23
Ruvu and Pangani basins showed an overall decrease in runoff, while Rufiji had an increase in runoff. The summary for runoff change due to doubling of carbon dioxide is given in Table 4. The reasons for an increase of
river runoff in the Rufiji basin can be attributed to an increase in rainfall of between 10 to 50 shown by the GCMs rainfall scenarios. This occurs during the period of high flows November-March. In addition to this,
Rufiji basin has relatively lower mean temperatures hence contributing to lower evapo-transpiration rates. The effect on runoff due to changed land use and urbanisation was not taken to account. These factors will influence
the hydrology of the basins.
Table 1.3. Summary of runoff impacts Mwandosya et al., 1998
Catchment Runoff change annual change in
1. Ruvu river at Morogoro Road bridge 2. Kikuletwa river at Weruweru
3. Pangani river ar Korogwe 4. Ruaha river at Mtera
5. Ruaha river at Kidatu -10
-9 -6
+5 +11
Coastal Processes
The coast of Tanzania experiences a semi-diurnal tide with two almost equal maxima and two minima during a lunar day 24.8 solar hours. There is a considerable rise and fall of water against the coast of Tanzania, which has
important geomorphologic and hydrodynamic repercussions when combined with the inter-tidal profile and the prevailing winds. While the ebb and flood tidal movements tend to be self-balancing on the open coast, individual
features will influence the extent of fluctuations. Tidal currents in estuaries and areas with islands and sand spits such as at Kunduchi are localised and important in their effect on coastal and offshore shelf topography.
Lwiza 1987 measured tide heights using a self-recording tide gauge, which he installed in the harbour of Dar es Salaam. He found the annual mean water level as 1367 mm, the maximum as 1487 mm and the lowest as 1264
mm above datum. The zero of the tidal staff at the tide gauge was assumed to be 1 metre below datum. The measured tidal currents for 30 days using drogues cast at about 800 metres from the shore. He found that during
the Northeast winds the current speed, averaged over one tidal cycle, was 0.25 ms and the maximum was 0.5 ms. There was, however, no big change in current speeds and direction during the Southeast winds. Ebbing speeds in
the Kunduchi-Manyema creek reached as much as 3.5 ms. The measured waves at Kunduchi Beach at about 1 metre below datum using an 8-metre graduated wooden staff for only ten days after which the staff was stolen.
Waves in other locations collected from visual observation. During the Northeast winds, he found that waves were low with an average height of 0.2 metres south of Mbezi River. The height increased northward such that at
Africana Hotel, it was about 0.4 metres. Between Bahari Beach and Ras Kiromoni, waves were observed to have heights of about 1.2 metres. Near Kunduchi, at the sand spit, waves were choppy and confused.
1.7 Resource utilisation 1.7.1
Over-exploitation of natural resources
Human activities, such as the exploitation of groundwater and hydrocarbons oil, gas and coal may cause subsidence of sedimentary basins. This subsidence would have the same effect as sea level rise. However, no
published information is available in this area.
1.7.2 Sediment supply to the coastal zone
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Interruption of sediment supply to the coastal zone is experienced in many areas along the Tanzania coastline and seems to occur in various forms. The construction of protective features at the source of littoral material may
disrupt the sediment supply. Such features include the construction of dams which normally traps a large quantity of sediments. In Tanzania dams are commonly constructed for production of electricity, irrigation and fisheries.
These dams exist in most of the rivers entering the Indian Ocean as also mentioned in Linden and Lundin eds., 1996. Table 5 summarises the main dams and rives where they are constructed.
Table 1.4: Major dams in Tanzania and the rivers where they are constructed.
Hydroelectricity Dam
River Status
Nyumba ya Mungu Pangani Falls
Hale Pongwe
Kidatu Mtera
Lower Kihansi
Lower Ruvu Ruvuma
Pangani Pangani
Pangani Wami
Great Ruaha Rufiji Tributary
Great Ruaha Great
Ruaha Ruvu
Ruvuma Multipurpose, built 1969, surface area 142 km
2
Built 1938 Built 1964, Multipurpose, Surface are 10 km
2
Surface area 600 km
2
Operational In progress
Proposed Proposed
Proposed Proposed
The extraction of sand from feed rivers and streams deprives the beaches the much needed sand and silt required for maintaining equilibrium. Since the beginning of the 1980s, there has been a boom in construction all over
Tanzania, especially in Dar es Salaam. The boom in construction is obviously associated with a great demand for construction material such as sand, cement, coarse aggregate and stone. A field survey conducted by Griffiths
1987 shows that a minimum of 100,000 m
3
of sand was extracted annually from the four streams Tegeta, Mbezi, Mlalakuwa and Kijitonyama that drain the hinterland of Kunduchi Beach. Nyandwi 1996 investigated
human activities including removal of beach material, removal of protection against wave breaking, and obstruction of sediment supply, that contributed to beach erosion problems on Zanzibar Unguja and Pemba
beaches. He reported the following results. Beach structures that were considered to contribute to erosion in Zanzibar were the Mkoani jetty, Mtoni jetty, Jambiani seawall and Maruhubi seawall. At Mkoani, the erection of
the jetty had led to concentration of wave energy leading to accelerated erosion. The jetty-like structure at Mtoni impeded longshore sand supply leading to intensive erosion at Maruhubi. Although the Jambiani seawall had
been effective in protecting a section of a beach from erosion, the effect had shifted to the northern end of the wall where loss of land amounted to about 10m in width in 19 years since 1977. At Maruhubi, erosion at the ends of
the seawall was reported to continue unabated.
The effect of sand mining was also mentioned at Unguja Ukuu, where a 5m wide coastal strip had been lost due to shoreline erosion between 1997-98, trees were also uprooted in the process. A similar process was expected at
Chuini where sand extraction was also taking place. The cutting of mangroves and its effect on erosion was also discussed. At Mbweni beach, the area was experiencing serious erosion. Mangroves usually acted as buffer
25
against wave battering and their removal aggravated the problem. Lastly, erosion was also linked to poor planning. There was a trend of erecting beach hotels and residential houses directly on or very close to the
beaches. The hotels at Uroa, Chwaka and the bungalows at Uroa were cited as examples.
Construction of buildings on dunes and eroding cliffs also decrease sediment supply to the littoral zone. A survey conducted by Dubi and Nyandwi 1999 shows that some structures such as residential houses and hotels are built
directly on the beach and some of them on potentially unstable landforms, such as sand dunes.
Coastal protection methods are varied and take many forms. A method of protection can comprise one or more options that fall under three categories, viz. protect, retreat and accommodate. Table 6 shows some of the options
falling under the three categories.
However, in an attempt to control erosion in Tanzania, coastal structures such as groynes and walls, whose design leaves a lot to be desired, have been erected. Such structures change the sediment dynamics regime and realign the
shoreline. Some of the typical protection measures employed in Tanzania include stone and masonry seawalls in Tanga and Bagamoyo, groynes, concrete and masonry seawalls in Dar es Salaam, Zanzibar, Lindi and Mtwara.
1.7.3 Sea-level rise
