Note for Hydraulics and Irrigation Design - HID By shivaraj k

Topics ( 6 )

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Hydraulic Structures and Irrigation design & drawing 10CV65 VTU SOLUTION PAPERS UNIT 1 Reservoir Planning 1. Explain the investigation for reservoir planning? (June 2012, June 2013) Investigations for Reservoir The following investigations are usually conducted for reservoir planning. 1. Engineering surveys 2. Geological investigation 3. Hydrologic investigations 1. Engineering surveys Engineering surveys are conducted for the dam, the reservoir and other associated works. Generally, the topographic survey of the area is carried out and the contour plan is prepared. The horizontal control is usually provided by triangulation survey, and the vertical control by precise levelling. (a) Dam site For the area in the vicinity of the dam site, a very accurate triangulation survey is conducted. A contour plan to a scale of 1/250 or 1/500 is usually prepared. The contour interval is usually 1 m or 2 m. The contour plan should cover an area at least upto 200m upstream and 400m downstream and for adequate width beyond the two abutments. (b) Reservoir For the reservoir, the scale of the contour plan is usually 1/15,000 with a contour interval of 2 m to 3 m, depending upon the size of the reservoir. The area-elevation and storageelevation curves are prepared for different elevations upto an elevation 3 to 5m higher than the anticipated maximum water level (M.W.L). 2. Geological investigations Geological investigations of the dam and reservoir site are done for the following purposes. (i) Suitability of foundation for the dam. (ii) Watertightness of the reservoir basin (iii) Location of the quarry sites for the construction materials. 3. Hydrological investigations The hydrological investigations are conducted for the following purposes : (i) To study the runoff pattern and to estimate yield. Dept of Civil Engg, SJBIT Page 1

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Hydraulic Structures and Irrigation design & drawing 10CV65 (ii) To determine the maximum discharge at the site. i.Run off pattern and yield The most important aspect of the reservoir planning is to estimate the quantity of water likely to be available in the river from year to year and seasons to season. For the determination of the required storage capacity of a reservoir, the runoff pattern of the river at the dam site is required. If the stream gauging has been done for a number of years before the construction of the dam, the runoff pattern will be available from the record. It is generally assumed that the runoff pattern will be substantially the same in future also. The available record is used for estimating the storage capacity. The inflow hydrographs of two or three consecutive bad years when the discharge is low are frequently used for estimating the required capacity. However, if the stream gauging records are not available, the runoff and yield have to be estimated indirectly by the empirical (or) statistical methods. These are : (i) Runoff expressed as a percentage of rainfall. (ii) Runoff expressed as a residual of rainfall after deducting losses due to evaporation, transpiration and ground water accretion. (iii) Run off expressed as a function of mean annual temperature and rainfall. (ii) Maximum discharge The spillway capacity of the dam is determined from the inflow hydrograph for the worst flood when the discharge in the river is the maximum. Flood routing is done to estimate the maximum outflow and the maximum water level reached during the worst flood. The methods for the estimation of the maximum flood discharge are: (i) Empirical relations mostly correlated with the catchment area (ii) Statistical methods (iii) Unit hydrograph method (iv) Flood frequency studies Usually for big reservoirs, a 1000 years flood is taken for spillway design. Selection of Site for a Reservoir A good site for a reservoir should have the following characteristics: 1. Large storage capacity The topography of the site should be such that the reservoir has a large capacity to store water. 2. Suitable site for the dam A suitable site for the dam should exist on the downstream of the proposed reservoir. There should be good foundation for the darn The reservoir basin should Dept of Civil Engg, SJBIT Page 2

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Hydraulic Structures and Irrigation design & drawing 10CV65 have a narrow opening in the valley so that the length of the dam is small. The cost of the dam is often a controlling factor in the selection of a site for the reservoir. 3. Watertightness of the reservoir The geological conditions of the reservoir site should be such that the reservoir basin is watertight. The reservoir sites having pervious rocks are not suitable. The reservoir basins having shales, slates, schists, gneiss, granite, etc. are generally suitable. 4. Good hydrological conditions The hydrological conditions of the river at the reservoir site should be such that adequate runoff is available for storage. The catchment area of the river should give high yield. There should not be heavy losses in the catchment due to evaporation, transpiration and percolation. 5. Deep reservoir The site should be such that a deep reservoir is formed after the construction of the dam. A deep reservoir is preferred to a shallow reservoir because in the former the evaporation losses are small, the cost of land acquisition is low and the weed growth is less. 6. Small submerged area The site should be such that the submerged area is a minimum. It should not submerge costly land and property. It should not affect the ecology of the region. Monuments of historical and architectural importance should not be submerged. 7. low silt inflow The life of the reservoir is short if the river water at the site has a large quantity of sediments. The reservoir site should be selected such that it avoids or excludes the water from those tributaries which carry a high percentage of silt. 8. No objectionable minerals The soil and rock mass at the reservoir site should not contain any objectionable soluble minerals which may contaminate the water. The stored water should be suitable for the purpose for which the water is required. 9. Low cost of real estate The cost of real estate for the reservoir site, dam,dwellings, roads. railways, etc. should be low. 2. Explain the zones of storage in a reservoir? (July 2012,June2013) A large number of terms are commonly used for reservoir planning. These terms are defined below. It may be noted that various terms are sometimes used to indicate the same quantity. 1. 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. The effective storage of the reservoir is computed upto the full reservoir level. The FRI is the highest level at which water is intended to be held for various uses without any passage of water Dept of Civil Engg, SJBIT Page 3

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Hydraulic Structures and Irrigation design & drawing 10CV65 through the spil1way. In case of dams without spillway gates, the FRL is equal to the crest level of the spillway [Fig 3.5(a)]. However, if the spillway is gated, the FRL is equal to the level of the top of the gates. 2. The full reservoir level is also called the full tank level (FTL) or the normal pool level(NPL). Normal conservation level (NCL) It is the highest level of the reservoir at which water is intended to be stored for various uses other than flood. The normal conservation level is different from the FRL as the latter may include a part of the flood. However, if there is no storage for flood upto FRL, the normal conservation level and the FRL become identical. 2. 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. The maximum water level is higher than the full reservoir level so that some surcharge storage is available between the two levels to absorb flood.The maximum water level is also called the maximum pool level (MPL) or maximum flood level (MFL). 3. Minimum pool level The minimum pool level is the lowest level up to which the water is withdrawn from the reservoir under ordinary conditions. The minimum pool level generally corresponds to the elevation of the lowest outlet (or sluiceway) of the dam. However, in the case of a reservoir for hydroelectric power, the minimum pool level is fixed after considering the Dept of Civil Engg, SJBIT Page 4