Hydrological studies of a tropical reservoir site, Nigeria

Journal of Afrwan Earth Sctences. Vol 3. No 4. pp 409-415. 1985

I1731-7247/85 $3 [)0 + 0 00 © 1985 Pergamon Pre~ Ltd

Printed m Great Britain

Hydrological studies of a tropical reservoir site, Nigeria E. H. ALABO D e p a r t m e n t of Cwd E n g m e e r m g . Univers.ty of Science and Technology, Port Harcourt, Nigerta

( Recetved 29 A p r d 19841 Abstract--The study area, a potential reservotr site, is sttuated in the tropical belt and was chosen by an earher worker after geophysical and geotechmcal investigations. T h e area is underlain by the P r e c a m b n a n Basement Complex T h e aim of the present study is to determine whether the site could provtde adequate water for a proposed reservoir T h e chmate of the area is of marked wet and dry seasons, with a tropical ramforest-type of vegetation T h e catchment area Is about 744 40 km 2 T h e population of the c o m m u m t i e s ts presently about 528,169 (1963 census), and it is esttmated to be approxtmately 600.000 by the year 2005. This paper provides an e s u m a t l o n of the water need of the expected populatton Owing to scanty hydrological data for the O w e n a River, tts expected discharge was extrapolated by comparison with the ne~ghbourmg Shasha Rwer From calculations, the following results were obtained' (I) expected avadable water wlthm the catchment area, 256 75 x 10~ m ~ per year, (2) expected reservoir capactty. 13 73 x lip m ~, (3) average discharge of the O w e n a Rwer at the investigation point, 2 91 m ~s -~ over a period of one year; (4) expected water need of the c o m m u n m e s (of 600,000 persons), 4 38 x 10n m a per year, about 0 5 m ~s-i O n the basra of the expected excess water over d e m a n d , losses through evapo-transplratton and seepage, as well as the possible avadabdtty of budding matertals within the area, it is r e c o m m e n d e d that the creation of an earth d a m reservoir is feasible

INTRODUCTION THE object of this work is to determine whether or not the site chosen by an earlier worker (Abe-Ajakaiye 1975) through geophysical and geotechnical investigations can provide enough water to create the desired reservoir. This is a contribution towards the continuing search for adequate water for rural and urban communities in Nigeria. The reservoir, if and when created, will supply water to towns and villages around Ondo, Owena, Akure, Idanre and Ile-Oluji in Ondo State of Nigeria (Fig. 1). Most towns and villages in the area obtain supplies of water from rivers, streams, springs and hand-dug wells, most of which dry up during the dry season (January-April) (Western Nigeria Water Corporation, Ibadan 1964a). The study area lies between latitudes 7°N and 7°30'N and between longitudes 4°50'E and 5°15'E in the Ondo State of Nigeria (Fig. 1). Data for the project were from various sources: rainfall and evapo-transpiration figures from the Agrometeorological Bulletin and the Annual Summary of Observations (1963-1970 and 1959--62); stage and discharge records from the Western Nigeria Water Corpor-

ation, Ibadan and population figures from the Institute of Population and Manpower Studies, University of Ife, Ile-Ife. From this, the expected population for 30 years from 1973 (i.e. 1973-2003) was computed (Table 1), based on a 3% increase for every 10 years. Both the catchment area and reservoir capacity were calculated to determine available water within them. The water requirement of the commumties, as well as that expected to be lost through evapo-transpiration and seepage, and silting were also determined. Fteid trips were made for geological mapping as well as for observation of the river and parts of its basin. Water consumption was estimated at about 40.0 litres per head per day (this will, however, mcrease following social and industrial development in the future), based on comparison with the average of 81.0 litres per head per day for Ibadan (Western Nigeria Water Corporation 1964a). The figure 40.0 litres per head per day was, and is still considered, adequate because most of the towns and villages are rural, smaller and less developed socially and industrially than Ibadan. Based on the above estimates and the computed population figures, water need for the different years is calculated (Table 2).

Table 2. Population and estimated water need

Table 1. Population a c c o r d m g t o years (based on 1963 Census) Year

Population

Year

Population

Esttmated water need (Iday -I )

1973 1983 1993 2003 2005

544,014 560,334 577,144 594,458 597,921 = 600,000

1973 1983 1993 2003 2005

544,014 560,334 577,144 594,458 600,000

21,760,560 22,413,360 23,085,760 23,778,320 24,000,01)0

409

410

E . H . ALABO

\ KWARA

\

\

STATE

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e*

.J

C \

\

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7° 3

i

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Ftg. 1. Map showmg study area nn Ondo State, Nngerm.

Higmahsed

~P-/I

Gneiss

Undtfferenf iafed

Bamc

Schist

,111 •

o

Quartz -schtsf

;q °

and

Quartzite

+o

iGl

Ch

Granite

Gnetss

Biofit e

and

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Hefa-lnfrusives

Geological

Boundary

GeoIoglcal

Boundary

--Structural --

--

ABCD

10 i

5 i .......

0 iii

5

10

I

I

Trend

Fautf Section

HornbLende Granite

of

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Fig. 2. Geological sketch map of Owena study area.

F ig 3

Approximate Uncertain Line

Hydrological studies of a tropical reservoir site, N~geria

411

"P13'

7"1

&O0 I ....

O I

4.UU I

t~U I

ILuu /

i o u u r, el t e:. I

Fig 3. Showing Dam site, and reservoir area (broken hne)

GEOLOGY, CLIMATE AND VEGETATION The geological sketch map of the area (Fig. 2) shows that the area is covered by six rock groups of the Nigerian Basement Complex, i.e. migmatitic gneiss, undifferentiated basic schist, granitised gneiss, quartzite, biotite and biotite-hornblende granite and charnockites. The migmatite is predominant, covering over three quarters of the total area. As in most tropical regions, the surface is covered in most places by a few metres of lateritic and non-lateritic overburden, with very low coefficient of permeability. Aerial photographs and topographical maps reveal linear sections along the Owena River, suggesting structural control of the river channel. Shallow seismic investigations carried out (Abe-Ajakaiye 1975) confirmed the above by revealing the presence of a fault along the river channel at the proposed dam site. The lowest part of the study area is the Owena River Valley itself, with a height of about 228.7 m above the Nigerian Ordinance datum. From the valley floor, the land rises eastwards and westwards. In the centre are low isolated hills and inselbergs, between which are

narrow stream channels (Fig. 3). The Owena River flows in a rocky valley, except in the area around the village of Owena, where there are very narrow strips of alluvium deposited during the floods of July, September and October. In the far south, around the proposed dam axis (Fig. 3), the valley floor considerably widens, while the area immediately east and west of it are fairly flat, relative to the rest of the area. The predominant and perennial Owena River originates from about 29.0 km NNE and NW of Ogotun village, below which three major tributaries join at different places to flow southwards as one river, the Owena (Fig. 4). In general the Owena River flows in a north-south direction, with a dendritic pattern (Fig. 4). The Owena area, in the humid tropics, under the "A" climate of K6eppen, typically has two seasons--a dry season with marked water deficit and a wet season with a substantial water surplus. The dry season is characterised by an almost complete absence of precipitation and a relatively high rate of evapo-transpiration. The rains, usually heavy and stormy, have an average duration of between 30 and 45 minutes, and last from April to October. Because of the short duration, aided

412

E . H . ALABO

average annual total of 1555.73 ram, while Owena alone has an annual average figure of about 1543.08 mm (based on figures for 12 years, 1959-1970) (Table 3, Agrometeorological Bulletin and Annual Summary of Observations). Temperatures, as in parts of Ondo, Oyo and Ogun States, are constantly high, ranging from a mean maximum of about 32°C in August. Available data on evapo-transpiration show that the mean daily rates for Ondo and Ibule Akure are 2.73 and 2.10 mm, respectively (Agrometeorologlcal Bulletin). With O n d o and Akure at two extremes of the work area, the average figure for the two locations would give a fairly reliable mean daily rate of about 2.415 mm for the area. This gives a mean annual rate of evapo-transpiration of 881.40 mm (about 22.66 m 3 s -l) from the catchment area of 744.40 km 2.

C A T C H M E N T AREA AND THE RESERVOIR

Fig, 4 0 w e n a

Rwer catchment area

by the relatively impermeable iateritic overburden, runoff rate is high, with only a small fraction retained by vegetation. Rainfall figures for five stations within the Owena catchment area, i.e. Ondo, Owena, Akure, IleOluji and Effon Alaiye, show a great variability in amounts (Table 3). These stations together have an

The catchment area of the Owena River at the new O n d o - A k u r e road bridge is about 744.40 km 2 (Fig. 4). The area around the village of Owena has an average annual rainfall of 1534.08 mm. Of the 12 years data, two years (1963 and 1968) have figures above the average (1959.0 and 1850.0 mm, respectively), while the other ten have figures below the average, with 1964 and 1969 having the lowest of 1435.0 mm each. Based on the average figure of 1555.73 mm of rain per annum for the area, about 1141.93 x 106 m 3 of water is expected each year within the catchment area. Of this amount about 57.5% will be taken up by evapo-transpiration annually, leaving a total of about 42.5% as available water. From this, an estimated 20% will be lost through infiltration and/or seepage and'silting (Western Nigeria Water Corporation, Ibadan 1964b). During the relatively less wet years, as in 1964 and 1969, the expected water for a rainfall of 1435 mm per year was 1068.19 × 106 m 3.

Table 3 Record of annual rainfall for five stations m and around thc study area A n n u a l rainfall (mm) Place Ondo S.W. Owena --Owena

N E Owena Ile-OlujJ W Owena Effon Alalye N N.W. Owena

07°7'N 286,151m (94)A S L. 07°12'N 228 6 m (750)A.S L 07°17'N 365.76m (1200)AS L 07°14'N 243.84 m (800')A.S L. 07o38'N 609.6m (2000)A.S.L.

1959

1960

1961

1962

1963

1738 1562

1575

1485

1481

1966

1967

1582

1968

1969

2813

1656 1755.33

2595 1859

1477

1556

1965

1575

1477

1379

1964

A n n u a l average (over I 1 years) 1970 (mm)

1444

1390

1470

1575 1482 1435

1984

1329

1295 1436 1485

1408

1427

1617 1435 1850

1291

1735

1534 08 1544 1465

1441

1489 58 1423

1407

1420

1420

1414

1764

1263

1394

1414

1318

1771)

1405

1451 00 1671

1426

Average for the area = 1555.73 ran.

1429 1470

1849 1662

1429 1270

1466 1470

1659 1784

1548.67

Hydrological studies of a tropical reservoir site, Nigeria

I

J

413

I

~3 ?-

~2 w

oJ c~n

I

2 3 h 5 6 7 ~ 9 I'0 ~I 12 13 Discharge m3/sec

1'4-

15 I'6 17 I'@ I'9 20 21 2'2 2'3 ~ >

Fig 5 Ratmg curve for the Shasha Rwer at Gbongan, Ondo state

H o w e v e r , after evapo-transp~ration and seepage losses, there will still be enough water w~thin the catchment area. It was observed that the O w e n a River does not appreciably increase in volume during the first rains (April, May, June), but does so from after July (Fig. 6). The p h e n o m e n o n could be due to a n u m b e r of factors. O n e of these is that a good proportion of the early rains goes to fill the small pores of the soil, the large fractures and fault zones, and infiltrate to replenish the ground water storage especmlly around the river channel. I m p o u n d i n g the O w e n a River by a dam will create a reservoir 2220 m long whose water surface is expected to be 251.15 m above the Nigerian Ordinance datum. The reservoir is expected to cover an area of 1.40 km 2 (Fig. 3), and hold about 13.78 × 106 m 3 of water. This a m o u n t of water is in excess of d e m a n d , and expected losses through evapo-transpirat~on and seepage together. D u e to inadequate hydrological data for the O w e n a catchment at Owena, data for the Shasha were c o m p a r e d and some workable data for the O w e n a River were obtained by extrapolation, since the O w e n a River is similar in character to the Shasha River. From a rating curve (Fig. 5), and using available gauge readings, the Shasha River's discharge rates were obtained (see Table

(a) Catchment area and dtscharge rate (total mean dady)

(b)

"

Shasha River

=

673.40 km 2

O w e n a River

=

744.38 km 2

Shasha's discharge

=

Owena's discharge

=

744.38 673.4----~× 30.92

=

34.18 m3 s -1 .

Rainfall and d&charge rate (total annual mean) Shasha

=

1,500 m m y-t

O w e n a River

=

1,534.08 mm y-i

Discharge for Owena

=

1,534.08

"

× 30.92

1,500.00

= (c)

30.92 m 3 s-1

31.62 m3s -l .

Expected water in catchment and discharge rate (total mean daily) Shasha

=

1010.1 × 106m 3

5).

Owena

=

1141.93 × 106m 3

Since the size, geology, topography and vegetation of the catchment areas of the two rivers are similar (Table 4), a p p r o x i m a t e discharge rates were calculated for the O w e n a River. From such calculations as shown opposite the figures of Table 6 were obtained. Based on the c o m p u t e d average, the Owena River discharges 2.65 m 3 s -I more than the Shasha River. But 2.65 m 3 s -I = 8.6% of Shasha's discharge, therefore to obtain the m e a n daily discharge for the O w e n a River, Shasha's mean daily figures should be multiplied by a factor of 1.086.

1010.10 × 106m 3

=

30-92m3s-I

1141.93 x 106m 3

=

1141.93 ~ × 30.92 1010.10

=

34.95 m 3 s- 1.

Average total mean daily discharge from Owena River =

33.58 m 3 s -I.

E. H. ALABO

414

Table 4. Comparison of Shasha Rtver at Gbongan wzth Owena River at Owena Factors compared

Shasha River

Owena River 7o12,N 744 40 Basic schist, migmatite gneiss complex, gramte gneiss and quartztte

Mean annual rainfall Total mean discharge over a year (m 3s-i )

7"28'N 673.40 Pegmatised, pelitic and basic schists, migmatite gneiss complex and granite gneiss 1500mm or 1.5m 30.92 (measured) by gauging

Mean average discharge over a year

2.58 (measured)

1534.08 mm or 1.534 m 33 58 (calculated by comparison with the Shasha Rwer) 2.91 (calculated)

15.57 (measured)

16.90 (calculated)

0 40 (measured)

0 43 (calculated)

Latitude Catchment area (kin z) Geology

(m~s-I) Highest mean d~scharge over a year (m3s -I ) Lowest mean discharge over a year (m-~s-j )

Table 5. Records for Shasha River. Gbongan, of average mean discharge over a period of one day for one year (1970) in m3 s-i Date

Jan.

Feb.

Mar.

Aprd

May

Jun.

I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

0.399 0.402 0 399 0.405 0 408 0.420 0 420 0.570 0 570 0.630 0 690 0 810 0.990 I 065 I.I10 1.290 1.350 I 350 1.350 1 350 1.320 1.185 1.020 0.990 0.960 1.050 1.185 1.290 1.170 I 050 1.050

0.990 0.990 0.990 0.990 0.990 0.990 0.990 0.990 0.960 0.960 0.960 0.960 0.930 0.900 0.780 0.750 0.690 0.630 0.540 0.510 0.510 0.525 0.525 0.510 0.510 0.510 0.495 0.495

0.495 0.495 0.495 0 420 0.420 0.420 0.420 0 420 0 420 0 420 0.420 0.420 0.420 0.420 0 420 0 411 0.411 0.411 0.411 0.495 0.510 0.570 0.960 1.080 1.110 1.290 1.440 1.470 1.530 1.530 1.560

1 575 1.575 1.575 1.575 1.575 1.575 1.575 1.575 1.530 1.470 1.470 I 470 1.440 I 350 1.230 1.230 1 215 1 215 1 170 I.I10 1.050 1.050 1.020 1.020 0.900 0.900 0.900 0.900 0.900 0.885

0.870 I).870 0.840 0 840 0.840 0.780 0 750 0 750 0 840 0 870 0.900 0.960 0.990 0.990 0.990 1.065 1.080 1.080 1.170 1 170 1 185 1 185 1 230 1.260 1.260 1.230 1.230 1.260 1.230 1.185 I 230

0.911 1 350 0.339

0 780 0.990 0.495

0.700 1.560 0 411

1.260 1.575 0.885

1.036 1.260 0.750

Mean Max Min.

Jul.

Aug.

Sept.

Oct.

Nov.

Dec.

1.260 1.260 1.335 1.335 1.350 1.410 1.515 1.740 1.740 1,740 1.710 1,830 1.860 2.133 2.265 2,265 2,460 2,505 2.745 3.030 3.150 3.150 3,300 3 720 3 900 4.800 6.000 7.230 8.730 6.900

9 630 8 610 7.650 6.900 6.000 5.115 4 800 4.650 4.215 4.050 3.540 3.420 3.090 3.030 3 030 3 030 3.030 2.880 2.730 2.505 2.133 2.010 1.830 1 800 1.740 1 350 1.050 0.930 0.930 0.900 0.900

0.900 0.900 0 930 1.020 1 050 1.050 1.065 1.170 I 230 I 230 1.230 1.230 1.230 1.170 1.050 1.020 1.020 0 960 0 960 0.930 , 0 900 0.900 0.840 0.780 0.780 0 780 0 690 0 690 0.690 0.690 0.690

0.750 1.230 1.800 1.800 I 800 l 620 1 335 1.260 1 260 1 335 1.335 1 500 1.020 1.335 4 830 10.410 14.100 15 570 15.300 15.300 15.570 14 550 14.100 14.700 9.750 9.630 9 330 9 210 9.000 8 730

8.160 7.515 7 650 7 815 8.100 8.100 8.100 8.160 8.430 8.430 8.550 8.550 9.000 9.330 9.630 9 930 10 200 10 410 10.410 10.200 9.630 9.630 8.730 7 650 7 350 7.230 7.230 6.900 6.900 6 810 6.810

6 900 4 800 4.440 3.960 3.540 3.090 2.730 2.610 2 460 2.010 1.830 2.880 2.965 2 700 2.700 2.700 2.580 2.505 2 133 2 010 I 800 1.800 1.740 1.410 1.230 1 185 1 170 1.065 1.065 1.050

0.990 0.990 0.990 0.990 0.990 0.990 0.990 0 990 0.990 0 990 0.990 0.990 0.930 0.930 0.930 0 930 0 900 0.810 0.810 0 750 0.750 0.690 0.630 0.630 0.630 0.615 0.615 0 615 0.570 0.540 0.570

2.946 8 730 1.260

3.467 9.630 0.900

0.960 1,230 0 690

6.981 15.570 0 750

8 430 10.410 6.810

2.499 6 900 1.050

0.949 0.990 0 570

Annual mean daily discharge = 2.580 m 3 s -I.

Table 6. Computed discharge figures for Owena River, Owena Village. Average mean discharge over a month for one year (1970). (Extrapolation by comparison with records for Shasha River) (m 3 s -I ) Date

Jan.

Feb.

Mar.

April

May

Jun.

Jul.

Aug.

Sept

Oct.

Nov.

Dec.

Mean Max. Min.

0.99 1.50 0.43

0.85 i.07 0.54

0 76 1.70 0.45

1.37 1.71 0.96

1.12 1.18 0.81

3.02 9.05 1.37

3.76 10.05 0 98

1.04 1.33 0.75

7.06 16.09 0 81

9.15 11.03 7.04

2.07 7.05 1.14

1.03 1.07 0.62

Annual mean daily discharge = 2.91 m3 s -I

Hydrological studies of a tropical reservoir site, Nigeria

8 7

DISCUSSION AND CONCLUSION

F1ean

Oady Discharge For

,/~

River

Shasha Af Gbongan

:

/

/

/

5

4 3 2 1

~o E

i

,

,

i

j

F

M

A

t

,., j

T=me

j

k

I

I

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I

s

o

N

o

Honfhs

In

>

¢•10• Hean

Oai(y Discharge For

River

0WEJ_A Figurenor From

/)~

9

Neasurementbut By Comparison /

87

With

River Shasha

1 o,

/

/

/

mand ,

F

.

N

.

.

.

.

A H J J Time in Honths

415

k'S

Fig. 6 Dischargecurves for Shara and Owena Rivers compared

The proposed reservoir level at the 251.15 m contour should have an area of 1.04 km 2 and an estimated capacity of 13.78 x 106 m 3. This will store water far in excess of demand, and losses through evapo-transpiration and seepage. The choice of the reservoir site is governed by four factors: (1) adequate geophysical and geotechnical results; (2) adequate reservoir capacity; (3) the reservoir will be removed from population centres since no town or village is expected to be flooded, and (4) the ease with which construction materials will be obtained and moved to the site. However, the reservoir will submerge parts of the Owena-Lafiagi, the Owena-Onipepeiye and the O n i p e p e i y e - A g o Belio roads. Since the lengths of the roads to be submerged are not great, the construction of subsidiary roads will be easily done as the cost will be minimal. Apart from water for domestic purposes, the availability of adequate water may stimulate small-scale industrial projects, especially now that Akure is a capital city. In addition to the above, irrigation of farmlands during the waterless dry seasons may give rise to double harvest in the area. An earth dam is considered most suitable for the size of the river and its valley. It is also very likely that the lateritic materials may contain clay fractions suttable as core fills. In conclusion, it ts recommended that the construction of an earth dam across the river at Owena is feasible.

Acknowledgements--The author is deeply indebted to Dr Klaus A hydrograph for the Owena catchment area based on the computed figures (Fig. 6) shows an average lowest mean discharge over a year of 0.8 m 3 s -I and an average peak of 9.2 m 3 s- 1for the months of March and October, respectively. For the same year, a minimum discharge of 0.43 m 3 s -1 and a maximum of 16.90 m 3 s -! were computed for four different days of January and September. In general, the O w e n a River is expected to have a mean average discharge of 2.91 m 3 s -] over a period of one year (Table 6). From separate calculations, expected demand for water is 0.5 m 3 s -l. Thus, the Owena River's discharge far exceeds the demand except for the rates of 0.43 m 3 s -1 for two days in January and of 0.45 m 3 s -1 for four days in March. However, these six days of belowdemand discharge will obviously be made up for by the many rates above the demand value.

.~S

3:4-B

Wirth who supervised this project at the Department of Geology, Umversity of Ire My thanks go to Prof O S Adegoke of the Department of Geology, Umversttyof Ire, Ile-lfe, for valuable early discussions, which stimulated this write up

REFERENCES

Abe-Ajakalye, ! T. I. (1975) Geotechnical survey of the course of River Owena for the possible location of a (multipurpose) dam Unpublished MSc Thesis, Universityof Ire, Nigeria Nigeria, MeteorologicalService(1959-62), Annualsummaryof observations, Lagos. Pub , Nigerian Meteorological Services Nigeria, Mmistry of Natural Resources and Research (1963-70) Agrometeorologlcal Bulletm, Lagos. Pub, Nigerian Meteorological Services. Western Nigeria Water Corporation, lbadan (1964a) Design Report of Ikare, Ondo, ldanre, and Akure Water Supply (Unpublished manuscript ) Western Nigeria Water Corporation, Ibadan (1964b) Design Report of [gbare-Oke Water Supply