Gully Morphological Assessment and Anthropogenic Determinants in Urban and Rural Settlements of Upper Imo River Basin (UIRB) South Eastern Nigeria

Abstract

Gully erosion have been proven to vary both in terms of size and number in urban and rural settlements and is caused by factors related to both human and natural phenomenon. This study assessed the gully morphological development in an urban and rural settlement with the view of finding the settlement that is more vulnerable to gully erosion development. It investigated gully development in the urban and rural settlements of UIRB; gully morphological features in settlements; and the anthropogenic factors of gully development in urban and rural settlements. A reconnaissance survey was carried out in 2016, while a detailed field survey on the sampled locations and gully sites was carried out from 2017 till early 2019. Two types of data were employed in this study. They were primary and secondary (archival) data. The secondary data set used for this study was obtained from maps (topographical and lithological) and satellite imageries obtained from USSG. Primary data used for the study were grouped into two-gully morphological data and anthropogenic data. The two data sets were obtained through field measurement, digital image analysis, and a questionnaire survey. The morphological data considered are gully length, width, depth, area slope, and elevation, while anthropogenic data involves human economic activities. The sampling technique employed was randomized systematic sampling. Three major factors considered in the course of sampling were settlement type, the number of second-order basins located within the various settlements of the upper Imo River basin, and several gullies located in the settlements of UIRB. Considering these factors 18 gully sites were randomly sampled from gullied second-order basins. Data analysis makes use of percentages, measures of variations, t-tests, and levene statistics. Results of the analysis showed that gully erosion in UIRB develops both in urban and rural settlements. 61.1% of the gullies develop in rural settlements, while 39.9% 0f the gullies develop in urban settlements. The total length, width, and area of the sampled gully sites in Upper Imo River were 6213.2 m, 1931.1, and 330,005.6 m² (330 km²) respectively. Out of these values, the total length, width, and area found in the rural area was 3,539.2, 863.5 m, and 91,767 m² (91.8 km²) respectively, while the total length, width, and area in the urban area was 2,674, 1,067.9 m and 238,238.4 m² (238.2 km²) respectively. The slope where gullies are located in rural areas ranges between 84.4-89.8%, while the slope where gullies are located in urban areas of UIRB ranges between 89.4-89.7%. The elevation where gullies develop in rural areas falls between 55-142 m, while the elevation where gullies are located in urban areas ranges from 55-89.9 m. In terms of number, more gullies are fun in rural settlements than in urban settlements, but in terms of intensity and threat, they are higher in rural areas than in urban areas. Failed road construction was the major factor of gully erosion initiation and development in the urban areas, while sand mining was the major factor of gully erosion in the rural areas of the upper Imo River basin. The two major contributors to gully erosion development in the urban area were failed road construction and sand mining, while the two least contributing factors of gully erosion in the urban areas of the study area were stone mining and farming activities.