Drivers of Surface Processes on the Bama Beach Ridge: A Quantitative Analysis of Rainfall, Vegetation and Sediment Transport

Abstract

This study investigates the combined influence of rainfall characteristics, vegetation dynamics, and runoff generation on sediment yield on the Bama Beach Ridge (BBR) in the Sudano–Sahelian zone of northeastern Nigeria. Field observations were carried out on a 0.002-ha artificial catchment located on a 5% grazing slope during the 1990 and 1991 rainy seasons. Rainfall was measured using an automatic siphon-type rain gauge (German model), while runoff and sediment yield were monitored with a discharge recorder and sediment collection tank. Grass growth and density were assessed using fixed quadrats. A total of 107 discrete rainfall events were recorded, with approximately 71% delivering less than 10 mm of rainfall and producing no runoff. Runoff occurred only when rainfall exceeded threshold values of 20 mm in amount and 15 mm h⁻¹ in intensity. Event-based analysis indicates that maximum rainfall intensity is the primary control on runoff generation and sediment transport, accounting for 82% of runoff variability. Partial correlation analysis further shows that rainfall intensity, the timing of events within the rainy season, and grass density together explain 93% of the variation in sediment yield. Sediment production was highest at the beginning of the rainy season, when vegetation cover was sparse and surface materials were loosely consolidated, and declined as vegetation density increased. Total sediment yield in 1991 was 3,095.62 g, corresponding to a mean erosion rate of 15.49 g m⁻² yr⁻¹ (154.9 kg ha⁻¹). The transported material was predominantly sandy (75–85%), with very fine sand forming the largest fraction. These findings highlight the importance of rainfall intensity thresholds and vegetation recovery in controlling hillslope erosion in semi-arid savannah landscapes and provide insight relevant to erosion management and geomorphic modeling.