John laing, mass media and misisi areas
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1.2climateThe city experiences a sub-tropical climate that is strongly seasonal. It has three distinct seasons, namely:
2geologyRocks underlying the city of Lusaka consist of schists interbedded with quartzites and dominated by thick and extensive sequences of marbles (Fig. 3), with the latter being generally referred to as the Lusaka Dolomites or Lusaka Limestones. 
Fig. 3: Geologic map of the city of Lusaka 2.1Important Geological StructuresThe main structures in the area trend SE – NW and they appear to have developed as a result of repeated overthrusting (Fig. 4), which coincides with the main SE –NW trend. Associated with these tectonic activities are three sets of joints, whose presence and those of other discontinuities, has enabled water to be transmitted through the rock mass, causing differential and preferential dissolution of the marbles and the subsequent development of an integrated and well-developed system of conduits and solution channels. Development of cavities in these rocks appears to follow the main fracture-direction from south-east to north-west as depicted in Figure 5.  
Fig. 4: Schematic model showing repeated thrusting in the Lusaka area (Modified after Nkhuwa, 1996). Fig. 5: A display of well-developed system of conduits and solution channels in the Lusaka marbles 
2.2Evidence for major geologic structuresGenerally, evidence for the presence of major geologic structures, particularly discontinuities in these rocks, is exhibited by some recognisable surficial features, such as lineaments and the association of sinkhole-distribution with zones of high lineament density. The regional scale relationship of karst features to lineaments is further evidence of the important role fracture zones have assumed in the process of solution weathering. Fig. 6 shows the distribution of sinkholes and the dispersion of their orientation demonstrates the effect of complex folding in the Lusaka area, while the apparent correlation of sinkhole distribution with zones of high lineament density is depicted in Fig. 7.   
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Fig. 6: Karst geomorphology of the Lusaka marble plateau   
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Fig. 7: The relationship between lineaments and sinkhole manifestations. After Nkhuwa, 1996 The presence of these discontinuities has enabled water to be transmitted through the rock masses, causing differential dissolution in the meta-carbonates and the subsequent development of an integrated system of conduits and solution features. This has riddled the terrain with collapse and subsidence sinkholes. Other than converting the meta-carbonate rocks into an important and comparatively cheap source of water supply, such characteristics have also transformed this terrain into one with specific and highly complex hydrogeologic and environmental conditions that now pose several potential hazards to the city aquifers. In turn, these conditions impose enormous restrictions on land use practices of this city terrain. 3RainfallThe arrival of the rainy season from about late September lasting through to May is generally coincident with a number of air-circulation patterns that influence the Lusaka region. The advent of these patterns, consisting of the Angolan Low, the Zaire Air Boundary (ZAB) and the Intertropical Convergence Zone (ITCZ), introduces air that is moist, unstable and confluent or convergent. Although these patterns generally arrive at different times, they usually overlap with and influence each other. The Lusaka plateau receives an annual average of 800 mm of rainfall. Fig. 8 shows the annual rainfall for the period 1971/72 to 2000/2001, with 819.7 mm/a representing the average rainfall. Table 1 depicts the monthly rainfall distribution during the same period. 
Fig. 8: Distribution of the average annual rainfall over the Lusaka plateau (1991/92 – 2000/01) Table 1: Average monthly precipitation (in mm) for the Lusaka area [1971/72 - 2001/02]
From Figure 6, one important aspect of the annual rainfall variability relative to the average annual rainfall on the Lusaka plateau is the tendency for sequences of relatively wet and dry years to occur. It is apparent from this diagram that 1971, 1973, 1974, 1975, 1977, 1979, 1980, 1988, 1989, 1990, 1996, 1998, 1999 and 2000 represent wet years. The years 1972, 1976, 1978, 1981, 1982, 1983, 1984, 1986, 1987, 1991, 1992, 1993, 1994, 1995, 1997 and 2001 are dry, with all but four occurring in the 1980s and 1990s. Their dryness, particularly in the 1990s, appears severe both in terms of amount and duration. |