Sedimentation assessment of a small reservoir at afaka forest reserve, kaduna, nigeria

Journal of Environment and Earth Science www.iiste.org
ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online)
Vol. 3, No.9, 2013
183
Sedimentation Assessment of a Small Reservoir at Afaka Forest
Reserve, Kaduna, Nigeria
Onwuegbunam D.O.1*
, M.A. Oyebode2
, Onwuegbunam N.E.1
, Maikano S.1
and Waziri C.H.1
1. Federal College of Forestry Mechanization, Forestry Research Institute of Nigeria, Afaka, Kaduna, Nigeria
2. Agricultural Engineering Department / Institute for Agricultural Research, Ahmadu Bello University, Zaria,
Nigeria
*E-mail of the corresponding author: donancy2001@yahoo.com
Abstract
The sedimentation assessment of a small reservoir constructed in 1987 at the Afaka Forest Reserve, Kaduna,
Nigeria was carried out between 2004 and 2013. The procedure for the assessment involved the peripheral
survey as well as bathymetric survey of the reservoir to determine its water surface area and mean water depths,
by means of a hand-held Global Positioning System, GPS (Model: GPSmap76CSGARMIN) and a 4m leveling
staff, respectively. The study showed that the reservoir storage capacity has decreased, due to sediment build-up,
from its initial design capacity 16400m3
to 10665 m3
, implying a storage loss of about 35%. A linear relationship
was established between the reservoir storage capacity and the age of the reservoir and the result shows a rate of
change (decrease) in the reservoir storage capacity of 221m3
yr-1
. This implies that the reservoir storage capacity
would eventually reduce to zero at reservoir age of 76.5, if desilting is not carried out. For economic
justification, it is recommended that the reservoir be desilted, at least, every 15 years by a capacity of 3280m3
(which is equal to reservoir depth of 0.66m), equivalent to 20% of the design capacity, to ensure continuous life
span of the reservoir. Results of reservoir sedimentation assessment are necessary tools that serve as guide for
safe design of reservoirs for various catchments.
Key Words: Reservoir sedimentation, Storage capacity, Assessment, Catchment, Afaka Kaduna
1. Introduction
Small reservoir (small earth-fill dam or micro-dam) has been defined as a barrier constructed across a river or a
natural stream to impound water, the dam height measured from lowest point of natural ground level being less
than 15m or up to 25m provided the capacity is less than 2 million cubic meters and does not cause significant
damage in life and properties to downstream environment in case of breach (NBCBN 2005). An understanding
of the quantity of sediment deposit in a reservoir is necessary for effective reservoir and basin management.
Sedimentation affects both the useful life and aesthetic quality of a reservoir. Most reservoirs are designed to be
usable for 50 to 100 years, before they are rendered useless by sedimentation (Hotchkiss 1995). The rate of
reservoir sedimentation can be accelerated by natural or human modifications to the watershed (Ongkosongo et
al. 1992; Renwick 1996).
Water being a scarce or limited resource, its efficient use is necessary for increased agricultural production per
unit volume of water, per unit area of cropped land. Hence, following rainfall events, the enormous amount of
water that is often wasted annually as runoff discharged into rivers far away from farm areas can be harnessed
and stored in reservoirs. Depending on the quality, the water can be useful for domestic purposes, irrigation,
potable water for livestock and fish rearing during the dry seasons. Runoff water harvesting and storage into
small reservoirs is a common practice in many rural communities in Africa like Kenya, Uganda, Ethiopia, Egypt
and Uganda (McCartney et al. 2002; NBCBN 2005).
The reservoir under study (Fig. 1) was designed and constructed in 1987 to be able to store 16400m3
of runoff at
full capacity. It is a micro-dam, having capacity less than 2 million cubic meters as defined by NBCBN (2005).
The water source is a runoff stream which drains all storm flow from within the catchment. The annual runoff
volume which can be stored in the reservoir has been evaluated to be in excess of the reservoir; implying that the
reservoir could be dredged to harness more runoff. The reservoir inflow persists even after rainy season to
January when base-flow ceases (Onwuegbunam 2008). Thereafter, the reservoir volume reduces continuously as
a result of use, as well as losses due to evaporation. Presently, the minimum reservoir capacity before recharge in
the following rainy season is at dead storage.
Reservoirs serve a number of different functions but one of the largest is to maintain an area’s water supply. As
means of conserving water for useful purposes during the dry or scarce periods in Kaduna State, Nigeria, the
government has constructed several dams among which are Kufana, Sabon Sarki, Pambegua, Fatika, Matari,
Likarbu and Kuzuntu earthdams; Gimbawa, Kangimi, Shika, Kubani and Bagoma dams (DIS 2013). These dams
are constructed mainly for irrigation, community water supply and recreation purposes. So far, massive siltation
has been reported at Bagoma and Gimbawa dams as well as Matari earthdam (DIS 2013). Inefficient reservoir
and watershed management often lead to massive reservoir sedimentation.

Vol. 3, No.9, 2013
184
The construction of dams or reservoir blocks the flow of sediments downstream recharging streams. This leads
to downstream erosion of the sedimentary depositional environment and increased sediment build-up in the
reservoir. While the rate of sedimentation varies for each dam and each river, eventually all reservoirs develop
reduced water storage capacity due to the exchange of storage space for sediment (McCully 1996). Diminished
storage capacity results in decrease ability to produce hydroelectric power, reduced availability of water for
irrigation, and if left unaddressed, may ultimately result in the expiration of the dam or river (Gregory & Jiahua
1998).
Siltation study for Afaka reservoir was not carried out since construction in 1987, until 2004. The sedimentation
study became necessary because of the continued reduction in the reservoir storage capacity. Accumulation of
sediment from upstream agricultural land may shorten the lifetime of a reservoir and reduce its long-term
benefits (Lee et al. 2013). Progressive monitoring of reservoir siltation is necessary information for the
prediction of storage losses and the probable economic life of reservoirs. The main objectives of this study were:
(i) to evaluate the annual rate of siltation in a small reservoir at the FRIN-JICA afforestation project of Afaka,
Kaduna, Nigeria using bathymetric surveys, (ii) to predict the life span of the reservoir based on observed
parameters.
2. Materials and Methods
2.1 Description of the Study Area
This study was carried out on a small reservoir constructed in 1987 at Sabon Afaka, Kaduna, Nigeria, by the
Japanese International Corporation Agency (JICA) in collaboration with the Trial Afforestation Project (TAP) of
the Forestry Research Institute of Nigeria (FRIN). The reservoir was constructed primarily for the purpose of
nursery plantation irrigation and forest fire fighting. The study area is located between latitude 100
33/
N - 100
41/
N and 070
26/
E - 070
28/
E.
The climate of Sabon Afaka is characterized by a clear distinction between dry and rainy seasons. The rainy
season lasts from mid-April to early October. The climate of this area can be categorized into three main
seasons: the warm rainy season, the cool dry season and the hot dry season (FAO 1971). The mean annual
rainfall is 1266.0mm based on annual rainfall record of forty three years (1969 – 2012) (NIMET 2012). The
general vegetation of the area is classified under the Isoberlinia Savannah or the Northern Guinea Savannah
vegetation characterized by woodland consisting of different layers rather less distinct than those of the forest
(Barbour et al. 1982).
The reservoir catchment has been evaluated to be 25.5ha, with a maximum travelling distance of 762.5m over
the catchment. The elevation over the maximum distance is 21.6m, corresponding to a slope 0.028
(Onwuegbunam et al. 2008).
The reservoir catchment is part of the FRIN-JICA afforestation project, with dominantly sandy clay loam soil
textural classification based on 1987 survey by the Afforestation project (FRIN-JICA 1991). Up to 25cm depth
of the top soil profile has been eroded by surface runoff leaving sandstone outcrops protruding in some parts of
the reservoir catchment (Sobowale 2006).
2.2 Bathymetric Surveys
The reservoir storage capacity for each of the observation years was estimated as a function of the water surface
area and the corresponding mean depth of water. The reservoir perimeter at full storage, just before spilling, was
traversed with the aid of a hand-held Geographical Positioning System, GPS (Model: GPSmap76CSGARMIN)
and plotted using AUTOCAD software. The reservoir surface area was subsequently obtained from the plotting
of the peripheral coordinates. The depth of the reservoir was measured at grid intervals of 20m by 20m along and
across its surface respectively, using floats, anchor and a 4m leveling staff. Hence a mean value was determined
for all the grid points.
2.3 Reservoir and Siltation Parameters
The reservoir and siltation parameters were computed using the formulae presented in Table 1 (Adwubi et al.,
2009; Aynekulu et al., 2009).

Vol. 3, No.9, 2013
185
Table 1
Formulae used for calculating the sedimentation parameters
Parameter Equation Equation Number
Reservoir storage capacity (RSC) = × ℎ (1)
Sediment volume (m3
) = − (2)
Sedimentation rate (tyr-1
) =
×
(3)
Sediment yield (tyr-1
) =
× ×
×
(4)
Area specific sediment yield (tha-1
yr-1
) = (5)
Where,
Ar = water surface area (m2
), h = mean depth of reservoir (m), RSCi = reservoir storage
capacity at an initial year, i (m3
), RSCi+n = reservoir storage capacity n years after i, (m3
), SV =
sediment volume (m3
), SR = Sedimentation rate (tyr-1
), = Bulk density (tm-3
), TE = Trap
efficiency (%), AR = Age of reservoir (yr), ASY = Area-specific sediment yield (tha-1
yr-1
),
SY = Sediment yield tyr-1
, A = Reservoir catchment area (ha)
2.4 Estimation of reservoir trap efficiency
Trap efficiency (TE) is the proportion of the incoming sediment that is deposited, or trapped, in a reservoir or
pond (Verstraeten and Poesen, 2000). To determine the average sediment yield from the contributing
watersheds, the weight of deposited sediment must be adjusted for the reservoir sediment TE. The calculation
proposed by Brown (1943) was used to estimate the TE of the reservoir, as follows:
= 100[1 −
. " #
$
%
] (6)
D is a coefficient with values ranging from 0.046 to 1 and a mean value of 0.1 (Brown 1943). The value of TE
depends on D, which also depends on a reservoir’s characteristics. The D value for this study was estimated to be
1 based on the curve developed by Brown (1943) which relates TE to a capacity–watershed area ratio (C/W).
Brown (1943) suggested that values for D are close to 1 for reservoirs in regions with smaller and more variable
runoff and for those that hold back and store flood flows.
3. Results and Discussion
The perimeter view of the reservoir shown in Fig. 1 is an irregular shape. The irregularity has been as a result of
reservoir bank erosion by contributing runoff streams as well as deposition of silts on the banks of the reservoir.
Fig. 2 shows a portion of the reservoir embankment worn out by the erosive force of incoming runoff stream.
Non-uniformity of reservoir depth was also observed, showing that the extent of siltation is not the same
throughout the reservoir volume.
The reservoir storage capacities, sediment volumes, loss in reservoir storage, bulk density of sediments, age of
reservoir, sedimentation rates, sediment yield and the catchment area-specific yield were determined (Table 2).
No sedimentation data existed for the reservoir between 1987 and 2004. It was ascertained from the Project
authority that desiltation of the reservoir had not been carried out since the time of construction.
The area-specific sediment yield varied over the years with maximum value of 14.0 tha-1
yr-1
occurring in 2013
while the minimum, 4.6 tha-1
yr-1
occurred in 2010. The main factors that contributed to the variation are the
annual rainfall in terms of amount and intensity as well as catchment vegetation and land use over each period of
observation. The catchment land use has been mainly for woodlot development, cultivation of arable crops and
free range grazing by the Fulani cattle.
Table 3 shows the variation of SR and ASY with the periodical annual rainfall. It was observed that SR and
ASY increased with the periodical mean annual rainfall. However, the relationship is not defined as there are
other factors that affect siltation rates, including the intensity of the rainfall, catchment slope, catchment soil

Vol. 3, No.9, 2013
186
characteristics and nature of the deposited sediments. High intensity rainfall is more effective to cause sediment
yield from watershed because detachment susceptibility of soil particles from ground surface is severely affected
by the impacting force of falling raindrop during rainfall. The available rainfall data of the catchment was in
terms of total rainfall amount; rather than intensity.
The reservoir catchment comprised Eucalyptus woodlot of about 70% canopy density (Fig. 3a) and cultivated
areas (Fig. 3b). Sheet and rill erosion have been predominant within the exposed parts of the catchment. Bank
erosion has led to the exposure of the upper zone root system of the Eucalyptus trees along the reservoir bank.
The ASY values are within the range of 3 to 49tha-1
yr-1
with a mean value of 19tha-1
year-1
as obtained by
Tamene (2005) in related studies in Tigray, Ethiopia. The mean global and mean African yields have been
estimated to be 15tha-1
yr-1
and 9tha-1
yr-1
, respectively (Adwubi et al. 2009), while SY ranges between 1.4 to
33tha-1
year-1
for different basin sizes ranging from 15 km2
to 70,000km2
(NEDECO 1997).
Fig. 4 shows a linear relationship between the reservoir storage capacity and age, with R2
value of 0.994. The
equation of the line is expressed as:
= −221 + 16401.4 (7)
Where,
RSC = Reservoir storage capacity (m3
), AR = Age of reservoir (year)
Hence, the rate of change (decrease) of reservoir storage, RSC with age, AR is given by the slope, 221m3
yr-1
.
One implication of this relationship is that the reservoir would have been completely silted up (zero storage
capacity) when its life span is 76.5 years. According to (IJPR, 1988), the useful life of a reservoir is terminated
when its capacity is reduced to 20% of the design capacity. This also implies that the reservoir useful life could
terminate in the next 15 years.
4. Conclusions and Recommendations
The study shows that about 35% of the reservoir storage capacity has been covered with sediments within a
period of 26 years, and an eventual total sedimentation at 76.5 years, which is a serious problem that undermines
the economic life of reservoirs and the associated water uses. Sediment accumulation from the upstream
agricultural and woodlot land may shorten the lifetime of the reservoir thus reducing its long-term benefits. The
rate of sedimentation was found to increase with periodical mean annual rainfall, though other factors affecting
sedimentation were not precisely considered. The study was conducted for only FRIN-JICA reservoir in Afaka.
However, it has been ascertained that there are several similar micro-dams within Kaduna State, some of which
are almost completely silted up. Since dams are constructed for their economic usefulness, progressive reservoir
sedimentation eventually amounts to total economic loss when the reservoirs can no longer perform. Soil erosion
control measures within catchments should be undertaken regularly as erosion has been identified as the basic
means of sediment detachment and transportation into the reservoir. Adequate data on reservoir sedimentation
should be taken as means to guide designers on proper working design of reservoirs for long-term benefits.
References
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Vol. 3, No.9, 2013
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Fig. 1 Perimeter survey of FRIN-JICA Reservoir at Sabon Afaka, Kaduna, Nigeria
Fig. 2. A portion of eroded reservoir embankment showing shape irregularities

Vol. 3, No.9, 2013
189
Fig. 3a. Part of the reservoir catchment comprising Eucalyptus woodlot
Fig. 3b. Cultivated portions of the reservoir catchment

Vol. 3, No.9, 2013
190
Fig.4: Reservoir maximum storage over time as affected by sedimentation
Table 2
Estimated reservoir storage and sediment parameters
Year RSC
(m3
)
SV
(m3
)
SL
(%)
ρb (tm-
3
)
AR
(yr)
SR
(tyr-1
)
SY
(tyr-1
)
ASY
(tha-
1
yr-1
)
1987 16400 - - - 0 - - -
2004 12654 3746 22.8 1.20 17 264.0 264.0 10.4
2007 12083 4317 26.3 1.04 20 197.9 197.9 7.8
2010 11734 4666 28.5 0.97 23 116.3 116.3 4.6
2013 10665 5735 35.0 0.96 26 356.3 356.3 14.0
Table 3
Mean periodical annual catchment rainfall and sedimentation rates
Year 1987-2004 2004-2007 2007-2010 2010-2013
R (mm) 1211.6 1093.0 972.9 2081.6
SR (tyr-1
)
ASY (tha-1
yr-1
)
264.0
10.4
197.9
7.8
116.3
4.6
356.3
14.0
R = Mean periodical annual rainfall (mm)
Age of reservoir, yr
30.0025.0020.0015.0010.005.000.00
Reservoirmaximumstorage,m3
16000.00
14000.00
12000.00
10000.00

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