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GHANA HYDROLOGICAL NETWORKS H. W. A. K. SACKEYFIO Chief Hydrological Engineer and K. KRISHNAMURTHY Senior Engineer, Hydrology Ghana National Construction Corporation, Accra 1. INTRODUCTION 1.1. In any expanding modem economy, exploitation of natural resources is of primary importance. As "Water" is one of the main natural resource, extensive efforts are being made in every country to-day to harness the water potential for the benefit of the people. 1.2. For economic and optimum utilization of water resources, a determination of the extent and availability of surface and ground waters is the first requisite and this in turn devolves round hydrometeorological data. Long term hydrological and meteorological data, characteristics of watersheds provide the basis for all water studies and determines the major aspects of hydrological and hydraulic design of any water utilization project. 1.3. Hydrology has correctly been defined to embrace the full life history of water and provide basic information needed for the planning, design and operation of all water conservation and utilization projects. A detailed study of all the elements of the hydrological cycle and furthering the knowledge goes a long way in economic appraisal of water projects. 1.4. The purpose of this paper is to bring out the development of the hydrological network in Ghana, its relation to the existing and proposed river valley projects and to indicate briefly the adequacy or otherwise of the network. Another paper presented by Ghana gives in detail the meteorological network. However some references are made in this paper to the meteorological aspects. A comparison is also made with the prevalent or recommended standards by international organizations. 2. BACKGROUND INFORMATION 2.1. Ghana has a geographical area of 91,843 sq. miles and the distribution by river basins is given in Appendix I. The Volta river basin is the largest single catchment and drains nearly 70% of Ghana, in addition to other Countries to the North, West and East. 2.2. Appendix II gives the details of drainage areas by sub-basins and lengths of the rivers from source to mouth. 2.3. The entire Volta catchment is low lying and is generally below 600 ft. m.s.l. except for two escarpment uplands rising up to 2,900 ft. In the south east corner of Ghana, the Volta river flows through the Akwapim scarp, a mountain range extending from just north of Accra in a northeasterly direction through Togoland. The western 253
Transcript
Page 1: GHANA HYDROLOGICAL NETWORKShydrologie.org/redbooks/a067/iahs_067_0253.pdfGHANA HYDROLOGICAL NETWORKS H. W. A. K. SACKEYFIO Chief Hydrological Engineer and K. KRISHNAMURTHY Senior Engineer,

GHANA HYDROLOGICAL NETWORKS

H. W. A. K. SACKEYFIO

Chief Hydrological Engineer and

K. KRISHNAMURTHY Senior Engineer, Hydrology

Ghana National Construction Corporation, Accra

1. INTRODUCTION

1.1. In any expanding modem economy, exploitation of natural resources is of primary importance. As "Water" is one of the main natural resource, extensive efforts are being made in every country to-day to harness the water potential for the benefit of the people.

1.2. For economic and optimum utilization of water resources, a determination of the extent and availability of surface and ground waters is the first requisite and this in turn devolves round hydrometeorological data. Long term hydrological and meteorological data, characteristics of watersheds provide the basis for all water studies and determines the major aspects of hydrological and hydraulic design of any water utilization project.

1.3. Hydrology has correctly been defined to embrace the full life history of water and provide basic information needed for the planning, design and operation of all water conservation and utilization projects. A detailed study of all the elements of the hydrological cycle and furthering the knowledge goes a long way in economic appraisal of water projects.

1.4. The purpose of this paper is to bring out the development of the hydrological network in Ghana, its relation to the existing and proposed river valley projects and to indicate briefly the adequacy or otherwise of the network. Another paper presented by Ghana gives in detail the meteorological network. However some references are made in this paper to the meteorological aspects. A comparison is also made with the prevalent or recommended standards by international organizations.

2. BACKGROUND INFORMATION

2.1. Ghana has a geographical area of 91,843 sq. miles and the distribution by river basins is given in Appendix I. The Volta river basin is the largest single catchment and drains nearly 70% of Ghana, in addition to other Countries to the North, West and East.

2.2. Appendix II gives the details of drainage areas by sub-basins and lengths of the rivers from source to mouth.

2.3. The entire Volta catchment is low lying and is generally below 600 ft. m.s.l. except for two escarpment uplands rising up to 2,900 ft. In the south east corner of Ghana, the Volta river flows through the Akwapim scarp, a mountain range extending from just north of Accra in a northeasterly direction through Togoland. The western

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limb of the scarp runs in a northwesterly direction through Mampong (Ashanti) reaching heights up to 2,600 ft. and forms the southwestern boundary of the Volta drainage basin. Most of the Volta river basin in Ghana is underlain by sedimentary rocks of the Voltain System and that of the river basins in S.W. Ghana are underlain by metamorphosed sedimentary rocks and granites of the Birimian (Middle Pre-Cambrian) System. 2.4. The Volta basin embraces three major zones of vegetation. The coastal belt extending up to 70 miles inland is covered with secondary scrub and a few large trees. Beyond this is the rain forest belt and the vegetation north of 7 ° 15 ' latitude consists of rolling savannah and grassland.

2.5. Out of the drainage area of the rivers in S. W. Ghana, most of the northern one third (south of Akwapim range of mountains) is between 500 ft. to 1,000 ft. above mean sea level. Though the land slopes gently from north to south there are several hills and ranges rising from 1,500 to 2,000 ft. The southern areas are generally low elevation being up to 500 ft above sea level. Most of the area comes under moist deciduous forests with the rain forest concentrated in the S.W. corner of Ghana.

2.6. The climate in Ghana is mostly determined by the movement of two main prin­cipal air masses, the southerly monsoons and the northerly trades. The boundary where these two masses converge is the Intertropical boundary. The monsoon air masses come from the south Atlantic ocean anticyclone and generally have a long history of sea trek before reaching the west African coast. Generally, the monsoon air mass is heavily moisture laden and convectively unstable when passing northwards over Ghana. The Northerly Trades, which are derived from the Azores anticyclone extending over the Sahara desert, passes over the desert before reaching the northern portions of the Volta catchment and Ghana as a north easterly wind known locally as the Harmattan.

2.7. Resulting from the general movement of the rainbelt which is closely associated with the seasonal movement of the l .T.B. following it in a northerly direction during April to mid September and preceding it in a southerly direction from mid September to December, southern Ghana experiences two rainy seasons and northern Ghana only one rainy season. Due to this, all of the rivers in southern Ghana reach two peaks during the year, one in June-mid July and one in mid September-October.

3. A LOOK AT THE PAST

3.1. Prior to 1951, there was practically no water utilization in the country except for domestic water supply. However during 1928, the erstwhile Public Works Depart­ment, and the then Railway and Harbours Administration established a network of river gauging stations only in S.W. Ghana, presumably to give an idea of the range of river rise and fall, for the construction of roads and railways. In 1942 most of these gauges were abandoned due to shortage of staff during the second World War period.

3.2. Unfortunately, none of these gauges were connected to any bench mark; however, by 1950, all the gauges had been washed off. Except for qualitative studies and analyses, the data collected then is of no use to any rational water studies.

3.3. The number of river gauges for observing water levels was only 6 in 1950. Apparently, the then requirements of the authorities did not necessitate installation of more river gauges.

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4. THE TRANSITION PERIOD

4.1. During the period 1950-55, the necessity of utilising the water resources was felt and this was the period when harnessing the Volta Waters for hydro-powerwas conceived. Dictated by the needs of planning and the different climatic regions and drainage systems, a skeleton network of gauging stations was established to obtain the then immediately needed information for planning the huge Volta dam. Most of the stations were of primary nature for continuous water level data. Occasional flow metering observations at representative stations were made to compute the runoff.

4.2. At the same time, a Hydrological Branch in then Public Works Department was established (in 1954) and the work of establishing gauging stations, particularly in S.W. Ghana was initiated. Certain rivers and streams were chosen initially according to the type of data required and in some cases only to meet the needs of mining companies.

4.3. By 1st January, 1955, the network had increased to 22, but the distribution was far from satisfactory. Many controlling factors still persisted and no systematic or scientific approach was made.

4.4. The position continued to improve in the succeeding years but it was only in 1959-60, a systematic approach was made on network planning and selection and operation of stations.

5. THE PRESENT NETWORK

5.1. Within the limitations of staff and funds, most of the major streams were selected and a network was planned on the basis of

(a) Data required by

(i) River Valley projects ;

(ii) Agricultural needs;

(iii) Domestic and Industrial requirements.

(b) Data required for a scientific and systematic study of the Hydrological cycle

(i) On rivers of major importance;

(ii) On rivers in different climatic regions and rainfall trends;

(iii) On major and minor tributaries.

5.2. For the rivers in Ghana, the hydrological data collected by the network consists of

(i) Water levels — observed once to thrice daily, every three hours during floods at selected stations, and more frequent observations as necessary.

(ii) Flow measurements — To cover the full rise and fall of the river. Measurements made either by 2 point method or by measuring velocities at every 0.1 depth along each vertical.

(iii) Suspended load — At selected stations

(iv) Chemical quality of Water — At most of the stations, samples collected during floods, dry season etc.

(v) Tidal effects — mostly in the delta of the main rivers.

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5.3. Network design in Ghana

5.3.1. The development of a network design in Ghana was firstly influenced by the then proposed Volta river development and subsequently was based on a rational study of climatic regions, types of rivers and flow pattern, population density and accessibility to sites.

5.3.2. The present network consists of 98 stations for observation of water levels, out of which metering observations are normally carried out at 50 stations. At all the metering stations water samples are collected for physical and chemical analyses.

Water level observation stations 98 Primary 50 Secondary 36 Tidal 12

Flow metering stations 50 Primary 42 Secondary 8

There are other temporary gauges not included in the above network since these stations are for specific periods.

5.3.2. Distribution of network by river basins

Name

Volta basin Black Volta White Volta Oti Volta (below the cofluence of the Black Volta and the White Volta, and the Oti)

River Basins in South Western Gha­na: Bia, Tano, Ankobra, Pra, Ochi, Ayensu, Densu and coastal rivers

All river basins in Ghana

Area sq. miles

63,982 13,555 17,685 6,260

26,482

24,820 91,843

Number of stations

62 10 17 5

30

36 98

1 1 1 1

1

1 1

Density

in 1 030 sq. miles " 1 356 " " 1 040 " 1 252

" 883

" 690 " " 1 065

5.3.3. Distribution of network by climatic Regions

Most of northern Ghana experiences only one rainy season and the rivers rise steadily to the major peak and recede; whereas in Southern Ghana, there are two rainy seasons with two peaks in flow.

Northern Ghana. 36 stations 1 in 1 300 sq. miles (46,945 sq. miles) Southern Ghana. 62 stations 1 in 675 sq. miles (41,857 sq. miles)

5.3.4. Distribution of network by population density

5.3.4.1. The population of Ghana according to the 1 961 census is 6,726,815 which

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works out to a density of 73 persons per sq. mile. Omitting the 3 major cities, the density of population is in the order of 62 persons per sq. mile i.e. 24 persons per sq. km. 5.3.4.2. The number of gauging stations pet square km. in Ghana is 0.41 per 1,000 sq. km.

5.3.4.3. The analyses of network versus population density in each of the major watersheds is indicated in the table below:

Basin

Volta Basin

Black Volta White Volta Oti Volta

(below the outfall)

River Basins in South Western

Ghana

Area

Sq. ml.

63,982

13,555

17,685 6,260

26,482

24,820

Sq. km.

150,500

35,500

45,750 16,200

68,600

64,400

Total population

2,698,248

436,305

828,137 207,433

1,226,373

3,001,609

Density of population

Per sq. ml.

42.24

32.19

46.83 33.14

46.30

121.10

Per sq. km.

16.30

12.41

18.10 12.80

17.89

46.80

No. of gauging stations

62

10

17 5

30

36

Gauging stations

per 1,000 km.

0.41

0.28

0.37 0.31

0.44

0.56

5.3.5. Distribution of network by utility needi

It is generally recognized that though a sp irse network can be considered sufficient to make an appraisal of the water resources of a country, a more dense network is essential to plan any major water utilization project.

In Ghana, the Akosombo dam was finally authorised for construction in 1961, on the basis of the available hydrological data and the then possible extensive water studies. The hydrological data was from a network of about 10 stations (one station with water level data for 25 years and the other stations with about 10 years data with same flow observation data). The total catchment area of the Volta river U/S of the dam site is 152,090 sq. miles out of which 89,830 sq. miles are outside Ghana. Not much water level data could be obtained from the drainage area outside Ghana. Considering the watershed in Ghana, the density of the network works out to 1 in 6,400 sq. miles or 15,560 sq. km. which from any existing standard is considered insufficient for planning.

During the past 6 years, the network was increased considerably and the behaviour of the main river and its major tributaries have been closely watched during the past 3 to 4 years. Analyses on the basis of latest data have revealed that the original network provided very reliable details on the water resources, though many interesting features have been revealed pertaining to smaller watersheds within the major river basin.

Many other major projects are in various stages of planning, design and are awaiting consideration and construction authorization. The network, hydrological and meteoro­logical, seems to have satisfied the requirements of the major projects. This has been

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facilitated to a certain extent by the largeness of the catchments. Sufficiently long term precipitation data was available, though the number of stations with more than 25 years data is not fully adequate.

The above details is not meant to have complaceny of the adequacy of the network. For the needs of many smaller projects, no reliable data is provided by the network even now. In view of this the network is continuously being enlarged. 5.3.6. It is not the intention to go into details of metering stations network but mention may be made of the procedure. All the main rivers are metered at selected stations (primary stations) regularly considering the drainage area and inflows between two

^ 120

3

kl

Fig. 2 — Existing and proposed hydrological network.

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stations. Immediately after the confluence of two major tributaries, metering is effected and the selection of primary and secondary stations has been based on the reliability of interpolating inflows from minor drainage areas or considerably large drainage areas wherein the population density is less and access is difficult.

5.3.7. At almost all the stations, water samples are collected regularly for chemical and physical analyses. Where the river is subject to tidal effect, special network is in operation to study the movement of salinity.

5.3.8. Figure 1 indicates the river basins in Ghana and the distribution of the hydro-logical network. Figure 2, gives the rate of rise during the past 3 decades and projected planning stations is shown against population density.

otNSirr ptGCÉnr

J _ _ i _ _ L 2 5 /O 20 50 IOO 2oO

P O P W A / / O N of-^'T/rr pus s'a. x/ioMfre?

Fig. 3 — Comparative areal densities of stream flow gauging stations.

6. RELATION OF GHANA'S NETWORK WITH RECOMMENDED STANDARDS

6.1. The W.M.O. Working Group on "Guide to Hydro-meteorological Practices' in their recent report (Ref. chapter 3) have advocated certain standards.

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W. M, O. standard Actual in Ghana

Ghana 1 station for 1 station for (Flat region of tropical zone) 1000-2500 sq. km. 2380 sq. km.

6.2. In flood control series No. 15 (E.C.A.F.E.) Mr. Langbein, U .S .G.S . in his lectures on Hydrological Networks has excellently dealt with the various requirements, necessities and possibilities of the networks. In relation to his analyses of comparative areal densities of stream-flow gauging stations, Ghana's network plots very well. Figure 3 gives the details. (Original sketch taken from Mr. Langbein's paper as pro­duced in the F. C. Series Journal No. 15) In Ghana it works out to a density of 0.41 gauging station per 1,000 sq. km. against a population density of 24 per sq. km. The areal density is almost 50% adequacy on this diagram. It is well above the "reasonable objectives" and the "minimum".

6.3. There are many views expressed by other authors on the minimum, maximum and optimum network. Ghana's network as developed during the last decade comes up fairly well.

7. ADEQUACY OF GHANA'S NETWORK

7.1. The analyses shown in the previous paragraphs indicate 50% adequacy. This is literally true but practically the situation is somewhat different.

7.2. Study of the distribution of population in the various major subcatchments coupled with the type of rainfall pattern gives a slightly more better picture. In specific areas the adequacy is more than 70% whereas in some areas it is less than 50%.

7.3. In 1961, with the commencement of the construction of the dam across theVolta river, the necessity of river forecasting came to the forefront. Naturally, the existing network had to be utilised. The problem of hydrological forecasting was tackled suffessfully. A network of 17 reporting stations together with data from a few secon­dary stations provided the forecasting office with the daily water levels and flow. Basic water studies made earlier with all possible combinations of correlations,analy-tical relationships, time lags, detentions etc. were utilised.

7.4. Though the scanty network proved fairly adequate for forecasting, there were some large areas uninhabited but within 24-48 hours time of concentration from the dam site. It may be of interest to the participants to know that a network of reporting raingauge stations on the periphery of this 8,000 to 9,000 sq. miles was chosen and the daily reports of rainfall together with some control water levels and weather forecast conditions were used to forecast the influence of local deposition. This worked satisfactorily and at least provided 24 hours flood warning.

7.5. Though it is considered preferable for the raingauge network and stream flow network to be under one agency, for effective forecasting, two different organizations are controlling in Ghana. Excellent liaison and close co-ordination exist between these two agencies: the Meteorological Services and Hydrological Services, and the networks are modified and regularly increased to meet the increasing needs and demands of water utilization schemes.

7.6. In Ghana the network is also distributed to evaluate the amount of water entering from outside countries and at definite distances along the river. At some places auto­matic recorders are functioning.

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7.7. For small river basins or part of basins with drainage areas less than 200-500 sq. miles, the present network is however inadequate.

8. PROGRAMMED DEVELOPMENT

8.1. With Ghana's active participation in the International Hydrological decade, extensive programme proposals have been outlined, one of which is the gradual increase in the hydrological network and study of representative and experimental basins.

8.2. Many aspects of surface water, ground water and atmospheric water are proposed to be studied. Ghana has kept pace with the development and proposes to continue intensifying her efforts to study all the parameters of the hydrological cycle.

8.3. It is proposed to increase the hydrological network by an addition of 5-10 stations per year and realise a dense network of about 150-200 stations within the next few years. The network will be designed to meet the multifarious needs of small and big projects alike.

9. CONCLUSION

9.1. In conclusion, it may be mentioned that the present network design in Ghana was based on immediate needs of development projects, to give reliable data on all the major rivers and tributaries, to ensure derivation of well correlated relationships between rainfall and flow in different drainage areas, to meet the needs of hydrological forecasting etc. Though the adequacy is nearly 50% this has been developed in the past decade particularly in the past 5 years.

9.2. Certain modifications to establish a peripherial network, around very thinly populated areas with poor communication facilities, for deriving analytical and rational relationships did prove successful and may be adopted where necessary. Where funds and staff are no question, radar, wireless sets and other automatic equip­ment for all purposes can be installed.

9.3. The network is programmed to be increased for reliable data gradually every year in coordination and close liaison with interested agencies in Ghana, throughout the International Hydrological Decade. Already a representative basin (280 sq. miles) has been established and 5 river gauges have been installed. It is hoped that an optimum tending towards maximum network will be achieved during the next 10 years.

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APPENDIX 1

River basins of Ghana

Ghana has a geographical area of 91,843 sq. miles and the distribution by river basins is as below:

Name of River Basin

1. Volta 2. Bia 3. Tano 4, Ankobra 5. Pra 6. Ochi-Amissa 7. Ochi-Nakwa 8. Ayensu 9. Densu

10. Subri-Amansuri 11. Butre 12. Kakum Saruwi 13. Todzi 14. Aka and Around Keta Lagoon 15. Minor coastal and other streams

Total

Drainage area sq. miles

63,982 2,500 5,742 3,267 8,953

528 580 660 985 325 180 380 720 665

2,376

91,843

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APPENDIX 2

Rivers and river basins of Ghana-details

S. No.

1.

2.

3. 4.

5. 6. 7, 8. 9.

10.

11. 12. 13. 14.

River

White Volta

Black Volta

Oti Volta

Bia Tano Ankobra Pra Ochi Amissa Ochi Nakwa Ayensu Densu Todzie Minor Coastal streams

Tributary

White Volta

Red Volta Kulpawn Nasia Mole

Black Volta

Oti Volta Proper

(from the con­fluence of White and Black Volta)

Pru Daka Sene Asukawkaw Obosum Dayi Afram Alabo

Total for Volta River

Total area of Ghana

Catchment area-sq.

In Ghana

7,512

266 6,050 1,982 1,875

17,685

13,555

6,260 9,676

3,405 3,542 2,478

593 1,322

680 4,370

416

26,482

63,982

2,500 5,742 3,267 8,953

528

580 660 985 720

3,926

91,843

miles

Outside Ghana

14,479

6,351 1,930

— —

22,760

43,980

21,840

1,087

163

1,250

89,830 Not

available 464

— — —

— — — 140

Total

21,991

6,617 7,980 1,982 1,875

40,445

57,535

28,100 9,676

3,405 3,542 2,478 1,680 1,322

843 4,370

416

27,732

153,812

6,206 3,267 8,953

528

580 660 985 860

3 641

Length from source to mouth-miles

In Ghana

349

30 173 116 84

385

372 299

178 259 107 55 76 64

199 59

94 382 171 226

57

56 71 77

102

Outside Ghana

289

166

434

213

24

16

Not available

Total

638

196 173 116 84

819

585 299

178 259 107 79 76 80

199 59

382 171 226

57

56 71 77

102


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