2.1 Sediment Bypassing and Off-Channel Reservoir Storage
Sediment bypassing diverts part of the incoming sediment-laden waters around the reservoir, so that they never enter the reservoir at all. Typically, the sediment-laden waters are diverted at a weir upstream of the reservoir into a high-capacity tunnel or diversion channel, which conveys the sediment-laden waters downstream of the dam, where they rejoin the river (Figure 2). Normally the weir diverts during high flows, when sediment loads are high, but once sediment concentrations fall, water is allowed into the reservoir. (A variant of this approach may involve the use of a bypass that diverts sediment-laden waters already in a reservoir.)
Figure 2. (a) Conventional reservoir, which traps incoming sediment, contrasted to alternative configurations for bypass of sediment-laden flood flows around the storage pool: (b) bypass off-stream storage, wherein a diversion dam in the river diverts water to the off-channel reservoir during times of clear flow but does not divert when suspended sediment concentrations are high, and (c) a sediment bypass channel or tunnel, which during times of high water and high sediment concentrations, diverts flow from the river upstream of the reservoir, passing it around the reservoir and into the downstream channel.
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The ideal geometry for sediment bypass is one where the river makes a sharp turn between the point of sediment collection and the point of sediment reintroduction to minimize the length of the conveyance device and take advantage of the relatively steeper gradient for gravity flow (Figure 2c). Where that ideal condition does not exist, the technique is most practical where the reservoir is relatively short, as there must be sufficient gradient to drive the transport of sediment through the diversion tunnel or diversion channel. At Nagle Dam in South Africa, the river takes a sharp bend at the reservoir, providing an ideal “short cut” for the bypass, with steep slopes [Annandale, 1987].
Overall, Japan and Switzerland are the leading countries for sediment bypass tunnels: in Japan, three are in operation and two under construction; in Switzerland, six are in operation [Vischer et al., 1997; Auel et al., 2010] (Table 1). The oldest sediment bypass tunnel in Japan was installed at the municipal water supply reservoir Nunobiki dam near Kobe city 8 years after completion of the dam in 1900. This bypass scheme has successfully diverted coarse sediment for more than 100 years as described by Sumi et al. . At the Miwa and Asahi dams in Japan, the rivers are sufficiently steep that a straight tunnel has adequate gradient to carry most sediment load downstream of the dam [Sumi et al., 2004; Suzuki, 2009; Sumi et al., 2012]. Miwa Dam (on the Mibu River, in the Tenryu River basin) was built in 1959 with 30 million m3 (Mm3) storage capacity. Subsequent deposition of 20 Mm3 of sediment has prompted expensive sediment removal efforts. To prolong the reservoir's life, a 4.3-km-long sediment bypass tunnel and diversion weir at the upstream end of the reservoir were constructed in 2005 (Figure 3). The dam and diversion tunnel operate such that during the rising limb of a flood, sediment-laden flows are diverted into the bypass tunnel, but the tunnel inlet is closed on the falling limb of the flood so the clear waters can be stored (Figure 4) (assuming the commonly observed hysteresis curve of higher sediment concentrations on the rising limb). The system is successfully routing sediment downstream, the efficiency being a function of the magnitude of the flood and the timing of the operation, and with no impacts detected on the downstream ecology in the 7 years after the scheme's inception [Sumi et al., 2012]. Asahi Dam on the Shingu River was built in 1978; sedimentation problems motivated the 1998 construction of a sediment bypass with a 13.5-m high diversion weir and 2350-m long tunnel. By 2006, sediment bypassing through the tunnel had avoided a cumulative 750,000 m3 of sediment deposition [Mitsuzumi et al., 2009].
Table 1. Characteristics of Successful Sediment Bypass Tunnels in Japan and Switzerland
|Name of Dam||Country||Date Constr||Tunnel Shape||Tunnel Cross Section (BxH) (m)||Tunnel Length (m)||General Slope (%)||Design Q (m3 s−1)||Design Velocity (ms−1)||Annual Operation Frequency (days/a)|
|Nunobiki||JP||1908||Archway||2.9 × 2.9|| 258||1.3|| 39|| 7||—|
|Asahi||JP||1998||Archway||3.8 × 3.8||2350||2.9||140||12||13|
|Miwa||JP||2004||Horseshoe||2r = 7.8||4300||1||300||10||2–3|
|Matsukawa||JP||2015||Archway||5.2 × 5.2||1417||4||200||15||—|
|Koshibu||JP||2016||Horseshoe||2r = 7.9||3982||2||370|| 9||—|
|Egshi||CH||1976||Circular||R = 2.8|| 360||2.6|| 74||10||10|
|Palagnedra||CH||1974||Circular||2r = 6.2||1800||2||110||13||2–5|
|Pffaffensprung||CH||1922||Horseshoe||A = 21 m2|| 280||3||220||14||ca. 200|
|Rempen||CH||1983||Horseshoe||3.5 × 3.3|| 450||4|| 80||12||1–5|
|Runcahez||CH||1961||Archway||3.8 × 4.5|| 572||1.4||110|| 9||4|
|Solis||CH||2012||Archway||4.4 × 4.68|| 968||1.8||170||11||1–10|
Figure 3. Diagram of sediment bypass system for Miwa Dam, Mibu River, Japan. A check dam traps coarse sediment, and a diversion weir diverts flows with high suspended sediment concentrations into a bypass tunnel.
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Figure 4. Sediment bypass at Miwa Dam June 19, 2010: (a) hydrographs for inflow into the reservoir and for flow through the bypass tunnel, and (b) suspended sediment concentrations (mg/l) at the diversion weir and in the bypass tunnel. [Modified from Kantoush et al., 2011]
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In Taiwan, the sediment-plagued Shihmen Reservoir on the Dahan River, will be retrofit with a sediment bypass, taking advantage of the sharp river bend at the reservoir [Wang and Kondolf, 2014; Water Resources Agency (WRA), 2010). Sediment bypasses are expensive because of the cost of the tunnel, but have many advantages, in passing sediment without entering the reservoir, and without interfering with reservoir operation. In case of coarse sediment bypassing, an anti-abrasion design for tunnel bottom surface is essential for minimizing long-term operation costs, as described by Visher et al.  and Sumi et al. .
An alternate approach to sediment bypass is to build off-channel reservoir storage, such that the diversions from the weir are clear-water diversions, while sediment-laden water is left in the river to pass downstream (Figure 2b) [Morris and Fan, 1998]. Similar to sediment bypass, there needs to be sufficient gradient to drive flow through diversion channels or tunnels to the off-channel storage feature. One advantage of this approach is that all bed load can be excluded from the reservoir. Simulations using daily data from streamflow and sediment gages in Puerto Rico indicate that it is possible to exclude between 90% and 95% of the total sediment load from an off-stream reservoir, thereby prolonging reservoir life by a factor of more than ten as compared with an on-channel reservoir on the same river [Morris, 2010]. The intake structure can be designed to present a much smaller impediment to the migration of fish species than a dam, and downstream river morphology is maintained because sediment load and flows capable of transporting sediment are not impaired. The rate at which water can be diverted to the off-channel storage reservoir is limited to the capacity of the diversion channel, so this approach is less suited to flashy streams in semi-arid zones where water flow is concentrated in floods. Under appropriate hydrologic conditions, even a diversion of relatively modest capacity may result in firm yields close to those achieved by an on-channel reservoir.
Off-channel storage could be more widely used than has been the case. In run-of-river hydropower projects, turbines run at full capacity during the wet season when streamflow exceeds the plant's design capacity. During the dry season, an off-channel reservoir can provide a small live storage volume, to store inflow over a 24-h period for delivery to turbines during the hours of peak demand. For example, the recently designed San José project in the Andes Mountains of Bolivia is fed by eight intakes, has 125 MW capacity with 600 m of gross head, and requires a 0.35 Mm3 regulating reservoir to provide 6 h of peak power (Figure 5). Off-channel storage was ideal to provide peaking power at this site, because vertical canyon walls made site access difficult for construction of a main stem dam, and the high load of large bed material (up to 1 m diameter) presented an unfavorable situation for sediment management. Coarse sediment (>0.15 mm) will be removed by desanders prior to entering the regulating reservoir, and finer sediment trapped in the pool will need to be excavated after several years.
Figure 5. Schematic layout of 125 MW San José hydroelectric project, Paractia River, Bolivia, incorporating an off-stream regulating reservoir for pondage and desanders to remove the coarse sediment abundant in the streams.
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The Cameguadua and San Francisco off-stream reservoirs in the Cauca River basin near Manizales, Colombia, have operated successfully for many years. These two reservoirs have a total installed capacity of 197 MW at five power stations; they are fed by seven intakes, and accumulated fine sediment is removed by dredging.
2.2 Sediment Sluicing
Drawdown routing, or sluicing [ICOLD, 1999], involves discharging high flows through the dam during periods of high inflows to the reservoir, with the objective of permitting sediment to be transported through the reservoir as rapidly as possible while minimizing sedimentation. Some previously deposited sediment may be scoured and transported, but the principal objective is to reduce trapping of incoming sediment rather than to remove previously deposited sediment. One advantage of this approach is that deposition in the reservoir is minimized and the sediment continues to be transported downstream during the flood season when sediment is naturally discharged by the river. Finer sediments are more effectively transported through the reservoir than coarse sediments.
Sluicing is performed by lowering the reservoir pool prior to high-discharge sediment-laden floods (Figure 6). This approach requires relatively large capacity outlets on the dam to discharge large flows while maintaining low water levels and the required velocities and transport capacity. These outlets need not be at the very bottom of the dam, and at some sites with smaller storage volumes, tall crest gates can be used for this purpose.
A drawdown and sluicing strategy may be employed at reservoirs of all sizes, but the duration of sluicing depends on the watershed size and the time scale of flood events.
For dams of small watersheds with rapidly rising floods, the reservoir may be drawn down only for a period of hours. In other cases, such as dam sites with small storage volumes for daily regulation (pondage), the reservoir may be held at a low level during the entire flood season to maximize sediment pass through while continuing to produce power and using a desander to protect hydro-mechanical equipment from the abrasive sediment that is mobilized by sediment sluicing. In storage reservoirs on large rivers the reservoir may be held at a low level for a period of many weeks at the beginning of the flood season and filled with late-season flows.
By virtue of passing the rising limb of the flood, which generally contains higher sediment concentration than the falling limb of the flood hydrograph, sluicing is consistent with the Chinese strategy to, “release the muddy flow and store the clear water” [Wang and Hu, 2009]. In China, sluicing has most-famously been implemented at the Three Gorges dam where prolonged seasonal drawdown during the early part of the flood season is designed to maximize flow velocity and sustain sediment transport through the reservoir, and also mobilize some of the previously deposited sediment. The reservoir level is raised later in the season to fill storage for sustaining releases during the low-flow season (Figure 7). The objective is to sustain the natural patterns of flood and sediment discharge along the river, while producing power and assisting navigation. This strategy to stabilize reservoir capacity is best suited to narrow reservoirs. The Three Gorges Reservoir, e.g., is about 600-km long but does not exceed 1.5 km in width, and it has a high-discharge capacity at the dam.
Reservoirs trap less sediment when the flood-detention period is reduced, and a change in the reservoir operating rules to minimize flood-detention time, especially on the rising limb, can reduce sediment trapping at a very low operational cost. While sluicing operates most effectively in long narrow reservoirs, benefits can also be achieved in storage reservoirs having other configurations. For example, the John Redmond reservoir in Kansas (USA) has a nearly circular configuration, a large flood control pool, and a small water conservation pool. Analysis of historical operations during 48 flood events plus modeling showed that a measurable increase in sediment throughput could be achieved by making relatively minor changes to the operating rule, while still maintaining downstream flood control targets [Lee and Foster, 2013]. Compared to the conventional reservoir operation, “the altered scenario purposefully minimized reservoir elevation and residence time through larger, more rapid releases of water after periods of high inflows,” resulting in measurably decreased trap efficiency [Lee and Foster, 2013:1437]. This reduction in sediment trapping efficiency is achieved without any structural modifications, by simply including a sediment management objective in the reservoir operating rule.
2.3 Drawdown Flushing
In contrast to sluicing, whose aim is to pass sediment without allowing it to deposit, drawdown flushing focuses on scouring and re-suspending deposited sediment and transporting it downstream. It involves the complete emptying of the reservoir through low-level gates that are large enough to freely pass the flushing discharge through the dam without upstream impounding, so that the free surface of the water is at or below the gate soffit (Figure 8). While flushing can be undertaken in reservoirs having any configuration, because the flushing channel will typically not be wider than the original streambed, flushing will recover and maintain a substantial fraction of the original reservoir storage only in reservoirs that are long and narrow.
The best scenario for flushing is to establish river-like flow conditions through the reservoir upstream of the dam, which is favored by the following conditions: narrow valleys with steep sides; steep longitudinal slopes; river discharge maintained above the threshold to mobilize and transport sediment; and low-level gates installed in the dam [Morris and Fan, 1998]. Flushing is best adapted to small reservoirs, and on rivers with strongly seasonal flow patterns [White, 2001].
Flushing differs from sluicing in two key respects [Morris and Fan, 1998]. First, as discussed above, flushing focuses on the removal of previously deposited sediments, instead of passing incoming sediments through the dam. Secondly, (and consequent to the first) is that the timing of sediment release to the downstream channel may be different from that of the sediment inflow into the reservoir, and the difference is greatest if flushing is conducted during the nonflood season. Flushing can release large amounts of fine sediment to the downstream channel during periods of relatively low flow, when the river is unlikely to have sufficient energy to transport the sediment downstream. The accumulation of sand and finer sediment on the bed can have substantial impacts on the river ecology, and if the deposits are sufficiently large it can also impact the channel's capacity to convey floodwaters. Flushing during the flood season also has the advantage of having greater discharges available, with more erosive energy, and incoming sediment can also be carried through the dam as well as the sediment being eroded and resuspended from reservoir deposits [Morris and Fan, 1998].
Flushing has been successfully implemented in many dams globally, such as: Unazuki and Dashidaira dams in Japan [Kokubo et al., 1997; Liu et al., 2004; Sumi and Kanazawa, 2006], Sanmenxia dam in China [Wan, 1986; Wang et al., 2005], Cachi Dam in Costa Rica [Jansson and Erlingsson, 2000], and Genissiat Dam on the Rhône River in France [Thareau et al., 2006], and recommended as the only sediment management measure feasible in terms of public acceptance and cost for Gavins Point dam on the Missouri River [US Army Corps of Engineers, 2002].
For flushing to be successful, the ratio of reservoir storage to mean annual flow should not exceed 4%, because with larger storage the reservoir cannot be easily drawn down Sumi  (Figure 9). Because flushing flows need to pass through the low-level outlet without appreciable backwater, it may not be feasible to use large floods which exceed low-level gate capacity as flushing events.
Figure 9. Plot of flushing projects from diverse environments showing that successful cases are characterized by impoundment ratios of 0.4 or less. That is, reservoir storage capacity divided by mean annual runoff (inflow to the reservoir) should be less than 0.4.
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Sediment deposited from flushing can have significant environmental impacts, especially if flushing is carried out during nonflood season and sediments remain on the bed of the downstream channel. Ecologically important pools can fill with sediment, gravel, and cobble riffles can be buried in finer sediment, and fine sediment can clog the bed, thereby eliminating surface-groundwater exchanges, smothering eggs, and clogging the void spaces between stones used as habitat by aquatic invertebrates and larval fish. Even a small release of sediment (i.e., a small fraction of the river's natural annual sediment budget) during the river's base-flow period can have large impacts because the sediment cannot be transported downstream. On the Kern River, California, sand was flushed from a small diversion dam during base flow in 1986 in anticipation it would be transported away the next winter. However, a series of dry years followed, and the flushed sand remained on the bed for several years because the river did not experience a sufficiently large flow to transport it away [Kondolf and Matthews, 1993].
As a general rule, flushing sediment-laden water through the power house is not recommended because it can cause abrasion of the turbines. Sand in particular will quickly destroy turbines. The Zhengzhou workshop presentations included reports of some cases in which fine sediments were successfully passed through powerhouses, but any such flushing scenario must be carefully monitored so that the penstocks can be shut off before sand is mobilized. However, as experienced at Nathpa Jhakri, India, even silt with a high-quartz content (70%–80%) can destroy turbines within months. It is therefore important to assess the mineral content of sediment and susceptibility of the hydro-mechanical equipment to damage, and to stop power production when the reservoir level drops to the point that abrasive sediment may be eroded from the reservoir delta and carried into the power intake.
The main challenges are to sustain the largest possible reservoir storage volume over the long term under drawdown flushing operations, while minimizing adverse downstream environmental impacts as described by Gerster and Rey  and Staub . There is a trade-off between frequent flushing with its frequent power losses and less frequent flushing operations. Generally, more frequent flushing (e.g., annually) has less downstream impacts because it delivers sediment to the downstream channel, where it is needed for river health, more often and in small pulses. This reduces the potential for sediment pulses to overwhelm the river's transport capacity and aggrade the channel. Opening of the gates gradually and at appropriate times such as high flows (e.g., the rainy season or snowmelt season) will lessen the impacts of change in sediment concentration on the downstream environment [Sumi et al., 2009]. Another consideration is consolidation of cohesive sediments. With time, cohesive sediments can “set up” and develop a hardened surface that requires heavy equipment to break up and push into the flushing current. Regular reservoir flushing can reduce or interrupt consolidation of cohesive sediments and aid in fine sediment removal. It is particularly important to be able to release a flow of clear water after flushing to mobilize sediment and carry it further downstream. This may be in the form of a natural flood hydrograph, or an additional release from the dam with the reservoir at a higher level so that sediment is no longer being scoured.
Flushing will not solve all sedimentation problems. Not only is there the limitation imposed by the limited width of the flushing channel with respect to the overall width of the reservoir, but there is also the problem posed by the limited hydraulic energy that can be generated with flushing. Thus, flushing discharges may efficiently remove fine sediments, but coarse sediments transported into the reservoir by large floods will continue to accumulate without being removed by lower discharge flushing flows. In Cachi Reservoir (Costa Rica) and Hengshan Reservoir (China), coarse-grained deltas are prograding downstream toward the dam despite regular sediment flushing [Morris and Fan, 1998].
In some cases there is no clear-cut transition point between reservoir drawdown for sluicing and for flushing, since drawdown for sluicing can scour and mobilize deposits, just a flushing does. Flushing and sluicing may be combined in a seasonal reservoir operation, wherein the pool is emptied and outlet gates are opened at the beginning of the rainy season to allow high flows to pass through the empty reservoir, carrying their incoming sediment as well as eroding stored sediment. This approach is employed in some Chinese reservoirs. For example, the Sanmenxia dam on China's Yellow River remains empty for over 2 months during the first part of each flood season, allowing sediment-laden floods to flush out sediment deposited during the previous year, and also allowing sediment-laden floods to pass through the reservoir [Wang et al., 2005].
Seasonal operation has also been used at the Jensanpei Reservoir in southern Taiwan, which was built in 1938 to supply water to a sugar mill, which operated only part of the year [Huang, 1994]. Through the early 1950s, the reservoir was rapidly filling with sediment, and lost 4.3 Mm3 of its original 7 Mm3 capacity (Figure 10), but beginning in 1955, the dam was operated to pass sediment through a seasonal drawdown approach. The reservoir would be drawn completely down and the outlets left open for the first 2.5 months of the rainy season (Figure 11). During this time, inflowing floods could transport most of their sediment through the reservoir without depositing it, and they could also scour sediment already deposited. Midway through the rainy season, the outlet gates were closed, and the reservoir began impounding water for processing sugar cane, which is harvested between November and April. However, by the late 1990s, because of economic changes, the sugar mill was no longer used, and the site around the reservoir was developed for tourism. For tourism, the drawn-down reservoir was considered unattractive, and the seasonal drawdown operation was abandoned from 1998. As a result, sediment began to accumulate in the reservoir until 2013, when the operators resumed seasonal drawdown and sediment pass through after finishing repairs to the sluice gate, which had become nonfunctional due to the sedimentation and lack of maintenance for the years without drawdown (Figure 10). The dam was also raised twice (in 1942 and 1957) to increase reservoir capacity from the original 7 Mm3 to 8.1 Mm3, but the benefit of this was minor compared to the benefit of seasonal sediment pass through. Jensanpei is an example of a combination of sluicing (allowing inflowing floods during the first half of the rainy season to pass through the reservoir without depositing their sediment loads) and flushing (scouring sediment deposited). It worked because the reservoir could be drawn down seasonally without affecting its functions.
Figure 10. Deposition within Jen-San-Pei reservoir in Taiwan, 1938 to present. Prior to 1955, no sediment management was conducted and the reservoir filled rapidly, but beginning 1955 seasonal drawdown and sediment passing maintained reservoir capacity. From 1998 to 2012, the pass-through operations were stopped, allowing sediment to accumulate. Pass-through operation was resumed in 2013. [Modified from Huang, 1994 and extended to present using data from Water Resources Agency of Taiwan]
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Figure 11. Seasonal operation of Jen-San-Pei reservoir in Taiwan to pass sediment. The reservoir was drawn completely down and the outlets left open for the first 2.5 months of the rainy season, to allow inflowing high flows to transport their sediment through the reservoir and scour sediment already deposited. Midway through the rainy season, the outlet gates were closed, so the reservoir could impound water to process sugar cane, harvested from November to April. [Redrawn from Huang, 1994]
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2.4 Flushing Sediment for Dams in Series
In flushing sediment through a series of dams, simultaneous flushing can be accomplished by releasing the flushing pulse first from the upstream reservoir. Just before that pulse reaches the next downstream reservoir, its lower level gates are also opened to pass the sediment. After finishing the sediment flush, the reservoirs are refilled and clear water released from upper level gates to flush the downstream channel of deposited sediment.
A notable example of management of dams in series is the operation of 19 dams on the Rhône from the Swiss border to the Mediterranean Sea, whose operation is coordinated by the Compagnie Nationale du Rhône with two dams upstream in Switzerland. Except for Genissiat Dam on the Upper Rhône, all are run-of-the-river dams that operate by short-circuiting the “old river” with a straight canal, leaving abandoned meander bends with greatly reduced flows, some of which have been the loci of ecological restoration efforts [Stroffek et al., 1996]. With availability of storage in Lake Geneva, sediment is managed in reservoirs and channels of the Upper Rhône by flushing, such that the opening of gates is coordinated from dam to dam as a pulse moves downstream. However, on the Lower Rhône, storage is lacking, and while it would theoretically be possible to coordinate flushing with high tributary inflows, disruptions to navigation must be arranged a year in advance, so flushing is not attempted, and instead, sediment is removed mechanically [Compagnie National du Rhône, 2010].
“Environmentally friendly flushing” from Genissiat Dam limits the potential impacts of flushing on downstream aquatic life, water supply intakes, and restored side-channel habitats. This approach is of particular interest because this flushing is conducted under extremely strict restrictions on turbidity and suspended sediment concentrations, not to exceed 5 g/l on average over the entire operation and not to exceed 15 g/l over any 15-min period [Thareau et al., 2006]. The dam is equipped with outlets at three levels: a bottom gate, an outlet halfway up the dam, and a surface spillway. Concentrations are controlled by mixing waters with high sediment concentrations from the bottom of the water column with enough “cleaner” water from higher in the water column (normally via the mid-level outlet) to stay within the required concentrations. Genissiat Dam receives high sediment loads from Verbois Dam upstream, which is flushed to avoid sedimentation and consequent backwater that could flood parts of urban Geneva. In four decades of flushing every 3 years, an estimated 23 million tons of sediment could have deposited in the reservoir, but only 4.5 million tons have actually deposited. The operation is costly to the Compagnie National du Rhône, which engages a staff of about 400 over approximately 10 days, at a cost of about €1.4 million (based on the 2003 flushing, [Thareau et al., 2006]). Nevertheless, to remove an equivalent volume (1.8 million tons in 2003) by dredging would have been far more costly.
On the Kurobe River, Japan, Dashidaira and Unazuki dams are operated in coordination, with high runoff triggering flushing of the upstream dam and sluicing through the downstream dam [Kokubo et al., 1997; Liu et al., 2004; Sumi and Kanazawa, 2006] (Figure 12). The basic sequence of operations is to draw down the reservoir water level, maintaining a free-flow state over several hours (the duration being determined by the amount of sediment to be flushed), and then allowing the reservoir water level to recover. In July 2006, a free-flow condition was continued for 12 h to flush out an estimated 240,000 m3 of deposited sediment (Figure 13). The flushing/sluicing operation is followed by release of a clear-water “rinsing” flow to remove accumulated sediment from the channel downstream.
Figure 12. Kurobe River, Japan. Map of drainage basin and location of major reservoirs (a), and longitudinal profile showing all reservoirs (b).
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Figure 13. Coordinated flushing on reservoirs in the Kurobe River, Japan, 1–3 July 2006. Precipitation (a), inflow and outflow hydrographs and reservoir stage for Dashidaira (b) and Unazuki (c) dams, and resultant suspended sediment concentrations (d).
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2.6 Turbidity Current Venting
Turbidity (or “density”) currents are important in the transport and deposition of sediment in reservoirs worldwide. Turbidity currents form when inflowing water with high sediment concentrations forms a distinct, higher density current that flows along the bottom of the reservoir toward the dam without mixing with the overlying, lower density waters. If the bed of the reservoir is highly irregular, with protruding features that would break up the flows and cause turbulence, turbidity currents may not sustain themselves. However, turbidity currents occur in many reservoirs, and it is often possible to allow this dense, sediment-laden water to pass through outlets in the dam, a practice referred to as “venting” of turbidity currents (Figure 14). This can be undertaken as a sediment management technique, even at large reservoirs where other techniques, such as reservoir drawdown, are not feasible. Some dams have been able to pass half of the inflowing sediment load by venting turbidity currents, but the technique is possible only in cases where the turbidity current has sufficient velocity and turbulence to maintain particles in suspension and the current can travel all the way to the dam as a distinct flow, where it can then be passed downstream [Morris and Fan, 1998].
Facilities for the venting of turbidity currents should be provided at every project where turbidity currents are anticipated to convey substantial amounts of sediment to the dam. Advantages of turbidity current venting are that it delivers suspended sediment to downstream reaches during the floods when the sediment would naturally be delivered, and that it does not require reservoir drawdown or otherwise significantly impact reservoir operations.
Both Sanmenxia and Xiaolangdi Reservoirs on the Yellow River vent turbidity currents, along with flushing to discharge sediments, and the Yellow River Institute of Hydraulic Research has developed a new formula to predict the formation of plunge point for density currents, which can help in selection of optimal dam sites for density current venting, and criteria for design and operation of reservoirs to create effective density currents. With installation of a curtain (typically a sheet of geotextile hanging vertically from the water surface, suspended from flotation tanks and secured in place by a cable and anchor system, extending partway down the water column to force flow underneath), it may be possible to vent density currents at higher outlets on the dam, avoiding problems of clogging low-level outlets.
2.7 Dredging and Mechanical Removal of Accumulated Sediments
Accumulated sediments can be removed by suction using hydraulic pumps on barges with intakes. If cohesive sediments have “set up,” cutter heads may be required to break up the cohesive sediments. Dredging is expensive, so is most often used to remove sediment from specific areas near dam intakes. If there is sufficient hydrostatic head over the dam, it can create suction at the upstream end of the discharge pipe to remove sediment and carry it over the dam as a siphon. This hydrosuction is typically limited to reservoirs less than 3 km in length, and to low elevations, where the greater atmospheric pressure facilitates the function of the siphon. In China, hydraulic suction machinery is commonly used to stir the sediment within the reservoir with hydraulic and mechanical power, then to discharge the highly concentrated sediment-laden water out of the reservoir through siphons by the help of water head difference between upstream and downstream of the dam.
If a reservoir is completely drawn down, mechanical removal can be employed using scrapers, dump trucks, and other heavy equipment to remove accumulated sediments. While still costly, mechanical removal is commonly less expensive than hydraulic dredging, and can remove coarser sediments, but it requires the reservoir to be drawn down far enough to expose coarse sediment. Mechanical removal is best adapted to reservoirs that remain dry for parts of the year such as flood control reservoirs. Cogswell Reservoir on the San Gabriel River, California, was mechanically dredged in 1994–1996, with 2.4 Mm3 removed and taken to a nearby upland disposal site, at a cost of $5.60/m3 (or $6.47/m3 if planning and permitting are included) [Morris and Fan, 1998]. Another 2.55 Mm3 has been identified as requiring excavation following a 2009 wildfire that increased erosion in the catchment [Los Angeles County Department of Public Works (LACDPW), 2012].