an engineering geological characterisation of tropical clays - GBV

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above sea level from Jomo Kenyatta International Airport to Wilson Airport; and 1740 - 1800 m
above sea level in the north-western parts of the study area; respectively (Survey of Kenya, 1990).
More than 80% of the study area, including the southern half, has therefore an altitude of between
1540 m and 1740 m above sea level. An average elevation of about 1650 m above mean sea level is
recorded for the study area; with the lowest elevations (1540 - 1580 m above sea level) in the southeastern
(around Beacon Ranch Farm) and north –eastern parts of the study area. The highest
elevations (1740 - 1800 m above sea level) occur at the far western and north-western sections of
the study area.
The main drainage in Nairobi, including the study area, is consequent upon the regional topography
and prevailing slope of the volcanic rocks. The streams are generally easterly-flowing, but a few
cases of drainage and flows to the south-east and south also occur (Morgan, 1967; Survey of Kenya,
1990; Fig. 1.1). The north-western and western parts of the study area bordering the Kikuyu
highlands are characterised by streams that show features of having been deeply incised and to have
re-excavated their former courses. According to Saggerson (1991), this could be attributed to the
continued uplift of the Rift region during the late Tertiary, and probably early Pleistocene; and also
to the continual deposition of lavas and tuffs onto an easterly - inclined surface. This is especially
observable in the Nairobi river valley and its main tributaries of Kerichwa Kubwa and Kerichwa
Dogo, as well as in the Mathare, Gitathuru and Ngong river valleys. Lavas, welded tuffs and ash
flows therefore poured down the valleys causing their inundation and choking with volcanic rock.
Lateral erosion was limited, and the erosional effect of the tuff-flows was mainly concentrated in
the stream-beds where channelling and corasion were at maximum, causing re-excavation of the
stream valleys.
The uplifting and deposition of volcanic materials have therefore given rise to streams that are
characterised by young valleys with steep gradients and narrow and/ or sharp V-shapes in the northwestern
and western parts of the study area. In addition, the rapid down-cutting, together with the
relatively soft character of the younger volcanic rocks have resulted in the streams flowing in
generally parallel courses, with limited instances of river capture (Gregory, 1921; Morgan, 1967).
Many parts of the study area receive rainfall amounts of around 900 mm per year, with the rainy
seasons concentrated in the periods of mid-March to May; and mid-October to mid-December
(Kenya Meteorological Department, 2001). The streams are most active during these months of the
year when rainfall is heavy, and headward erosion of gullies and tributaries is common. The
transported load is mainly a result of erosion of the thick soil cover, and this frequently gives rise to
flowing streams of red mud (Survey of Kenya, 1990).
The study area is characterised by two main drainage basins. The Nairobi river and its tributary
valleys dissect and drain the northern and north-western parts; while the tributaries of the Athi river
drain the southern and south-western parts of the study area, respectively (Fig. 1.1). The main
tributaries of the Athi river include the Sosian, Mokoyeti, Donga and Mbagathi valleys; which are
generally medium – wide rivers, with gentle side-slopes and flat bottoms. Aerial photo -
interpretation and topographic surveys show a generally wide drainage pattern over the study area,
with distances of 1 to 6 km between adjacent channels. The drainage and flow direction are
generally consequent upon the local topography and slope direction, with the Nairobi river basin
draining generally eastwards, while the flow in the Athi river basin is mainly to the south and southeast.
A smaller drainage basin occurs mid-way across the study area, and consists of the Ngong
river and its tributary of Motoine valley, both of which flow generally eastwards. However, apart
from the Nairobi river and Athi river and their main tributaries, most of the other stream channels
are seasonal and dry during the hot months of the year (Saggerson, 1991; Survey of Kenya, 1990).