Analysis of Hydrological Characteristics: A Case Review of the Niger Delta

Abstract

This paper reviews the significant contribution of hydrologic continuity in the development, evolution and fate of the Niger Delta. Formed from the aggregation of sediments from a drainage catchment area of over 2 million km 2 , the Niger Delta has evolved into a prolific sedimentary basin with a dense network of rivers and creeks. The ecological system is in a fragile state with fresh and saline water ecosystems maintaining a dynamic equilibrium. Rainfall typically varies from 2500 - 4000 mm per year compared to average annual evaporation of about 1500 mm, resulting in net antecedent moisture. Over 70% of the rainfall occurs in 4 months between May and September, creating a potential for flooding. Water level varies from less than 1.5 m in the estuaries to about 8 m at the apex of the delta, with Nun River having a slightly higher elevation compared to Forcados River, implying that canals connecting both rivers at the same reach would experience water movements from Nun to Forcados River. Similarly, water level gradients vary across the stretch of the rivers from 8.6 to 9.5 × 10 -5 (cm/km) between Onitsha and Asamabiri, 7.5 to 7.7 × 10 -5 (cm/km) on the Forcados River downstream of Asamabiri, and from 7.4 to 7.6 × 10 -5 (cm/km) on the Nun River. At peak flood, about 23 × 10 3 m 3 /sec of runoff enters the Niger Delta. At the same time, about 16 × 10 3 m 3 /sec discharges from the estuaries into the ocean in a semi-diurnal tidal flow leaving a surplus that accumulates to cause flooding. The delta thus acts like a sponge, absorbing the shear amount of flow and releasing slowly stored water through at least twenty-one outlet estuaries into the Atlantic Ocean. The shear amount of discharge overwhelms the capacity of the network of distributaries and predisposes the delta to flooding. Assessment of the capacity of the rivers to evacuate pollutants showed that rivers to the west of Brass which are fed by Forcados River exhibited relatively stronger flux efficiencies prior to 1998 but are presently weaker compared to rivers fed by Nun. The rivers to the east between New Calabar and Imo Rivers exhibited very weak ebb tide asymmetry or net upstream flow largely because these rivers receive little or no freshwater influx from upstream sources. Flood and erosion are the major hydrological hazards in the region.