12 Sediment Management in River Basins: An Essential Element …
265
factor affecting the intensity of sediment transport, partition of the riverbed by a dam
affects the continuation of its transport. According to the theory of River Continuum
Concept (RCC) by Vannote [23] interruption of its continuity, unquestionably affects
the change of hydrological conditions. There are approximately 0.8 million dams in
operation on the larger half of the world’s rivers. Of these dams, approximately
40,000 are registered as large, meaning those that are at least 15 meters in height, or
with a reservoir capacity greater than 3 million m
3 [24]. Almost all large reservoirs
accumulate 98% of their river’s sediment, thus holding limit delivery to seas and
oceans.
Sedimentation of the debris in reservoirs creates many serious exploitation and
ecological problems [25]. The most important ones include siltation of the reservoir,
leading to the reduction of the reservoir capacity and eutrophication of its waters [18,
26–32]. The problem is successive filling the reservoir with sediments, which reduces
its capacity. For example, the Kulekhani dam in Nepal, commissioned in 1981,
whose lifespan was designed for 85 years, lost almost half of its dead capacity (i.e.,
approx. 12 million m
3 ) after just 8 years. The Cerron Grande reservoir in El Salvador
has functioned for only 30 years, instead of the originally assumed viability, which
was estimated at 350 years [33]. In order to reduce silting of large dam reservoirs,
sludge cleaning is carried out by controlled discharge of water into the lower site
by the bottom drains. This allows the release of liquefied bottom sediments from
the reservoir [34]. In total, each year, 30 million tons of sediment accumulated in
Xiaolangdi reservoir in Jiyuan created on the Yellow River are released in this way.
In the last dozen or so years, 390 million tons of sediments collected in the reservoir
were discharged along with the river’s waters [35]. The variability of hydrological
and operational conditions has a significant influence on changes in the backwater
reach of the reservoir. Its length on the Cymla´ nski Reservoir on the Volga is about
200 km, in the Dnieproges Reservoir on the Dnieper 70–80 km [36].
Undoubtedly, the accumulation of sediment in the reservoir affects the reduction
of its operating volume and total capacity [37]. According to the Hartung criterion,
the reduction of the reservoir volume to the level of 80% results in the loss of its
functionality. In order to unblock the riverbeds (mainly waterways) and to release
the reservoir’s capacity, dredging works are carried out. In Germany, for example,
more than 45 million m
3 of river sediment are dredging every year to provide safe
waterway depths [1]. In addition to practical reasons, sediments extracted from rivers
and reservoirs are a desirable raw material on the construction market (in particular
sand-gravel fractions) [38]. The decisive economic factor in this type of mining
branch is the cost of transporting the output to the customer, hence locating of the
collection sites is related to the proximity of the markets and major transport routes.
In addition, river gravel is usually of high enough quality to be classified as a product
suitable for use in the production of concrete from Portland cement [39]. The material
as a result of river transport is rounded, already granulated, well sorted and free of
impurities. The exploitation of aggregates from the river bed requires small financial
outlays [38]. In the case of mountain rivers, due to the ease of access to the aggregate
exposed in the form of rapids, it is enough to use mainly excavators and vehicles used
for transport. Alluvial sediments can also be exploited from neighbouring floodplains
265
factor affecting the intensity of sediment transport, partition of the riverbed by a dam
affects the continuation of its transport. According to the theory of River Continuum
Concept (RCC) by Vannote [23] interruption of its continuity, unquestionably affects
the change of hydrological conditions. There are approximately 0.8 million dams in
operation on the larger half of the world’s rivers. Of these dams, approximately
40,000 are registered as large, meaning those that are at least 15 meters in height, or
with a reservoir capacity greater than 3 million m
3 [24]. Almost all large reservoirs
accumulate 98% of their river’s sediment, thus holding limit delivery to seas and
oceans.
Sedimentation of the debris in reservoirs creates many serious exploitation and
ecological problems [25]. The most important ones include siltation of the reservoir,
leading to the reduction of the reservoir capacity and eutrophication of its waters [18,
26–32]. The problem is successive filling the reservoir with sediments, which reduces
its capacity. For example, the Kulekhani dam in Nepal, commissioned in 1981,
whose lifespan was designed for 85 years, lost almost half of its dead capacity (i.e.,
approx. 12 million m
3 ) after just 8 years. The Cerron Grande reservoir in El Salvador
has functioned for only 30 years, instead of the originally assumed viability, which
was estimated at 350 years [33]. In order to reduce silting of large dam reservoirs,
sludge cleaning is carried out by controlled discharge of water into the lower site
by the bottom drains. This allows the release of liquefied bottom sediments from
the reservoir [34]. In total, each year, 30 million tons of sediment accumulated in
Xiaolangdi reservoir in Jiyuan created on the Yellow River are released in this way.
In the last dozen or so years, 390 million tons of sediments collected in the reservoir
were discharged along with the river’s waters [35]. The variability of hydrological
and operational conditions has a significant influence on changes in the backwater
reach of the reservoir. Its length on the Cymla´ nski Reservoir on the Volga is about
200 km, in the Dnieproges Reservoir on the Dnieper 70–80 km [36].
Undoubtedly, the accumulation of sediment in the reservoir affects the reduction
of its operating volume and total capacity [37]. According to the Hartung criterion,
the reduction of the reservoir volume to the level of 80% results in the loss of its
functionality. In order to unblock the riverbeds (mainly waterways) and to release
the reservoir’s capacity, dredging works are carried out. In Germany, for example,
more than 45 million m
3 of river sediment are dredging every year to provide safe
waterway depths [1]. In addition to practical reasons, sediments extracted from rivers
and reservoirs are a desirable raw material on the construction market (in particular
sand-gravel fractions) [38]. The decisive economic factor in this type of mining
branch is the cost of transporting the output to the customer, hence locating of the
collection sites is related to the proximity of the markets and major transport routes.
In addition, river gravel is usually of high enough quality to be classified as a product
suitable for use in the production of concrete from Portland cement [39]. The material
as a result of river transport is rounded, already granulated, well sorted and free of
impurities. The exploitation of aggregates from the river bed requires small financial
outlays [38]. In the case of mountain rivers, due to the ease of access to the aggregate
exposed in the form of rapids, it is enough to use mainly excavators and vehicles used
for transport. Alluvial sediments can also be exploited from neighbouring floodplains