Wetland & Fish Ecology - Enviro Dynamics Namibia

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Note: *Would be permanent and serious if substrate is not replaced or new sediment
is not allowed to collect in the lined canal, meaning that plants and benthic fauna
will not be able to recolonise the newly deepened and lined channel. It would be
even better if the Okatana River channel were not lined but allowed to continue to
function naturally as an oshana.
Consider designing the new Okatana River channel in Oshakati in a way that keeps
it as natural as possible, with a very gradually sloping margin on either side of the
main channel, using a hollow brick structure allowing sediment to collect and plants
to regrow along these margins and that will make provision for the smaller seasonal
flows to flood temporarily into these margins alongside the main oshana channel. Be
sure to prevent any development, building activity within this flood-plain area. The
present design can be adapted by having a very gradual slope extending well
away from the main channel to simulate natural marginal flood conditions.
4.3 IMPACTS OF POLLUTION, LITTER AND SUBSTRATE DISTURBANCE
DOWNSTREAM
Although not picked up directly during the scoping exercise, the consultants have
added the impacts of substrate disturbance e.g. from excavation work at the site of
the dike construction or for the channel alongside where a route needs to be cut
through ridges and from the deepening of the Okatana River in Oshakati.
Two rather different types of water pollution may arise:
Firstly from contaminated runoff that collects in the storm-water and collects in the
Okatana River which is expected to finally collect all the storm-water in the city once
this system has been upgraded. In the older parts of the original town near the
airfield and hospital there are still wide, shallow ditches alongside the sidewalks that
were designed to collect and divert rain water through the town. Unfortunately with
the rapid expansion of the town since Independence, large parts of the town
particularly the rapidly growing informal settlement areas have no such provision.
Rain water will simply collect whatever else has collected within the runoff area and
so is very likely to become polluted with biological waste as well as chemicals and oil
spill that have collected on the roads. This will enter the Okatana River and flow
downstream.
Little organic enrichment does little harm and may be good, adding nutrients
(fertilizer) to the floodplains; this only becomes a problem at concentrations of
nitrogen and phosphates high enough to promote nuisance algal growth. Such
algal blooms and subsequent die off and decay of the algae can cause local
anaerobic conditions that could cause fish kills. Of course any broken sewage pipes
or flooding of sewage treatment works will also spill into the storm-water as will runoff
from ill-sited dump sites; these can cause serious eutrophication and result in algal
Draft Environmental Impact Assessment: Oshakati Flood Mitigation Project
Specialist Study: Wetland Ecology and Fish Ecology Imputs
July 2012