lines drawn using best fit by eye. The survey
generated water depth only but could not map
sediment thickness. The same procedure had
previously been used in Lake Naivasha to determine the volume of the lake (Hickley et al. 2002).
This indicates the need for a more robust method
like the use of multi-frequency acoustic profiling
system (APS). The multi-frequency survey would
be more useful in the combined measurement of
water depth and sediment thickness measurement.
This method is important since it also gives an
overview of the spatial distribution of sediment
within the lakes and reservoirs.
4 Importance of Bathymetric
Survey and Sedimentation
Assessment in Reservoir
Management
Developing these tools for reservoir management
is important in informing water managers of the
volume of water resources available. In successive surveys, change in depth-volume or deptharea relationships can be an indicator to reservoir
managers of a need to change water abstraction.
It also informs them of how much of the reservoir is affected by sedimentation. Successive
surveys provide an indication of the sedimentation rate within the lakes or reservoirs. This
approach was used by Wisser et al. (2013) to
give an indication of sedimentation rates of 1,024
reservoirs in the contiguous US and of 191
reservoirs to give an indication of the global
average. Changes in the depth-volume relationship or depth-surface relationship could also
indicate a change in the sedimentation status of
the reservoirs. This would call for intervention
measures either upstream or within the dam to
slow down the volume loss. Such measures could
then be evaluated economically, at least for
reservoirs used for hydropower generation as
demonstrated by Ngondjeb and Ayuk (2021). In
combination with dating, historical data generated can be used to analyse sediment yield
dynamic and relate them with various events in
the past such as extensive land use changes or
fire events and propose mitigation measure in
case of similar events in the future as demonstrated by Sang et al. (2015) and Allen et al.
(2011) in the case of an increase in sediment rate
after drought events.
The change in depth-volume relationship or
depth-surface relationship could also be an indicator of the need to change the approach used in
releasing water downstream or to the consumers.
Since this change indicates a change in volume, a
new approach to manage the available volume
would be critical.
5 Conclusions
There are many lakes and reservoirs in Kenya,
which are mainly used for water supply, irrigation, tourism, and hydropower generation.
However, the reservoirs’ functions are threatened
by sedimentation which leads to loss of storage
capacity, hence affecting the development and
economy of the country. However, little has been
done to understand the status of these lakes and
reservoirs, since most of these water bodies do
not have continuous monitoring of sediments
other than the water level at a gauging station. In
this study, BSS was successfully used to survey a
reservoir and a natural lake in Kenya. The results
from the sedimentation assessment showed that
the Ruiru Reservoir has lost about 14% of its
original volume due to sedimentation at an
approximate sediment accumulation rate of
1.75 cm/year over the past 67 years. These
results were used to provide critical tools and
data for the management of the reservoirs, which
were missing before these studies. The use of
BSS in sedimentation assessment plays a key
role in generating multifunctional historical data
of these reservoirs. However, the application rate
in Kenya is low where only three out of about 17
major reservoirs have been surveyed. Therefore,
there is a need for technical capacity building on
how these robust methods can be applied to the
already threatened water bodies as part of a
comprehensive monitoring strategy facilitating
integrated resources management.
90
J. Sang and C. Maina
generated water depth only but could not map
sediment thickness. The same procedure had
previously been used in Lake Naivasha to determine the volume of the lake (Hickley et al. 2002).
This indicates the need for a more robust method
like the use of multi-frequency acoustic profiling
system (APS). The multi-frequency survey would
be more useful in the combined measurement of
water depth and sediment thickness measurement.
This method is important since it also gives an
overview of the spatial distribution of sediment
within the lakes and reservoirs.
4 Importance of Bathymetric
Survey and Sedimentation
Assessment in Reservoir
Management
Developing these tools for reservoir management
is important in informing water managers of the
volume of water resources available. In successive surveys, change in depth-volume or deptharea relationships can be an indicator to reservoir
managers of a need to change water abstraction.
It also informs them of how much of the reservoir is affected by sedimentation. Successive
surveys provide an indication of the sedimentation rate within the lakes or reservoirs. This
approach was used by Wisser et al. (2013) to
give an indication of sedimentation rates of 1,024
reservoirs in the contiguous US and of 191
reservoirs to give an indication of the global
average. Changes in the depth-volume relationship or depth-surface relationship could also
indicate a change in the sedimentation status of
the reservoirs. This would call for intervention
measures either upstream or within the dam to
slow down the volume loss. Such measures could
then be evaluated economically, at least for
reservoirs used for hydropower generation as
demonstrated by Ngondjeb and Ayuk (2021). In
combination with dating, historical data generated can be used to analyse sediment yield
dynamic and relate them with various events in
the past such as extensive land use changes or
fire events and propose mitigation measure in
case of similar events in the future as demonstrated by Sang et al. (2015) and Allen et al.
(2011) in the case of an increase in sediment rate
after drought events.
The change in depth-volume relationship or
depth-surface relationship could also be an indicator of the need to change the approach used in
releasing water downstream or to the consumers.
Since this change indicates a change in volume, a
new approach to manage the available volume
would be critical.
5 Conclusions
There are many lakes and reservoirs in Kenya,
which are mainly used for water supply, irrigation, tourism, and hydropower generation.
However, the reservoirs’ functions are threatened
by sedimentation which leads to loss of storage
capacity, hence affecting the development and
economy of the country. However, little has been
done to understand the status of these lakes and
reservoirs, since most of these water bodies do
not have continuous monitoring of sediments
other than the water level at a gauging station. In
this study, BSS was successfully used to survey a
reservoir and a natural lake in Kenya. The results
from the sedimentation assessment showed that
the Ruiru Reservoir has lost about 14% of its
original volume due to sedimentation at an
approximate sediment accumulation rate of
1.75 cm/year over the past 67 years. These
results were used to provide critical tools and
data for the management of the reservoirs, which
were missing before these studies. The use of
BSS in sedimentation assessment plays a key
role in generating multifunctional historical data
of these reservoirs. However, the application rate
in Kenya is low where only three out of about 17
major reservoirs have been surveyed. Therefore,
there is a need for technical capacity building on
how these robust methods can be applied to the
already threatened water bodies as part of a
comprehensive monitoring strategy facilitating
integrated resources management.
90
J. Sang and C. Maina