Morphological characteristics of gullies in the Kakia-Esamburmbur catchment, Kenya

Abstract

Gully erosion has a global environmental, economic and social impact. Gully erosion in the Kakia-Esamburmbur catchment, Kenya, has led to the significant degradation of agricultural land resulting in the reduction of land productivity and land value. Prediction and modeling of gully erosion rate is complex and governed by locally-specific climatic and geomorphologic factors. The objective of this study was to investigate the morphological characteristics of gullies, develop an empirical gully volume prediction model and establish the topographic threshold of gully formation in the catchment. The study focused on active gullies on cropland and alongside roads. The gullies were divided into valley floor, hollow and roadside based on their topographic characteristics, and they represented 11%, 82% and 7% of the total gullies respectively. The results showed that the length (L), depth (D) and cross-section areas (CSA) of the gullies varied between 1.35m and 256m; 0.11m and 2.70m and 0.36m² and 8.54m² respectively. These characteristics differed between gullies based on their topographic location. Empirical models between gully Volume (V) and Length (L) and cross-section area (CSA) were developed. It was found that a combination of gully length and cross-section area were more accurate predictors than the individual morphological characteristics. Gully volume could be well estimated with an R² of 0.93 for valley floor gullies; 0.87 for hollow gullies and 0.99 for roadside gullies. The topographic threshold for gully initiation was  which indicated that the dominant process for gully erosion is overland flow. There was a substantial increase in gully volume from 1,801.5 m³ in 2009 to 13,553.31 m³ in 2022, indicating an average annual erosion rate of 979.32 m³/year driven by both natural and anthropogenic factors. The findings offer a practical method of predicting gully volume and identifying areas of gully initiation in the Kakia-Esamburmbur catchment and similar landscapes. This is an important decision support tool in planning of appropriate gully control measures.