the Nuanetzi and the Changane; it is to these that it owes its permanent volume of water for the flow from those joining it, its principal supplier being the Elephants River, a water-source which crosses a region of high rains, its hydrographic basin having a somewhat impermeable geological configuration.

It belongs to the hydrographical system of the African Continent and it is of the torrential rate of permanent volume.

The course of the water which takes the name of Limpopo River, is formed by the junction of the Marico and Crocodile Rivers which have their sources at an altitude of 1.500 metres to the west of the city of Pretoria.

The principal tributaries of the right bank, all with their sources in the Transvaal, from the source to the mouth of the Limpopo River are as follow: River Matablas , Pongola, Palala, Sand, Pafuri (flowing in close to Pafuri, already in Portuguese territory) and the Elephants River, the largest and most important which joins it within Mozambique after some 110 kms. On its left bank, the Limpopo receives large courses of water all with their sources in Rhodesia, the principal ones being:

River Notwani, Macloutsie, Tuli, Umtzingwane, Bubye, Nuanetzi (which has already flown about 50 kms in Mozambique) and the River Changane.

1.2 - Geological Aspect, Soils and Vegetation: In the Limpopo basin, formations are found which belong to different

systems, such as Karroo, Waterberg, Primitive System. The basin in South African and Rhodesian territory seems to be consti-

tuted of basaltic lava, Serie Ecca, siliceous detrital rocks (sandstone) of brown red and purple colours, formations of conglomerates, graphite and gneiss.

In Mozambique, the basin is mainly constituted of sedimentary formations. In a narrow area near the border, volcanic rocks are found, in the upper

course of the Limpopo River and Elephants River formations of the Cretaceous Era, in the rest, Quaternary formations with alluvium, sandstone, calcarium and sand deposits.

./.

144

The vegetation in foreign territory is mainly constituted of bush and

grass, of great density in the highlands, and mixed bush and grass plains. In Mozambique, the vegetation is of the bushy type and plains with some trees, level grass plains and large stretches of grassy land.

The predominant soils in our territory are: sandy in the coastal area, salty in the river vales, soils of mananga in the lower Changane and conglome- rates.

1.3 - Climate: In respect to the area situated in Mozambique, it appears that the

average annual temperatures are practically the same in almost all the basin, being 240 C with the exception of the north eastern side, where it goes as low as 220 C.

On the coastal and north-eastern areas, the average maximum daily tem- peratures are 300 and 320. C and in the central area 34Q.C.

The average temperature in the hottest month is 280 C. and the lowest 260.c., the annual variation of these averages being between 60 and 9Q.C.

The average temperature in the coldest month is 20% in the central area, and 18% in thê rest, while the coastal area has an average minimum in the coldest month of 12%.

The annual average relative humidity in the central area is 65%, increa2 ing to the north and south to reach the highest rate of 75%.

According to the classification of Koppen, the climate of the basin is in general the dryness of steppes with a dry season in winter, dryness of the desert in the area of Pafuri, dryness of the steppes in the south of the basin, and in the coastal area, tropical raininess of a savanna.

The predominant winds in the months of September to February are those from the East and, during the other months, almost entirely with predominance from the West.

In the whole basin, one finds that it is situated between the isothermics of 240 and 170 with the average temperatures of 200, 2003 and 2002 respectively for the areas of Beitbridge, Mapai and Trigo de Morais.

1.4 - Hydrological Occupation: Both the South African Republic and Rhodesia have a network of udometric

and hydrometric stations which, for the African Continent, can be considered dense: one pluviometer for 200 km2 and one hydrometric station for 4 o00 h2.

145

There are readings from 31 Rhodesian udometric stations and from 90 South African posts, the majority of which with more than 30 years of existence.

The more significant hydrometric stations in Rhodesia and South African not only for the area they cover and their locality, but for the extension of their records, are:

Rhodes ia :

South Africa:

- River Tuli - 4 144 km2 - Unzimgwane River - 2 533 km2 - Bubye River - 8 029 lan2

A3 MO7 - Eerste Poor - Groot Marico Rivier A2 M25 - Hardekool Bulti - Crocodile River A5 MO2 - Vischgat - Palala River A5 MO3 - Oxenham Ranch - Limpopo River A7 MO4 - Beitbridge - Limpopo River A7 MO3 - Zamenkomst - Sand River A9 MO1 - Schuinshoogte - Luvuhu River

- Liverpool - Olifants Rivier - (354) - Olifants Rivier - Manorvlei - Letaba River - Letaba Ranch - LetkaRiver - Driehoek - Blyde River

- 8 588 h 2 - 21 i09 lan2 - 2 341 h 2 - 97 850 km2 -180 O00 h 2 - 6 900 km2 - 912 lan2 - 42 352 km2 - 27 928 km2 - 668 km2 - 4 716 h 2 - 2 199 km2

In Mozambique, there are 33 udometric stations, some with a significant period of regular readings, and 12 stations for the measurement of water volu- me, some equipped with linmographs, the most significant of those with regular readings being those of Maçuço (66 O00 km2) and Tiobine (68 450 km2) on the Elephants River; Vila Trigo de Morais (340 O00 km2), Pafuri (235 930 km2), Mapai (246 O00 km2), Mohambe (342 780 h2), João Belo (407 970 km2) on the Limpopo River, and Chibuto (43 200 km2) on the Changane River.

As regards evaporation, there are 6 U.S. Class A Tina evaporemeters in Rhodesia, 20 Standard Symons in South Africa, and 5 U.S. Class A Tina evapore- meters and 7 Piche atmometers in Mozambique.

./.

146

Also in the Portuguese part of the basin are 3 lysimetric stations, 3 cli matological stations, 2 agronomic-climatological posts and 4 climatological posts.

2 - RAINS 2.1 - Introduction:

From the analysis of the normal isohyetal map, it is noted that the basin is within the ishoyetal extremes of 400 and 1.500 mm., the monthly distribution of rainfall being divided in a deficient way throughout the year,with a concentrg tion of about 85% of the total during the months from October to March inclusive.

To determine the average annual rainfall in the Limpopo basin as far as Mapai, three methods were used:- of the rainy districts; of the area of influes ce and the isohyetal figures.

2.2 - The method of the rainy districts The South African Republic is divided into restricted zones by rains of

average equality, certain rainy districts under the same principle also dividing the areas of Rhodesia.

In accordance with the udometric records for the period 1914/15 to 1963/ /64 (50 years), the average annual rainfall figures were determined in respect to the basin, taking into account the fall in each district.

From the period of 50 years, the average annual rainfall arrived at was

579 mm, its having been 582, 566 and 560 mm respectively during the past 10, 15 and 25 years.

2.3 - Method of Area of Influence In accordance with the records existing for the period 1954/55 to 1963/

/64 (10 years) and using the 71 udometric posts, the average annual rainfall was determined, the figure obtained being 514 mm.

2.4 - Isohyetal Method With the normal figures - 30 years - recorded at the udometric posts,.

isohyetal curves were interpolated, the areas between the adjacent isohyetal figures being thereafter measured.

The average figure for the rainfall in the basin thus obtained was 504m.

147

2.5 - Correlation of the methods: For the common period of 1954/55 to 1963/64 and 1955/56 to 1963/64 of

which the average annual rainfall figures are available, the correlation between the two methods was determined, the following equations having been obtained:

1954/55 to 1963/64 - x = 0,94y + 99 1955/56 to 1963/64 - x = 1,02y + 62

which give a lineal relation between them, the rates of the correlation being very significant (0,98 and 0,96) the figures arrived at showing only small dif- f erences .

2.6 - Resumé: ANNUAL RAINFfiLL

Averages :

Period of 50 years ........... 579 mm Period of 25 years ........... 560 mm Period of 15 years ........... 566 mm Period of 10 years ........... 582 mm Rainiest year ................ Rainiest year w/lOO year occ- rence ........................ 1 101 mm Driest year .................. Driest year w/lOO year OCCUT- rence ........................ 290 mm

971 mm (1924/25)

355 mm (1963/64)

3 - RUNOFF 3.1 - Elementary Principles:

In view of there not being any measurements of the volume of the Limpopo River in Portuguese territory, it was necessary to resort to comparative studies, taking as a basis the specific flowage in the various hydrometric stations, existing upstream in the region of Mapai.

./*

148

3.2 - Details of the Study Barrows, in his book "Water Power Ehgeneering" stated that "it is often

possible to consider, without serious error, that the specific volume of a river is similar to the successive contours along the same river". also advises that, whenever possible, comparisons and corrections should be esta blished, not only in respect to the rainfall, but also to the altitude, slopes, constitution and the rock formations of the soil.

The same author

Based on the measurements made at the Hydrometric stations of the Repu- blic of South Africa and Rhodesia which cover 195 841 km2, 79,6% of the respective basin in the area of the barrage of Mapai, we took into account the specific run- off year by year and we calculated the specific runoff of the locality under study.

Since 1963, efforts have been made to estimate the average annual runoff always using the methods of the specific volume but adopting various criteria.

The average annual figure obtained for the period of 12 years was 3 095 million cubic metres, which corresponds to the specific runoff of 12 580 m3 k2 - 1 (chart attached).

Thus we have: 3 13 700 m (k2)-1

13 658 m3 (k2)-1 Study in 1963 ( 6 years) specific runoff Study in 1965 ( 7 years) specific runoff Present study (12 years) specific runoff 12 583 m 3 (k2)-1

If we observe the sequence of the years in which measurements existed for the 3 studies realised, it will be noted that the period of 12 years includes an excessively dry year (1963-1964) and one high runoff (1966-67) while there were no measurements taken in 1965-66 at the fundamental station of study (Beit- bridge) due to the hydrometric station having been under water (floods in Februa- ry, 1966) and as a result of which, the figure now obtained is necessarily defec- tive.

The 1970 study covering a period of 15 years, places the specific average 3 2 runoff as 13 O00 m (k )-l.

For the 246 O00 km2 of the river basin, the figures obtained for the average annual runoff in the region of Mapai are respectively as follow:

1963 study 3 1965 study 3

.............. 3 370 million m

.............. 3 360 million m 3 3

1970 study .............. 3 200 million m Present study ........... 3 095 million m

./.

149

Any of these figures fall within the admissible limits based on the rate of the runoff observed at the stations of Beitbridge and Vila Trigo de Morais, the average of which is respectively 0,015 and 0,026.

Thus for a figure of 3 O00 million m3 and for the average rainfall of 579 nun., runoff coefficient is 0,021, which is within the observed limits.

Using the method of Coutagne only for the average annual figures, for

6 3 a rainfall of 579 mm as the average over 50 years, we arrive at a runoff of 2 969 10 of 2 999,7 10 m .

m and for 582 nun as the average for the past 10 years, the figure 6 3

Observing and trying all these ways and means, we shall adopt chart attached hereto for the annual runoff because, as they arise from direct mea- surements, they fall within all the estimated figures.

3.3 - Monthly Distribution The monthly distribution is based on a hydrometric station in the Repu-

blic of South Africa (Beitbridge), which already has years of sufficient read-

ings, its area being much like that of Mapai. Thus we haïe:

October ................ 0,5% November ................ 0,6% December ................ 4,6% January ................ 25,% February ................ 33,8% March ................ i8,i% April ................ 8,s May ................ 4,3% June ................ i,% July ................ 1,1% August ................ o, 9% September ................ 0,3% 10% -

4 - FLOODS The River Limpopo is typically torrential and as such, not only dries

during consecutive months as it is susceptible to exceptional floods. Many records of the volume of floods have been compiled in the Republic

of South Africa since 1915, always based on specific volumes and they obtained

. /.

150

measured details which extended to the area of Mapai, gave us the figures of

16 925 m3/s (1933) and 12 792 m3/s (1966). Thus, using various formulas, one can estimate the volume of floods for

return periods of 100 and 200 years.

100 years 200 years

Water affairs formula ............ 13 243 14 963 m3/s ........ 18 375 II Mimoso Loureiro formula 14 304

Fuller formula ................. 19 763 21 284 11

(Period 34 years and specific

Larivaille formula ............... - 15 375 'I

volumes measured)

The estimate is thus very difficult and depends a great deal on the type of barrage to be adopted.

The hydrograph of a maximum flood was also determined, based on the formula of Giandotti in the calculation of the times of concentration of the

peak and of the swell of the flood, and on the hydrographs of the floods record-

ed at the border (Pafuri) during the years 1955, 1958, 1959, 1966 and 1967. It was verified that, in the flood of 1966, there was agreement in the

calculated and observed times, because the calculation placed the figure at 153 hours and from observation, at 150 hours for the time of concentration, there being, however, a difference in the time of the swell of 600 with 724 hours (sketch attached).

5 - Evaporation and Solid flows

Using the details measured in Rhodesia, South Africa and Mozambique, we can place the resulting evaporation at Mapi as 1 344 mm. mes, the figures are few and vary greatly; med by the complex composition of the hydrographic basin, as for the same volu - me one obtain 21,8 kg/s and 97,l kg/s, without any meaning.

As to solid volu- as for the rest, definitely, confix

151

o n 0

N

N

1 I I

W s u o

a r

2

n

U

-I 4 3

z

z 4