| dc.description.abstract | This is a study on soil erosion and its impact. on small
reservoirs located in the Nguu Tatu catchment situated in the
northern parts of Mombasa District, Kenya. The main objective of
this study is the determination of the types, patterns and rates
of# soil erosion within a sub-humid tropical Nguu Tatu Catchment
and especially the assessment of the impact of soil erosion on
reservoirs water quantity and quality. The methodology applied on
this study involved three major approaches namely catchment erosion
survey, measurement of water and suspended sediment discharge and
finally reservoir sedimentation survey. Catchment soil erosion
surveys involved the determination of rates, patterns and magnitude
of soil erosion within the catchment by use of hills] ope plot
experiments, slope traverses and benchmarks employing the use of
erosion nails. Measurement of stream flow and corresponding
suspended sediment concentration provided information on sediment
discharge. The reservoir sedimentation survey involved the
establishment of transects across the reservoirs as well as digging
holes into the deposited sediments in order to establish rates of
reservoir siltation.
In this study, it has been established that high rates of soil
erosion occurs in this small catchment area experiencing a
combination of arid and semi-arid type of climate. The predominant
t.ypss of soil erosion was found to be gully/rill erosion and
sheetwashoccurring on sideslopes and in localities surrounding the
reservoirs that have been highly degraded. It was also found that
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ABS'l'RACT
This is a study on soil erosion and its impact:: on small
•. ,.reservoirs located in the Nguu Tatu catchment situated in the
northern parts of MombasaDistrict, Kenya. The main objective of
this study is the determination of the types, patterns and rates
of# soil erosion within a sub-humid tropical Nguu Tatu Catchment
and especially the assessment of the impact of soil erosion on
reservoirs water quantity and quality. The methodology applied on
this study involved three major approaches namely catchment erosion
survey, measurement of water and suspended sediment discharge and
finally reservoir sedimentation survey. Catchment soil erosion
surveys involved the determination of rates, patterns and maqn'i tude
of soil erosion within the catchmen-t by Lise of hills] ope plot
experiments, slope traverses and benchmarks employing the use of
erosion nails. Measurement of strenm flow and corresponding
suspended sediment concentration provided information on sediment
discharge. The reservoir sedimentation survey involved the
establishment of transects across the reservoirs as well as digging
holes into the deposited sediments in order to establish r at es of
reservoir si 1tat ion .
In this study, it has been established that high rates of soil
erosion occurs int.his small. catchment area experiencing a
combination of arid and semi-arid type of climate. The predominant
types of soil erosion was found to be gully/rill erasion and
sheetwash occurring on sideslopes and in localities surrounding the
reservoirs that have been highly degraded. It was also found that
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the flashy characteri.sti.cs of the ephemeral flows in the
catchment coincides with very high sediment transport rates. The
~.a'nnual sediment production rate was found to be highly variable on
seasonal basis. The rates of gully and rill erosion were measured
to be in the range of 0.24 m3/m2 to 1.39 m3/m2• The mean sediment
transport rates in the catchment is 5,957 tons/yr. The mean soil
erosion rate in the catchment was measured to be 0.7 cm/yr which is
equivalent to sediment production rate of 2,979 tons/km2/yr.
Factors that were found to influence soil erosion rate in the
catchment are mainly rainfall characteristics (Rainfall amount and
intensity), slope gradient and length I surface runoff depth and
percentage, ground cover and rock /soil type. The main land use
activity in the catchment is livestock grazing. However, the rate
of soil erosion var Le s depending on the intensity of Livestock
grazing which is the main land use type in the catchment. High
rates of soil erosion in the catchment are responsible for
siltation of the small reservoirs located with catchment. The rate
of reservoir siltation was measured to the range of 558 tons/yr and
tons/yr. The reservoir storage depletion rate in the
catchment was found to be 2.0% per annum with the or igina1 design
capacities having been reduced by 80%.It was estimated that for the
last4 decades, approximately 41,250 tannes of sediments have been
deposited into the main Nguu 'l'atureservoir wh.iLe 22 I 324 tonnes
nave been deposited into the other smaller Nguu Tatu reservoir.
Within the catchment it was established that ac.ceLe ra t ed soil
erosion is having serious implications on water resources
development in terms of reduction in design and storage capacities
of reservoirs, water quality deterioration, eutrophication, and
~decline in productivity of the land. It has been established that
if land degradation continues unabated, increased soil erosion in
the catchment and resultant high sediment production will pose a
serious threat on the marine ecosystems situated along the coast.
Thestudy furnishes vital scientific information on the impact of
catchment soil erosion on water resources management. The
information generated could be applied in designing soil and water
conservation programmes in areas of coastal Kenya which have
similar climatic and physical characteristics as Nguu 'I'a t.u
Catchment. It also provides a well tested methodology t.hat;
incorporates soil erosion assessment and reservoir siltation survey
that can be applied in other studies with the obj ect ive of
assessing the impact o'f soil erosion on water resources. The study
shedslight on the processes of soil erosion within a small subh~
M tropical catchment as compared to most of the other studies
that have concentrated on large river basins and whose findings
tendsto be too generalised. The study draws various conclusions
basedon the results of data analysis and offers suggestions that
cooldbe attempted to control the problem of soil erosion and its
associated impacts on small man-made lakes in the Nguu rratu
catchmentand other small catchments situat.ed in the coustal re qi.on
of Kenya. | en |
