Mass movements in the Aberdare catchment: a case study of the Kihuri landside

Abstract

The ,problem of landsliding has been addressed in various fora in which various recommendations have been made. Despite this, the triggering of these events seems endless. The purpose of this study was to determine the cause of the Kihuri 2004, Kihuri 1961 and Kawandia 2004 landslides and to establish their relationship with several factors namely; relief, climate, geology, soils, vegetation and land use. This study will cover some the various classifications of mass movements, their relationship with aspects such as geology and quick clay as well as their occurrence in Kenya. Kihuri in Othaya Constituency Nyeri District is located on the eastern foot slopes of the Aberdare ranges an area characterized by great topographic variability with steep slopes with gradients of over 30° and a dense dendritic river pattern which make the landscape particularly irregular. The high gradients on steep sided valleys suggest a correspondingly high potential for mass movement processes particularly during the wet season when soil pore water pressures are high. The volcanic terrain comprises basaltic agglomerate as the major rock type of the area and fertile residual andosols as the predominant soil type. Being a tropical climate zone, Kihuri receives mean annual rainfall and temperatures of 1500mm and 21°C respectively and as such the area is prone to moderate chemical weathering. Prior to human encroachment on forests, the study area used to be part of the now Aberdare forest. However, most of the natural vegetation has been cleared paving way for small scale tea farming. The fateful Kihuri landslide claimed the lives of five children on the wake of 1st May 2004 besides the destruction of land and property. Based on the theodolite survey method, the slide was 28.5m long and 19 m high. The width range was between 7.5m and 15m with the head scarp being the narrowest point. This rotational slide left three steep scarps with angles of slope of between 34° and 25°:Laboratory analysis of the soil samples collected strategically at the site revealed that with high moisture contents of 40 to 58%, over 90% of all the soil samples passed the +200 sieve size revealing that the samples were primarily fines (silts and clays). Further analysis using the hydrometer method placed the soils in the granulometeric nomenclature class of silty loams of high compressibility and plasticity. The liquid limit and plastic limit range of the six samples were 40.4 to 58 and 7.5 to 21 respectively. The unconfined compressive test value was 0.96kg/cm3 and deductions made from this indicated medium consistency of the 15 to 26 % clay content in the soils. The 1961 Kihuri landslide dimensions were mostly based on the extent of the eucalyptus tree species that now dominate the old scarp. Thirty six years later, the 25m long 45m wide area is extremely steep with an angle of slope of 49° and is free of any human activity. Moisture contents of the fine grained silty loams varied between 46 and 53.6%. The average liquid limit of the four, medium consistency clayey soil samples was 52 whereas the plastic index had a wide range of between 4 and 50. A simultaneous slide 20JIl long and 7.5m wide, triggered in Kawandia village approximately one-kilometer away from Kihuri, on the same fateful night of 1st May 2004. The average moisture content of the three silty loam soil samples was fairly high at 51% while the liquid limit values were 59,59 and 50 and plastic index 15,12,9.5. The unconfined soil strength was 0.47kgP with clay consistency being soft and thus prone to failure on a surface with a slope angle of 37°. The predominance of steep slopes, high rainfall, soil type and human activity were seen to play a crucial role in the triggering of the three studied landslides. Two management concepts; preservation of the natural vegetation and soil conservation practices such as terracing and benching were recommended to ensure landslide risk reduction in the highly unstable Kihuri area.