an appraisal of sediment transport measure ment in ghanaian...

Challenges in African uyarology and Water Resources (Proceedings of the Harare Symposium, July 1984). IAHS Publ. no. 144.

An appraisal of sediment transport measurement in Ghanaian rivers

S. A. AKRASI & N. B. AYIBOTELE Water Resources Research Institute (CSIR) , PO Box M32 , Accra, Ghana

ABSTRACT The measurement of sediment transport encounters many difficulties in Africa. This paper outlines the approach adopted in Ghana in a recent attempt to design and establish a sediment monitoring network. Taking account of the numerous ad hoc sediment measurements that had been carried out over the years in connection with water projects, it was first decided to collect and synthesize these available data. From this work it was concluded that the existing ad hoc measurements provided a certain amount of information about sediment discharge in Ghanaian rivers. It was recommended that this knowledge should not be ignored and that the new network should be designed to confirm doubtful results and to fill gaps in the existing knowledge. A less dense network costing less to set up and operate was therefore proposed.

Etude critique des mesures de transports solides effectuées sur les cours d'eau du Ghana RESUME La mesure des transports solides rencontre en Afrique de nombreuses difficultés. Cette communication indique l'approche adoptée au Ghana pour une récente tentative en vue de projeter et d'établir un réseau de surveillance des transports de sédiments. Tenant compte des nombreuses mesures de transports de sédiments occasionnelles qui avaient été effectuées pendant des années en relation avec nombreux projets hydrauliques, il a été d'abord décidé de recueillir et de faire la synthèse de ces données disponibles. A la suite de ce travail on est arrivé à la conclusion que ces mesures occasionnelles existantes fournissent une certaine masse d'informations sur les débits solides des cours d'eau du Ghana. On a recommandé que ces informations ne restent pas ignorées et que le nouveau réseau devrait être projeté de façon à confirmer ou non les résultats douteux et de combler les lacunes des connaissances actuelles. Un réseau moins dense coûtant moins cher a établir et à exploiter a donc été proposé.

INTRODUCTION

Ghana is situated in the western part of Africa and is located between latitudes 5°N and 11 N and longitudes 1°E and 3°W. The country is drained by two main river systems. These are the Volta

301

302 S.A.Akrasi S N.B.Ayibotele

River basin system, which comprises about 70% of the total land surface area, and the southwestern and coastal river basins system. The two main drainage systems are shown in Fig.l.

At present networks exist for some hydrometeorological and hydrological data collection on a continuous basis. Variables measured include rainfall, river stage and discharge, lake and reservoir levels and evaporation. Between 1937 and the present, 22 synoptic stations for observation and measurement of normal meteorological phenomena and 600 rainfall stations have been established. River gauging stations were established for the first time in 1954, and by 1969 there were 134 such stations (Ayibotele, 1969). At the end of 1983 the number stood at 206.

During this period a sediment discharge data collection network was not established, in spite of a number of efforts to do so. Lack of trained personnel, equipment, logistic support and the inability to identify an institution to be responsible for the task were responsible for this situation. In spite of this, ad hoc sediment discharge measurements have been made over the years in connection with irrigation, domestic and industrial water supply, flood control and hydroelectric project investigations.

About 18 months ago, the Water Resources Research Institute (WRRI) which had been interested in the subject since 1971, initiated a project to fill this gap in the hydrological data collection network system, and to establish a sediment data collection network to be operated on a continuous basis.

As part of this effort, it was decided firstly to collect and assemble all available data that had been generated over the years and to evaluate these data in order to see what information they provided concerning sediment discharge in Ghanaian rivers. Secondly, current and anticipated sedimentation problems in rivers which required immediate study were to be highlighted. Thirdly, all network design proposals which had been made in the past, but which had not been implemented were to be reviewed. Finally, using all the information gathered above, and having regard to present resources, proposals were to be formulated for a network which would fill gaps in the existing knowledge and also focus on immediate sedimentation problems.

This paper reports the findings of the first three parts of this project and the recommendations regarding the basis for the design of the network.

EXISTING SEDIMENT DISCHARGE DATA, PREVIOUS NETWORK DESIGN PROPOSALS, AND CURRENT SEDIMENTATION PROBLEMS

Existing sediment discharge data

The two sources from which data were collected, were reports on investigations of individual water projects, and reports on sediment measurements and analysis derived from planned networks.

Data from project investigations reports These were assembled for the Volta and the southwestern and coastal drainage basin systems respectively. For the Volta basin, the first reference to sediment

Sediment transport measurement in Ghana 303

Internat ional Boundary.

— •— — Drainage Boundary,

À Sediment Sampling Station

(FAO) Food and Agricul ture Organization

(HP,USSR)'Hydroproject',USSR.

(NK) Nippon Koei

(N) NEDECO

(A&T) Ayibotele and Tuffour-Darko

(HP,CZC) Hydroprojett; Czechoslovakia (AES&TAH) AESC

FIG.l River systems and sediment sampling stations.


discharge was a reported impression obtained through visual observation of river discharge during the investigation into the Volta River Project for a dam to generate hydroelectricity for alumina smelting at Ajena (Sir William Halcrow & Partners, 1950). The view was expressed that silting was unlikely to present any serious problem in the Volta River.

The next reference to sediment discharge was by the Netherlands Engineering Consultants (NEDECO, 1959a) who investigated the Angaw River for an irrigation project. They reported that sediment concentrations in the Angaw River were low and in the range 70-100 mg l-1. NEDECO (1961), again carried out investigations on the Lower Volta for a bridge on the river at Tefle, measured bed load transport, and reported that the maximum and minimum particle sizes at Tefle were 8.7 and 0.06 mm respectively. Between 1962 and 1963 suspended sediment discharges were measured at Bui on the Black Volta River for the Bui hydroelectric project. Five measurements were taken from October to December 1962 and 18 in 1963. The results

TABLE 1 Suspended sediment loads in the Volta basin measured in 1965 (FAO, 1967)

Station

Pwalugu Yarugu Wiasi Nangodi Ekumdipe Nasia Nakong Kalbuipe Benja Nakponduri Yendi Navrongo Garu Nabousongo

River

White Volta White Volta Sissili Red Volta Daka Nasia Sissili Sorri Kulaw Morago Daka Tano Tamne Pembik

Drainage area (km2)

57 41 11 10

7 4 6 2 1 1 1

460 940 970 650 100 400 810 640 870 460 140 650 410 400

Susp. sed. concentration: (mg 1' Max.

218 244 147

91 71 56 91 70

101 214

95 74

274 52

-1) Min.

43 37 18 36 11 10 29 10 39 26 18 3.5 34 27

Gauge (m)

Max.

6.7 2.4 5.7 3.0

10.0 6.3 1.7 3.4 3.2 1.0 3.3 0.5 0.8 1.5

reading:

Min.

2.5 0.8 1.2 0.9 5.5 3.3 1.3 1.7 1.3 0.3 2.0 0.3 0.2 1.1

TABLE 2 Results of suspended sediment sampling undertaken by Nippon Koei in 1966

Station River Drainage area (km2)

Susp. sed. concentration (mg I'1): max. Min.

River discharge (m^s*1):

Max. Min.

Susp. sediment yield (t year'1)

Max. Min

Pwalugu White 57 460 440 40 356.5 7.4 2 474 000 9271 Volta

Nasia Nasia 4 400 25 12 94.9 9.9 53 840 3760


were reported by Hydroproject USSR (1964). They gave the maximum sediment discharge as 255 kg s-1 and the minimum as 0.35 kg s-1. The maximum sediment concentration was determined as 206 mg l-1 and the minimum as 4.4 mg l-1. The sediment yield for 1963 was estimated to be 0.714 x 10 t, with an annual mean suspended sediment discharge of 22.2 kgs-1 and average concentration of about 45 mg l-1. From geological investigations, the sediments on the bottom of the Black Volta at the Bui site were reported to contain 70-80% coarse and medium grained sands.

The next and more significant sediment measurements in the Volta basin were carried out by the FA0 (1967). They were undertaken in connection with a land and water resources survey for the upper and northern regions of Ghana. Measurements were made at nine stations on the White Volta, two each on the Volta River proper and Oti River, and one on the Black Volta as shown in Fig.l. The results are shown in Table 1.

The last reported sediment measurements in the Volta basin were by Nippon Koei (1967a,b). Following their feasibility study on the Lower White Volta Development Project for irrigation, hydroelectric, flood control and domestic water supply, Nippon Koei reported on the results of sediment sampling and analysis at Pwalugu on the White Volta and at Nasia on the Nasia River. Table 2 gives the results.

In the southwestern and coastal drainage basin systems reference to sediment characteristics was made for the first time by Hydro-projeltt Czechoslovakia (1964). This was made in the report on the feasibility studies for hydroelectric projects at Tanoso and Hemang on the Tano and Pra Rivers respectively. Suspended sediment particles were described as consisting of silt with an effective mean diameter of less th an 0.08 mm and the bed load as comprising boulders, pebbles, gravels and sand. Nippon Koei (1967a) reported the next measurements of sediment discharges. These were carried out as part of their feasibility studies for the comprehensive development of the southwestern rivers. Measurements were carried out at two stations on the Tano River and at one each on the Ankobra, Pra and Ayensu Rivers. The results are shown in Table 3.

A comment on the sediment discharge of the Tordzle River, one of the rivers in the coastal drainage basin, was expressed by Wakuti (1968). In their feasibility study of the Avu-Keta Lagoon for flood control and irrigation, they reported that no transportation of sediment was observed, and that silting of the planned reservoir on

TABLE 3 Results of suspended sediment sampling undertaken in 1966, 1974 a.nd 1976 by Nippon Koei, AESC and WRRI

Station

Twi fo Praso Alenda Jamuro Preste<a. Manhia Nsuaem

River

Pra Tano Tano Ankobra Densu Ayensu

Drainage area (km2)

20 750 15 850 10 4O0

4 280 2 120 1 500

Susp. sed. concentration (mg 1"

Max.

385 44 70 59

658 470

- 1 ) : Min.

181 41 45 57 12

170

River discharge (m's-1

Max.

509.1 313.8 106.2 149.0 106.1

16.5

) : Min.

66.7 190.0

24.4 5.0 0.8 0.3

Susp. sediment yield (t year'1):

Max.

6 100 OOO 436 OOO 233 800 271 700

1 071 700 245 300

Min.

385 246

34 91

1

400 400 750 620 510 460

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The £>ed load rating curve for the Densu River at


the Tordzie River was not to be expected. Ayibotele & Tuffour-Darko (1974) carried out sediment measurements

and analysis on the Densu River at Manhia as part of the overall water resources assessment of that basin. Sediment rating curves for suspended and bed loads were established as shown in Figs 2 and 3. The bed load particle size was found to range from 0.67 to 0.3 mm, while the suspended load particle size ranged from 0.1250 to 0.O061 mm.

In 1974, the Architectural and Engineering Services Corporation (AESC) of Ghana in partnership with Tahal Consulting Engineers of Israel (TAHAL) carried out similar measurements on the Densu River at Manhia to estimate the amount of sedimentation to be expected in the new Weija Reservoir for the water supply of Accra-Tema (AESC, 1975). In 1977 the WRRI carried out sediment sampling and analysis at Nsuaem on the Ayensu River for the AESC, in connection with a proposed water supply reservoir on the river. The results of these measurements are also shown in Table 3.

Data measurements based on planned networks Between 1971 and 1976 the Water Resources Research Institute carried out sediment discharge measurements and analysis for both wash load and bed load on the southwestern and coastal rivers, namely the Ankobra, Pra, Nakwa, Amissa, Ayensu, Alabo and Tordzie. The rivers and stations were selected from the design of a sediment discharge, monitoring network which had been proposed by Ayibotele (1973). The exercise was not related to any specific water development projects, but was intended to be the start of sediment discharge monitoring on a continuous basis. The results were reported by Ayibotele & Tuffour-Darko (1979). While suspended and bed load rating curves were established from the data available for each station, regional rating curves were also developed for suspended load and bed load. These two curves are shown in Figs 4 and 5.

In addition to the rating curves, the particle size distributions were determined for both the suspended load and the bed load. This work could not be extended to the Volta drainage basin system because even for the southwestern and coastal river system the monitoring could not be continued due to lack of personnel and logistic support.

Previous network design proposals

The first effort to establish a planned network for continuous sediment discharge data collection was recommended in a report by the Nathan Consortium for Sector Studies (1969). The consultants were contracted to assess the surface water and groundwater resources of Ghajia to meet the demand for water for socio-economic development up to the year 2000. As part of their report they proposed a programme of sediment sampling beginning with an initial network of three stations on the Rivers Densu, Volta and White Volta. This was to be increased by three to four stations each year until an optimum network had been achieved. This recommendation was not implemented.

In an attempt to rationalize the then hydrometerological and hydrolo-gical data collection networks, and to plan for the inclusion of hydrolo-gical variables for which no data were being systematically collected (e.g. water quality and sediment discharge) the Water Resources


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INSTANTANEOUS SUSPENDED SEDIMENT DISCHARGE (Metric tons day -1)

FIG.4 The regional suspended sediment rating curve for southwestern and coastal rivers.

Research Institute made proposals for comprehensive new designs for hydrometeorological and hydrological data collection networks for Ghana. This work was reported by Ayibotele (1973). The proposals included a network for sediment discharge monitoring for each river basin. The network was dense and would have been expensive to set up and operate. A scaled down network which focused on the most important areas was formulated by Ayibotele (1974). It consisted of 46 stations to be set up over a 5-year period at a total investment cost of $33 000 (US). Apart from the partial effort to establish the network in the southwestern and coastal river basins which was mentioned earlier, the network was not established.

Current sedimentation problems

In 1979 the Water Resources Research Institute carried out a survey to assess the relevance of its research programmes and projects to the work of its user agencies, and also to establish the hydrological and water resources problems which they faced and which required investigation. This was reported by Ayibotele (1980). In the area of erosion and sedimentation problems, Table 4 summarizes the problems that the organizations wished to be investigated.


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FIG. 5 The regional bed load rating curve for southv/estern and coastal rivers.

ASSESSMENT OF DATA AND INFORMATION COLLECTED

Within the Volta basin system, sediment discharge measurements have been made at 15 stations on the White, Red and Black Voltas and on the Oti River. On the Volta River proper no measurements have been made on the right bank tributaries. The most significant work was done by the FAO, and this concentrated on the suspended sediment loads. It gave the maximum and minimum concentrations and the corresponding gauge readings, but did not analyse the particle size distribution. In the case of the work carried out by Hydroproject USSR at Bui, detailed analysis of sediment concentrations and the mean annual sediment discharge appears to have been undertaken, but no information is provided about the particle size distribution. In the case of bed load, no assessment was made of the discharge but the particle size was described. The work by Nippon Koei also focused on the suspended sediment. They determined concentrations and the corresponding annual sediment yield but did not determine the particle size distribution. No sediment rating curves have been developed either for the individual stations or for the region.

For the southwestern and coastal river basin system, measurements


TABLE 4 The sedimentation problems suggested for investigation in the survey

Organization Sedimentation problem

Ghana Highway Authority, Data required for construction of Oti Accra bridge and Jomuro bridge over Tano River

State Gold Mining Corp. , Determine effect of gold dredging on Tarkwa sediment discharges in the Offin River

Ghana Water & Sewerage Investigate sedimentation around the Corporation, Sunyani intakes of the Techiman, Sunyani ,

Acherensua and Teppa Water Supplies

Volta River Authority Determine general and point of maximum sedimentation in the Oti River

have also been carried out at 16 stations. In all cases attention focused on the suspended load. Only in the studies of AESC (1975) and Ayibotele & Tuffour-Darko (1974, 1979), was bed load considered. In addition both studies describe the particle size of the suspended sediment and the bed load. Again, both establish rating curves for the stations at which measurements were made, but Ayibotele & Tuffour-Darko (1979) went further to establish regional rating curves.

Compared with the Volta basin, it can be said that knowledge concerning sediment transport in the southwestern and coastal river basin system is better, in the sense that more work has been undertaken on bed load transport and on particle size distribution. There is, however, a need to know more about the sediment discharge at higher river flows. At present hardly any information is available about sediment discharge in the Volta downstream of Akosombo. The individual station rating curves, and the regional rating curves need to be confirmed by additional measurements and analysis.

The sediment problems mentioned by the water development agencies (Ayibotele, 1980) deal with problems that have arisen following construction of structures in rivers, and problems that are expected to arise after construction of such structures. The projects are, or will be, located on sections of rivers for which no information concerning their sediment discharge is currently available. Additional knowledge will be provided by investigations into these problems.

The network proposals of the Nathan Consortium for Sector Studies were modest and would have been easier to implement than the more ambitious proposals of Ayibotele (1973). However, the scaled down network proposed by Ayibotele (1974) was realistic and could have been set up if the funds and personnel had been available. Hence the two designs should form the basis for any new design proposals.

SUMMARY OF CONCLUSIONS AND RECOMMENDATIONS

A search through the records collected over the past 20 years has revealed a significant amount of data on sediment discharge in the rivers of Ghana. The data are, however, of varying degrees of quality


completeness and uniformity of presentation. This may be due to different methods of sampling and analysis, and also to the relative importance of the sedimentation problems anticipated in the water projects investigated.

Whatever their shortcomings, the data and other information that have been assembled constitute an important source of information about sediment discharges in Ghanaian rivers and cannot be ignored in any attempt to design and establish a network for monitoring on a continuous basis.

From an assessment of the data, it is clear that within the Volta drainage basin, and upstream of the Volta Lake, more is known about suspended sediment discharges than about bed load movement. However, information concerning the particle size distribution of the suspended load is lacking and needs to be obtained. There is also a lack of knowledge regarding what relationship, if any, exists between river runoff and sediment discharge per unit area of land. It is therefore recommended that the new network should first aim to fill these gaps . No information is currently available on the sediment discharges of several important tributaries of the Volta River (e.g. Afram, Obosom) which flow into the Volta lake. The new network should therefore build up knowledge for these sections of the Volta River.

For the southwestern and coastal drainage basins, existing information concerning suspended sediment and bed load transport is appreciable, although more is known about the suspended,load. However, it is clear that additional measurements at higher river discharges and further measurements to confirm the station and regional rating curves are required. It is recommended that any new network should be designed to address these issues.

The findings of the study of the current sediment problems facing water development agencies represent an important factor which the new network must take into account. As a basis for arriving at a network which address the above problems, it is recommended that the proposals of the Nathan Consortium for Sector Studies (1969) and of Ayibotele (1974) should be used and modified to take into account the above findings.

REFERENCES

AESC (1975) Accra-Tema Water supply Project - New Weija Dam Sedimentation study. Architectural and Engineering Services Corporation, in collaboration with Tahal Consulting Engineers Ltd.

Ayibotele N.B. (1969) An Historical Sketch of Water Resources Devel opment in Ghana .

Ayibotele N.B. (1973) Hydrological and Hydrometeorological Data Collection Networks for Ghana - New Proposals.

Ayibotele N.B. (1974) Plan of Implementation for Hydrological and Hydrometeorological Data Collection Networks for Ghana - New Proposals.

Ayibotele N.B. (1980) unpublished survey of relevance of current research projects.

Ayibotele, N.B. & Tuffour-Darko, T. (1974) Report on Sediment Measurements in the Densu River at Manhia.

Ayibotele, N.B. & Tuf f our-Darko, T. (1979) Sediment Loads in the

312 S.A.Akrasi $ N.B.Ayibotele

Southern Rivers of Ghana. FAO (1967) Land and Water Survey of the Upper and Northern Regions

of Ghana. Sir William Halcrow & Partners (1950) Preliminary Report on the

Development of River Volta Basin to the Gold Coast Government. Hydroprojekt Czechoslovakia (1964) Tanoso and Hemang Developments

Feasibility Study, vol. 1. Hydroprojekt Czechoslovakia in cooperation with Technoexport.

Hydroproject USSR Hydroproject (1964) Bui Hydroelectric station on the Black Volta River of Ghana, vol. 1.

Nathan Consortium for Sector Studies (1969) Sediment sampling Program for Ghana.

Nathan Consortium for Sector Studies (1970) Water Resources Development in Ghana, General Report.

NEDECO (1959a) Angaw Basin Project. NEDECO (1959b) River Studies of Niger and Benue. NEDECO (1961) Feasibility Report on Lower Volta Bridge at Tefle. Nippon Koei Co. Ltd. (1967a) The Comprehensive Development Project

of Water Resources in South Western Ghana. Nippon Koei Co. Ltd. (1967b) The Lower White Volta Development

Project. WRRI (1977) Report on Analysis of Sediment Samples From the Ayensu

River of Nsuaem. Wakuti (1968) Feasibility Study for Avu-Keta Project.

an appraisal of sediment transport measure ment in ghanaian...

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