MODELLING SEASONAL FLOOD IN THE DOWNSTREAM OF SHIRORO DAM, NIGER STATE, NIGERIA

ABSTRACT

The highest occurring natural disasters in Nigeria are attributed to flood with its attendant consequences on lives and properties. Nigeria witnessed an increased number of dam construction between 1970 and 1995 as an aftermath of the Sahelian drought of 1970-1972. Communities located downstream of these dams, are traditional areas of special importance for rural communities. The resulting population growth and degradation of natural resources in these areas, aggravates flooding and vulnerability. This study assessed the vulnerability and level of risk through flood modelling of these communities downstream of Shiroro dam, Niger State, Nigeria. The specific objectives include extraction of hydrologic layers from Digital elevation model (DEM), estimation of surface runoff and analysis of satellite imageries of the area from (1990-2016) and flood simulation of the study area. Meteorologic data (rainfall, temperature, relative humidity and evaporation), hydrologic data (inflow, reservoir elevation, average discharge and storage) satellite images for the period of 26 years (1990 – 2016),  DEM  were  acquired.  The  Hydrologic  Engineering  Center  (HEC)  Hydrologic  Modelling System (HMS) and River Analysis System (RAS), ARCGIS 10.3, ERDAS IIMAGINE and IDRIS Selva were employed for data analysis. Results revealed variation in hight in the analysis of the DEM of the study area from 48 m to 723 m, where 44.21% of the area is on lower elevation, 32.19% are moderate risk areas while 23.60% are low risk areas based on elevation. The surface runoff analysis reveals that the sub-basins actually aggravate the flooding in the study area with a discharge of 2459.3 (m3/s). The analysis of satellite images revealed a trade-off between vegetation and agricultural land. As the agricultural land increases vegetation decreases. Between 1990  – 2016 vegetation drastically decreased from 62.84% in 1990 to 28.34%, on the contrary, agricultural land shows significant increase during the said period from 22.43% to 50.98%. Built-up areas and bare ground /rock outcrops shows a slight increase from 3.77% to 6.37% and 8.93% to 12.34% respectively. Water body on the contrary continue to shrink from 2.03% to 1.87%. The analysis of the flood simulation revealed that at an altitude of 50 meter above sea level 1.787616 km2 (0.01%) of land area was inundated, similarly, at 100 meter above sea level 2729.162657 km2 (14.47%) of land area was inundated, furthermore, at 150m 6647.622904 km2 (35%) was inundated while, at 200m 9486.40261km2 (50.29%) was inundated. Generally, the result indicates the study area is vulnerable and at risk of seasonal flood. It is recommended that these findings should influence rational decision making with respect to resettlement and compensation and facilitate realistic flood sensitisation campaign on sustainable flood plain management for disaster risk reduction in the study area. The result should serve as a further knowledge that improve our understanding of the flood plain and its attendant advantages and disadvantages to the communities.

CHAPTER ONE

1.0    INTRODUCTION

1.1     Background to the Study

The environment is literally defined by Singh (2010) as the “surrounding and everything that affects an organism during its lifetime “. It further describes the environment as the sum total of water (hydrosphere), air (atmosphere) and land (lithosphere) interrelationships   among   themselves   and   also   with   the   other   living   organisms (biosphere) that exist on, and in, (land, air, and water), and property. Among all these constituents of the environment, man is the most active agent of change. Man modifies his environment to suit him or the situation at a given time. In this quest to satisfy his needs, man built dams to solve problems of municipal water supply, flood control, irrigation, navigation, sedimentation control, and for hydro-electric power generation. Uyigue (2006), defined a dam as ” a barrier constructed across a stream or river to impound water and raise its level for various purposes such as; generating electricity, directing water from rivers into canals for irrigation and water supply systems, increase river depths for navigational purposes, to control water flow during times of flood and droughts and to create artificial lakes for fisheries and recreational use.” In doing this, surface waters (streams, rivers, and lakes) are being interfered with. Hydroelectric dams are established all over the world for the generation of environment-friendly energy and are often regarded as developmental projects (Salami and Sule, 2010).

Flooding is a global phenomenon that affects both developed nations such as USA, China, Germany, Italy and Poland, and developing nations (Cosgrave, 2014). Parker and Silke (2010)  reported  that  in  October  2010,  Burkina  Faso,  Cameroon,  Guinea,  Chad,  Cote d’Ivoire, The Gambia, Ghana, Guinea Bissau, Mali, Mauritania, Niger, Nigeria, Senegal, Sierra Leone, Togo, Sudan and Benin were all affected by floods with Benin recording the highest number of 360,000 affected persons followed by Nigeria  with 300,000 affected persons. While the developed nations are not unaffected in terms of natural disasters, disasters cause severe consequences for the continued existence, self-esteem and livelihoods of all communities, predominantly the poor in developing countries (Parker and Silke, 2010). Consequently, increasing disaster risk is hindering sustainable development (UNISDR, 2011).

The  United  Nation’s  World  Water  Assessment  Programme  (WWAP),  reported  that global trends in natural disasters confirmed that floods, droughts and windstorms have been the most frequently occurring disaster events since 1900 (UN-Water, 2006). The report further revealed that the three accounted for 88.5% of the most catastrophic natural events with more than 83% of flood-related disasters occurring in Asia. The number of casualties per decade has shown a continuous decrease from nearly two million people in the 1960s to half a million in the 1990s Adikari and Yoshitani (2009) have disclosed that a comprehensive analysis of global and regional water-related disasters between 1980 and 2006 indicated a decline in the universal water-associated casualties while the figures of the affected people and estimated economic damages have increased. Asia is the most vulnerable region to water-related disasters, accounting for more than 45% of fatalities with more than 90% of the people affected by disasters. However, as a result of water related disasters in 1990, Africa recorded the highest number of fatalities accounting for more than 46% of the world’s total (Takara, 2014).

In Nigeria, an upsurge of dam construction was observed between 1970 and 1995 due to the effect of Sahelian drought of 1970-1972 (Uyigue, 2006). Shiroro Gorge Dam is one of the three hydro-electric power dams constructed in Niger State in 1990. Others are the Kainji, Jebba and dams constructed in 1969, 1984 and 2013 respectively. Communities located downstream of the dams often referred to as floodplains, are traditional areas of special importance for rural communities that present favourable conditions for human settlement, economic development, and assets for sustainable livelihood support (Patrick, 2009). In Nigeria, as in other developing nations such India and Zambia, these floodplains continued to  attract human populations, particularly in recent times due to a southward migration in the northern regions   in   response   to   climate   change,   land   degradation   and   security   challenges (Abdulkadir, 2011). The resulting population growth and degradation of natural resources (land, vegetation and water) in these areas, aggravates flooding and vulnerability of communities resulting to varying levels of disaster during the seasonal rains.

Floods can occur in rivers when the capacity of the river bank can no longer contain the inflow, water flows out of the river channel, particularly at bends and meanders. This often causes damage to lives and property. While flood damage can be virtually eliminated by moving away from the river banks, people have lived and worked by the water to seek sustenance and capitalise on the gains of cheap and easy travel and commerce by being near water as it is apparent in the riverine areas across the world (Adikari and Yoshitani, 2009). Thus, humans’ continuous persistence to live in and around areas threatened by flood is an evidence that the perceived value of living near the water exceeds the cost of repeated seasonal flooding Richter et al., (2010).

According to Reynaud et al. (2013) and Tawari-Fufeyin et al. (2015), flooding cannot be completely prevented, but the incidence of a flood needs not be considered a failure and, on the contrary, minimisation of losses may constitute a success. There are lessons to be learned from every flood and it is important to use them in preparing for the next flood. Once we accept that no flood protection measures can guarantee complete safety, a general  change of concept is needed to  reduce human vulnerability to  floods. The attitude of living in areas prone to flooding and accepting the flood in decision making seems more sustainable than totally striving to eradicate them (Reynaud et al., 2013). Despite all these, the hazard of, and damage caused by, flooding cannot be overemphasized in terms of loss of life, property, displacement of people and disruption of socio-economic activities as well as the loss of valuable agricultural land due to the attendant inundation of flood plains. Thus, there is the need for a proactive approach in dealing with this disaster.

In Nigeria for instance, four predominant flood types were observed by Salihu (2014) as Urban flooding (41%); Flash flooding (11%); River flooding (43%); and Dam flooding (5%)  with  the  menace  gradually  becoming  a  serious  environmental  problem.  The country recorded series of catastrophic flooding events during the past years (Abia in July 2001, Adamawa in April 2001, Akwa Ibom in March 2001 and in Niger 1999 and 2000 to mention but a few). According to Etuonovbe (2011) Lagos and Ogun States experience seasonal floods as a result of the release of excess water from Oyan dam. Also more than 5000 people were affected in two communities of Sagbama and Kokoluma Local Government Areas of Bayelsa State, due to the over flow of River Nun (Ogba and Utang, 2008). Similarly, several areas along the coast of the along major river valleys are also affected by floods every year.

In the North Central region however, Rivers Benue and Niger account for major river flooding (Salihu, 2014). Seasonal floods occur on many rivers, forming adjoining regions known as floodplains (Tawari-Fufeyin et al., 2015). In 2012 the region experienced the most devastating flood catastrophe by inundating larger parts of the flood plains with the confluence town of Lokoja and its surrounding communities as the most widely affected (James et al., 2013). In Niger State, the flood plains of Rivers Niger and Kaduna, have been identified by Jinadu (2014) as a permanent dwelling of over 350 smaller  communities.  The  downstream  communities  of  Nupeko,  Gurmana,  Ketso, Akare,  Wuyakede,  Muye,  Mawogi,  Dutsen,  Gusoro,  Lenfa-Kuso,  Jiffu,  Gusoro, Gbogifu and Sonlafi have a long record of seasonal riverine flooding as a result of river banks overflowing due to the release of water from the three hydroelectric dams in the State (Ikusemoran, et.al 2014; Jinadu, 2014; Musa et al., 2016).

An essential requirement for a substantial reduction of communities’ vulnerability to natural hazard noted by ISDR (2005) is flood modelling. Flood modelling, estimation, prediction, and forecasting became noticeable in the 1930’s and 40’s after the work of Sharman and Horton (1932) cited in (Ramirez, 2000). These led to the understanding of laws governing the hydrological processes and behaviour of floodwaters following improvements in observational capability based on progressively higher computational power. With the break-through in science and technology that brought about the development of computers, programmes and GIS, these models began to be tested and calibrated. Ramírez (2000) defined flood modelling and prediction as “a process of transformation of rainfall into a flood hydrograph, and to the translation of that hydrograph throughout a watershed or any hydrologic system”. This process generally involves the approximate description of rainfall-runoff transformation, based on the representation of the physical process involved. Thus, the need for flood modelling as a decision support system for sustainability of floodplains and other relevant socio- economic sectors such as agriculture is long overdue. Consequently, this work will incorporate GIS and remote sensing technology, that will integrate runoff from land use land cover map, slope and height to develop a flood modelling and forecasting scheme that will provide sustainable mitigation measures.

1.2 Statement of the Research Problem

In Nigeria, like other parts of the world, floods have often rendered millions of people homeless, injured several others and caused many losses in lives and property (Salami and Sule, 2010). In his address to the 18th session of the Conference of the Parties (COP18) to the United Nations Framework Convention on Climate Change (UNFCCC) 2012, in Doha, Qatar, the Director General, Nigerian Emergency Management Agency (NEMA) stated that, “the 2012 flood in the Country affected a total of 7.7 million people during which over 300 people were killed and 2.1 million others were displaced. Sources of livelihood worth billions of (Nigerian) Naira were destroyed due to the floods (UN, 2012)”. A classic example of this is the fate of communities downstream of the Shiroro hydropower dam, including Muregi, Gurmana, Jiffu, Gbara and Akare, which experience annual floods with submergence of farmlands as well as residential and institutional buildings Olukanni and Salami, (2012), Salami and Sule, (2010).

The highest stirring natural hazards in Nigeria is attributed to flooding, with great consequences on the lives and properties (Aderogba, 2012) cited in (Komolafe et al. 2015). Numerous Studies have been carried out in the study area on; hazard and vulnerability assessment using questionnaire, oral interview and direct field work (Jinadu, 2014), operational and environmental impact Usman and Ifabiyi (2012), Salami and Sule (2010)), Wahab and Adeola (1999), Olajuyigbe et al. (2012) and Olukanni and Salami (2012), river analysis and hazard mapping (Musa et al., 2016) and numerous others. Despite the fact that flood modelling approaches are crucial components of flood disaster risk reduction, because of its flexibility and data update capability (Nkwunonwo et al. 2015),  none  of  these  researches  have  been  carried  out  on  flood  modelling  and forecasting in the study area.

In addition, the absence of flood data and additional supplementary record for example, the absence of available record on discharge from the catchment sub basins of River Kaduna is a major setback in tackling flood menace in the study areaa. This is due to the fact that the discharge figures from the dam have not been taking into cognisance, the contribution of these catchments. The absence of a geospatial database for flood phenomena and the absence of flood models for Disaster Risk Reduction (DRR) and sustainable livelihoods particularly on the floodplains, despite annual frantic activities by the National Emergency Management Agency (NEMA), Nigerian Meteorological Agency (NIMET), Nigerian Hydrological Services Agency (NHSA) and other related agencies and parastatals.

Therefore, this research work seeks to fill this subsisting void by developing a flood modelling scheme that will integrate the use of geospatial technology (to analyse the impact of land use and land cover dynamics, geospatial database development such as flow direction, flow accumulation, stream definition and segmentation, catchment grid delineation and polygon processing, drainage line and slope) hydro-meteorological variables and discharge from catchment sub-basins of River Kaduna, as a proactive approach towards DRR to ensure livelihoods protection and guarantee sustainable development. Consequently, these have lead to series of questions such as;

1.3 Research Questions

1.  What hydrological layers are relevant for hydrologic modelling?

2.  What are the possible impacts of land use and land cover on surface runoff coefficients?

3.  What is the contribution of sub-basin on flood downstream?

4.  What will be the intensity of future flood?

This research effort therefore seeks to answer these questions

1.4 Aim and Objectives of the Study

The aim of this research is to employ geospatial technique in modelling flooding activities in the downstream of Shiroro dam in furtherance of disaster risk reduction in Niger State. To enhance this, the following specic objectives are being processed as follows:

i.     Extract hydrologic features (layers) for input into flood modelling from Digital Elevetion Model (DEM)

ii.     Analyse Land use and land cover (LULC) map of the study area from 1990 to 2016

iii.      Estimate surface runoff coefficient of the area using (LULC) map

iv.     Produce a model simulating river flood in the study area.

1.5 Justification of the Study

We cannot stop natural calamities, but we can and must better equip individuals and communities to withstand them. Former UN Secretary General, Kofi Annan (UN-HFA, 2005).

Despite the efforts of various researchers on the scale of inundation and impacts of flood in Nigeria particularly across the study area (mostly during the 1999 and 2012 floods) such as (Ojigi, et al. (2013), Ejemma et. al. (2014), Ejikeme et.al. (2015), Suleiman and Ifabiyi (2014), Musa et. al. (2016), Jinadu, (2014), Usman and Ifabiyi (2012), Salami and Sule (2010), Wahab and Adeola (1999), Olajuyigbe et al. (2012) and Olukanni and Salami (2012), one vital question that still remains unanswered is “what is the solution to the persistent flooding in the study area?”

The answer could be attributed to either the approaches employed by the researchers are grossly inadequate, lacking in data integration and update capability from the researchers or lack of will to implement the recemmondations of the research findings by the Government. This work attempt to answer the reasons from the researchers perspective.

Similarly, (de Almeida et al., 2012; de Moel et al., 2009; Merz et al., 2007) argued that a range of flood modelling approaches are fundamental constituents of flood risk reduction since they are capable of speedy, constant and routine simulation of flood data needed for flood hazard and risk assessment.

Additionally, the fact that Nigeria is one of the most populated countries of the world with population size projected at over 180 million people in 2016, 286 milllion by 2030 and 300 milllion people by 2035 (World Bank, 2016), couplllled with the assertion by the Global Network for Disaster Risk Reduction (GFDRR, 2016) that, the future population  increase  will  drive  future  flood  risk  further  emphasises  the  urgency  in seeking for ways of preparing human population in the country to adapt to floods.

In many countries with similar issues of rapid population increase, outstanding among them like China, India, Bangladesh and Vietnam, even though some of them having ‘not too well’ instituted flood management systems and flood modelling approaches for flood risk appraisal and mitigation (Huong and Pathirana, 2011; Renyi and Nan, 2002; Sayers et al., 2013). It will therefore be a developmental achievement for Nigeria to execute flood modelling given that in other parts of the world such a technique is demonstrated towards flood hazard and risk mapping. Thus, this study provide new approach towards adapting to flooding and to support other stakeholders’ attempts towards flood disaster risk reduction in Nigeria.

1.6 Scope of the Study

The scope of the study was limited to the application of geospatial techniques for modelling river flood and its associated impacts in the downstream of Shiroro Dam, Niger State, Nigeria encompassing selected communities from the downstream of Shiroro Dam, in Niger State, Nigeria.

The study did not cover all the communities downstream of the dam. Rather, since flood vulnerability of the communities downstream of Shiroro dam declines with increasing distance from the river channel and elevation (Ismail and Saanyol 2013) cited in (Komolafe  et  al.,  2015),  a  community  each  located  within  4-10km  from  the  river channel such as Akare, Ketso, Gurmana and Nupeko were selected. These communities were affected by recent floods and classified by Ikusemoran et al. (2014) as highly vulnerable to floods.

The LULC classes were grouped into five main groups, namely, agriculture, vegetation, water body bareground/rock outcrops and built-up areas because of their different effect on  river  discharge  Kabanda  and  Palamuleni  (2013).  Thus,  there  was  a  need  to distinguish agriculture from the general vegetation.

1.7 Description of the Study Area

This sub section described the study area under the following themes; location, climate, rainfall, relief and drainage, hydrology, economic activities, vegetation and land use.

1.7.1 Geographical location of the study area

The study area are the vulnerable communities located between Longitude 5020’E to7’10’E and Latitude 8045’N to 10015’N downstream of Shiroro Dam, Niger State, Nigeria. Figure 1 show the map of the study area. With a population of over 4 million people, Niger State has a total land area of 72,200.14km2. The Niger valley terrain covers 18,007.38km2 (24.94%), the plains cover 24,181.04km2 (33.49%), upland is 20616.09km2 (28.55%) while the remaining 9593.3km2 (13.01%) are made up of highlands (Mayomi et al,. 2014). Shiroro dam is situated 550 metres downstream of the confluence of Kaduna River with River Dinya its main tributary. The dam is built on River Kaduna that takes its origin around the west and North-West of the Jos Plateau in North-Central Nigeria from where it flows westward and southwest-ward. Rivers Koriga, Maarigna and Durimi are main tributaries of River Kaduna, (Ikusemoran et al., 2014).

1.7.2 Climate of the study area

Niger State experiences distinct dry and wet seasons with annual rainfall varying from 1,100mm in the northern parts to 1,600mm in the southern parts. The maximum temperature (usually not more than 350C) is recorded between March and June, while the minimum is usually between December and January (Niger State, 2012). The climate of the area is tropical and belongs to the tropical wet and dry (AW) of the Koppen’s climatic classification (Garnier, 1967) cited in (Eze, 2004). However, the creation of Shiroro lake has lea to a change in climatic conditions in and around the lake area (Eze, 2004).

1.7.2.1  Rainfall of the study area

Adefolalu, (1992) cited in (Yakubu, 2012) studied and summarized the rainfall patterns in Northern Nigeria into four periods within a year (January – March, May – July, August – September and November – December). Rainfall in January to March is very low ranging from 5mm in January to about 40mm in the extreme Southwest in March, by April rainfall of 70mm or more covers the North Central part which is the study area while the lowest value is 40mm in the extreme Northwest. Between May and July, the Shiroro lake watershed receives in excess of 100mm with peak value of about 280 – 300mm in July. There is no part of the North Central watershed that receives less than 180 – 200mm of rainfall in July, August and September, constituting the peak of rainy season within the study area. Rainfall amount is in excess of 300mm in the western half of the area. The highest rainfall receipt of over 400mm is to be expected in September during normal rainy year but can drop sharply to a maximum of 130 – 150mm during a low rainfall year. The onset of the seasonal rainfall is between 20 – 30th April and the Length of Rainy Season (LRS) is between 161 – 200 days.

1.7.3  Drainage pattern and relief in the study area

Shiroro Lake has about eight smaller tributaries contributing to its sum total capacity of about (8 x 10m3) inflow with Kaduna river being the major contributory of almost 70% of the total capacity (Eze, 2004). Rivers Muye, Sarkin Pawa, Guni, koriga, Durumi and Mariga are the major tributaries in the study area that feed the main river Kaduna. However, unlike most rivers in northern Nigeria, the river is a perennial river. The river flows  in  a  moderately  straight  course  in  the  upper  and  middle  stages  with  some consisting of a number of steep gradient valley steps, which are separated by elongation of others with low gradient. Its course is interrupted where it crosses hard rocks, and deep gorges have been cut across the area of more pronounced steeps in the valley, figure 1.3 showa the drainage map of the study area.

The topography is highly undulating and varied in height. Isolated hills of over 600m and low lands as low as approximately 50 m are common in the study area.While valleys in between can get as low as 4000m. The underling hills are made up of granite rocks while the lower terrains are dominated by schist and gneiss (Eze, 2004). Figure

1.4 shows the relief of the study area.

1.7.4   Hydrology of the study area

The two major rivers that traverse Niger State are Rivers Niger and Kaduna. River Kaduna being the major tributary of the Niger River in central Nigeria, rises from the Jos Plateau and travels a distance of about 350km before emptying into the reservoir (Garba et al. 2013). It flows through the Jos Plateau 29km southwest of Jos town near Vom and flows in a north-westerly direction to a bend 35km northeast of Kaduna town (Iloeje, 1982) cited in (Eze, 2004). It then takes up a southwesterly and southerly path before finishing its 340- mile (550-kilometre) flow to the Niger at Muregi, Shiroro lake was built on river Kaduna. The reservoir is about 12.8km wide and the regime of the river is characterized by the occurrence of the wet and dry season. Rainy season between April and  October,  peak  falls  between  August  and  September,  dry seasons  spell  usually between November and march (Eze, 2004).

1.7.5 Economic activities in the study area

The Gbagyi  communities  across  the  study area are  utilizing River  Kaduna’s  upper floodplains  for  rice  cultivation,  while  in  the  southern  plains,  in  Nupe  land,  rice, sugarcane production and fishing are the most important economic activities. In the vicinity of Bida, Edozhigi and Badeggi natural irrigation such as rice-growing is major economic activity, while in the floodplain in Shiroro, yam, cassava, sugar cane and guinea corn, are the main crops ( SLG, 1999, Niger State, 2012;).

1.7.6   Vegetation of the study area

The vegetation of the area is typical of that of savannah with patches of few woodlands mainly tress with little shrubs and grasses. The trees are short broad-leaved plants of up to 16.5m height while the grasses are between 1.5-3.5m high. The trees shed their leaves in the dry season in order to minimize loss of water by transpiration while the grasses have durable rooting system which remain underground such that even when they have been burnt after dry season bush fires, they regenerate again with the onset of the rain the following year (SLG, 1999; Eze, 2004). Figure 1.5 shows the vegetation of the study area.

1.7.7 Soil of the study area

Soil types within the study areas are dominantly hydrological soil group C (HSG-C): moderately high runoff potential (<50% sand and 20-40% clay) with some traces of HSG-B: moderately low runoff potential (50-90% sand and 10-20% clay) downstream, well drained shallow to moderately deep; the colour varies from very dark grayish brown to dark or strong brown to yellow red (SLG, 1999). The names of the soils of the Shiroro catchment area are derived from pre-existing Precambrian basement (complex rock) consisting of gneiss, granite, amphibole and schist (SLG, 1999). Figure 1. 6 shows the soil of the study area.

1.7.8 Land use of the study area

Because of the rich fertile land of the area, the predominant occupation of the people is farming while other inhabitants earn their living through fishing due to their proximity to the river. The area is endowed with abundant natural land and water resources. The occurrence  of  commercially  viable  mineral  resources  like  Gold,  Columbite  and Diamond have been proved, while it also ranked as a major producer of rice, yam, maize, cottons, beniseed, groundnut, millet and guinea corn in the State (SLG, 1999).

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