- Background of the Study
Sediments are fragmented loose earth particles which are detached, entrained and deposited within fluvial channels. They can exist in organic and inorganic form with varied shapes and sizes. Mostly, they originate from weathering activities. Running water is the most dominant method of transportation (solution, suspension, saltation, and traction)(Mcl,1996); followed by other surficial geomorphic processes such as mass movement, runoff processes and mechanisms of soil erosion (Inter-Agency Sedimentation Project,1963; Environmental Response Team (ERT)/U.S EPA1999; Edward et al. 1999; Joshua and Oyebanjo,2010). They are also from erosion activities going on within a drainage basin at different magnitude-localized or site specific and broad or diffuse scale (Sear, et al, 2003; Tamene, 2005).
River sediments exhibit some location differences because their composition is indelibly molded by the environment where they are deposited, though a common attribute of river sediments is that they function as a habitat at every level-from the least level of accumulation in a river bed down to deep sea because of their oxygen content which encourage anthropogenic activities (Garrison, 1999; Steven, et al 2015)
In a drainage basin, volume of sediment that passes through a channel in a given time is a function of many variables: cross-section of the channel and flow velocity (Singh, 2004); precipitation rate, land use pattern, clogging of the river channels (Jeje, 2001; Iloeje, 2004; Aper, 2007); undulating topography and soil erosion (Eze, 2008); textural class and clasts size of soil(White ,2001); land use pattern ( Sun et al,2014; Shi, et al 2014;); prevailing earth surface processes(Junsheng et al , 2015); human interventions ( Haifango et al,2015 ) and velocity of the river transport ( Leopold ,1964 ) e.t.c. This is because when the transport capacity of a river is overcome by the volume of sediment being conveyed, it leads to an accelerated deposition of sediment (Meyer et al. 1969; Haan et al. 1994; Manoj et al. 2010) which is pivotal to myriad of environmental challenges that obtains in a drainage basin.
Aptly, researchers (Gray et al, 1997; Leal, 2009; Kroes et al, 2010; Moulin et al, 2011; Gellis et al, 2014; Batson et al,2015) stated that excessive sediments amount to sedimentation, contamination of water quality, siltation of water resource facilities, poor function of irrigation schemes, high cost of maintenance of facilities and management of river resource channels, reduced light penetration thereby endangering the aquatic organisms and flooding e.t.c. Conversely, paucity of river sediments impedes the formation and development of wetlands-cum- flood plains, slowed processes of soil formation and scouring of river beds.
Given that sediments are resources (SedNet,2009) and our drainage basin is located within sedimentary rock formation geographically positioned in the humid tropical region where there is intense surficial geomorphic processes orchestrating from the fluvial processes (Ofomata and Umeuduji,1994), the nexus of this research is designed to characterize the stream sediments of Anambra drainage basin in the rainy and dry season. Thus, this present study will also provide insight on the implications of seasonality on sediments and such knowledge will go a long way in forestalling problems associated to overbank processes and sedimentation which drainage basins at the tropics are exposed to. Furthermore, our findings on the sediment characteristics at the rainy and dry season will provide a vista for the achievement of safe settlement planning for the rural basin dwellers inhabiting the drainage basin because they will be better informed on the best management options to adopt with respect to seasons. Thus, this will be effective in the formulation of basin management policies and improved understanding of textural and fluvio- geomorphic processes operating within the basin surrounding. This is because sediments originate mostly from soil erosion, runoff activities, anthropogenic and non-anthropogenic activities that present day environmental geo-hazards thrive on (Turner, et al. 1990; Eswaran et al. 2001; Manoj et al. 2010).
Following the above, river sediments study has been the focal point of most discussion but its characterization in-terms of seasonality has not been really anchored on. Researchers has tilted towards sediment -transport, particle size, sources, fluxes, e.t.c and undermined the aspect of seasonal characteristics. This study is set out to unravel the sediment characteristics of Anambra drainage basin in order to bridge the existing academic gap.
1.2 Statement of the Research Problem
Sediments ranging from consolidated to non consolidated have generated a lot of research interest mainly because sediments are resources (SedNet 2004;2009); facilitates the development of geomorphic features- terrain, floodplain, sand bars, delta, marshes, alluvium plains, landform and soil formation (Walling,2006); fundamental in maintenance of the morphological dynamics both at the river channels and stream beds, thresholds of sediment delivery at the different segements and conveyance capacities, substrates for tidal channels, inter-tidal flats, marsh plains formations (Fagherazzi 2012; Mariotti and Fagherazzi, 2013); provide insight about pollution status or contamination level of a river ( Yu et al,2000; Iwegbe et al,2007). They are also source of vital information and data for management of environmental problems pertaining to overbank processes and achievement of goals pertaining to distributive and efficient river water resource utilization through the design and construction of hydraulic structures such as dams, canals and irrigation schemes (Edward et al,1999). Sediment analysis provides a platform for advocacy on clean river environment, management, controls or standards on activities involving river sediment (Gray, et al, 1997).
Anambra basin exists within the Benue rift structure located in the West African Rift system (Fairhead and Okereke, 1987; Genik, 1992) and is the centre piece of most earth science studies in Nigeria because it is a hydrocarbon province (Okeke, 2014; Agagu et al, 1982; 1985; Akaegbu and Schmitt, 1998; Akande et al, 1988 and Nwajide, 2005). Interestingly, consolidated sediment -Ajalli sand stone has been focused in most researches (Simpson, 1954; Reyment, 1965; Hogue and Ezepue, 1977; Ladipo, 1986; Adekoya et al, 2011; Odigi and Amajor 2008; Adeigbe et al, 2009; Odumoso et al, 2013) whose contribution defined the depositional environment of our basin.
Unfortunately, surface stream sediments characteristics of the Anambra drainage basin have not occupied a prominent place in recent researches within our study area. Thus, the surface stream sediments have largely been untested and therefore not documented. Other available literature within the basin under study tilted towards: evaluation of surface rock and hydrocarbon potential of Anambra basin Ugochukwu(2010); hydrodynamic flow and formation pressures in the Anambra drainage basin(Uma and Onuoha 1997); depositional environment of Campano-Maatrichtian sediment in Anambra basin(Adeigbe et al, 2009); depositional environment of the Mid –Maatrichtian sediment Ajali sand stone in Anambra basin (Odumoso et al ,2013); heavy metal in water column of Anambra river (Ezeonyejiaku,2010); PAHs in freash water media; factorial effects and human dietary exposure risk assessment( Obiakor et al ,2014) which did not take cognizance of the sediment dynamics with respect to space and time. Since most of the existing studies orient towards other scientific enquires with little or no address to the sediment with respect to its elemental composition -trace or micro metallic composition, despite the continuous perturbation of the tranquility of our river by earth surface processes and man’s successive socio-economic activities which has grossly resulted to some unexpected impact, this present study is therefore considered very vital.
Physiographicaly, Anambra drainage basin lies entirely within a weakly consolidated cretaceous, tertiary and quaternary sedimentary rock formation with high susceptibility to sediment yield. Up till now, the surface stream sediment spatio –temporal variation has not been studied despite the fact that our drainage basin is located within the humid tropical region characterized with intense rainfall and our study area is traversed by a maze of rivers prone to overbank processes. This has made the basin vulnerable to flooding risks, sedimentation, menace of soil erosion, and other vagaries of environmental hazards associated with cycles of fluvio-geomorphic processes. To the above end, we concluded that a study on seasonal characteristics of the stream sediment is considered critical in understanding the sediment geo-chemical composition. An insight on the seasonal characteristics (dry and rainy season) of the stream sediment is important in basin planning and in the development of appropriate basin policy which is a fundamental aspect of river basin management. This study is therefore designed to give premium to the elemental composition of the stream sediments, sediment grain size distribution, physical characteristics of the basin regolith, sources of stream sediment, basin form and its implications
Strengthening the timeliness of this research, is the fact that, spatio –temporal changes of stream sediment is gaining currency (Alliot et, al, 2003; Lansard et al 2009, Cathlot et al,2010; Bourgeois et al 2011, Pastor et al,2011 b, Cathalot et al,2013; Paulo, et al 2015 ), however, to the best of our knowledge within our study area , there is apparently a limited research within the domain of stream sediment spatio-temporal analysis. Conversely, there is therefore no front to disaffrim that a new study which will utilize appropriately spatial and temporal sampling prcedure to present a holistic description of the basin understudy is not crucial at the moment. In this context, this study is anchored on examining the stream sediment characteristics of the Anambra drainage basin at the rainy season and dry season.