reservoir sedimentation: the case of the opa reservoir ...

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RESERVOIR SEDIMENTATION: THE CASE OF THE OPA RESERVOIR CATCHMENT, SOUTHWESTERN NIGERIA A. ADEDIJI

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Department of Geography, Obafemi Awolowo University, Ile-Ife, Nigeria Version of record first published: 01 Feb 2012.

To cite this article: A. ADEDIJI (2005): RESERVOIR SEDIMENTATION: THE CASE OF THE OPA RESERVOIR CATCHMENT, SOUTHWESTERN NIGERIA, South African Geographical Journal, 87:2, 123-128 To link to this article: http://dx.doi.org/10.1080/03736245.2005.9713835

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ABSTRACT This study aimed at determining the rate of sedimentationin Opa Reservoir since impoundment in 1978 with a view of ascertaininghow much of its volume has been silted up. The volume of sediment trapped in the reservoir since impoundment was estimated using the bathymetric survey method. Based on reservoir profiling, the volume and mass of deposited sediments since impoundment in 1978 were estimated at 1 313 397m3and 893 096 tomes, respectively.The rate of sedimentation and specific sedimentyield from the reservoir catchment wqe estimated at around 38 OOOt'yr and 575tAun2/yr,respectively. The annual rate of sedimentationobtained in this study highlights the need for more frequent bathymetric surveys especially after large floods to provide more information on the magnitude of annual sedimentation, and to help planners determine how to prolong the economic life of reservoirs.

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Introduction N.W. 263). The dam and the associatedreservoir were built on the Opa River within the confines of the University estate in 1978. The reservoir on the Opa River was approximately 2.5km long and 0.5km at its widest point. It had a total impounded volume of 2.81 million m3 of water on completion (Capital Project and Development Unit, OAU, Ile-Ife). It provides treated water to the UniversityCommunity and supports recreational fishing. The reservoir has successfully reduced flooding incidences downstream of the dam. The dam spillway is 25m wide and 55m long.

Accelerated erosion with attendant sediment transportation has been identified as an endemic problem in the tropics particularly in the humid and subhumid tropics (Rapp et al., 1972; Rapp, 1975; Jeje, 1987). Apart from causing the loss of valuable soil resources from farmlands in the affected drainage basins, it has resulted in accelerated reservoir sedimentation, with implications for severe reduction in the designed water storage capacity of reservoirs. This has been observed in reservoirs all over the tropics, particularly in southwestern Nigeria (e.g. Ado-Ekiti, Ede, Owo, Asejire). In fact, Jica (1994) observed that about 650 existing dams in Nigeria were rapidly being silted up. Among the consequences of severe reservoir sedimentation is shortage of potable water supply to the communities that these reservoirs were designed to serve.

The natural vegetation of the area is tropical rainforest characterized by emergents with multiple canopies and lianas. However, widespread and persistent practice of rotational bush farming, coupled with widespread cultivation of cash crops such as cocoa, kolanuts and plantains has led to the destruction of the original vegetation. In fact, as shown in Figure 2, the area around the reservoir is intensively cultivated with arable crops. The reservoir basin is underlain by granites, undifferentiatedschist-gneisses,pegmatilised schists,pegmatites and epidiorite. The Opa river channel at the university campus is on quartz-biotite schist (Konsadem Associates, 1987). The study area falls under Koppen's Af humid tropical rainforest climates. The mean daily minimum temperature is 25°C while the mean maximum is 33°C (Adejuwon and Jeje, 1975). Mean annualrainfdl is about 1 40Omm, with the miny season extending from April to October. The rainy season is marked by two maxima, in June/July and September/October.These two periods are separated by a short dry spell in August. The beginning and end of the rainy season are usually marked by high intensity thunderstorms.

Several studies have been carried out on reservoir sedimentation(e.g. Rapp et al., 1972; Rapp, 1975 in Tanzania; Adam, 1996 in Poland; William, 1996 in Miami, USA and White et al., 1996 in the southern Pennines, UK). Also, reservoir re-survey data provided by a large number of authors for over 300 impoundments worldwide are summarized in White's ( 1993) work. However, except for the pioneering work of Rapp et al. (1972) and Rapp (1975) in Tanzania, studies of water bodies and reservoir sedimentation in the tropics are rare. The volume and rates of reservoir sedimentation are still to be scientifically documented in most parts of the humid tropics, including southwestern Nigeria. Thus, this study sets out to provide some information on reservoir sedimentation in this part of Nigeria. This study is especially important since the Opa Dam Reservoir (the study reservoir) was impounded in 1978 and nothing to date has been establishedregarding its rate of sedimentation. The findings of this study will not only provide information on the volume of sediment deposited in the study reservoir to date, it will also provide an indication of the rate of reservoir sedimentation in the general study area. In addition, this study will shed light on the use of reservoir surveys as a means of deriving catchment sediment yields (e.g. Rausch and Heinemann, 1984) with the aim of overcoming the limitations inherent in short term suspended sediment sampling (see Walling and Webb, 1981).

Methods A land use/vegetationmap of the study area (Opa Reservoir Catchment) was compiled from SPOT images (SPOTXS IFE IMG, 1989) obtained from the Regional Centre for Training in Aerospace Surveys (RECTAS), OAU, Ile-Ife. Land uses were identified and classified from the SPOT images (1 420 by 1 420 pixels) based on colour, texture, shape and size using ILWIS 3.0 software (International Institute for Aerospace Survey and Earth Sciences, the Netherlands, 1997).

Study area The Opa Reservoir catchment extends from the Obafemi Awolowo University campus to Osu in Atakumosa and constitutes the study area (Figure 1). The study area is approximately 68km2in area. The area lies between latitudes 7"27N and 7'35" and longitudes 4"30'E and 4"40'E (Federal Survey Topographical sheets, Ilesha S.W. 243 and Ondo

Recent interest in sedimentary processes has focused attention on the application of radionuclides as geochemical tracers to determine sedimentation rates in aquatic environments (see Duck and McManus, 1994). The *l0Pbchronology in association with 137Cshas been used to determine the sedimentation rate. However, due to the absence of this dating facilityand technology 123


Figure 1: The study area and Opa Reservoir Catchment.

Reservoir

Basin Divide

A corer manufactured locally from a transparent plastic pipe with a diameter of 2.50cm was used to take core samples of deposited sediments in the reservoir. The diameter and height of sedimentslmud trapped in the plastic pipe was used to determine the volume of the cores. Samples of the deposited sediments were taken from ten different locations between the dam long axis and the end of the reservoir. The samples were taken to the laboratory for the determination of bulk density. Following oven drying, bulk density of the deposited sediments was calculated as:

in Nigeria, the rate of sedimentation of Opa Reservoir was determined using the bathymetric survey method as outlined by Heinemann and Dvorak (1965). In this regard, the profiling of the reservoir was carried out between February and March, 2002. A longitudinaltraverse was cut along the side of the reservoir and profiling was carried out at intervals of 50m. The profile lines were made parallel to the dam axis with the aid of a prismatic compass; full details of the procedures involved are documentedelsewhere (see Adediji, 2002). Depth measurement was conducted at 10m intervals along each of the profiles, with the exception of a few which were less than 20m in length. The values were then used to produce a bathymetric survey and cross-sectional diagram of the reservoir bed by plotting depth against distance from the edge of the reservoir.

Bulk density of deposited sediments - Wt. of oven-dried sediment (glcm’) Vol. of the core sampler The average bulk density (g/cm3)of samples of deposited sediments was used to determine the mass of sediment trapped in the reservoir since impoundment. This was done by simply converting the bulk density value in glcm’ to kg/m3 and subsequently the deposited volume in m3 was multiplied by weight of sediment in kglm’. The result obtained was divided by the total number of years since impoundment (i.e. 23 years) in order to determine the rate of sedimentation per annum (tonneslyear). The specific sediment yield of the reservoir catchment was obtained by dividing the annual sedimentation by the reservoir catchment area (68km2).

The volume of water (m3)between the profiles was calculated as the average of the cross sectionalarea of the profiles multiplied by length of interval (5Om) between all the profiles (50 profiles) across the reservoir and summed to obtain the current total volume of water in the reservoir. The current volume of water in the reservoir obtained was then substracted from the total impounded volume (2.81 million m3) when the dam was constructed. This was done to determine volume of sediment deposited (volume of reduction) in the reservoir since impounded. 124


Figure 2: Classification of SPOT-XS sub-scene of the study area.

The Opa river was gauged at the main point of inlet into the reservoir to monitor sediment inflow. In addition, the gauge at the dam spillway was used in order to determine sediment outflow from the reservoir outlet during the study period (i.e. between June, 1999 and June, 2000). Reservoir-sediment trap efficiency was also determined. This requires accurate measurements of all sediment transported into the reservoir as well as the sediment discharged through the spillway. Reservoirsediment trap efficiency (E), usually expressed as a percent, is the ratio of the weight of sediment entering a reservoir as to the weight that is trapped therein:

E =

Results and discussion Land usehegetation types The land usehegetation classification of Opa Reservoir Catchment from the SPOT images of the study area using ILWIS 3.0 software revealed the following merged classes of land usehegetation types (Table 1). Reservoir sedimentation Figures 3a-g and Figure 4 show the current cross-sectional shape of the reservoir bed and bathymetric map of the reservoir

Sediment inflow - Sediment outflow x 100 Sediment inflow Classes

Suspended iediment yield at the point of inlet and outlet of the reservoir was obtained using the sediment rating curve technique (see Walling, 1977). Streamflow discharge was monitored using a staff gauge at the point of inlet on Opa river and at the Opa dam spillway (outlet).The staff gauge reading was observed twice a day, in the morning and evening. Streamflow discharge was determined using the velocity-area technique. Streamflow velocity was measured with a Valeport BFM current meter but during flood condition when it proved difficult to use the meter, a surface float was used to determine the velocity (see Gregory and Walling, 1973).

Built-up

I G o a , kola,oil palm, etc. 1

I

The water samples taken during the streamflow discharge measurement were analysed for suspended sediment concentration using standard laboratory methods (see Davis and De Wiest, 1966). Determination of suspended sediments involved the filtration of each 200ml of stream water using Whatman filter paper and a vacuum pump assembly, oven drying, cooling in desiccator and weighing the sediment residue. 125

Cultivated tieldlfallow vegetation

I

Percentage (%) of the Study Area

Area Extent (km2)

8.46

5.75

I

38.81 23.73

'

I

26.39 16.14

Hydrophytes

3.73

2.54

Rock outcroplbare surface

2.84

1.93

I Sec. regrowthlrelictof forest I

20.29

I

13.80

Water-bodylreservoir

0.19

0.13

Non-classified point (reject)

1.95

1.33

Total

100

68.00


Figure 3: Cross-sectional diagrams of the reservoir bed obtained along the profiles.

twenty or so years is 38 830.26tlyr and 575.26t/km2/yr, respectively. The specific sediment yield obtained for this reservoir catchment, while lower than the values obtained (1 1 0 4 W y r ) from the reservoirs studied in the USA by White et al. (1996), the value is greater than those obtained from other parts of Africa (259t/km2/yr)and Asia (293t/km2/yr)by White (1993).

respectively, based on the data obtained through the profiling of the reservoir. Water depth at the sides of the reservoir is naturally shallower than at the middle of the reservoir. This is also an indication of current scouring of the reservoir bed in the rainy season in association with storm discharge. In addition, the shallow depth of water at the sides of the reservoir indicates that the edges of the reservoir are silted up. This confirms the observations of Bourman and Bamett (1995) in South Australia where it was found that the margins of the terminal lake on River Murray have silted up by lOdyr. The sediment trapped at the reservoir margin encourages the growth of hydrophytes such as ferns and water weeds among which are Pistia stratiotes, Scirpus cubensis and Rhnchospora corymbosa. Other common plants include Lemna spp., Wo@a arrhiza, Nymphaea spp; Ipomoea aquatica and Ceratophyllum demersum. In fact, these water weeds have virtually completely colonized the periphery of the reservoir. The effect of these aquatics is to increase the effective surface area of the reservoir and, through transpiration especially in the dry season, lower the water surface.

Sediment outflow through the Opa Dam Spillway during the study period was determined as 35 1.73t/yr and sediment inflow into the reservoir through the main point of inlet on the Opa River was estimated at 1 727.41tlyr. Therefore, using the sediment budget method, the total sediment retained in the reservoir was calculated at 1 375.68 tonnes. The reservoirsediment trap efficiency (E) is thus calculated at 79.6%. The sediment deposited in the reservoir through the main point of inlet was lower than the mean annual sedimentation of the reservoir estimated through bathymetric survey. This may be accounted for by soil eroded and transported fiom slopes around the reservoir as well as under-estimation resulting from the sediment rating curve used for estimating sediment yield at its main point of inlet on Opa river (see Walling and Webb, 1981). This further confirms the reservoir survey method as an accurate means of deriving catchment sediment yield and one that overcomes the limitations inherent in short term suspended sediment sampling (Rausch and Heinemann, 1984). As shown in Figure 2, bare surfaces are indicated by yellow colours on both sides of the reservoir reveal the sites where laterite was minedremoved for construction purposes. This could have led to high volume sediment loss from slopes into the reservoir. It is also evident from Figure 2 that, apart from bare surfaces, most of the area around the reservoir was intensively cultivated with arablelannual crops (cultivated field crops). This would

The volume of water in the reservoir at the time of measurement was calculated at 1 496 623.0m3,while the total impounding volume of water in the reservoir was 2.8 million m3, which implies that water had been reduced by 46.7% of its impounded volume. The estimated volume of deposited sediment since impoundment was 1 313 377.0m3. This volume of deposited sediments per km2 (839m3/km2/yr)obtained in this study compares to the sediment yield of 729m3/km2/yrobtained by Rapp (1975) from the Matumbulu reservoir basin in Tanzania. At a mean bulk density of 0.68g/cm3, the mass of deposited sediments (sediment trapped) since impoundment in 1978 is calculated to be 893 096 tonnes. Thus, the rate of sedimentation and reservoir catchment specific sediment yield for the past 126

Figure 4: Bathymetric survey map of Opa Reservoir.

likely increase sediment yield from slopes draining directly into the reservoir. As observed during the field survey, bush paths created by (illegal) fishermen would further contribute to the volume of sediment into the reservoir. In addition, the common farming practice in the area of using maximum tillage in the form of heap and ridge making enhances the generation of large volumes of runoff loaded with sediments.

Conclusion


This study presents a bathymetric survey of Opa reservoir with the aim of determining the volume and mass of sediment trapped in the reservoir since impoundment as well as the specific sedment yield of the reservoir catchment. The sedment inflow into the reservoir through the main point of inlet on the Opa River was determined at 1 727.41t/yr while sediment outflow over the Opa Dam Spillwaywas calculatedat 351.73Vyr. Thus, the total sediment mass trapped in the reservoir through its point of inlet during the study period was calculated at 1 375.68 tonnes. However, the result of the reservoir profiling shows that the volume and mass of deposited sediment since impoundment in 1978 were 1 313 377.0m3and 893 096 tonnes, respectively. Thus, the rate of sedimentation per annum was estimated at 38 830.26tlyr. The rate of annual sedimentation obtained in this study has further shed light on the need for more frequent bathymetric surveys especially after large floods to provide more information on the magnitude of annual sedimentation.Also, with the findings of this study on annual sedimentation, there is need to integrate anti-erosion measures before dam constructionif the life span of any proposed reservoir/ dam is to be prolonged. In this regard, the participation of the local population in anti-erosion management is an important requirement. Thus, the local population should be enlightened as to the consequences of accelerated erosion. In the case of the Opa reservoir, the university authority should urgently take measures to clear hydrophytes and other water weeds which have colonized the area around the periphery of the reservoir if its life span is to be prolonged.

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Adediji, A., 2002: Sediment delivery ratios in Opa reservoir catchment, southwestern Nigeria.Unpublished PhD thesis, Department of Geography, Obafemi Awolowo University, Ile-Ife, Nigeria. Adejuwon, J.O. and Jeje, L.K., 1975: Land element of the environmental system of Ife area. In: Ojo, A. (ed.), Environmental Resources Base Project. No. 2, Department of Geography, University of Ife, Ile-Ife.

Konsadem Associates, 1987: Feasibility report of the study and design of Opa River Flood Control Works. Onireke Printing Press, Ibadan, Nigeria, 106 pp.

Bowman, R.B. and Barnett, E.J., 1995: Impact of river regulation on the terminal lakes and mouth of the River Murray, South Australia. Australian Geographical Studies, 33 (l), 101-115.

Lahlou, A., 1996: Environmental and socio-economic impacts of erosion and sedimentation in North Africa. International Association of Hydrological Sciences (IAHS), Special Publication, 236,49 1-500.

Duck, R.W. and McManus, J., 1994: A long term estimate of bedload and suspended sediment yield derived from reservoir deposits. Journal of Hydrology, 159,365-373.

Rapp, A., Murray-Rust, D.H., Christinsson, C. and Berry, L., 1972: Soil erosion in four catchments near Dodoma, Tanzania. Geografiska Annaller, 54A (3-4), 255-318.

Gregory, K.J. and Walling, D.E., 1973: Drainage Basin, Form andprocess: a Geomorphological Approach. Edward Arnold, London, 158 pp.

Rapp, A,, 1975: Soil erosion and sedimentation in Tanzania and Lesotho. Ambio, 47 (4), 154-163.

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Renwick, W.H. 1996: Continental scale reservoir sedimentationpatterns in the United States. InternationalAssociation of Hydrological Sciences (IAHS), Special Publication 236,5 13-522.

White, P., Habadz, J.C. and Butcher, D.P., 1996: Sediment yield estimates from reservoir studies: an appraisal of variability in the southern Pennines of the United Kingdom. International Association of Hydrological Sciences (IAHS), Special Publication 276, 164-173.

Walling, D.E. and Webb, B.W., 1981: The reliability of suspended load data. International Association of Hydrological Sciences (IAHS), Special Publication 133, 177-194. White, P., 1993: The reliability of suspended sediment yield estimates from reservoir studies, an appraisal of variability in the southern Pennines of the United Kingdom. Unpublished Ph.D. thesis, University of Hundersfield,U.K


A. Adediji Department of Geography Obafemi Awolowo University Ile-ge Nigeria

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