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PHYSICAL SETTING

Posted by on 2/6/2003 11:28:46 AM |

PHYSICAL SETTING

Geology: Rivers State lies on the recent coastal plain of the eastern Niger Delta. Its surface geology consists of fluvial sediments. This includes the recent sediments transported by Niger River distributaries and other rivers, such as Andoni, Bonny and New Calabar. These materials deposited as regolith overburden of 30m thickness are clays, peat, silts, sands and gravels.

University of Port Harcourt ( Main Entrance )
University of Port Harcourt ( Main Entrance )

The depositional sequence exhibits massive continental sand stones overlying an alternation of sandstones and clays of marginallly marine origin, but eventually grading downwards into marine clays. Sands, by far, form the largest group of rock types in Rivers State, while mud constitutes all the polluted brackish waters of the riverine areas. However, peat constitutes the various vegetal and animal remains that lies in bogs and shallow pits. The gravel and pebbles form the last unit of the subsurface rocktype, and are usually found at the base of the river channels.

Relief: The land surface or Rivers State can be grouped into three main divisions: the fresh water, the mangrove swamps of AkukuToru, Abualdual,AsariToru, Degema, Okrika, OguBolo, Bonny, Andoni and Opobo Local Government Areas; and the Coastal Sand ridges zone. The freshwater zone is the plain that extends north wards from the mangrove swamps. This land sur face is generally under 20m above sea level.

National Fertiliser Complex, Port Harcourt
National Fertiliser Complex, Port Harcourt

This lower Niger floodplain has a greater silt and clay foundation and is more susceptible to perennial inundation by river floods. The value of the mean thickness appreciates upward to about 45 m in the northeast and over 9m in the beach ridge barrier zones to the southwest. The flood plain is a homo clinal geomorphic structure whose trends west wards and southwards are broken in many places by small hogback ridges and shallow swamp basins (Aisuebeogun, 1995). The southern part is affected by great tidal influence.

Most water channels in the freshwater zone are bordered by natural levees, which are of great topographical interest and of great economic importance to the local people for settlements and crop cultivation. The upland is undulating to the hinterland and sandwiched with NWSE and EWW direction ridges and attain a maximum height of 30m above sea level at Okubie, to the southwest. The narrow strip of sandy ridges and beach ridges lie very close to the open sea.

The soils of the sandy ridges are mostly sandy or sandy loams and supports crops like coconut, oil palm, raffia palm and cocoyam. Fourteen of the twenty three LGAs of the State are located on the upland with varying heights between thirteen to 45m above sea level. These include Ogoni, lkwerre LGAs, Ahoada, Abual/Odual, 0gba/ Egbema/Ndoni LGAs and Port Harcourt LGAs. The drier upland area of Rivers State covers about sixty one percent while riverine area, with a relief range of 2m to 5m, covers about thirty nine per cent of the State. The entire topography of the State is also characterised by a maze of effluents, rivers, lakes, creeks,lagoons and swamps crisscrossing the lowlying plains in varying dimensions.

Drainage: Drainage is poor, being lowlying, with much surface water and a high rainfall, of between 3,420 mm and 7,300 mm.

Risonpalm Oil Palm Estate, Ubima
Risonpalm Oil Palm Estate, Ubima

Thus, almost all riverine LGAs are under water at one time of the year or another. Again, some areas of the State are tidally flooded, while others are seasonally, thus limiting agricultural practices and nucleated/urban settlement development that would have enhanced social welfare facility provision. The State is drained by two main river systems, i.e. freshwater systems whose waters originate either outside or wholly within the coastal lowlands, and tidal systems confined largely to the lower half of the State.

Drainage densities of rivers within the state have typical value of 1.5 km and sinuosity ratios are in excess of 1.9, indicating that the meandering channels are tortuous. These systems have a general downstream increase in width and velocity, especially in the freshwater zones. The State is drained by the BonnyNew Calabar river systems and by a maze of effluent creeks and streams. River bank levees are prominent and valley side slopes are very gentle and experience a great deal of erosion and accretion. All the rivers enter into the sea through wide estuaries.

Soils: There are three major soil groups in Rivers State, namely: the marine and fluvial marine sediments; the mangrove swamp alluvial soils; and freshwater brown loams and sandy loams. The marine and fluvialmarine sediments are found in the wet coastal region. The soils are organic in nature and essentially sandy in texture.

Some consist of mud mixed with decayed organic matter. The mangrove swamp alluvial soils are found in the northern part of the coastal sediments zone. They are brownish on the surface, sometimes with an unpleasant and offensive odour. The soils of the swamps are rich in organic matter in the top layer, but contain too much salt especially in the dry season.

The third soil group, the brown loams and sandy loams are found in the fresh water zone of the delta. The levees which form the common land forms of this zone are made up of rich loams at their crests, changing to more acidic and more clayed soils along their slopes.

Climate: Rainfall in Rivers State is seasonal, variable, and heavy. Generally, south of latitude 05N, rain occurs, on the average, every month of the year, but with varying duration. The State is characterised by high rainfall, which decreases from south to north. Total annual rainfall decreases from about 4,700 mm on the coast to about 1,700 mm in extreme north of the State. It is 4,698 mm at Bonny along the coast and 1,862 mm at Deqema.

Rainfall is adequate for allyearround crop production in the State. The duration of the wet season is not less than 330 days, of which a great number is rainy days (days with 250 mm or more of rain). For Port Harcourt, the rainy days are about 182. Mean maximum monthly temperatures range from 28C to 33C, while the mean minimum monthly temperatures are in the range of 17C to 24C.

The mean monthly temperature is in the range of 25C to 28C. The mean annual temperature for the State is 26C. The hottest months are February to May. The difference between the dry season and wet season temperatures is only about 2C. Relative humidity is high in the State through out the year and decreases slightly in the dry season (Salawu 1993).

Vegetation: The "upland" area was originally occupied by rainforest which has been drastically modified by human activities. In most places, economic trees, particularly oil palm, have been preserved and thus the sobriquet for this vegetation as "oil palm bush." The riverine area is divisible into three main hydrovegetation zones namely, the beachridge zone, the saltwater zone and the freshwater zone.

The beachridge zone is vegetated mainly by fresh water swamp trees, palms and shrubs on the sandy ridges and mangroves in the intervening valleys or tidal flats. The saltwater zone is the tidal flat or swamps vegetated by the red stiltsrooted mangrove (Flhizophora racemosa) and two other species of mangrove.

The outliers of raised alluvial ground or coastal plain terrace within the swamps are vege tated by tall forest tree species and oil palm. The freshwater zone is mainly the Upper and Lower Delta ptainsfloodplains of the Niger, having fresh water forest trees which are the edaphic variants of the rainforest. The Abura tree, oil palm, raffia palm, shrubs, lianas, ferns and floating grasses and reeds are the typical vegetation.

Ecological Problems: Deforestation is among the ecological problems confronting the State, as mass deforestation of both mangrove and rain for est is extensive. In fact, in some parts of the State, derived Savannah exists. Rivers State is a State of physical difficulties, such as lowlying terrain riddled with an intricate system of natural water channels; too much surface water and a high rainfall; unin habitable mangrove swamps and some parts of the state suffer from inaccessibility.

The character of Rivers State relief, drainage and geology poses much problem to resource exploitation and economic development. Other ecological problems include severe beach erosion associated with sea level rise due to global climatic change; annual inundation by river floods; salty soils especially in the dry season; too much leaching of soil fertility due to excessive rainfall; and suceptibility of settle merit sites along the creeks to creek erosion.

Oil spills and gas flares with associated thermal, air, water surface and aquifer pollution, caused by oil exploration and production, are taking a toll on the agricultural output of the land, fisheries, vegetation and wildlife.




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