Volume 6 hydro power 6 13 – long term sediment management for sustainable hydropower

Volume 6 hydro power 6 13 – long term sediment management for sustainable hydropower Volume 6 hydro power 6 13 – long term sediment management for sustainable hydropower Volume 6 hydro power 6 13 – long term sediment management for sustainable hydropower Volume 6 hydro power 6 13 – long term sediment management for sustainable hydropower Volume 6 hydro power 6 13 – long term sediment management for sustainable hydropower

F Rulot, BJ Dewals, S Erpicum, P Archambeau, and M Pirotton, University of Liège, Liège, Belgium

© 2012 Elsevier Ltd All rights reserved

6.13.6.1 Measures Addressing Driving Forces

6.13.6.2 Measures Addressing Pressures

6.13.6.4 Measures Addressing Impacts

6.13.6.5 Numerical Modeling for Sustainable Sediment Management

6.13.6.5.1 Modeling scales and approaches

6.13.6.5.2 The WOLF modeling system

6.13.6.5.3 Case study: Alpine shallow reservoir

6.13.6.6 Assessing Sustainability of Hydropower Projects

This reservoir sedimentation process in turn has a number of important consequences The reduced available reservoir capacity undermines water supply and hydropower production Flood control effectiveness is also decreased, and conditions may ultimately

be reached in which the dam would be overtopped during an extreme flood Operation of low-level outlets, gates, and valves is disturbed, while the extra pressure acting on the dam as a consequence of sediment deposition may affect dam stability The abrasive action of sediment particles can roughen the surfaces of release facilities and cause cavitation as well as vibrations Downstream of the dam, degradation can undermine the foundations and also deteriorate dam stability Sedimentation also affects water quality

As the life span of a dam is determined by the net sedimentation rate and since many existing major reservoirs are approaching a stage in which sediments clog low-level outlets, it is a key priority to take sedimentation into better consideration in the planning, design, operation, and maintenance of dams and reservoirs

One way of preserving reservoir storage is to remove sediments out of the reservoir For example, under favorable conditions, it is possible to flush sediments through outlet works within the dam This technique can be applied both to existing dams (with adaptations) and to new dams However, the technique is only effective depending on site-specific conditions and is not applicable

Comprehensive Renewable Energy, Volume 6 doi:10.1016/B978-0-08-087872-0.00620-X 355

Table 1 Worldwide rate of reservoir sedimentation

Region

Inventoried large dams

Storage (km³)

Annual percentage storage loss by sedimentation

Hydroelectricity produced with respect to potential for hydroelectricity

in all geographical areas An alternative consists in building more dams to replace the depleting storage of the existing stock However, there are less and less suitable dam sites available, and many new dam projects are considered as leading to serious environmental and social consequences Moreover, between half and one percent of the worldwide storage capacity of dams is lost annually as a result of reservoir sedimentation, resulting in the need to build approximately 400 dams every year just to compensate for lost storage capacity [1] In addition, the global demand for water is increasing at a rate even higher than the rate of population growth In contrast, the commissioning of large dams tends to decrease with time, as shown in Figure 1

Worldwide storage in reservoirs reaches almost 6815 km³ Seventy percent of the existing world stock of reservoir storage is situated in America, northern Europe, and China Sedimentation rate can be expressed as the percentage of total original reservoir volume lost each year; this rate depends on the geographic region as shown in Table 1 Biggest annual loss of storage occurs in China with 2.3% of storage lost by sedimentation There are also significant differences between the regional averaged rates and the rates for individual reservoirs, showing high spatial variability For example, data gathered from 16 reservoirs in Turkey give a mean annual rate of storage loss of 1.2%, but the rates for individual reservoirs ranged from 0.2% to 2.4% [3], confirming that the problem is indeed highly site-specific

As the industrialization of nations increases worldwide, power consumption is growing Hydropower turns out to be an increasingly attractive alternative ever to generate electricity The percentage of hydroelectricity actually exploited in 2005 as

Commissioning of large dams by decade, 20th Century

compared to the existing potential, as shown in Table 1, reveals that the potential of hydroelectric energy available is largely used in Europe and North America whereas it is far from being totally exploited in Africa

6.13.1.2 The DPSIR Framework

It is very important to understand the complex problem of sedimentation management along with its causes and con­sequences For the assessment of such environmental problems, the European Environment Agency (EEA) recommended the use of a specific framework, developed by the National Institute of Public Health and Environment (RIVM), which distinguishes driving forces, pressures, states, impacts, and responses It is known as the Driver, Pressure, State, Impact, Response (DPSIR) framework Figure 2 shows the DPSIR model [4] According to the DPSIR framework, there is a chain of causal links starting with ‘driving forces’ that exert ‘pressures’ on the environment and, as a consequence, the ‘state’ of the environment changes This leads to ‘impacts’ that may elicit a societal ‘response’ The response provides feedback to the driving forces, pressures, state, and/or impacts through adaptation of curative action A driving force is an anthropogenic activity that may have an environmental effect, like agricultural or industrial human activities The pressures account for the direct effects of the driving forces As an example, industrial human activities can cause pressures like gas emissions or waste generation The state is the condition of the water body resulting from both natural and anthropogenic factors It is the physical, chemical, and/or biological state of the water For example, the state of the water becomes acidic due to industrial wastes and emissions The impacts are the environmental and/or human health effects of the pressure(s) Finally, the responses are the measures taken to improve the state of the water body For example, water acidity could be reduced if agriculture is managed in a more environmentally friendly manner The DPSIR framework is applied in the following sections with regard to the specific issue of long-term sediment management for sustainable hydropower generation

6.13.2 Driving Forces

Driving forces can be seen as independent, autonomous, ‘outside’ forces directly or indirectly affecting a dependent system For dams or reservoirs principal natural driving forces are geology, slope, climate, and/or vegetative cover Main human driving forces are modifications in land use like urban development, deforestation, and agriculture, as well as drainage density The features of the soil change, and therefore the behavior of erosion and deposition also changes For example, urban development in the catchment area of a dam often reduces the sediments supply Driving forces depend on the surveyed problem and may not be the same for all hydropower dams A good example is deforestation Deforestation may play an important part in flood generation because when rain falls in a geographic site where deforestation has happened, water is no longer absorbed and the runoff seriously erodes the soil Deforestation occurs mainly in Amazonia, south Asia, Indonesia, and central Africa; for example, the 183 km² catchment of Ringlet reservoir in Malaysia has been gradually changed from forests to plantations and holiday facilities, which has resulted in a dramatic increase of the amount of sediment since the mid-1960s (Figure 3)

6.13.3 Pressures

Pressures are direct stresses deriving from the anthropogenic system (i.e., caused by humans, like deforestation) and natural systems, and affecting the natural environment Particle input and transport, bottom and bank erosion, and resuspension are the principal pressures Sedimentation is a more general term used to describe these pressures Distribution, frequency,

Table 2 Sediment yield from gauging stations worldwide

6.13.3.1 Variability in Pressures

The amount of sediment exported by a basin (drainage network) over a period of time is referred to as ‘sediment yield’ Obviously, it

is always less than the amount of sediment eroded within a watershed, owing to redeposition prior to reaching reservoirs ‘Sediment delivery ratio’ is the ratio of delivered sediment to eroded sediment ‘Specific sediment yield’ is the sediment yield per unit area

6.13.3.1.1 Variation in space

Sediment yield is highly variable over space In some cases, even a small part of the landscape unit contributes a disproportionate amount of the total sediment yield For instance, intensive local logging leads to a substantial increase in sediment yield Sometimes, the yield ratio between a logging zone and a ‘normal’ one can reach several hundreds Hence, knowledge of the spatial variation in yield is required to focus yield reduction efforts on the landscape units that deliver the maximum amount of sediments

The wide variation in specific sediment yield in the global data set is also reflected at all levels of analysis: national, regional, and within-watershed The phenomenon is highly site specific Specific sediment yields typically vary by up to 3 orders of magnitude depending on the geographic region

6.13.3.1.2 Variation in time

Estimates of long-term sediment yield have been used for many decades to size the sediment storage pool and estimate reservoir life However, the models that estimate long-term sediment yield are not accurate for floods, as most of the sediment is exported from watersheds during this relatively short period of the year For instance, Santa Clara river basin in Southern California is reported to have discharged 50
106

tons of sediments during a single flood event, which represents more than 700 times the measured average annual load In the United States, more than half of the annual sediment load is discharged in only 1% of time Thus temporal variation is also a key factor to study

Techniques for evaluating sediment yield depend on the choice of time horizon Very long-term trends in sediment yields appear after decades and can usually be correlated with human activities in the watershed As an example, it has been reported [3] that the Piedmont area of the eastern United States was completely deforested by the mid-1800s, leading to increased erosion rates and sediment yield After 1920, erosion rates declined because hillside farms were abandoned and revegetated naturally, while soil-conservation methods were implemented in the remaining farms Despite the high erosion rate of soil over a 150-year period, the sediment delivery ratio remained as low as approximately 5% because eroded sediments were deposited further downstream in channels and on floodplains

Long-term trends can be visualized by constructing a cumulative-mass curve for water and sediment Figure 4 gives a better idea

of trends than a timewise plot because it helps compensate for runoff variability The dotted curve accounts for an equivalent system

in which flushing is employed, thus decreasing the cumulative suspended sediment discharge In applying regional curves to a particular study site, care must be taken to consider local features such as upstream reservoirs, land use, and topographic or geological conditions that may depart from regional norms

A cyclic seasonal variation can be observed in specific regions of the world For example, seasonal variations of erodibility were observed in Nepal because of monsoon and vegetation cycles In some other regions, wind plays an important part, transporting sediment from ridges into depressions where it becomes available for fluvial transport

Short-term trends are attractive for describing a flood or storm situation Usually, a suspended solid concentration (C) versus discharge (Q) plot is used to represent the short-term phenomenon As shown in Figure 5 and Table 3, storm or flood events can be divided into three categories The characteristics correspond to the forms of the C–Q graphs observed in Figure 5 [6]

Class I represents cases for which sediment concentration responds immediately to a variation in discharge In the graph, discharge is just a scaling of the sediment concentration This implies that sediment supply through the flood is uninterrupted and sediment concentration should be directly related to hydraulic factors alone Class I occurs not very often, but these curves are widely used for their simplicity

In contrast, Class II occurs commonly This pattern is usually observed when sediment concentration reaches a peak value before discharge as shown in Figure 5 Under certain conditions, it also happens that sediment concentration and discharge peak simultaneously Three causes can lead to clockwise C–Q loops:

• The sediment accumulated or the easily erodible material in the watershed is washed out when water discharge increases a little bit, and sediment load supply decreases over the duration of the event because sediment becomes less readily erodible

• During the event, prior to the peak of water discharge, sediment supply from the bed becomes limited because of the development of an armor layer

• Spatial variations in rainfall and erodibility across the watershed can concentrate sediment discharge from areas of high sediment production near the catchment area outlet before the peak of discharge

Class III occurs when soil erodibility is high and erosion is prolonged during flood or as a result of specific rainfall and erodibility distributions across the watershed In such cases, occurrence of the peak of the sediment concentration curve is delayed

Cumulative water discharge

Table 3 Classification of C–Q graphs

6.13.3.2 Measurement Techniques

There are two approaches for measuring sediment yield:

• inspection of the sediments volume deposited in the reservoir

• continuous monitoring of fluvial sediment discharge

The main advantage of the first method is its accuracy because the construction of a reservoir eliminates problems of missed

or underreported events at fluvial gauge stations The main advantage of the second method is the good description of spatial and temporal patterns; it is thus easy to identify and diminish the sediment yield These two strategies are detailed below

6.13.3.2.1 Reservoir survey

The first reservoir survey method is bathymetric mapping, which is often combined with local surveys to determine the grain size of the deposits and enables verification of computational or mathematical models Generally, reservoir measurements may be performed at intervals of 5–20 years; it depends essentially on budgetary constraints, rate of storage decrease, and management requirements However, unscheduled surveys may be called for after a major flood or other phenomena that lead to a surge in sediment yield Surveys should also be conducted downstream of the dam More than 20 years of surveys may be needed to get a reliable trend in long-term sediment accumulation There are two main techniques to compute reservoir volume: the range line and contour surveys The original volume of reservoir is often computed using the contour method based on preimpoundment topographic mapping

The widely used range line method uses a system of cross sections where depths are measured Each range line is tied to the initial elevation–capacity relationship of the reservoir reach corresponding to that range and provides the base against which all future surveys will be compared This sequence is repeated at regular time intervals, and an elevation–capacity relationship with respect to time can be plotted The range line method is less accurate than the contour method because the latter entails a complete survey of the reservoir The use of global positioning system (GPS) facilitates these measurements today

An alternative method for drawing a contour map of the reservoir is used when the pool is often drawn down or emptied When the water level decreases, photographs of the reservoir may be taken from an aircraft or a satellite at different stages, gradually drawing the contour map of the reservoir

The rate of sediment accumulation can also be determined by measuring the depth of deposition above an identifiable and datable layer of 137Cs This is a toxic radioactive isotope of cesium It is water-soluble but penetrates only a short distance into clayed soil As the half-life of 137Cs is approximately 30 years, it can be used as a dating tool Nevertheless, 137Cs is a toxic element, so its production is forbidden Therefore, if this method is to be employed, the watershed must be impacted by uncontrolled events such

as large fires, volcanic eruption, or Chernobyl-like radioactivity that produce or have produced 137Cs The datable horizon is limited

in time Indeed, before the year 1954, which corresponds to the first atmospheric nuclear testing, 137Cs never appeared in measurable amounts [7] Another limitation is that the procedure does not work when the reservoir is drawn down Moreover, it

is necessary to take several samples from a number of locations to reliably map deposition thickness because of the uneven deposition in reservoirs An isotope of lead 210Pb is another radioactive element sometimes used to measure sediment deposition

6.13.3.2.2 Fluvial data

Sediment-rating curves are one of the widely used tools for the estimation of sediment discharge in a river based on fluvial data Over a given period, data (obtained by gauging stations) are plotted in an instantaneous discharge–concentration relationship These graphs are often in log–log scale A sediment-rating curve from several years of field data that include sampling of flood events can be applied to a long-term discharge data set to estimate long-term sediment yield There are different procedures to be considered for the development of accurate rating curves Regression techniques very often incorporate bias if data are too widely scattered In such cases, data of a particular kind of runoff event should be gathered (e.g., seasonal runoff) It is thus important to back test a rating relationship by applying it to the original stream flow data set to ensure that it correctly computes the total load Sometimes a multiple slope is also necessary to have accurate values at high discharge

If sediment concentration data can be measured frequently, the use of sediment-rating curve becomes redundant; sediment load can be computed directly as the product of discharge and concentration at short intervals The main advantage of this method is that time variation can be accurately represented; for instance, looped rating curves can be detected In highly variable hydrographs, for example, rivers, short sampling intervals are required to accurately track sediment yield

Turbidity measurement can give an idea of the suspended sediment concentration Turbidity is the term used to describe the reduction in water clarity due to particulate matter suspended in solution The attenuation (reduction in strength) of light passing through a sample column of water gives a measure of its turbidity An automatic pumping sampler is used to take samples at short intervals, but laboratory costs are high Moreover, it is possible that the sample bottles are filled before the end of the peak event, resulting in high undercounting errors When pumped samples and turbidity measurement are analyzed together, it represents a viable strategy for improving the quality of sediment discharge data There is no direct correlation between turbidity and suspended sediment concentration; hence errors are unavoidable However, turbidity data can be recorded every few seconds and averaged As

a matter of fact, errors can be reduced if local sensors take into account the suspended sediment concentration at several points over

a cross section of the river

The method discussed above can measure the amount of suspended sediment For bed load, the method is obviously different because riverbed particles are usually bigger than suspended ones Bed load samplers directly measure the load of particles moving along the bed The main difficulty in measuring bed load is the highly irregular rate of bed load transport even at a constant discharge Another challenge is that the bed load transport is multidirectional; it can also occur in the transverse direction of the flow Hence, sampler efficiency is defined in such a way that sampled transport rate divided by sampler efficiency determines the true transport rate Sampler efficiency is determined by calibration in a hydraulic flume in the laboratory and varies as a function of grain size and transport rate The method widely used for measuring the transport distance and transport rate of sediment is the marking of stones (painting, embedding magnets) in one section of the stream and relocating them repeatedly during a certain period of time This method can prove appropriate to determine the condition of initiation of motion in different areas of the streambed Methods for collecting the grains depend on their size and the depth of water Coarse grains (gravels, cobbles) are collected by hand if not too heavy, whereas smaller grains (sand) can be sampled with a mechanical system if the river is shallow and the velocity remains moderate

6.13.3.2.3 Modeling

Neural network models are numerical models rather than experimental ones They are nonlinear black boxes that establish a link between input data (stream flow, rainfall, temperature, and other parameters from gauging stations) and output data (sediment concentration) by training their internal algorithms and their weighting scheme The main advantage of this type of modeling

as compared to sediment-rating curves is that sediment concentration can be correlated with several inputs It is thus easier to show the effects of any one parameter on sediment concentration Several approaches exist for this method One approach is to use these models to develop rating relationships based on channel hydraulic characteristics Another approach is to predict suspended

sediment concentration or discharge based on channel with the help of watershed and hydraulic parameters or watershed parameters alone

Another way to compute sediment yield is to use spatial modeling Spatially distributed data may be analyzed to compute the yield of both water and sediment from the watershed based on observation of the soil, hydrologic input parameters, and land use, and the output data (sediment load and runoff) are routed to the watershed exit Thus, the main disadvantage of this method is that the problem highly depends on watershed data The next step consists in coupling the empirical erosion prediction model with a sediment delivery module to simulate sediment yield Alternately, models that simulate both sediment detachment and transport processes may be coupled with fluvial routing procedures to simulate sediment yield As an advantage, several land use scenarios can be compared to identify areas where erosion control would provide the highest benefit

6.13.4 State

The state accounts for the environmental conditions of the system It corresponds to a description of the system subjected to pressures and driving forces Here, the amount of sediment trapped in the reservoir, the reservoir sediment deposition, and its geometry describe the state of the system Another important point developed below is the expected future evolution of the reservoir

6.13.4.1 Capacity Loss

When a tributary enters an impounded reach, flow velocity decreases and the sediment load begins to deposit The volume of the sediment deposited in a reservoir depends on the trap efficiency of the reservoir and the density of the deposited sediment Trap efficiency is the percentage of sediment load that stays in the reservoir over a given period of time It depends highly on the fall velocity of sediment particles, the shape and size of the reservoir, and the variation of flow through the reservoir This parameter is computed with the help of graphics There are two evaluation methods widely used The first one was developed by Brune [8] for large-storage reservoirs The trap efficiency is given as a function of the ratio of reservoir capacity to average annual inflow The capacity of the reservoir is taken at the mean operating pool level for the period to be analyzed For smaller reservoirs, Churchill [9] developed a specific trap efficiency curve

Though it is possible to estimate the sediment deposition in the reservoir based on these empirical formulae, if the anticipated sediment accumulation is larger than one-fourth of the reservoir capacity, trap efficiency should be determined for incremental periods of the reservoir life because trap efficiency generally decreases with time

Periodic reservoir surveys are often considered as one of the most suitable methods for the determination of sediment yield from

an upstream watershed The volume of sediment trapped in a reservoir during a period between two surveys is simply the difference

in reservoir volume between these two surveys The difference between the original capacity (water volume) and the actual gives a global estimation of the loss of storage in a reservoir (Figure 6) In association with the difference in area, Figure 6 also gives some insight into the distribution of sediments at a given elevation

The average percentage values of annual loss of storage due to sedimentation vary gradually between 0.5% and 1% Except for China where the mean loss of storage per year reaches 2.3% (Table 1), storage loss generally tends to grow faster in smaller reservoirs than in larger ones Today, the number of dams and reservoirs commissioned worldwide tends to decrease, and the rate of loss of storage is not counterbalanced by the newly available storage

0

0

Delta-coarse

sediment deposit

Turbidity current

Fine sediment deposit

Clear water

abrupt reduction in grain size; it also corresponds to the downstream limit of bed material transport in the reservoir Upstream limit is not well defined, and sediment deposits extend into the river After the delta section, there may be a plunge point if turbidity currents take place in the reservoir Turbidity currents are flows of water and very fine sediments (< 100 μm) driven by the difference in density between clear water and sediment-laden water The ‘bottomset bed’ consists of fine sediments, which are deposited beyond the delta

by suspension and turbidity currents Under specific conditions, such as floods, reservoir drawdowns, and slope failures, coarser sediments may be transported further downstream into the reservoir It is thus possible to observe several layers of different grain sizes near the dam Although longitudinal deposition patterns can have different shapes, depending on pool geometry, discharge, grain size characteristics of the inflowing load, and reservoir operations, the most typical pattern is well represented by Figure 7

Regarding lateral depositional pattern, the deposition in a cross section of the reservoir occurs first in its deepest part and subsequently spreads out across the submerged floodplain to create broad flat sediment deposits (Figure 8; stage I)

Sedimentation rates may be alternatively expressed by means of reservoir half-life, which is the time required to lose half of the original capacity of the reservoir In contrast, ‘reservoir life’ is defined as the time between the construction of the dam and the total filling of the usable storage pool preceding the abandonment of the structure Three successive stages may be distinguished

Stage 3: Full sediment balance This is the stage when long-term sediment inflow counterbalances long-term sediment outflow This balance is obtained when sediments can be transported beyond the dam or artificially removed (e.g., flush)

Most of the dams are designed to work in the continuous sediment trapping mode, but some reservoirs have been designed to achieve sediment balance, such as Three Gorges reservoir on China’s Yangtze River designed to reach full sediment balance after about 100 years

Sediment management can postpone the filling of the reservoir It is also possible to increase the capacity of the reservoir Capacity history curves may be drawn to visualize historical and anticipated changes in usable storage volume under different management strategies

Table 4

Locations Impacts

Sedimentation impacts Above the normal pool Bed aggradation Higher level flooding Higher groundwater levels Impaired navigation

Pool area Reduced conservation and flood control pool volumes Clogging of intakes Abrasion of structural equipment Environmental impacts Increased static load on the dam

Below the dam Channel incision Bank erosion Lower groundwater level Scouring below the dam

Adapted from Morris GL, Annandale G, and Hotchkiss R (2008) Reservoir sedimentation In: Marcelo HG (ed.) Sedimentation Engineering: Processes, Measurements, Modeling, and Practice American Society of Civil Engineers 110: 579–612 [3]

Floodplain Fine materials

Below the dam, trapping of sediments leads to an incision in the channel A general decrease in water level is thus observed and the following impacts are noticed: tributaries’ degradation, destabilization and undercutting of streambanks, undermining of structures like bridge piers Net erosion below the dam occurs only if there is a sediment deposit below the dam Other environmental effects are also observed in tributaries below the dam, as a result of lower groundwater levels, such as dewatering

of wetlands During the first decade of reservoir operation, erosion of the river below the dam will be limited by the formation of an armor layer, preventing the erosion of finer sediments by clogging them with coarse ones

These environmental problems are at the root of public’s and environmental organizations’ opposition to the construction of new reservoirs Nevertheless, if long-term impacts of the reservoir are taken into account from the early stages of dam design, they can be mitigated to a great extent by means of a proper management scheme

6.13.6 Responses

In man-made reservoirs, constructed for water supply, irrigation, flood and low-flow control, or hydropower generation, both the loss in storage capacity and the location of deposits are a concern These reservoir sedimentation issues would be solved if watershed erosion could be stopped, or at least controlled, and sediment yields drastically reduced This may, however, turn out to be economically nonfeasible and would create other problems such as upstream river bed degradation and scouring

In contrast, authorities in charge of reservoir sustainability may implement responses which, in line with the DPSIR framework, may be targeted toward any component of the causal chain, between driving forces and impacts

Therefore, possible responses may be classified into a number of categories, depending on which stages of the DPSIR chain they affect Among other possibilities, sediment-control measures are related to driving forces, sediment bypass is linked to pressures, whereas sediment dredging or flushing are oriented toward the state of the reservoir

As a result of the complexity and natural variability of the involved sedimentation processes (such as the influence of turbulence or grain sorting) and site-specific parameters, there is no single measure generally suitable for solving sediment management concerns Therefore, an optimal site-specific strategy needs to be developed For this purpose, a comprehensive understanding of the fundamentals of sediment transport, erosion, and deposition is a prerequisite Very valuable is also a thorough quantitative knowledge of the sedimentation processes that take place on the site, which requires suitable measurement devices and monitoring programs For practical purposes, a wide range of possible responses needs to be reviewed to lead to a cost-effective and sustainable sediment management strategy, usually involving a combination of several carefully selected measures The optimal combination of sediment management measures may vary in time during the life of the reservoir and depends mainly on the purposes of the reservoir, its hydrological size (capacity vs inflow), and site-specific environmental challenges

This section provides an overview of responses for mitigating sedimentation and its impacts, including both standard practice approaches and more advanced techniques These responses are classified depending on the component of the DPSIR chain they address

6.13.6.1 Measures Addressing Driving Forces

From the perspective of mitigating sedimentation in reservoirs, reducing soil erosion in the catchment may appear as the ideal solution, though difficult to successfully implement in practice It typically involves measures such as terracing or suitable agricultural practices, as well as structural measures such as bank protection and slope reduction using sills in thalwegs

Experience shows that it may be particularly effective in small catchments or catchments with confined intensive erosion-producing areas, while being economically unrealistic if the reservoir has a large drainage area

Furthermore, implementation of such measures requires cooperation of a potentially large number of landowners throughout the basin Obtaining the commitment of all the influencing parties may be difficult, because reduced reservoir sedimentation usually benefits other parties than those who own and exploit upstream land Nevertheless, involvement of stakeholders may be facilitated by the numerous side-benefits of reduced soil erosion, including enhanced soil fertility, water quality, and state of the environment

In some specific areas, wind erosion may play a part and thus needs to be addressed by appropriate measures including increasing vegetative cover and construction of wind barriers

6.13.6.2 Measures Addressing Pressures

For a given set of driving forces, measures addressing pressures tend to reduce net sediment inflow into the reservoir, by means of upstream sediment retention, sediment bypass, or sediment routing

of the current empirical methods could also result from the fact that they disregard the flow pattern when estimating trapping efficiency of the basin