Environmental Engineering Reference
In-Depth Information
Figure 5.5. Pollution flux at a given
location of a stream for a storm with
duration T
2
- T
1
.
separate sampling point and its various slopes. In addition, the type of cover for each
specific area was evaluated as a percentage of the total drainage area. These parameters
would determine the runoff flow pattern-the steeper the surfaces the faster the runoff can
drain to its outlet and the higher the impervious cover - the larger of the volume of the
runoff generated by the specific rainfall event.
The SA-SCS method uses curve numbers, where the initial curve number selection is
based on the soil and land characteristics only. The second step of the method includes
the determination of the different soil characteristics and was executed based on the soils
classification in Zimbabwe (Nyamapfene 1991). The study area was subdivided into three
major soil types:
• The first soil type is Kalahari sand of Permian to Triassic age together with that of
alluvial Holocene age, soils that are relatively youthful. These were to be found mostly
in the built up part of the study area. The residential areas consist of this type of
geological formation, but the fact that they have been compacted over time for many
years, has resulted into a soil type of slow infiltration rates and moderate to high
runoff potential. The home industry and commercial areas, gravel street roads also fall
under this category. This category is classified as soil group C in the hydrological soil
groups.
• The second soil type comprises of the vertisoils. These consist of clayey soils. The area
with such characteristics are subject to some degree of water logging and as a result
develops structure and cracks which may be confined to the top 30 to 50 cm in most
areas. This type is mainly found along the stream area, where informal agriculture
takes place and consists of a combination of the soil groups B and C that is classified
as B/C.
