Morphometric and hydraulic geometry assessment of a gully in se nigeria

Abstract

Gully erosion represents one of the most devastating form of land degradation in the Sedimentary Formations underlying south eastern Nigeria leading to both onsite and off- site adverse effects on the environment. In this paper, a surveyed section of a second-order gully located in Uyo, Akwa Ibom State, SE Nigeria is analyzed. Along the gully, 20 cross-sections were established and measured with a Rover GNSS and the Base GNSS. The sections were located at variable distances, placing them in areas where active erosion was evident. In total, 19 field measurements were carried out, and the geometric characteristics of 19 cross-sections were obtained. Morphometric analyses were carried out in 421m surveyed Udo Inwang segment of the 860m long main gully. The results indicated that Udo Inwang gully was a mature gully, measured gully depth varied from 1.1m to 29m, while the shoulder or top width ranged from 26m to 98m. It is worth noting that the bed width is typically narrow ranging from 4m to 33m wide. The cross-sectional profiles were indicative of U-shape to V- shape, all with very narrow bed width and steeply sloping gully sides, which are feature of gullies developed on very loose and incoherent soils that slumps/collapses on exposure. The total volume of soil lost from the watersheds was 480,376.26m3, which translated to 18,593.64 tons/ha and the average rate of soil loss has appeared to be 476.76 ton/ ha /year. The annual average growth rate of the gully length was 22m/year. Hence, the average growth rate of gullies in the study watershed with 22 m/ year-1 laid under catastrophic or destructive type of gully erosion.