an engineering geological characterisation of tropical clays - GBV

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recorded for Nairobi and its adjacent areas over a period of 25 years are 32,8° C and 3,9° C,
respectively. A summary of temperature statistics over the last 50 years for Nairobi and adjacent
areas is presented in Table 1.1 (Kenya Meteorological Department, 2001).
Nairobi and the present study area are also characterised by wet and dry seasons which show
marked differences in the amounts of precipitation. The average annual rainfall is about 900 mm;
but the actual amount in any one year may vary from less than 500 mm to more than 1300 mm per
year (Kenya Meteorological Department, 2001). The distribution of rainfall is bimodal, with long
rains being registered in the months of mid - March to May ( main rainy season, with 137 to 195
mm per month); and short rains from mid - October to mid - December (secondary rainy season,
with 77 to 114 mm per month). The dates when the rainy seasons actually start and end are very
variable, i.e. the beginning and end of a wet season are therefore usually not well defined. The two
rainy seasons coincide approximately with the time of changeover of the south-west monsoon and
the north-east monsoon currents which affect eastern Africa (including Kenya) in April and
November, respectively. The dry season is experienced from mid - December to mid - March. The
amount of rainfall generally tends to increase with increasing altitude, from about 500 mm per year
around Kitengela (1520 m above sea level) located about 8 km to the south of the study area; to just
over 900 mm per year around the Nairobi city centre (1660 m above sea level) in the northern part
of the study area, respectively. The precipitation is mainly in the form of convectional rainfall,
commonly occurring as late afternoon showers (Survey of Kenya, 1990; Kenya Meteorological
Department, 2001). A summary of average rainfall (mm) for each month of the year, based on
records for 50 years is given in Table 1.2.
Average
Rainfall
(mm)
Mean relative humidity
values (%)
9:00 am 3:00 pm
Bright sunshine
(average hours
per day)
Months
January 48 79 45 8,9
February 48 74 37 9,5
March 115 82 43 8,7
April 195 86 53 7,0
May 137 85 55 5,7
June 42 85 59 5,8
July 15 83 53 4,4
August 21 85 53 4,2
September 24 82 50 5,9
October 52 80 47 7,0
November 114 81 57 7,0
December 77 83 54 7,9
Year 74 78 51 7
Table 1.2. The average rainfall (mm) based on records for 50 years, mean relative humidity values
(%) and amount of bright sunshine (average hours per day); for each month of the year, for Nairobi
city and the present study area (Kenya Meteorological Department, 2001).
Nairobi and its environs are also located within the path of predominantly easterly winds. The
winds are near the ground and blow across the area , generally between north-east and east from
October to April; and between east and south-east from May to September. Strong winds with
speeds of 30 to 40 km/h are common during the dry season, just before the start of the long rains in
mid - March; and especially from mid-morning to early afternoon. Wind speeds of 15 to 25 km/h
are also recorded during other times of the year; while nights are commonly characterised by light
winds.
Nairobi city and its adjacent areas are some 400 km away from the sea and do not therefore
experience the rather unpleasant humid heat that usually characterises tropical coastal towns. A very