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Dams and Reservoirs -Hydraulics engineering
- 1. Hydraulics Engineering Dams and Reservoirs Dr. M. Mubashir Qureshi
- 2. What is a dam? ◼ A dam is a barrier built across a stream, river or estuary to hold and control the flow of water for such uses as drinking water supplies, irrigation, flood control and hydropower generation etc.
- 3. What is a dam?
- 4. Benefits of Dams ◼ The benefits of dams are usually to the advantage of humans. They may include: Irrigation Hydroelectric production Flood control Recreational opportunities Navigation Industrial and Domestic water supply Aeration of water ◼ For animals the benefits may include: Larger numbers of fish and birds in the reservoir Greater habitat diversity
- 5. Purpose Distribution of Dams 48.60% 17.40% 12.70% 10.00% 5.30% 0.60% 5.40% Irrigation Hydropower Water Supply Flood Control Recreation Navigation and Fishing Others Source: International Commission on Large Dams (ICOLD)
- 6. Disadvantages of dams Changes in temperature and flow in the river downstream from the dam Loss of flowing water habitat and replacement with standing water (reservoir) habitat Interruption of animal movements along the course of the river Possible alteration of the fish community in the region of the river Interruption of genetic exchange among populations inhabiting the river course Reduction in the delivery of river nutrients to downstream section of the river because of entrapment by the reservoir The loss of the floodplain habitat and connectivity between the river and bordering habitats upland
- 7. Classification of dams: Dams are classified on several aspects, some of the important aspects are as follow: 1) Based on Hydraulic Design: ➢ Over flow dams (e.g. concrete dams) ➢ Non over flow dams (e.g. embankment dams) 2) Based on Structural Design: ➢ Gravity dams ➢ Arch dams ➢ Buttress dams 3) Based on Usage of Dam: ➢ Storage dams ➢ Diversion dams ➢ Detention dams
- 8. Classification of dams: 4) Based on Construction Material: ➢ Concrete / Masonary dams ➢ Earthfill dams ➢ Rockfill dams ➢ Earthfill rockfill dams ➢ Concrete faced rockfill dams (CFRD) 5) Based on Capacity: ➢ Small dams ➢ Medium dams ➢ Large dams
- 9. The choice of dam is decided upon by examining foundation conditions, load strains, temperature and pressure changes, chemical characteristics of ground water and possible seismic activity. Followings are the important factors considered for the selection of type of dams: Selection of dam Type: 1) Topography 2) Geology and nature of foundation ➢ Bearing capacity of the underlying soil ➢ Foundation settlements ➢ Permeability of the foundation soil 3) Availability of construction materials 4) Spillway location 5) Safety considerations 6) Earthquake zones 7) Purpose of dam and economics 8) Aesthetic considerations 9) Life of the Dam
- 10. Following the important factors considered for the selection of site for a dam: Site selection of a dam: 1) Catchment characteristics 2) Length of dam 3) Height of dam 4) Foundation conditions 5) Availability of suitable Spillway location 6) Availability of suitable construction materials 6) Storage capacity 7) Construction and maintenance cost 8) Access to the site 9) Options for diversion of river during construction
- 11. Following the important factors considered for the selection of site for a dam: Site selection of a dam: 10) Compensation cost for property and land acquisition 11) Quality of water 12) Sediment transport 13) Environmental conditions
- 12. Types of Storage Dams Reservoirs are readily classified in accordance with their primary purpose, e.g. irrigation, water supply, hydroelectric power generation, flood control etc. Dams are of numerous types and type classification is sometimes less clearly defined. An initial broad classification into generic groups can be made in terms of the principal of construction material employed: ✓ Embankment Dams: Constructed of earth-fill and/or rock-fill. Upstream and downstream face slopes are similar and of moderate angle, giving a wide selection and high construction volume relative to height. ✓ Gravity Dams: Constructed of mass concrete. Face slopes are dissimilar, generally steep downstream and near vertical upstream and dams have relatively slender profiles depending upon type Note: Embankment dams are numerically dominant for technical and economical reasons, and account for over 85-90% of all dams built
- 13. Types of Storage Dams Concrete Dams Gravity Dam Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight. These are typically used to block streams through narrow gorges. Buttress Dam Buttress dams are dams in which the face is held up by a series of supports. It can take many forms -- the face may be flat or curved. Arch Dam Cupola or Double Arch An arch dam is a curved dam which is dependent upon arch action for its strength. Arch dams are thinner and therefore require less material than any other type of dam. ◼ Embankment Dams Earth-fill Dam ◼ Earth-fill dams, also called earthen, rolled-earth or simply earth dams, are constructed as a simple embankment of well compacted earth. Rock-fill Dam Rock-fill dams are embankments of compacted free-draining granular earth with an impervious zone. The earth utilized often contains a large percentage of large particles hence the term rock- fill is used.
- 14. Types of Storage Dams Embankment dam Gravity dam Arch damButtress dam
- 16. Large Dams: World Register Statistics (ICOLD 1998) Group Type ICOLD* Code % Embankment Dam Earth Fill TE 82.9 Rock Fill ER Concrete Dams including Masonry dams Gravity PG 11.3 Arch VA 4.4 Buttress CB 1.0 Multiple arch MV 0.4 Total Large Dams 41413 ICOLD= International Commission of Large Dams ICOLD defines large dams as dams exceeding15 m in height or in case of dams of 10-15m, satisfying one certain criteria e.g a storage volume in excess of 1x106 m3 or a flood discharge capacity of over 2000 m3/s
- 17. Large Dams: World Register Statistics (ICOLD 2000)
- 23. Panoramic Views of Dams Tarbela Dam: Earth and Rockfill Dam Warsak Dam: Gravity dam Mangla Dam: Earthfill
- 24. Panoramic Views of Dams Gordon Dam: Arch Dam Hoover Dam: Arch Dam
- 25. Panoramic Views of Dams Hume Dam: Buttress dam Coolidge Dam: Buttress dam Le Prele Dam: Buttress dam Itaipu Dam: Buttress dam
- 26. Merits & Demerits of Embankment Dams Merits: It may be equally suitable at sites in wide valleys and relatively steep-sided gorges. Its adaptability to a broad range of foundation conditions, ranging from competent rock to soft and compressible or relatively pervious soil foundations. The use of natural material, minimizing the need to import or transport large quantities of processed materials or cement to the site. Subject to satisfying essential design criteria, the embankment design is extremely flexible in it ability to accommodate different fill materials, e.g earth-fills and/or rock-fills if suitably zoned internally The construction process is highly mechanized and is effectively continuous. Unit cost of earth-fill and rock-fill i.e cost per m3 is lower compared with the concrete dams.
- 27. Merits & Demerits of Embankment Dams Demerits: Inherent great susceptibility to damage or destruction by overtopping, with a consequent need to ensure adequate flood relief and separate spillways. Vulnerable to concealed leakage and internal erosion in dam or foundation.
- 28. Merits & Demerits of Concrete Dams Merits: Concrete dams are suitable to the site topography of wide or narrow valleys alike, provided that a competent rock foundation is available at shallow depth. Concrete dams are not sensitive to overtopping even under extreme flood conditions. Can accommodate a crest spillway, if necessary over entire length, provided that steps are taken to control downstream erosion and possible undermining of the dam. Outlet pipe work, valves, and other ancillary works are readily and safely housed in chambers or galleries within the dam body.
- 29. Merits & Demerits of Concrete Dams Demerits: Concrete dams are relatively demanding with respect to foundation conditions, requiring sound rock. Concrete dams require processed natural material of suitable quality and quantity for aggregate and the importation to site and storage of bulk cement and other materials Traditional mass concrete construction is labor intensive and relatively discontinuous and require certain skills e.g. formwork, concreting etc. Complete unit cost for concrete dams, i.e. cost per m3 is relatively higher compared with the embankment dams. This is seldom counter-balanced by the much lower volumes of concrete required in a dam of given height.
- 31. Pros and Cons of Earthfill and Rockfill Dams Pros: Raising is possible Adoptability in seismic zone Economical Can be Built on any foundation type Relatively Easy availability of Material Less skilled labor is needed Rapid speed of construction due to recent development ◼ Cons: ◼ Damaged by heavy rainfall, floods & overflow ◼ Maintenance cost is high ◼ Silt problems are relative more ◼ Sudden failure of dam ◼ Not suitable for narrow gorges ◼ Cannot be constructed for large height ◼ Very regular supervision and instrumentation is needed to study behavior of dam.
- 32. Pros and Cons of Gravity Dam Pros: Suitable for narrow gorges with steep side slopes, areas with heavy rainfall Overflow conditions Can be built to any height provided that foundation is stable and strong. Low maintenance cost Cost can be reduced by reducing the thickness of sections Outlets can be provided at various elevations Failure is not sudden ◼ Cons: ◼ Stable and strong foundation is required ◼ Availability of construction material and skilled labor ◼ Raising of dam is difficult ◼ Not recommended in seismic zones
- 33. Pros and Cons of Buttress Dam Pros: Lesser concrete is needed which make it economical Structurally stable especially against sliding, Lesser pressure on foundation & lesser uplift pressure Can be built on permeable foundation Powerhouse can be accommodated within space of buttresses Future extension is possible Maintenance cost is low Speed construction Lesser problems of temperature ◼ Cons: ◼ Reduced stabbility against lateral thrust and earthquakes ◼ Rich mix of concrete, reinforcement and formwork ◼ More susceptible to damage and failure under military attacks. ◼ Highly skilled labor needed ◼ Sensitive to deterioration of concrete ◼ Closer supervision is needed.
- 34. Pros and Cons of Arch Dam Pros: Suitable for narrow gorges with lesser base width Structurally stable, Problem of uplift is less ◼ Cons: ◼ Highly skilled labor needed ◼ Good quality formwork is required ◼ Low speed of construction
- 35. Spillway Outlets and Ancillary Works ❑ Dams require certain ancillary structures and facilities to enable them to discharge their operational function safely and effectively. ❑ In particular, adequate provision must be made for the safe passage of extreme floods and for controlled draw- off and discharge of water in fulfillment of the purpose of the reservoir. ❑ Spillways, outlets and ancillary facilities are incorporated as necessary for the purpose of the dam and appropriate to its type.
- 36. Spillway Outlets and Ancillary Works ❑ Spillways: ❑ The purpose of spillway is to pass flood water safely downstream when the reservoir is full. ❑ It has two principle components ❑ Spillweir ❑ Spillway Channel (Its purpose is to conduct flood flow safely downstream of dam) ❑ The Spillways can be ❑ Uncontrolled (Normally) ❑ Controlled ❑ The Spillway capacity must accommodate the maximum design flood. ❑ Note: Concrete dams normally incorporate an overfall or crest spillway, but embankment dams generally require a separate side- channel or shaft spillway structure located adjacent to the dam.
- 40. CHUTE SPILLWAYS Auxiliary Spillway of Tarbela Dam Service Spillway of Tarbela Dam
- 41. SHAFT SPILLWAY Shaft spillway at Ladybower Reservoir
- 42. SIPHON SPILLWAY Siphon Principle Typical Siphon Spillway
- 43. STEPPED SPILLWAY Stepped Spillway at Gold Creek Dam
- 44. SUBMERGED SPILLWAY Submerged Orifice type Spillway at Mangla Dam
- 45. Spillway Outlets and Ancillary Works ❑ Outlet Works: ❑ Controlled outlets are required to permit water to be drawn off as is operationally necessary. ❑ Provision must be made to accommodate the required penstocks and pipe works with their associated control gates or valves. ❑ Readily accommodated in body of gravity dams ❑ Provided external control structure or valve tower in embankment dams. ❑ A bottom discharge facility, of reasonably high capacity, is provided in most of dams to provide additional measure of control and to allow emptying of reservoir.
- 46. Spillway Outlets and Ancillary Works ❑ River Diversion: ❑ Necessary to permit construction to proceed in dry conditions ❑ An outlet tunnel may be adapted to this purpose during construction and subsequently employed as a discharge facility for the completed dam. ❑ Alternate of such tunnels can be cofferdams. ❑ Cut-offs: ❑ Used to control seepage around and under the flank of dams. ❑ Embankment cut-offs are generally formed by ❑ Wide trenches backfilled with rolled clay, ❑ Grouting to greater depths ❑ Grout Screen cut-offs in rock foundations
- 47. Spillway Outlets and Ancillary Works ❑ Internal Drainage: ❑ Seepage is always present within the body of dam. Seepage flows and their resultant internal pressures must be directed and controlled. ❑ In embankment dams, seepage is effected by suitably located pervious zones leading to horizontal blanket drains or outlets at base level ❑ In concrete dams vertical drains are formed inside the upstream face, and seepage is relieved into an internal gallery or outlet drain. ❑ In arch dams, seepage pressure in rock abutments are frequently drained by purpose built system of drainage adits
- 48. The tunnels inside the dam for control of seepage and monitoring structural stability Seepage Control in Concrete Dams
- 49. Spillway Outlets and Ancillary Works ❑ Internal Galleries and Shafts ❑ Galleries and shafts are provided as means of allowing internal inspection, particularly in concrete dams. ❑ These can be used to accommodate structural monitoring and surveillance purpose.
- 50. Internal gallery at concrete-gravity dam inspected by D'Appolonia. Internal Gallery in Concrete Dams
- 51. How are dams used? ▪ Dams contain outlets that allow enough water to pass through for irrigation, water supply or power generation. ▪ They may include special gates with stepped pools, locks or fish ladders for the upstream or downstream passage of migratory fish. ▪ The sluices or gates drain silt, which accumulates behind the dam. If not drained, additional pressure can occur and fill up the reservoir. ▪ Spillways discharge excess amount of water due to rain or landslides and this water is diverted alongside the dam.
- 52. The Principal stages involved in site appraisal and leading to selection of the optimum dam site and type of dam for a major project are shown in indicated schematic diagram Stages for Dam Site Appraisal
- 53. Stages for Dam Site Appraisal
- 54. Forces on Dams Primary Loads: are identified as those of major importance to all dams irrespective of type, e.g. water and related seepage loads and self weight loads Secondary Loads: are universally applicable although of lesser magnitude or alternatively are of major importance only to certain type of dams (e.g. thermal effects within concrete dams) Exceptional Load: are so designated on the basis of limited general applicability or having a low probability of occurrence. (e.g. tectonic effects, or the inertial loads associated with seismic activity)
- 55. Forces on Dams Primary Loads: are identified as those of major importance to all dams irrespective of type, e.g. water and related seepage loads and self weight load. (a): Water Load: Hydrostatic distribution of pressure with horizontal resultant force. (b): Self Weight load: Determined w.r.t. an appropriate unit weight for the material. For simple elastic analysis the resultant is considered to operate through the center of the section. (c): Seepage Loads: Equilibrium seepage patterns will establish within and under the dam, e.g. in pores and discontinuities, with resultant vertical loads identified as internal and external uplift respectively.
- 56. Forces on Dams Secondary Loads: are universally applicable although of lesser magnitude or alternatively are of major importance only to certain type of dams (e.g. thermal effects within concrete dams) (a): Sediment load: Accumulated silt etc. generate a horizontal thrust, considered as an equivalent additional hydrostatic load with horizontal resultant. (b): Hydrodynamic wave load: Transient load generated by wave action against the dam. (not normally significant) (c): Ice Load: Ice thrust may be significant in more extreme climatic conditions. (not normally significant) (d): Thermal Load: (concrete dams), Internal, generated by difference associated with changes in ambient temperatures and with cement hydration and cooling. (e): Interactive effect: Internal, arising from relative stiffness and differential deformations of dam and foundation. (f): Abutment hydrostatic load: internal seepage load in abutment rock mass ( This is of particular concern to arch and cupola dams)
- 57. Forces on Dams Exceptional Load: are so designated on the basis of limited general applicability or having a low probability of occurrence. (e.g. tectonic effects, or the inertial loads associated with seismic activity) (a): Seismic Load: Horizontal and vertical inertia loads are generated with respect to dam and the retained water by seismic disturbance. (b): Tectonic Loads: Saturation, or disturbance following deep excavation in rock, may generate loading as result of slow tectonic movements
- 58. Combination of Loads A dam is designed for the most adverse combinations of loads as have reasonable probability of simultaneous occurrence. For construction conditions dam is completed, reservoir is empty, no tail water i. With earthquake forces ii. Without earthquake forces For normal operating conditions, reservoir full, normal weather tail water conditions, normal uplifts and silt load i. With earthquake forces ii. Without earthquake forces For flood discharge conditions, reservoir at max flood level, all spillway gates open, tail water at flood levels normal uplifts and silt load
- 59. Safety Criteria Safety against Overturning. Safety against Sliding Safety against Crushing Safety against Tension Dams are not designed to take any tension load. Stabilizing Moment FOS Overturning Moment = max 6 1 W e P B B = + ( )tanW u FOS P − = min 6 1 W e P B B = − W= weight of dam u= uplift force Φ= coefficient of internal friction tanΦ= 0.8 P= Horizontal Force f= Allowable stress in concrete 2 25 30 /f kg cm= −
- 60. Numerical problem: A concrete gravity dam has the following dimensions: ▪ Max water level = 305 m ▪ Bed level of river = 225 m ▪ Crest level = 309 m ▪ D/s face slope starts at 300 m ▪ C/L of drainage galleries at 8m d/s of u/s face ▪ Uplift pressures: at Heal = 100 % at Toe = 0 % at drainage gallery = 50 %
- 61. Numerical problem: ▪ Density of concrete = 2400 kg/m3 ▪ No tail water ▪ Consider self weight, hydrostatic pressure and uplift pressure Calculate: ▪ Max. vertical stresses at heal and toe of the dam. ▪ Major principal stress at toe of dam and intensity of shear stress on horizontal plane near the toe. ▪ Check the stability of dam for the given conditions
- 62. Numerical problem: 12m α 920 m 908 m W.L 915 m 4 3 B.L. 825 m
- 65. What is a Reservoir? ▪ It is an area developed by water body due to construction of dam. Tarbela Dam JungHua Dam (Taiwan)
- 66. Classification of Reservoirs? Storage Reservoirs: Storage reservoirs are also called conservation reservoirs because they are used to conserve water. Storage reservoirs are constructed to store the water in the rainy season and to release it later when the river flow is low Flood Control Reservoirs: A flood control reservoir is constructed for the purpose of flood control. It protects the areas lying on its downstream side from the damages due to flood. Retarding Reservoirs: A retarding reservoir is provided with spillways and sluiceways which are ungated. The retarding reservoir stores a portion of the flood when the flood is rising and releases it later when the flood is receding.
- 67. Classification of Reservoirs? Detention Reservoirs : A detention reservoir stores excess water during floods and releases it after the flood. It is similar to a storage reservoir but is provided with large gated spillways and sluiceways to permit flexibility of operation. Distribution Reservoirs: A distribution reservoir is a small storage reservoir to tide over the peak demand of water for municipal water supply or irrigation. The distribution reservoir is helpful in permitting the pumps to work at a uniform rate. It stores water during the period of lean demand and supplies the same during the period of high demand Multipurpose Reservoirs: The are constructed for more than single purpose Balancing Reservoirs: A balancing reservoir is a small reservoir constructed d/s of the main reservoir for holding water released from the main reservoir.
- 68. Basic Terms and Definitions
- 69. Basic Terms and Definitions
- 70. Basic Terms and Definitions Full reservoir level (FRL): The full reservoir level (FRL) is the highest water level to which the water surface will rise during normal operating conditions. Maximum water level (MWL): The maximum water level is the maximum level to which the water surface will rise when the design flood passes over the spillway. Minimum pool level: The minimum pool level is the lowest level up to which the water is withdrawn from the reservoir under ordinary conditions. Dead storage: The volume of water held below the minimum pool level is called the dead storage. It is provided to cater for the sediment deposition by the impounding sediment laid in water. Normally it is equivalent to volume of sediment expected to be deposited in the reservoir during the design life reservoir.
- 71. Basic Terms and Definitions Live/useful storage: The volume of water stored between the full reservoir level (FRL) and the minimum pool level is called the useful storage. It assures the supply of water for specific period to meet the demand. Flood/Surcharge storage: is storage contained between maximum reservoir level and full reservoir levels. It varies with spillway capacity of dam for given design flood. Bank storage: is developed in the voids of soil cover in the reservoir area and becomes available as seepage of water when water levels drops down. It increases the reservoir capacity over and above that given by elevation storage curves. Valley storage: The volume of water held by the natural river channel in its valley up to the top of its banks before the construction of a reservoir is called the valley storage. The valley storage depends upon the cross section of the river.
- 72. Basic Terms and Definitions Yield from a reservoir: Yield is the volume of water which can be withdrawn from a reservoir in a specified period of time. Safe yield (Firm yield): Safe yield is the maximum quantity of water which can be supplied from a reservoir in a specified period of time during a critical dry year Secondary yield: is the quantity of water which is available during the period of high flow in the rivers when the yield is more than the safe yield. Average yield: The average yield is the arithmetic average of the firm yield and the secondary yield over a long period of time. Design yield: The design yield is the yield adopted in the design of a reservoir. The design yield is usually fixed after considering the urgency of the water needs and the amount of risk involved.
- 73. Storage Capacity of Reservoirs Whatever may be the use of a reservoir, its most important function is to store water during floods and to release it later. The storage capacity of a reservoir is, therefore, its most important characteristics. The available storage capacity of a reservoir depends upon the topography of the site and the height of dam. To determine the available storage capacity of a reservoir up to a certain level of water, engineering surveys are usually conducted. The storage capacity and the water spread area at different elevations can be determined from the contour map
- 74. Storage Capacity of Reservoirs (a) Area-Elevation Curve: From the contour plan, the water spread area of the reservoir at any elevation is determined by measuring the area enclosed by the corresponding contour. Generally, a planimeter is used for measuring the area. An elevation-area curve is then drawn between the surface area as abscissa and the elevation as ordinate. (b) Elevation-Capacity Curve: The storage capacity of the reservoir at any elevation is determined from the water spread area at various elevations. An elevation- storage volume is plotted between the storage volume as abscissa and the elevation as ordinate. Generally, the volume is calculated in Mm3 or M ham. The following formulae are commonly used to determine the storage Capacity.
- 75. Storage Capacity of Reservoirs It is the usual practice to plot both the elevation-area curve and the elevation- storage curve on the same paper.
- 76. Storage Capacity of Reservoirs The following formulae are commonly used to determine the storage capacity 1. Trapezoidal formula: According to the trapezoidal formula, the storage volume between two successive contours of areas A1 and A2 is given by where h is the contour interval. Therefore the total storage volume V is where n is the total number of areas.
- 77. Storage Capacity of Reservoirs 2. Cone formula: According to the cone formula, the storage volume between two successive contours of areas A1 and A2 is given by ◼ 3. Prismoidal Formula: According to the prismoidal formula, the storage volume between three successive contours is given by The prismoidal formula is applicable only when there are odd numbers of areas (i.e. n should be an odd number). In the case of even number of areas, the volume up to the second last area is determined by the prismoidal formula, and that of the last segment is determined by the trapezoidal formula.
- 78. Storage Capacity of Reservoirs 4. Storage Volume from cross-sectional areas: In the absence of adequate contour maps, the storage volume can be computed from the cross-sectional areas of the river. Cross-sectional areas (a1, a2 ... etc) are obtained from the cross sections of the river taken upstream of the dam up to the u/s end of the reservoir at regular interval d. The volume is determined from the prismoidal formula. The formula is applicable for odd number of sections
- 79. Reservoir Sedimentation Reservoir sedimentation is filling of the reservoir behind a dam with sediment carried into the reservoir by streams. The flow of water from the catchment upstream of a reservoir is capable of eroding the catchment area and of depositing material either upstream of the reservoir, or in the still water of the reservoir. The nature of the material in the catchment area and the slope of the catchment area and the inlet streams are a factor, as is the nature of the ground cover. Heavy rainfall falling on erodible material on a steep slope with little ground cover resulting from overgrazing or wildfire is a recipe for substantial sediment transport and significant reservoir sedimentation
- 80. Reservoir Sedimentation The loss of storage is only one deleterious effect of sedimentation in reservoirs; others are Increased flood levels upstream of the reservoirs Retrogression of river bed and water levels downstream of the dam The elimination of nutrients carried by the fine sediment Effect on reservoir water quality The reservoir capacity can be preserved by Minimizing the sediment input into the reservoir Maximizing the sediment through flow The recovery of storage For Preliminary studies, Trap Efficiency Curve is used (Brune 1953)
- 82. Sediment Management Maximum efforts should water should be released so that less sediments should retain in reservoir. Following options are: ❑ Catchment Vegetation ❑ Construction of coffer dams/low height barriers ❑ Flushing and desilting of sediments ❑ Low level outlets / sediment sluicing
- 84. Wooden barriers
- 85. Wooden barriers
- 86. Stepped watershed for sediment control
- 87. Flushing of sediments from reservoir
- 88. Mechanical desilting from reservoir