Geomorphic Threshold for Gully Initiation in Different Geographical Environments of Wanjoga River Catchment, Tana Basin, Kenya

Abstract

Gullies in semi-arid region are important in landscape modification, degradation and increased overland flow affecting geomorphic thresholds of an area. Gullies generate about 95% of global sediment load, important in landscape modification, degradation and increased overland flow in semi-arid regions, but little is known on geomorphic factors that increase ecological fragility increasing gully initiation. To address the problem, landscape regions of accelerated geomorphic processes must be determined. The study aimed to establish topographical thresholds and geomorphic factors which increase landscape fragility in gully head positions in different geographical regions. Gully heads were analyzed by detailed field surveys from 10 m up and down-slope position. Drainage area contributing to gully was demarcated from the point overland flow was assumed to reach the gully head based on water visible flow-lines while gully head slopes were determined by use of clinometers. Gully threshold concept was applied to identify the critical slope ( S ) and drainage area ( A ), using appropriate S - A relation ( S = a A b ) and verified using ANOVA. The empirical S - A threshold relation S = 0.383 A -0.397 , R 2 = 0.0321 (upper-segment), S = 0.174 A -0.032 , R 2 = 0.498 (mid-segment), S = 0.23 A -0.020 , R 2 = 0.088 (lower segment), represented approximate critical slope-drainage area for gully initiation and regions of dominant geomorphic processes, above which gully initiation was likely to occur. Negative b values represent an areas more dominated by overland flow over sub-surface processes. Coefficient of correlation multiple R = 0.7055 (70.55%) Mid-segment indicated strong relation slope-drainage area for gully initiation. ANOVA analysis p = 0.01, 0.004 and 0.4498 for upper, mid and lower segment respectively revealed stronger relation between independent and dependent variables. p > 0.05 indicated regions influenced by more factors than slope and drainage area. Thus, slope-drainage threshold relation line can be applied in the semi-arid environments to locate vulnerable sites of dominant geomorphic processes which should be checked for gully conservation.