Relationships between average gully depths and widths on geological sediments underlying the Idah-Ankpa Plateau of the North Central Nigeria

This study investigated the relationships between the average gully depth and the average width on two of the three geological sediments underlying the Ida-Ankpa Plateau of the North Central Nigeria. Gully length (L), average depth (D), and average width (W) were measured on 37 and 5 gully samples formed respectively on the Ajalli Sandstones' (AS) and the Upper Coal Measures' (UCM) geological sediments. From these, the average volume (V), average cross sectional area (A), and average form factor (W/D) were computed. These sets of variables (L, D, W, V, A, and W/D), a total of six for each gully, were analysed using correlation analysis and the sample bivariate regression to examine the relationships between D and W on the two geological units. Results show that D correlates poorly, positively, and significantly with W on the highly erodible AS (R = 0.565, P < 0.01), whereas the relationship is stronger, positive, and significant on the more resistant UCM (R = 0.997, P < 0.01). On the AS, D is a poor predictor of W with the relationship: W1 = 3.670 + 0.355D1 (R2 = 0.319), whereas it is a near-perfect predictor on the UCM (W2 = 1.667 + 0.689D2, R2 = 0.994). The results of the study suggest that for gullies cut on deep, homogeneous formations, the correlation of D with W diminishes with increasing vulnerability to erosion of the sediments, whereas the effectiveness of D as a predictive tool of W increases with the resistance of the sediments to erosion.