equipment, and poor accessibility of various
lakes and reservoirs (Dost and Mannaerts 2008).
However, there are currently efforts, in collaboration with international partners, to address
these limitations.
2 Reservoir Sedimentation
Assessment Techniques
Various pre-and post-impoundments reservoir
survey techniques can be used to determine the
volume and sedimentation rate in reservoirs. The
pre-impoundment techniques involve establishing the potential reservoir volume during the
construction of the reservoir. Before the 20th
century, water depth was measured manually
from a boat using a sounding line and lead
weights. At the tip of the weight, an adhesive
was applied to indicate sediment level has been
reached (Jakubauskas 2008). In some cases,
where water is shallow, graduated poles were
used. Another technique used to survey sediment
distribution in the reservoir is the use of a spud
survey. Spud survey estimates the thickness of
sediment and it can even be used in areas where
previous sediment thickness information was not
available. In this method, a sounding weight is
used to confirm the surface of the sediment
(Ritchie and McHenry 1985; Morris and Fan
1998). The method works well in areas where
sediment thickness is up to 4 m (Morris and Fan
1998). The spud is dropped into the water and
allowed to fall vertically until it penetrates the
deposited sediments. The spud has grooves
where sediments enter, ensuring it is not washed
away as the spud is being retrieved from the
water. The method is easy, but the accuracy of
data depends on the operator’s experience. In
addition, the method is more applicable to small
reservoirs.
Another method that can be used for reservoir
assessment is ranging. The method uses permanent ranges in the reservoir especially those that
are re-surveyed at regular intervals. It is thus
useful in computing the volume difference
between the surveys. To get very accurate results
the method can be used together with bathymetry
survey or aerial photographs, where photogrammetry is used to define contour lines.
Photogrammetric techniques or airborne lasers
are useful especially when the reservoirs are dry
or where a high drop of water level is noted
(Morris and Fan 1998). Initial reservoir volume
is determined from pre-impoundment topographic maps and aerial surveys. To improve
accuracy, the method is used together with
ground-truthing activities. The use of aerial survey technique is illustrated by Wooldridge
(1984) who used pre-impounded aerial photographs of 1965 and a sediment assessment of
1982 to determine sediment accumulation in the
Kamburu reservoir, in Kenya. However, the
aerial survey method is limited in its use since it
can only be used before the impoundment of the
reservoir or in cases where the reservoirs are
completely empty. Finally, other options use
Light Detection and Ranging (LIDAR).
Although advances in remote sensing have
enabled the use of LIDAR data in reservoir surveys, the method was reported to be very
expensive and time consuming (Heyman et al.
2007), thus it cannot be easily used in regular
surveys.
A need exists for a fast and cheap methodology that can be used for sedimentation assessment. This would benefit developing and some
developed countries where over 60% of reservoirs do not have baseline sedimentation data.
Modern technology that allows simultaneous
operation of multiple transducers (i.e. collection
of multiple transducer data separated by acoustic
wavelength) is important. This makes it possible
to collect spatially and temporally correlated
acoustically independent data. An example of
such a low cost methodology is the use of a
Bathymetric Survey System (BSS). The BSS
combines multi-frequency Acoustic Profiling
System (APS) and a vibe-coring system (Fig. 2).
The use of independent multi-frequency APS
means that surveyors can utilize higher frequency
acoustics to calculate water depth, while simultaneously utilizing the sediment-penetrating
capability offered by lower frequency to map
post-impoundment sediment thickness (Dunbar
et al. 1999). The BSS works in three frequency
86
J. Sang and C. Maina
lakes and reservoirs (Dost and Mannaerts 2008).
However, there are currently efforts, in collaboration with international partners, to address
these limitations.
2 Reservoir Sedimentation
Assessment Techniques
Various pre-and post-impoundments reservoir
survey techniques can be used to determine the
volume and sedimentation rate in reservoirs. The
pre-impoundment techniques involve establishing the potential reservoir volume during the
construction of the reservoir. Before the 20th
century, water depth was measured manually
from a boat using a sounding line and lead
weights. At the tip of the weight, an adhesive
was applied to indicate sediment level has been
reached (Jakubauskas 2008). In some cases,
where water is shallow, graduated poles were
used. Another technique used to survey sediment
distribution in the reservoir is the use of a spud
survey. Spud survey estimates the thickness of
sediment and it can even be used in areas where
previous sediment thickness information was not
available. In this method, a sounding weight is
used to confirm the surface of the sediment
(Ritchie and McHenry 1985; Morris and Fan
1998). The method works well in areas where
sediment thickness is up to 4 m (Morris and Fan
1998). The spud is dropped into the water and
allowed to fall vertically until it penetrates the
deposited sediments. The spud has grooves
where sediments enter, ensuring it is not washed
away as the spud is being retrieved from the
water. The method is easy, but the accuracy of
data depends on the operator’s experience. In
addition, the method is more applicable to small
reservoirs.
Another method that can be used for reservoir
assessment is ranging. The method uses permanent ranges in the reservoir especially those that
are re-surveyed at regular intervals. It is thus
useful in computing the volume difference
between the surveys. To get very accurate results
the method can be used together with bathymetry
survey or aerial photographs, where photogrammetry is used to define contour lines.
Photogrammetric techniques or airborne lasers
are useful especially when the reservoirs are dry
or where a high drop of water level is noted
(Morris and Fan 1998). Initial reservoir volume
is determined from pre-impoundment topographic maps and aerial surveys. To improve
accuracy, the method is used together with
ground-truthing activities. The use of aerial survey technique is illustrated by Wooldridge
(1984) who used pre-impounded aerial photographs of 1965 and a sediment assessment of
1982 to determine sediment accumulation in the
Kamburu reservoir, in Kenya. However, the
aerial survey method is limited in its use since it
can only be used before the impoundment of the
reservoir or in cases where the reservoirs are
completely empty. Finally, other options use
Light Detection and Ranging (LIDAR).
Although advances in remote sensing have
enabled the use of LIDAR data in reservoir surveys, the method was reported to be very
expensive and time consuming (Heyman et al.
2007), thus it cannot be easily used in regular
surveys.
A need exists for a fast and cheap methodology that can be used for sedimentation assessment. This would benefit developing and some
developed countries where over 60% of reservoirs do not have baseline sedimentation data.
Modern technology that allows simultaneous
operation of multiple transducers (i.e. collection
of multiple transducer data separated by acoustic
wavelength) is important. This makes it possible
to collect spatially and temporally correlated
acoustically independent data. An example of
such a low cost methodology is the use of a
Bathymetric Survey System (BSS). The BSS
combines multi-frequency Acoustic Profiling
System (APS) and a vibe-coring system (Fig. 2).
The use of independent multi-frequency APS
means that surveyors can utilize higher frequency
acoustics to calculate water depth, while simultaneously utilizing the sediment-penetrating
capability offered by lower frequency to map
post-impoundment sediment thickness (Dunbar
et al. 1999). The BSS works in three frequency
86
J. Sang and C. Maina