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Environmental Sensitivity Atlas for Coastal Area of Kenya, March 2006

 
Contents and overview, Environmental Sensitivity Atlas for Coastal Area of Kenya, March 2006
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12
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2.1 CLIMATE
The Kenyan coast runs in a south-westerly direction from the Somalian
border in the north, at 1° 41'S to 4° 40'S at the border with Tanzania. It
lies in the hot tropical region where the weather is influenced by the great
monsoon winds of the Indian Ocean. Climate and weather systems on the
Kenyan coast are domi-nated by the large scale pressure systems of the
western Indian Ocean and the two distinct monsoon periods.
From November/December to early March, the Kenyan weather, particular-
ly at the Coast, is dominated by the Northeast Monsoon (Kazkazi) which is
comparatively dry. During March and April the monsoon winds blows in
an east to south-easterly direction (Kuzi) with strong incursions of mariti-
me air from the Indian Ocean bringing heavy rains. During the months of
May, June, July and August, the South-easterly Monsoon influence gradual-
ly sets in and the weather becomes more stable with dull and comparatively
cooler temperatures. Between September and November, the Northeast
Monsoon gradually re-establishes itself and by December the northern
influence is dominant once again.
2.1.1 Rainfall
A relatively wet belt extends along the entire Indian Ocean coast of Africa
and annual rainfall on the Kenyan coast follows the strong seasonal pattern
outlined above. The main rains come between late March and early June
with the rain-fall decreasing from August. Some rain occurs between
October and November but from December, rainfall decreases rapidly once
again to a minimum during January and February. Mean annual total rain-
fall ranges from 508 mm in the drier, northern hinterland to over 1,016
mm in the wetter areas south of Malindi. Relative humidity is comparative-
ly high all the year round, reaching its peak during the wet months of April
to July.
2.1.2 Wind
The windiest time of the year at the Kenya Coast is during the Southeast
Mon-soon from May to September, while the calmest months are March
and No-vember when the winds are also more variable in direction. Wind
records from Lamu, Malindi and Mombasa show a consistent daily pattern
whereby wind strength (in knots) drops during the night and is always less
at 0600 hrs than at 1200 hrs. This pattern is less pronounced in Lamu
which also tends to be the windiest place on the Coast at 0600 hrs. Overall,
it would seem that Mombasa is the windiest, but the strongest winds are
likely to be experienced in August in Malindi.
The monsoons
2
The Coastal Environment
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2.2 HYDROLOGY
2.2.1 Rivers and Catchments
The hydrology of the coastal region of Kenya can best be viewed by exami-
ning the drainage patterns of both perennial and seasonal rivers draining
into the western Indian Ocean basin. There are two main perennial rivers
namely the Tana River and the Sabaki River which originates from the
Highlands around Mt. Kenya and Nairobi. Discharge from both rivers is
highly seasonal, characteristic of dry land rivers, which can deliver over 80
% of their annual sediment loads within a period of a few days at the onset
of heavy rains (Dunne, 1979; Obura, 2001)
The Tana River is the longest in Kenya being approximately 850 km in
length and it has a catchment area of 95,000 km
2
. An average of 4,000 mil-
lion m
3
of freshwater and some 3 million tonnes of sediment are discharged
annually. It enters the ocean about halfway between Malindi and Lamu,
near Kipini, into Ungwana Bay. However, before it does, and about 30 km
upstream, it gives off a branch which leads to the complex of tidal creeks,
flood plains, coastal lakes and mangrove swamps known as the Tana Delta.
The Delta covers some 1,300 km
2
behind a 50 m high sand dune system
which protects it from the open ocean in Ungwana Bay. (Figure 2-5)
The Sabaki River is the second longest with a length of 650 km and a
catchment area of 70,000 km
2
extending into the south-eastern slopes of
the Nyandarua Range in central Kenya. The Sabaki River discharges 2,000
million m
3
of freshwater and 2 million tonnes of sediment annually into
the sea through the Sabaki estuary north of Malindi. (Figure 2-9)
There is also a number of semi-perennial and seasonal rivers such as the
Mwache, Kombeni, Tsalu, Nzovuni, Umba, Ramisi, Mwachema and Voi,
all of which drain into the coastal region from arid and semi-arid catch-
ments. The Ramisi River, which arises in the Shimba Hills forested area,
discharges 6.3 million m
3
of freshwater and 1,500 tonnes of sediments
annually intOFunzi -Shirazi Bay in the southern part of the Kenya coast.
The Umba discharges 16 million m
3
of freshwater intOFunzi - Shirazi Bay
while the Mwachema and Mwache rivers discharge 9.6 million m
3
and 215
million m
3
of freshwater annually, respectively.
2.2.2 Coastal lakes
There is a number of lakes in the Kenya coastal region with the majority
found in the Tana Delta. Most of these lakes are quite small and shallow
and are typical oxbow lakes, remnants of the various meanders of the Tana
River. Some of the lakes, especially the smaller ones, show swamp characte-
ristics. These lakes are either recharged through ground water seepage or by
the periodic flooding of the Tana River.
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Environmental Sensitivity Atlas for Coastal Area of Kenya
Tana River
Sabaki River
Other rivers
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2.3 GEOLOGY AND GEOMORPHOLOGY
The Kenyan coastal environments are set in a passive continental margin,
the evolution of which was initiated by the break-up of the mega continent
Gondwanaland in the Lower Mesozoic. The initial opening of the Indian
Ocean was preceded by doming, extensive faulting and down warping simi-
lar to that observed in the modern Great Rift Valley of East Africa. These
tectonic movements formed a North-South trending depositional basin.
During the Mesozoic, this basin was exposed to numerous marine incur-
sions and by the Jurassic, purely marine conditions are thought to have exi-
sted. The coastal range running parallel to the coastal zone appears to have
been uplifted through faulting during this time.
Throughout the Tertiary, the coastal areas experienced further faulting and
extensive continental erosion. The older Cretaceous deposits were totally re-
moved in many areas. The present coastal configuration, however, evolved
during the Pleistocene to Recent times, a period marked by numerous fluc-
tuations in sea level.
Three physiographic zones are observed on the Kenya coastal zone. The
Nyika lies at 600 m above the present sea level and represents the higher
ground covered by the Duruma sandstone series and older rocks to the
west. The Foot Plateau occurs at an elevation between 140 m and 600 m
above the present sea level. This coincides well with the relatively younger
Jurassic rocks. The Coastal Plain, the lowest step, rises from sea level to 140
m. On average, this belt increases from a few kilometres wide in the sout-
hern sector, to over 40 km in the north. The geomorphology of the Coastal
Plain is dominated by a series of raised old sea level terraces. Most of the
coastal environment and the modern shore configuration follow the 0-5 m
and the 5-15 m sea level terrace complexes.
Due to its evolutionary history, the principal rocks observed along the
Kenyan coastal margin, are of sedimentary origin and range in age from
Triassic to Recent. The Duruma Sandstone series, the oldest formation, is
represented by the Mariakani and the Mazeras sandstones which were depo-
sited under subaqueous, deltaic, lacustrine or possibly neritic conditions
that prevailed before the opening of the Indian Ocean. The Upper Meso-
zoic is represented by marine limestones and shales with occasional horizons
of sandstones and early limestones. Cenozoic to Recent rocks comprises
mostly of marls and limestone's, and is represented by the sandstones, clays,
conglomerates and gravels such as the Marafa beds. Quaternary representa-
tives include windblown Magarini Sands, limestones, cemented sands and
coral sands. Recent unconsolidated windblown sands, beach sands and clays
overlie the older units.
Kenya has a coastline of over 600 km, but the exact figure depends on the
extent to which small islands are included in the measurements. The Ken-
yan coastal region is generally lowlying and characterised by the extensive
fossil reef which lies a few metres above present sea level. The coastal plain
is backed in the interior by a line of hills that rarely exceed 300 m except in
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Sedimentary rocks
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southern parts where the Shimba Hills reach an altitude of around 1,000 m
above sea level. Further inland the Taita Hills rise to an elevation of 1,500
m above sea level.
Soils of the coastal region show considerable variety. The porous parent
rocks of sedimentary origin, generally give rise to soils of low fertility.
However, patches of highly productive soils have been observed in areas of
alluvial deposits. The principal soil types in the region include a narrow
strip of coastal sands towards the north where it is permeated by narrow-
bands of grumosolis brown clay soils. The soil south of Lamu is composed
of bialternate bands of loams beyond which the grumosolis are permeated
by thick bands of ash and pumice soils.
The shoreline in most of the region apart from the Malindi area is receding
as a result of coastal erosion.
Living coral reefs occur along most of the Kenyan coast. A fringing reef co-
lonizes the shallow parts of the continental shelf along most of the Kenyan
coastline to a depth of around 45 m and at a distance of between 500 m
and 2.0 km offshore, except where river systems create conditions of low
salinity and high turbidity which limit coral growth. Estuaries and deltas are
instead characterized by extensive mangrove forests.
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Environmental Sensitivity Atlas for Coastal Area of Kenya
Erosion
Figure 2-1 Photo taken south of Chale island at Funzi looking north against Diana and Mombasa.Typical
coastline showing the fringing reefs typical for the southern part of the coastline. The small white beach
and the northern part of Chale island is a famous tourist resort.
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The width of the continental shelf off the Eastern African coast varies mar-
kedly throughout the region, but it is generally quite narrow. Kenya, with a
coastline of about 600 km, has an estimated continental shelf area of about
19,120km
2
. Of this, some 10,994 km
2
is considered trawlable. South of
Malindi the continental shelf extends only 5 km offshore, whereas north of
Malindi, in the North Kenya Banks, the edge of the shelf is about 60 km
offshore (UNEP 1998).
2.3.1 Classification of the Kenya Coastline
Many attempts have been made worldwide to classify coastlines regarding
their vulnerability to oil pollution damage. They are usually based on the
geomorphology, the degree of exposure to wind and waves, and may also
take other conditions into account. Most of these systems are inspired by
Gundlach and Hayes (1978) and the IMO publication on "Sensitivity
Mapping for Oil Spill Resonse" (IMO/IPIECA 1994)
The classification adopted for this Atlas also builds on these systems. In
addition to the coastal geomorphology and degree of exposure the ecologi-
cal value and biodiversity of the particular stretch of coastline is taken into
account. This classification comprises seven categories or types, each of
which is described below in terms of its geomorphology, ecological value
and vulnerability.
1.
Mangrove swamps
Moderate tOFully sheltered from wind and wave action and mainly
intertidal; a productive environment very sensitive to oil pollution.
2.
Sheltered rias
Steep to moderate sloping sedimentary structure inside the creeks;
reduced wind and wave actions; a basis of fossil reefs often observed.
Moderate biological productivity
3.
Tidal flats
Mainly mud flats exposed during low tide; subject to reduced wave
energy due to protection from a reef or inside creeks; high biological
productivity
4.
Sheltered sand beach
Mainly beaches of coral sand sheltered behind a reef; subject more to
wind than wave actions; low biological productivity.
5.
Exposed sand beach
Long stretches of beach often with windblown dunes; the beach
is not protected by a reef and open for wind and wave actions; low
biological productivity.
6.
Steep cliff
Usually steep dipping to near vertical walls of fossil reef; exposed to
wind and wave actions and sometimes protected by a shallow reef;
low to medium biological productivity
7.
Coral reefs and reef flats
Intertidal to sub tidal and subject to significant wave action; highly
productive and very susceptible to oil pollution
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The shelf
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The classification of the respective stretches of the coastline may include
two or three of the types above. However, in determining the classification
of a particular stretch of coast, it must be remembered that there is no defi-
nitive and precise measure of qualities such as value and importance.
Neither is there an accurate measure of vulnerability. The classification is
therefore a comparative one and one which is derived from the collective
opinion, experience and technical judgement of experts from the region.
2.4 OCEANOGRAPHY
2.4.1 Coastal currents
There are four oceanic currents affecting the Kenyan coast. These are the
South Equatorial Current, the East African Coastal Current, the Equatorial
Counter Current and the Somali Current. The westward moving South
Equatorial Current divides into two branches once it reaches the African
coast at Cape Delgado. It gives off the Mozambique Current which flows
southwards, and the East African Coastal Current which flows north-east-
wards, parallel to the coast. The East African Coastal Current flows north-
wards all the year round at least as far as Malindi. During the Southeast
Monsoon it continues beyond Malindi northwards, joins with the Somali
Current and continues right to the Horn of Africa ( Figure 2-3 ). During the
Northeast Monsoon (November to March), however, the northward extent
of the East African Coastal Current is more restricted. At this time it meets
and joins the southward flowing Somali Current (which changes direction
under the influence of the monsoon) with this convergence taking place
anywhere between Malindi and north of Lamu, depending on the strength
of the monsoon in any particular year. The two streams then turn eastward
and flow offshore as the Equatorial Counter Current. ( Figure 2-2)
The Somali Current is the only one that reverses its direction of flow under
the influence of the monsoon. It flows in a south-westerly direction at
about 1.5-2.0 knots with the Northeast Monsoon (November to March).
While during the Southeast Monsoon (April to October), the Somali
Current reverses its flow and increases its velocity to around 2.0-2.5 knots.
It now appears as the north-wards extension of the East African Coastal
Current which still arises from the onshore South Equatorial Current. At
this time of the year, the Equatorial Counter Current is not so distinctive
from the general Southwest Monsoon Drift at the lower northern latitudes
of the Indian Ocean. The net onshore currents result in the sinking of sur-
face waters along most of the Kenyan coast. The exception is near Kiunga
where some mild upwelling is thought to occur during the Northeast
Monsoon (UNEP, 1998).
2.4.2 Tides
Kenyan coastal waters are characterized by semi-diurnal tides - approximate-
ly two tidal cycles for every 24 hour period. Except for limited periods in
the year, however, the levels of high and low water of each successive tide
differ appreciably from the corresponding tide before and the tide follo-
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Environmental Sensitivity Atlas for Coastal Area of Kenya
Figure 2-2 Currents in January during the
Northeast Monsoon.
Figure 2-3 Currents in July during the
Southeast Monsoon.
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wing. The tides can therefore be designated as mixed semi-diurnal tides.
Spring tidal variations (Brakel, 1982) in Eastern Africa can be up to 4.0 m,
with average variations within the 2.5-3 m interval.
The reference port for tidal observations in Kenya is Kilindini (Port of
Mombasa) where the maximum tidal range does not usually exceed 3.8 m.
Tidal range for Malindi is 2.0 m for neap tide and 2.9 m for spring tide.
There is a lag in the tidal state which increases with distance north along
the Kenyan coast. Malindi is normally 5 minutes after Kilindini while
Lamu is about 40 minutes behind.
Deviations from the predictions in tide tables are influenced by barometric
pressure, onshore winds and oceanic swell. However, the lowest tides occur
persistently during the Northeast Monsoon since they combine with the
prevailing winds to drive water offshore.
2.4.3 Sea temperature and salinity
Sea surface temperature and salinity also vary with the monsoon season.
The highest temperatures of 28-29°C occur following the Northeast Mon-
soon in the months of March and April. The lowest sea surface temperature
occurs in August and September with a minimum of 24°C.
During the Southeast Monsoon the shifting of ocean currents brings Pacific
Ocean water of high salinity into the South Equatorial Current while du-
ring the Northeast Monsoon the South Equatorial Current draws water of
low salinity from the Malay Archipelago. These changes in turn result in
changing salinities of the East African Coastal Current waters. A further
influence on salinity is the incidence of rainfall, especially the heavy rains of
March to May when the discharges from all major river systems as well as
all the more minor seasonal ones are at the maximum. As can be expected,
offshore waters are influenced mainly by the oceanic currents and surface
water salinities in Kenyan coastal waters vary from a minimum of 34.5
to a maximum of 35.4 . The influence of the river outflow is contained
mostly in inshore areas by the prevailing wind conditions and much wider
variations in salinity do occur at the local level.
2.5 COASTAL ECOSYSTEMS
2.5.1 Introduction
Terrestrial, inter-tidal and sub-tidal ecosystems usually forms an interdepen-
dent continuum, but can often be divided into easy recognizable zones or
habitats dominated and even physically structured by a few keystone species
(mangrove forests, seagrass beds) or classes (corals).
2.5.2 Rocky coast & man made structures
Much of the Kenyan cost is formed by low, about 4 - 6 m high limestone
coral cliffs. These are fossilized coral reefs build during Pleistocene more
than 100,000 years ago. They are exposed by the current sea level and are
now eroded by the force of the waves resulting in an irregular and rugged
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Geomorphology
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appearance. The wide reef flats usually found in front of the cliffs can be
regarded as these fossil reefs "levelled off" by the wave actions in combina-
tion with the current reef building activity (Figure 2-4) . Usually the cliffs
make up the middle and upper part of the intertidal zone, and with a reef
flat or sandy beach below.
Rocks in the upper part of intertidal zone have a sparse biological activity
with mostly unicellular algae and a fauna of chitons and limpets and some
amphibian crustaceans. Sub tidal rocks and manmade hardsurface structures
as piers and wharfs may develop a richer flora and fauna resembling the
conditions found on reefs.
Exposed cliffs are regarded as less sensitive to oil pollution than most other
habitats because of the sparse biological activity and because the wave expo-
sure make the surface self cleaning to some extent. The rugged surface of
the eroded ancient reef may reduce the self cleaning ability and leaves oil in
cracks and cavities, but manual clean up with low or high pressure flushing
are possible on these surfaces.
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Environmental Sensitivity Atlas for Coastal Area of Kenya
Biology
Sensitivity to oil spill
Figure 2-4 The ancient reefs are soft and undercut by wave actions.
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2.5.3 Sandy Beaches and Dunes
Two types of sandy shores are present along the Kenyan coast.
Gentle to steep sandy beaches without protection from a reef. The beach is
often backed by one or a series of wind-blown sand dunes. The sand may
be of terrestrial origin and supplied by the larger rivers (Tana, Sabaki). This
type of beach is found around Sabaki River mouth, and from Ngomeni
through Ungwana Bay to Lamu Island.
Gently sloping beaches sheltered behind a fringing reef are common along
the coast south of Ungwana Bay. The sand is often white calcareous sand of
marine origin (coral sand). Diani Beach and Watamu Beach are typical
examples.
Species diversity on sandy beaches is usually low. On the higher parts of the
beach, above the high water line, only a few burrowing crabs and amphi-
pods are usually found. The density and diversity of crabs, bivalves, poly-
chaetes and other marine invertebrates increases in the intertidal, but
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Geomorphology
Biology
Figure 2-5 Ungwana Bay. A wind blown sand dune separates the short steep beach from the huge Tana
River delta.The beach is exposed to the sea without the protection from a reef.
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remain low compared to most other habitats. Stranded debris in this zone
may attract a variety of foraging waders and other birds.
In protected beaches the lower intertidal zone will usually be covered with
seagrass which also covers most back reef lagoons.
In exposed beaches the waves prevent seagrass growth in the upper subtidal
zone and the sand remains low in biological value.
Sandy beaches may serve as nesting sites for marine turtles. Green turtles,
hawksbill and the rare olive ridley all nests on Kenyan beaches. While green
turtles are found nesting on suitable beaches all along the coast, hawksbill is
found in Kiunga, Malindi, Watamu, and Funzi, and olive ridley only along
beaches near Malindi, Watamu and Mombasa.
Fine grained sandy beaches are less sensitive to oil pollution because of the
relative sparse biological activity, and because they are relatively easy to
clean. This holds especially for exposed hardpacked sand beaches, (Figure 2-
5)
, but also in more delicate surrounding (Figure 2-6) is the beach the least
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Environmental Sensitivity Atlas for Coastal Area of Kenya
Sensitivity to oil spill
Figure 2-6 Sheltered sandy beach behind a protective reef.The sand is white coral sand, and intertidal
seagrass beds are seen in front of the beach and on the reef flat.
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sensitive habitat. Oil does not penetrate deep intOFine sand and can be
removed either manually or by use of front loaders or other heavy machine-
ry. If the oil is not removed it may form a dense pavement on the beach.
Coarse sand or gravel makes a difference. The oil can sink deep into the
sand and can be impossible to remove or clean. The oil weathers slowly
deep in the sand and the beach will be seeping oil for an extended period.
Coarse sand or gravel not common along the coast of Kenya.
Turtles nests high on the beach, usually out of reach of high water and the
stranded oil, but the cleanup operation and use of heavy machinery may
ruin the nests. And turtles on their way to the nest and broods on their way
to the sea may be caught in the oil. Turtles nest during a period of several
months, so only a smaller part of a population may be at risk at any given
time.
2.5.4 Corals reefs and reef flats
The wide reef flats seen in front of the cliffs can be regarded as the fossil
reefs "levelled off" by the wave actions in combination with the current
active reef growth at the reef crest and at the outer slope facing the ocean.
This structure is found more or less continuously from Malindi southwards
to Chale island. Sometimes the cliff is replaced by sandy beaches, and some-
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Turtles
Geomorphology
Figure 2-7 Part of the reef flat north of Mombasa.The waves are breaking at the reef crest and along the
coast are seen contrasting sandy beaches and rocky cliffs.
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times the continuity is interrupted and the reef flat is degraded - usually at
the outlet from creeks or small rivers. The width of the reef flat can be more
than 2 km. South of Chale and to the Tanzanian border, the coastline is
more degraded and the reef is discontinuous and broken into islands and
patch reefs. The distance to the coast (and hence to terrestrial sources of silt
and pollution) makes some of these reef - Kisite Island and others - the best
and most diverse in Kenya.
On the other hand, the Sabaki and especially the Tana River plume reduce
or prevent coral growth within a greater area.
Coral reef communities in Kenya are found from about mean sea level to a
depth of 20-25 m. Coral reefs are among the most diverse ecosystems in the
sea. With corals as the keystone species, the rich diversity includes almost all
other groups of marine flora and fauna as macroalgae, fish, molluscs, crusta-
ceans etc. The extent, size, and diversity of the coral reef communities
decrease northwards along the Kenyan coast due to discharge of sediments
from the large rivers and to influence from the Somali current. Corals are
slow growing organisms and very slow colonizers. The Kenyan reefs have
not yet recovered from the damage they suffered from the increased tempe-
rature during the El Niño event in 1998.
The reef flats are usually a mixture of hard and soft substrate and typically
with a distinct back reef lagoon close to shore. Most of the reef flat is inter-
tidal and with only few corals. Instead, there is a dense mixed vegetation of
seagrass on the soft sediment and seaweed on the hard substrate. The high
abundance of fish and invertebrates is an important source of food and
income for the local communities.
Most of the reefs are subtidal and therefore sheltered from direct contact
with the oil slick. But the reef crest is usually exposed at low tide, and (the
few) intertidal corals are killed immediately by contact with oil. But the
deeper parts of the reef may be endangered as the waves break on the crest
and fine oil droplets are dispersed in the water column. Corals are filtering
organisms and the oil droplets are toxic. Recovery of a damaged reef may be
a matter of decades, and restoration techniques are usually not very suc-
cessful.
The reef flat and its organism is vulnerable and sensitive to oil pollution,
and is difficult to clean. Waves and tides helps cleaning but not without
great damages because the exposed area can be very large.
2.5.5 Rias
Rias are drowned river valleys. They are usually estuaries cut by the river in
sedimentary materials and filled during sea level rise. They are characterised
by rather steep slopes of sedimentary material along part of the coastline.
They includes the well known habitats of sand, mudflats and mangroves,
but are usually found as narrow bands along the coast and sheltered from
wave. Kilifi Creek is the most typical ria along the Kenyan Coast. (Figure 2-
8)
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Environmental Sensitivity Atlas for Coastal Area of Kenya
Biology
Sensitivity to oil spill
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The typical ria has a steep slope or a sheltered cliff often with a narrow sub
tidal muddy beach with a few mangrove trees. The sheltered environment
has only little self cleaning capacity, but it is possible to clean from the sea
side because of the narrow extent.
2.5.6 River mouths and estuaries
Rivers transport sediment from inland and have formed large gentle sloping
floodplains as seen around Sabaki River, in the Tana River delta and in the
creeks of the Lamu archipelago. Because of the gentle slope, the marine
tidal impact and hence the zone with fluctuating salinity can reach far
upstream from the river mouth. This is the estuarine zone. The mouths of
the smaller rivers are often hidden behind mangrove creeks whereas Sabaki
and Tana rivers have pronounced mouths and a clear estuarine zone.
Few species except for the mangroves are adapted to low or fluctuating sali-
nities. The biodiversity is therefore low within the estuary. The density, on
the other hand, is usually very high because of the continuous supply of
food and nutrient from the river. The high density of bivalves, snails and
other benthic invertebrates usually attracts a wealth of birds.
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Sensitivity to oil spill
Geomorphology
Biology
Figure 2-8 Rias are creeks or estuaries developed from drowned river vallies.They are sheltered from
waves and wind, and include often narrow bands of mangrove, sand and mudflat and sheltered rocky
coasts.
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The tide can bring an oil pollution far into an estuary and up the river and
in combination with a high density of food items, the sensitivity "per
metre coastline" is very high. Only in the seasonal high flow situations are
there a fair flushing and selfcleaning capacity.
2.5.7 Mangroves
Kenya has about 500 km
2
of mangrove forest. The largest areas are in Lamu
district where protective islands, a gentle relief, and slightly estuarine condi-
tions have favoured a lush forest cover of more than 300 km
2
. Other impor-
tant areas are in the Tana River delta and the area north of Ngomeni. The
oceanic coast between Ungwana Bay and Gazi is too steep and too exposed,
and only the creeks of Mida, Kilifi and Mombasa holds significant mangro-
ve stands. In the south, the bays of Gazi, Shimoni and Vanga also holds
large and important mangrove areas.
Mangroves have a high productivity as they profit from nutrients from both
land and sea, and mangrove detritus are often the main source of energy
fuelling the estuarine food webs. Mangroves are favoured by fine grained
nutrient rich sediment, and are therefore often associated with estuaries and
other freshwater outlets where fine grained organic materials settles along
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Environmental Sensitivity Atlas for Coastal Area of Kenya
Sensitivity to oil spill
General
Geomorphology
Figure 2-9 Sabaki River Estuary at high tide.The estuary is a bird area of international importance.
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with mineral particles. Mangroves are not only favoured by the existing geo-
morphological conditions - they enhance sediment accretion by trapping
sediment among their aerial roots. The intricate networks of knee-, stilt-, or
prop roots reduce current speed and enhance sedimentation of mineral and
organic particles.
Accordingly, clear cutting or otherwise killing of mangrove may cause coa-
stal erosion and sediment transport, with possible adverse effects on
neighbouring sensitive habitats as seagrass beds and coral reefs.
Kenya has 8 species of mangrove trees, or 9 if the Xylocarpus mollucensis is
included. Rhizophora mucronata is the most common and prominent species
along the Kenyan coast, easily recognisable from the arching stilt roots sup-
porting the trunk. Sonneratia alba and Avicennia marina are common speci-
es and among the first colonizers of new areas. Especially Avicennia is a
robust species found as a primary colonizer in the front row along the sea-
ward site and often also on the high shore on the landward side of the man-
grove where the soil salinity is too high for most other species except maybe
for Lumnitzera racemosa . Heritiera littoralis is very rare in Kenya and is only
found sporadically in the Tana River Estuary and near Gazi.
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Kensea
Biology
Figure 2-10 Dense mangrove forest north of Lamu.
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While the diversity of species of mangrove trees is limited, the physical
three dimensional complexities of the mangroves above and below the water
surfac creates a multitude of niches suitable for a vast diversity of other
organisms. In particular the aquatic communities are diverse, and include a
multitude of algae, seagrass, crabs, shrimps, bivalves, fish, etc.
Mangroves are adapted to growth in salt water by several important featu-
res. In the present context, the adaptation to growth in anoxic and acidic
sediment in the presence of sulphide is important. Aerial roots are promi-
nent features of mangroves. Stilt-, prop-, and knee roots as well as pneumat-
ophores, does not only to support the tree. The sediment under a mangrove
stand is usually strict anoxic and exhibits toxic concentrations of hydrogen
sulphide and other reduced components. The root itself needs oxygen for its
metabolism. The aerial roots which are exposed once in every tidal cycle,
take up oxygen from the atmosphere, and ventilate the subsurface part of
the root. If the oxygen uptake by the roots is blocked by e.g. prolonged
inundation or by a smear of oil, the roots and subsequently the tree may
die.
Adaption
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Environmental Sensitivity Atlas for Coastal Area of Kenya
Figure 2-11
Rhizophora mucronata is the most common and prominent species along the Kenyan cost,
easy recognisable from the arching stilt roots.The black band high on the stem and branches marks a
recent oil spill.
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Mangroves are of vital economic importance throughout the tropics, since
they provide spawning and nursery grounds for the fish and shrimps, they
protect the shores against erosion, and they provides firewood and building
material as well as a long list of other natural products.
Unsustainable forestry, i.e. logging without replanting is the major threat,
but also conversion to salt work evaporation dams and seepage from the
dams to the surrounding mangrove have caused loss of mangrove forest.
Shrimp farms are a future but realistic threat.
Oil can be acute toxic to mangrove. Especially refined products with high
concentrations of water soluble aromatic hydrocarbons are highly toxic.
Smothering of the aerial roots by oil by heavier oil types may hinder proper
ventilation of subsurface parts and lead to suffocation and stress end eventu-
ally death.
Hydrocarbons in the sediment may stay toxic to trees and propagules for
years. Oiled debris and sediments may leach oil for months.
Clean up operations may prove to be extremely difficult due to the low
energy environment of mangrove forests. Manual cleaning is hampered by
the depth and impenetrable nature of the mangrove forest. Flushing and
absorbing booms may be the only means.
28
Kensea
Threats
Sensitivity to oil spill
Clean up
Figure 2-12 Seven years after a spill of 5.000 tonnes of oil, the mangrove in Makupa creek has only star-
ted to recover.
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Natural regeneration can be a long process, and depends on a proper supply
of propagules. Replanting is a relatively simple and usually successful tech-
nique.
2.5.8 Intertidal mud flats
Sheltered mangroves in creeks or bays are often fringed by a broad intertidal
mudflat. The open mudflat can either be in balance or the result of man-
grove logging or it can be an accreting mudflat where the mangrove forest
progression is lagging behind.
Intertidal mudflats are accreting areas with high productivity and a high
density of marine invertebrates as mussels, snails and crustaceans. The
diversity is usually low, but they are very important feeding grounds for
aquatic birds.
The mudflats are sensitive to oil spill themselves They are difficult to clean
mechanically without downmixing the oil into the sediment. The often
close connection with mangrove forest makes the clean up even more diffic-
ult - for both habitats. The invertebrates have a high regeneration rate, but
the sediment may be unsuitable due to oil remains.
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Environmental Sensitivity Atlas for Coastal Area of Kenya
Regeneration
Geomorphology
Biology
Sensitivity to oil spill
Figure 2-13 Mudflats are sheltered accreting areas, often in front of mangrove forests. Mida Creek.
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2.5.9 Seagrass beds and Seaweeds
Seagrass grows mostly on sandy to sandy-muddy sediments from the interti-
dal zone down to a depth of 20 m or more. In the oil spill context the
seagrass beds on the reef flats, in the back reef lagoons, and in shallow man-
grove creeks are the most interesting, but seagrass on the outer slope of the
reefs and most probably also in suitable areas in Ungwana Bay are impor-
tant too. Suitable areas mean a proper substrate and enough light. Enough
light means approximately down to the depth of visibility (Secchidepth), i.e.
from more than 20 m on the reefs far from the coast to a few metres close
to the mouth of the large rivers.
Seagrasses have an important function as stabilizers of soft substrates. The
network of rhizomes and shelter from the leaves protect the sediment surfa-
ce from erosion
At least 12 species of seagrass are found along the coast of Kenya, but with
the Thallasodendron ciliatum as the dominant species on the reef flats. This
species grows even on coral rubbles. The fauna associated with seagrass is
very rich, mostly because of the complex physical structure of the seagrass
bed creates. It has an important role as spawning and nursery grounds for
many fish species. Few species feed directly on seagrass, but may utilize
debris of degrading leaves and waders forage in the beach cast of dead sea
grass leaves
30
Kensea
Geomorphology
Biology
Figure 2-14 Sea grass may grow from the intertidal zone to more than 20 meters depth.
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Two endangered species feed on seagrass, the green turtle and the dugong.
Seagrass in the intertidal zone especially on reef flats are at risk in case of oil
spill. Seagrasses are killed by the smothering of oil but may also be uprooted
during cleanup actions. Removal of seagrass beds may start coastal erosion.
Seagrasses are surprisingly slow colonizers, and restoration of destroyed beds
may take from years to decades.
Seagrass distribution along the Kenyan coast has not been mapped, and is
therefore not included in the sensitivity index, except as a part of the reef
flats.
2.5.10 Endangered species
Marine Turtles
All of the five species of marine turtles found along the East African Coast
are alsOFound in Kenya. Three of them nest on Kenyan beaches: Green
turtles, hawksbill and more rarely the olive ridley
Hunting of turtles is illegal in Kenya, and the biggest threat is poaching and
unintended bycatch by trawlers. Oil spill will not pose a serious threat to
marine turtles in Kenya. They are sensitive if smothered, but will usually
have the ability to escape. Stranded oil and the cleanup activities may local-
ly threat turtle nests. Most turtle nests are known by position by local
NGO's and can in most cases be relocated in case of an oil spill.
Dugongs
The dugong is one of the most endangered species along the African coast
and is almost extinct in Kenya. It is on the IUCN Red List of Vulnerable
species. It is occasionally sighted or caught in fishing net. Most observations
are from the southern coast close to the Tanzanian border and from the
Lamu district, but there is no pattern in the observation indicating a stable
and viable population.
Dugongs feeds solely on some of the smaller seagrass species which are usu-
ally found close to the mangrove forest or on the outer reef slope. The
dugongs are not limited by available food but by hunting and unintended
bycatch. Dugongs are to scarce to be taken into account in oil spill contin-
gency planning, and although they can be harmed by the oil, they will nor-
mally be able to escape.
2.5.11 Migratory and Other Coastal Birds
Coastal birds concentrate usually on intertidal areas. Especially mudflats,
but also estuaries, reef flats, and beach cast of seagrass debris may attract
waders and gulls. Terns are often attracted to shallow or turbulent waters.
The Kenyan coast does not host any globallythreatened species but e.g. the
Mida Creek area, and the Kiunga reserve hosts important numbers of crab-
plovers, and roseate tern nests on several of the smaller islands. The coast
host a number of Palearctic immigrant waders during the Palearctic winter
between September and March.
Although waders and terns are less sensitive to oil pollution than swimming
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Environmental Sensitivity Atlas for Coastal Area of Kenya
Sensitivity to oil spill
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32
Kensea
sea birds as e.g. auks and ducks, an oil spill near important bird areas, may
affect the birds directly (Oil contact) and indirectly by disturbing the fee-
ding ground.
Figure 2-15 Gazi fish landing site.
2 - KenSea, Environ. Sensitivity Atlas for Coastal Area of Kenya