International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
Spatial Variation of Suspended Sediment
Transport on Omi River, South-Western Nigeria
A.A. Adegbola#1, O. S. Olaniyan*2
#Department of Civil Engineering, LadokeAkintola University of Technology, Ogbomoso,
*2Department of Civil Engineering, LadokeAkintola University of Technology, Ogbomoso,
Abstract
grid reference longitude 3º 5E, latitude 7º 20N. It is situated at
Prediction of rate of sedimentation from any river
an average height of 200m above sea level, drained by three
discharge is important in the design of flood mitigation, dam-
major river basins (Ogunpa, Ona and Ogbere) and surrounded
reservoir and hydraulic structures.River Omi is an un-gauged river
by secondary rainforest as well as a savannah. Spatially, it
located within Ido Local Government Area of Ibadan,South-Western
sprawls over a radius of 12-15 km and experiences a mainly
Nigeria.
Spatial variation in suspended sediment transport was
examined based on turbidity records and measured suspended solids
tropical climate with an estimated annual rainfall of about
1250 mm [1].
from seven stations along 14.1 km reach of the river. The
It has a tropical wet and dry climate, with a lengthy
downstream pattern of suspended sediment concentrations shows an
increase in total suspended solid due to predominant farming
activities in the dry season. It was observed that erosion of the
wet season and relatively constant temperatures throughout
the year. The rainy season span from March through October,
farmland might be a significant source of wash load during the rainy
though August break is always attributed with lower or no
seasons. The upstream tributaries were discovered to be the major
precipitation. This scenario almost divides the wet season into
source of sedimentation. Some additional sediments were observed
two different wet seasons. The dry season runs from
to be added at the midstream, possibly due to the weekly economic
November to February forms the city’s dry season, during
activities at Omi Adio market. The river produced a sediment load of
-1
which Ibadan experiences the typical West African harmattan
-2
1.31 kg /km /year.
Keywords—Turbidity,
[3]. The study area (Ibadan) is shown in Figure 1.
Total
Load,
Flood,
Wash
Load,
Suspended Sediment
The objectives of this study include:
(i)
To measure turbidity value across the across
River Omi.
I. INTRODUCTION
Ibadan (Oyo State, Nigeria) is the largest city in West
(ii)
River Omi in a rapidlyurbanizedcommunity.
Africa and the second largest in Africa, with land size
covering an area of 400 km2. The city is located on geographic
To assesssedimenttransportingcapability of
(iii)
To develop the relationshipbetween discharge,
gauge and sedimentload
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
1978 to 1982 and gauge data from 2007 to 2009. Others are
turbidity, suspended solid, gauge and average velocity data
from 2011 to 2012. There were cases of missing data for some
years due to lack of keeping appropriate record by the
monitoring authority, thus synthetic data were generated to fill
up. Landsat image of the catchment was also used with the
Arch Map software to better understand the study area.
Sediment samples were taken twice in a week and this was
increased during high flood events when turbidity changes
more quickly. A relationship between turbidity and suspended
sediment concentration was established at each location by
sampling a wide range
of turbidities and sediment
concentrations over a year. These were all significant to
the0.05 level. Most storm events have duration of at least 2 to
3 days so, while the daily sample may underestimate actual
peak values, turbidity events will not be completely missed. In
all cases, suspended sediment concentrations were determined
using standard light-weight hand held US DH-48 sediment
Figure 1 Map of part of Ibadan showing Sampling Stations on River Omi
sampler. The procedure used was in line with [5]. The
suspended sediment concentration was computed thus:
II. MATERIALS AND METHODS
Turbidity meter, ultrasonic flow meter, calibrated rope,
=
meter rule and filtering apparatus with 0.45µm filter, were
{(
)
used in the study. The turbidity of the water sample from the
field was measured and later filtered. The filters were weighed
}(
(
)×
)
(1)
=
before and after oven drying. Ultrasonic flow meter was used
/1000/1000/1000 =
to measure the velocity of the river at the surface. Discharge
Where:
records were available from a network of continuous gauging
SSC = Suspended Sediment Concentration (mg/l)
stations. The available discharge and gauge data were within
III. RESULTS AND DISCUSSION
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Sediments are trapped at station Z2 (Aba nla) due to flood
There is great variation in the monthly sediment
control plant within the zone. Sediment concentrations were
concentrations as shown from Table 1. From the statistical
relatively low at the midstream due to trapped sediment by
analysis, the Total Suspended Solids (TSS) effect on each
plant and accumulated debris in the channel. The land use
measuring points are equal since its P value: (0.226)
0.05.
pattern across River Omi is shown in Figure 3. The upstream
Similarly, the monthly variation differs significantly from
stations are dominated by agricultural lands which are sources
each measuring station as its p value: (0.01)
0.05. This is
of sedimentation during rainy season. The urbanized part of
evident in the results obtained from August, 2011 to August,
the catchment is close to the midstream while the downstream
2012.
is a degraded zone with less agricultural activities.
Flow is dominated by flood events that occurred in
In the lower river, at Igisogba and Atere, there were
Ibadan on August 26th, 2011. There are many peaks in
concentration peaks only at dry season characterized by low
suspended sediment concentration values due to wash load
flow pattern. Farming activities, fishing, washing and social
from flooding. A number of general patterns can be observed.
activities on the river may account for higher sediment
Low flow storm responses are more peaked and high sediment
concentration at dry season. As the intensity of precipitation
concentrations are of shorter duration at River Omi. Sediment
increases, the sediment concentration increases as well. The
response
peakedness
increases
within
upstream
and
sediment concentration increases from (0.4 to 6.13) mg/l from
downstream from August to December, 2011 except during
January to August, 2012.
This implies that discharge is
flooding. During flooding (between September-October,
among the deterministic factors of sediment transport on River
2011), the sediment concentration peakedness was recorded at
Omi.
the mid stream and then decreases downstream significantly
Sediment loads have been calculated based on the
with increasing attenuation of the storm responses. Sediment
data
from
April,
2010
till
December,
2012.
They
concentration was at its peak only at upstream from January to
weredetermined using the total daily discharge and the
August, 2012 with the values ranging from (2 - 7.4) mg/l.
suspended sediment concentrations from weekly turbidity
In contrast to the low flow pattern, concentrations
readings. Inevitably errors will result from the use of a single
increase from the upstream stations to midstream and then
daily turbidity value, especially during flood events when
decrease considerably in the two downstream stations, as
sediment concentration is changing more rapidly. The
shown in Figure 2. At Lade, Aba Nla and Railway stations
magnitude of these errors was tested during the August 26th,
(upstream), the more pronounced peaks were associated with
2011 flood in Ibadan, and was relatively small.
flow pattern and farming activities that span across the year.
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Figure 2 Total Suspended Solids across River Omi
Many of the sediment concentration peaks were
associated with minor discharge events where flow was
.
derived from tributary runoff and the resulting mass of
sediment transport was relatively small. Upstream trends in
sediment load and downstream trend in discharge value are
shown in the annual sediment budget and flows for the rivers
based on existing and measured data. Care must be taken with
these results because sedimenttransport in the River Omi is
highly variable and two years of record may not accurately
characterize longer term trends.
TABLE I: Statistical Analysis of monthly Suspended
Sediment Concentration
Analysis of Variance (ANOVA)
SOURCE OF
VARIATION
SS
DF
MS
F
P
VALUE
P
CRITICAL
ROW
1351.763
5
270.353
1.434
0.2267
2.3828
COLUMN
6919.55
11
629.05
3.337
0.00142
1.96754
ERROR
10368.43
55
188.517
TOTAL
18639.744
71
Figure 3 Land-Use Map of River Omi Catchment Area
120 Suspended Sediment Concentration Solids (SSC) of
River Omi
Aug,11
100
Sept,11 record with coefficient of regression of 0.973 from existed
Oct,11
data. The peak discharge is mostly between Augusts to
Nov,11
Dec,11 September from 1978 till date. The gauge reading has
80
SSC (mg/l)
The daily gauge reading relates well with discharge
60
40
Jan,12 decreased by 36.3percent from 1980 till date (Figure 4-6).
20
Feb,12
0
-20
0
2
4
6
Station (Z)
8
Total Suspended Load on River Omi is seasonal in nature. In
March,12
Nigeria, there exist rainy seasons between (April-October) and
April,12
May,12 dry seasons between (November-March). There is a breakage
in rainfall in August. Although, this pattern may vary due to
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changes in weather and human factors. Generally speaking,
Total Suspended Load (TSL) decreases downstream from
3500
3000
Daily Guage against Discharge Records
1978
January to August, 2012.
The sediment concentration at
midstream tends to increase at high discharge within June to
August, 2012.
Although, average velocity tends to decrease
Discharge(CFS)
2500
1979
y = 1.648e1.454x
R² = 0.973
2000
1500
1980
1000
1981
500
1982
0
0
downstream from 0.75 m/s from upstream to 0.4m/s at
2
4
6
8
10
Guage Reading (m)
midstream, downstream average velocity varies from (0.31 –
0.4) m/s. Scouring of the benthic layer may not be visible
Fig. 4 Daily Gauges against Discharge
downstream River Omi ranges from (0.6 - 1.5) m. During
flooding at peak discharge (August and September, 2011),
total suspended solid at the midstream was extremely high.
Discharge(CFS)
downstream at such mild discharges. The average depth
This is due to high discharge which erodes river channel and
3500
Discharge Measurements
3000
2500
2000
1500
1000
500
0
-500 0
5
10
flood plain. The contributing tributaries on the river also
across the year
1978
1979
1980
1981
15
1982
Months
discharge at midstream which added to total suspended value.
This additional wash load is responsible for the peak value of
Fig. 5 Discharge Measurements across the Year
associated with irrigation abstractions. The magnitude of this
loss cannot be accurately calculated but an approximate figure
may be derived.
Guage Reading(m)
TSS (123.2 mg/l). Some of the decrease in load was
9
8
7
6
5
4
3
2
1
0
Guage Reading across the Year
1978
1979
1980
1981
1982
0
5
10
15
2007
2008
Months
Fig. 6Gauge Readings across the Year
The
spatial
variations
in
both
sediment
concentrations and loads show a consistent pattern except
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during flooding. Both are relatively high at upstream and
IV. CONCLUSIONS
River Omi showed considerable spatial variation in
decreases downstream depending on the season. During the
rainy season, the sediment concentration is at the peak at
the transport of sediments through its 14.1 km reach studied.
upstream and tends to decrease downstream. It was observed
Sediment concentrations displayed two patterns:
that the mid sediment concentration at midstream in August
(a) A downstream decrease in sediment concentrations during
was arbitrarily higher than upstream value. This was due to
periods of low flow;
the recent dredging exercise on the river by the present Oyo
State government in July 20th, 2012. The dredged materials
(b) An increase upstream, then a decrease downstream during
flood events;
were deposited close to the river channel which could be
The rapid development of infrastructures in Omi
eroded at high discharge.
Adio area of Ibadan may be responsible for the dramatic
The reverse is the case at dry season (Octoberincrease in the surface runoff and higher sediment delivery
December) where the sediment concentration tends to increase
into the Omi River. Activities within the vicinity of the basin
downstream due to irrigation practice, farming, domestic and
such
as,
agriculture,
deforestation
and
mining,
also
social activities. From January to March, 2011, the sediment
contributed to a large extent on the increase in sediment
concentration was at peak at upstream of River Omi. This
volume.
trend continued in April and May, 2012 with sediment
The relationship between discharge (Q), gauge (h)
concentration peakedness recorded at upstream.
and sediment load (y) on River Omi watershed is : (
) =
In contrast, the low flow sediment concentration
pattern shows a progressive downstream decrease during dry
(0.8659 ln
+ 2.866)
( /ℎ) .
season and early part of rainy season. At midstream where
sediment increases, the geometry corresponds to the region
with all the major tributaries. It is steeper, and drains
agricultural land. At downstream, where sediments decreased,
there are few tributaries and relief is much steeper. The most
important decrease in sediment is through deposition of
sediment on the flood plain. There is a considerable increase
in gradient downstream River Omi with a resulting increase in
energy. Also, there is a major widening of the flood plain at
midstream, which narrowed down at the downstream.
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The flood sediment loads have a similar pattern to the
flood concentrations. The annual sediment loads are
dominated by flood sediment transport. Most of the
sedimentsare generated in a limited area from the farming
activities at upstream. It would then be transported only a
short distance to be deposited on the flood plain at midstream
which continues downstream. Sediment transport is minimal
over much of the lower low energy section of River Omi.
While this pattern differs from that reported by Thorns and
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
Walker, 1991, it is consistent with the other research in
Nigerian basins, as reported by Adeogun et al. (2011).
ACKNOWLEDGMENT
The authors gratefully acknowledge the technical
staff of the Department of Civil Engineering, Ladoke Akintola
University of Technology, for helping with the laboratory
work of this project. Our project students that have helped for
field sampling are also appreciated. The Oyo State Water
Corporation is acknowledged for providing the available
discharge record on the river and its tributaries.
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[1]
[2]
[3]
[4]
[5]
A.A. Adegbola and O.S. Olaniyan. “Temporal Variation of Suspended
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Transport in Kubanni Watershed in Northern Nigeria using Grain size
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Environmental
Engineering
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3(5),
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102-108.
http://www.academicjournals.org/ijwree.
I.J. Goldface and Irokalibe, Water Management in Federal and Federal
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River omi South Western Nigeria”, LAP Publisher, Germany, 2012.
L. J. Olive and W. A. Rieger, Stream suspended sediment transport
monitoring – why, how and what is being measured? In: Erosion and
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