Flood Control Structure: Influences on the Bacita ...

Flood Control Structure: Influences on the Bacita Sugar Cane Plantation at Downstream of Kainji and Jebba Dams in Nigeria Salami Adebayo.Wahab* and Adedeji Abdullah Adeola** Department of Civil Engineering, University of Ilorin, P.M.B 1515, Ilorin, Nigeriia [email protected] *, [email protected] , [email protected] ** Abstract: The occurrence of flood has great effect on the Bacita sugar plantation as a result of sudden release of water from both Kainji and Jebba dams when the reservoirs are full and when the tributaries and rivers carry flows in excess of their capacities which led to the damage of irrigation and water conveyance structures, sugar cane loss and additional cost of maintenance. The cost of rehabilitation in 1994, 1998 and 1999 due to the effect of flood was about N1.3 billion ($10.8 million). Due to the huge amount of money being spent as a result of damages from flood, a structural control structure (Levee) is suggested and designed based on the flood level data collected at three gauging stations near Bacita. The maximum flood levels were selected and fitted with various probability distributions. For the purpose of prediction, the design of the control structure was based on flood level with a return period of 100 years. The predicted flood levels are 81.56 m.a.s.l, 76.34 m.a.s.l and 77.07 m.a.s.l, while the datum levels are given as 79.40 m.a.s.l, 71.51 m.a.s.l and 71.30 m.a.s.l for Oshin, Belle and Yelwa stations respectively. The height of the levee embankments together with the free board of 1.50 m.a.s.l were obtained approximately as 3.70 ma.s.l, 6.33 m.a.s.l and 7.27 m.a.s.l for Oshin , Belle and Yelwa respectively. Keywords:

Sugar cane plantation, flooding, flood level, flood control structure, cost of rehabilitation

1. INTRODUCTION Civilization has always developed along rivers whose presence guaranteed access to and from the sea coast, irrigation for crops, water supply and power generation. The many advantages have always been counter balanced by danger of flood (Adeniyi, 1973 and Wilson, 1979). Flood may be defined as a discharge which exceeds the channel capacity of a river and then proceed to inundate the adjacent flood plain. Such high flows are normally caused either by intense or prolonged rainfall (Framji, 1976). The flow regime of the River Niger below the Jebba dam is governed by the operations of the Kanji and Jebba hydroelectric schemes and runoff from the catchments. The annual ‘white floods’ event which usually sets in July and peaks in September does not maintain the same frequency as almost every four years the flood sets in with greater velocity, this lead to the dam overflowing its banks. When the authority of PHCN (Power Holding Company of Nigeria) opens the spillway of the dams to let out torrents of water, this in turn creates flooding and other social impact to the downstream communities and projects. Lawal and Nagya (1999) stated that the occurrences of flood at Mokwa, Rabba and its environs in 1997 and 1998 destroyed properties worth over Five hundred million Naira and submerged several houses, farm land and crops. Human activities on the Kainji – Jebba dam led to intense runoff and havoc at the sugar plantation in the years 1994, 1998 and 1999. This caused destruction of sugar cane plantation by inundation of sugar cane field, irrigation pumps and submerging of farming areas occupied by the villages. Bolaji (1999) stated that most of Nigeria’s food crops are produced in the middle belt of the country particularly around the lower Niger axis where two third of the country’s dams are located. With the annual flooding of the lower Niger, most of Nigeria’s staple foods such as cowpea, rice, maize and vegetable production are threatened. The floods have multiple effects as the local dwellers do not just loose income on their investment, they hardly have enough to eat and this affect their livelihood. According to Lawal and Nagya (1999) and Bolaji (1999) the havoc wreaked by the flooding of the lower Niger in 1998 and 1999 also has its effect on social services to the people of the area. For example schools in about 32 and 52 villages were submerged in the flood of 1988 and 1999 respectively, while schooling for fleeing villagers remain disrupted and the hospitals were not spared by flood.

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2.

METHODOLOGY

2-1 Description of study area The Nigeria Sugar Company (NISUCO), was established in 1957. The company is located on longitude 9 o North and latitude 5 o east. Bacita town is about 120 Km north of Ilorin, the Kwara State capital, Nigeria. The location of the study area and the flood plains of river Niger downstream of Jebba dam are presented in Figure 1.

Figure 1 Map of Nigeria showing location of the study area and flood plain at downstream of Jebba dam

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2-2 Collection of data Flood level data was collected at the Irrigation and Agronomy Unit of the Agriculture Department of the Nigerian Sugar Company, Bacita. The flood levels were collected for three gauging stations near the study area; they include Oshin, Belle and Yelwa. The flood levels (maximum and minimum) are available for a period of thirty four years (1972 - 2005). The variations of the minimum and maximum flood levels are presented in Figures 2 – 4. The maximum flood level data was fitted with the various probability distributions for the prediction of flood of 100 year return period required for the design of flood control measure. Other report/material was also collected from which the flood problems faced by the company were studied and identified.

74.5 74.0

Flood level (m)

73.5 73.0 72.5 72.0 71.5

Max. flood level Min. flood level

71.0 70.5 70.0 Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Time (month)

Fig. 3 Variations of Maximum and Minimum flood level for Belle near Bacita (1972 - 2005)

76.0

Flood level (m)

75.0

74.0

73.0

72.0

Max. flood level Min. flood level

71.0

70.0 Jan

Feb

Mar

Apr

May

Jun Jul Aug Time (month)

Sep

Oct

Nov

Dec

Fig. 4 Variations of Maximu and Minimum flood level for Yelwa near Bacita (1972 -2005)

2-3 Causes of Flood The flooding experienced at the sugar cane plantation of the company is caused when the reservoirs of both Kainji and Jebba dams are exceeded by runoff from the catchment areas of the river which cuts across many countries like Mali, Niger and Nigeria. The only alternative left to protect the dams from collapse is to discharge more water to the downstream of the dams. Other factors that caused flood at the sugar cane plantation during the raining season include:

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(a) Excess rainfall leading to extraordinary runoff (b) Lack of proper drainage system and drains of inadequate capacity (c) Silting up of natural drains and riverbeds by sediment due to erosion in the catchments (d) Highly meandering and unstable rivers, changing of course thereby causing flood. 2-4 Effect of Flood on the Sugar Company This can be enumerated as follows: * It leads to destruction of sugar cane on the field, which in turn affects productivity * It leads to extra spending in the reconstruction of the conveying channels that are damaged during the flooding * It requires more labour, and more expensive in clearing the conveying channels that are blocked during the flood. Salami (2007) reported that in 1994 over 2,260 ha of sugar cane fields were flooded and remained innundated for over six weeks. The flood damaged irrigation and water conveyance structures, washed away the existing flood protection embankments, roads and caused displacement of settlements and communities along River Niger. The 1994 flood at the sugar cane plantation has a total cost effect of N368 million due to cane loss, rehabilitation of irrigation facilities and replanting. In the 1998 the episode was repeated with a total loss N 366 million to the company. In the 1999 the flood problem re-occurred and led to a total loss of N 394 million. As from 1999 the occurrences of the flooding become an annual event, with the attendant loss of sugar cane, infrastructure and equipment. Tables 1 and 2 indicate the losses in respect of flooding of sugar cane fields and cost of replacement of irrigation structures, while Table 3 shows the cost of rehabilitation of the plantation in respect of materials for field and preparation, replanting, ploughing, crop husbandry and maintenance for each year under reference. Table 1 Cost of Sugar cane losses during 1994, 1998 and 1999 flooding S/N

Flood year

Total hectares under cane

Total hectares flooded

Estimated cane loss(tones)

1. 1994 5,298 2,260 143,000 2. 1998 3,336 2,200 143,000 3. 1999 2,925 2,300 146,000 Source: Nigeria Sugar Company, Bacita (1N = 0.0073 $, 2008)

Cost of cane per tone (N)

Total cost (N) million

1,200 1,200 1,200

171.60 171.6 187.68

Table 2 Cost arising from destruction of irrigation infrastructure and accessories (a)

1994 flood

S/N

Structure

Quantity Destroyed

Unit cost (N) 680,000 200

1. 2. Total

Irrigation pumps Protective dyke

10 No. Concrete.60,200 m3

1. 2. Total

Irrigation pumps Protective dyke

(b) 1998 flood 8 No. 650,000 Concrete 56,000 m3 200

(c) 1999 flood 1. Irrigation pumps 15 No. 600,000 2. Protective dyke Concrete 60,200 m3 200 3. Turn out gate Concrete 25 m3 8,000 Total Source: Nigeria Sugar Company, Bacita (1 # = 0.0073 $, 2008)

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Total cost of construction (N) million 6.80 12.04 18.84 5.20 11.20 16.40 9.00 12.04 0.20 21.24

Table 3 Cost of rehabilitation of the destroyed sugar plantation (a) 1994 flood S/N

Activities

A B C D E

Hectares (Ha)

Cane for re-planting Reploughing Fertilization Weed control Crop maintenance Sub-Total (b) 1998 flood S/N Activities

2,200 2,200 2,200 2,200 2,200

A B C D E

2,200 2,200 2,200 2,200 2,200

Cane for re-planting Reploughing Fertilization Weed control Crop maintenance Sub-Total (c) 1999 flood S/N Activities

Hectares

Hectares

A B C D E

Total required for replanting 17,600 No. 2,200 Ha 22,000 bags 30,800 litres 2,200 Ha

Cost per unit (N)


Cost per unit (N)


Cost per unit (N)

Cane for re-planting 2,300 Reploughing 2,300 Fertilization 2,300 Weed control 2,300 Crop maintenance 2,300 Sub-Total Source: Nigeria Sugar Company, Bacita (1 # = 0.0073 $, 2008)

1200 27000 2,000 1,700 520

1,200 27,000 2,000 1,700 520

1,200 27,000 2,000 1,700 497.40

Total cost (N) (million) 21.12 59.40 44.00 52.36 1.144 178.024 Total cost (N) (million) 21.12 59.40 44.00 52.36 1.144 178.024 Total cost (N) (million) 22.08 62.10 46.00 54.74 1.144 186.064

2-5 Flood Control Measures The control measures which can be used include: Non-structural measures and Structural measures. Structural measures * Improvement of storage reservoir to enhance drainage capacity so as to ensure that the flood can safely discharge without exceeding the permissible level in the river. * Construction of retarding basin in hilly and sub-hilly areas in order to accommodate more volume of runoff coming from the catchments areas for certain period of time during the critical storm period to when it has reduced and stored volume can be emptied at a particular rate. * Increasing infiltration by natural and artificial recharge as a result of increase in population and human activities * The diversion of the runoff through another subsidiary channel of the same river or to select diversion channels * The improvement of the conveying capacity of the channel would also reduce the peak flood of the river and thus serves as a flood control measures. Non-structural measures This represents an administrative measure of flood plain regulation and management. * This involves flood forecasting and flood warning, based on observation of rainfall and river gauge reading in the upstream catchments areas. It is possible to forecast the magnitude and time of occurrence of flood at any downstream point in a river.

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* With modern communication system like the telephone, radio, microwave, radar and artificial satellites, it is then possible to instantly transmit the data observed in the upstream of the catchments area to centrally located flood forecasting stations. Design of structural control measure The total loss due to the effect of flood on the company is too enormous and there is need to find a lasting solution to the problem. The structural facility that can be used to control the flood is levee. The design of the levee is based on the predicted flood level having 100 yrs return period and the procedure involves Fitting of distribution model The maximum flood levels were selected for the three gauging stations and ranked according to Weilbull plotting position. The corresponding return period were determined and plotted against the maximum flood level in order to fit a probability distribution for the prediction of future occurrence of the flood. The fitted curves for Oshin, Belle and Yelwa lead to the development of the model equations are presented in equations 1 – 3. a) Oshin gauging station

Ho max  0.0439 ln(Tr )  81.354

(1)

Hbmax  0.605 ln(Tr )  73.556

(2)

Hy max  0.756 ln(Tr )  73.585

(3)

b) Belle gauging station

c) Yelwa gauging station

where: Homax is the maximum flood level at Oshin gauging station; Hbmax is the maximum flood level at Belle gauging station; Hymax is the maximum flood level at Yelwa gauging station and Tr is the return period Levee for Oshin sugar cane plantation The flood level having 100 years return period was predicted to be 81.56 m with equation (1) and the datum level is given as 79.40 m from the data collected. This implies that the highest water level at Oshin is the difference between the predicted height and datum levee i.e. (81.56 – 79.40 = 2.16 m). For safety purpose there is need for an additional freeboard of 1.50 m (Framji 1977, Falade 1998, and Afini 2002). Thus, the design height of the levee embankment is approximately 3.70 m. The top width of levee is 3.0 m for embankments higher than 2.0 m and the slope not steeper than 1.2 (Framji, 1977). The design shows that the structure is stable since the factors of safety are greater than 2 and the eccentricity is less than 1.83. A typical flood levee section is presented in figure 5. Levee for Belle sugar cane plantation The flood level of 100 year return period was predicted to be 76.34 m with equation (2) and the datum level is given as 71.51 m from the collected data. This implies that the highest water level at Belle is the difference between the predicted height and datum levee i.e. (76.34 – 71.51 = 4.83 m) and with an additional freeboard of 1.50 m, the designed height of the levee embankment becomes 6.33 m. The top width of levee is 3.0 m the design shows that the structure is stable with factor of safety higher than 2 and the eccentricity is less than 1.83. Levee for Yelwa sugar cane plantation The flood level having 100 year return period was predicted to be 77.07 m with equation (3) and the datum level is given as 71.30 m from the data collected. This implies that the highest water level at Oshin is the difference between the predicted height and datum levee i.e. (77.07 – 71.30 = 5.77 m) and with an additional freeboard of 1.50 m the designed height of the levee embankment becomes 7.27 m. The top width of levee is 3.0 m and the design shows that the structure is stable with factor of safety higher than 2 and the eccentricity is also less than 1.83.

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W 0.5m

3m

3.5m

3.5m

0.5m

0.5m

11m

Figure 5 A typical structural element of a levee embankment

3. RESULT AND DISCUSSION The study involves collection of flood level data and information concerning the extent of flood hazards at the NISUCO, Bacita. The causes of the flood were majorly identified as a result of release of excess water from the Kainji and Jebba dams and the runoff from the adjoining catchments. The effect of flood that occurred in 1994, 1998 and 1999 are studied and the cost incurred due to the floods were discussed. A total of 34 years record of maximum and minimum flood stage data were obtained for three gauging stations (near Bacita). The variations of the collected data are presented in Figures 2 – 4. From Figure 2, it can be observed that two peaks flood level occur at Oshin gauging station. The first peak occurs in the month of June while the second peak occurs in the month of September. The highest flood level at Oshin is 81.38 m and the datum level is given as 79.40 m. From Figure 3, it can be observed that at Belle gauging station the peak flood occur in the month of September and the value of the maximum flood level is 74.14 m, while that of the datum level is given as 71.51 m. Also from Figure 4, the maximum flood level at Yelwa occurs in the month of September as 74.87 m and the datum level is given as 71.30 m. For the purpose of curve fitting, the return periods for the maximum flood level at each gauging station were estimated and plotted against the corresponding maximum flood level. The fitted curves for Oshin, Belle and Yelwa gauging stations lead to the developed model equations used to predict flood level of 100 years return period which was used in conjunction with the datum level for the estimation of the height of a levee. The predicted flood levels are 81.56 m, 76.34 m and 77.07 m for Oshin, Belle and Yelwa respectively. The design of the structural flood control levee was based on the predicted flood level. The heights of the embankments (H) were determined as 3.70 m, 6.33 m and 7.27 m for Oshin, Belle and Yelwa respectively. Other information concerning the levee sections is summarized in Table 4. 4.

CONCLUSION AND RECOMMENDATION

This study has highlighted the causes, effect and control of flood at the Nigeria Sugar Company, Bacita. The identified causes include sudden release of water from the two hydropower dams located at the upstream of the study area. Many damages have occurred to the sugar cane plantation of the company as a result of flood in the years 1994, 1998 and 1999 which led to a total rehabilitation cost of N1.3 billion. A structural control measure was suggested and designed. The structural control measure suggested is a levee and its design was based on the predicted flood level of 100 years return period. The implementation of the suggested control measure would help in mitigating the flood problem at the study area. Based on the outcome of the study, the following recommendations are made: 7

* The construction of the levee should be implemented in order to alleviate the effect of flood as related to the total loss and other hazards at the sugar cane plantation. * The management of Sugar Company should always liaise with the management at PHCN at Kainji and Jebba to regulate the release of water especially during the months of August – October. * Flood routing along the Niger River need to be carried out to study flood from Kainji to Jebba and Bacita, a stretch of about 130 Km along the Niger River channel. This would enable the prediction of flood waves. * With the established routing model, forecasting of flood waves would be easier and the downstream water users can be informed adequately so as to avoid damages to the farm lands, houses and sugar cane and other crops. Table 4 Design summary of levee section Location

Oshin Belle Yelwa

Height of embank ment H, (m)

Slope

3.70 6.33 7.27

1:2 1:2 1:2

Weight W,(KN)

695.17 1107.8 1321.83

Top width T (m)

3.0 3.0 3.0

Base width, B (m)

Hydrostatic force (KN) Horiz ontal (Hf) 23.74 117.8 194.68

11.0 11.0 11.0

Verti cal (Vf) 21.58 48.07 61.80

Uplift force, Uf (KN)

118.70 264.38 339.92

Factor of Safety Over turning 4.2 2.83 4.5

eccentricity (e)

Sliding

6.37 4.92 6.76

0.41 0.31 0.31

5. REFERENCES Adeniyi, E.O. (1973). Downstream impact of the Kainji Dam in Kainji Lake. A Nigerian Man-made Lake. Vol. 2. Mabogunje, A.L (ed.). NISER, Ibadan Nigeria. Afini, A.E (2002). Flood Dyke design for flood control at Tada-Shonga Rice Irrigation Project, Kwara State, Report presented to the Design and Planning Unit, Lower Niger River Basin Development authority, Ilorin. Pp 24 -30 Bolaji, T (1999). Sacked by the Almighty River Niger, Newspaper Publication. The Guardian on Sunday, October 23, 1999. pp 4 -5 Falade, O.K (1998). Design of the retarding basin across Maafa stream, B.Eng Project report, submitted to the Department of Agricultural Engineering, Federal University of Technology, Minna, Nigeria. Framji, K.K (1976). Flood control in the World, Printed in Indian at the Central electric press, Delhi. Pp 4 – 6 Framji, K.K (1977). Manual of flood control and practice, International Commission for Irrigation and Drainage (ICID). Printed in India at Central electric Press, Delhi. pp 10 Lawal, A.F and Nagya, M.T (1999). Socio-Economic effects of flood disaster on farmers in Mokwa Local Government Area, Niger State, Nigeria. A paper presented at the 33 rd Annual Conference of Agricultural Society of Nigeria held at National Cereal Research Institute, Badeggi, Niger State, Nigeria. pp 1 – 10 Salami, A.W (2007). Operational performance of water management models for a hydropower system under reservoir inflow forecasts. A Ph.D. Thesis report presented to the Post graduate School, University of Ilorin, Ilorin, Nigeria. th Wilson, E.M (1979). Engineering Hydrology. 4 edition, Macmillan Press Limited.

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