Environmental Engineering Reference
In-Depth Information
5 NUTRIENTS VARIATIONS IN THE URBAN STORM WATER OF
THE CITY OF HARARE
Considering the advanced stage of eutrophication of Lake Chivero, nutrients
contributions to the Lake itself and its tributaries is an important issue, which needs
priority attention in the water resources management practice. In this context, the
investigation of nutrients concentrations, and their seasonal variation in the urban storm
water, should not be underestimated. This section evaluates the status and temporal
variations of nutrients in the urban drainage, based on data from the water quality
monitoring program of the city of Harare during the period 1995-2000. It discusses the
results in a broader context, considering the impacts of the present regulatory instruments
and the implementation of the monitoring process and pollution abatement measures, in
order to provide environmentally sustainable development.
5.1
Methodology and study area
The monitoring program of the city of Harare, regarding urban drainage water quality,
includes two major drainage channels. The Coventry Road Drainage Channel is an earth
ditch 2.5 m wide and 1 m deep, which discharges into the Marimba River and is referred
in this section as DR1. The Central Business District Channel (CBD) is referred as DR2.
It is a concrete pipeline discharging into Mukuvisi River through a culvert box with a
combined cross section, the bottom part is a rectangular channel of 2m width and 0.35 m
depth, the top part is a trapezoidal section with side slopes 1:1 and a depth of 3.2 m. DR1
collects runoff from an area of about 11 km
2
of a mixed land use pattern, including a
medium density residential areas, commercial and institutional buildings (schools, hotels,
offices), and an industrial area. The former comprises of light industrial enterprises,
including meat processing, dairy production, washing materials, automotive enterprises
and a variety of small to medium businesses, offices and storage facilities. About 20 % of
the whole drained area, in the downstream part of the channel, consists of open
undeveloped areas and informal agriculture plots.
DR2 collects the runoff from the town center (CBD-approximate area of 3.3 km
2
),
where the land use is predominantly commercial, including high story buildings and
medium-density residential areas. It is characterized by a high percentage of impermeable
surfaces and intensive traffic. About 20% of the drained area consists of developed open
spaces as parks and a golf course. The location of the drainage areas of the two channels
is shown in Fig.3.4. The urban drainage system of Harare has numerous discharge points,
but only these two channels have been included in the monitoring program of the city
because of the relatively high value of the drained area, compared to other discharge
points, and the specific type of urban land use practice.
The parameters, describing nutrients variations, included in the city's monitoring
program are phosphates, ammonia and nitrate. In addition, variations of pH have been
included. The planned frequency of sampling was once per month, but at the end of the
study period it has been reduced drastically. During 1999 and 2000 only few
measurements for the whole year have been executed and for this reason, the available
data has been averaged for both years. Analytical methods used are based on the Standard
methods of examination of water and wastewater (1989). Phosphates were directly
