Environmental Engineering Reference
In-Depth Information
• The high storm flow potential, low infiltration rates and permeability have resulted in
classifying tarred street roads and storm drains in the soil group D.
The results of the third step of the method application are shown in Table 5.4, which
summarizes the soil type and the percentage of the area covered as adopted for the
purpose of the study. Once the soil type and group were known, the model soil groups,
associated with each sampling point, were adopted from SCS hydrological soil groups,
and the initial curve numbers for each land use pattern, including the cover and treatment
were selected (Table 5.5). Then the weighing factors were applied to account for the
areas covered by soil type, cover and land use in the whole catchment area. The sum of
these weighted initial curve numbers represents the initial composite curve number for
the catchment area.
After the determination of the initial composite curve numbers follows the adjustment
of the initial curve numbers, with respect to the soil moisture conditions of the area prior
to the rainfall events. The rainfall data for the study area was obtained from the
Meteorology Department, Harare, and is presented in Figure 5.6. A significant feature of
the rainfall is its unreliability in terms of rainfall height and duration. The onset of the
rains, which is critical, is rather unpredictable. The variation from year to year is such
that in some years, as little as quarter of the arithmetic mean may fall while in other years
200-300% of mean may fall.
Table 5.4. Adjusted soil type distributions in the
study area.
Catchment component
Soil
group
% Area
covered
Area covered
10
6
m
2
Weighing
factor
Pervious
Rain fed row crop
B/C
22
1.430
0.22
On plot gardens
B
5
0.325
0.05
Open spaces, parks
A
5
0.325
0.05
Impervious
Roofs
C
43
2.827
0.43
Tarred streets, roads and home
industry
D
12
0.761
0.12
Commercial
C
11
0.715
0.12
Gravel street roads
C
1.5
0.0975
0.015
Table 5.5. Initial composite curve numbers.
Sampling Point
% Area covered
Weighing factor
Initial composite CN
A
21.6
0.216
27.2
B
45.8
0.458
29.155
