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