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