Unit 4 Reservoir Planning Part One1.pptx

Details about Reservoir Planning

1. Presented By : Prof. S. R. Satpute Indira College of Engineering and Management Department Of CIVIL Engineering Unit 4 Reservoir Planning Hydrology & water resources engineering 1

2. The water required for drinking purposes, Industrial purposes, Irrigation, Navigation and generation of hydropower (Electricity) is mostly drawn from the surface source (i.e. river or lake). During low flow in the river (ex. Summer season, Drought), it may not possible to meet the above demands of the consumers. Reservoir is , an enlarged natural or artificial lake, pond or   impoundment created using a dam to store water. Water storage reservoirs may be created by constructing a dam across a river, along with suitable appurtenant structures . INTRODUCTION 2

3. Reservoirs are also meant to absorb a part of flood water and the excess is discharged through a spillway. Impounding reservoirs, into which a river flows naturally. Impounding or storage reservoirs are intended to accumulate a part of the flood flow of the river for use during the non-flood months. Service or balancing reservoirs, which receiving supplies that are pumped or channeled into them artificially. These are relatively small in volume because the storage required by them is to balance flows for a few hours or a few days at the most . INTRODUCTION 3

4. TYPES OF RESERVOIRS Storage Reservoirs Flood Mitigation or control reservoirs Distribution reservoirs Multipurpose Reservoirs 4

5. Storage Reservoirs or Conservation Storage reservoir is also called conservation reservoir because they are used to conserve water. The pool of water created by the construction of a dam across a river is called reservoir. When the reservoir remains in water supply during the rainy season and releases it during dry period as per requirement, it is called storage or impounded reservoir . 5

6. 6 Storage Reservoirs

7. Flood Mitigation or Control reservoirs Flood control reservoirs collect water at times of high flow and then release it slowly over the course of the flood and during the subsequent days or weeks. It is a reservoir constructed to store the portion of the water so as to protect the area on the downstream side from flood damage. Flood control reservoir are further sub classified as i ) Detention basins reservoirs ii) Retarding reservoirs 7

8. 8 Flood Mitigation Reservoir

9. Distribution reservoirs or service Distribution reservoirs, also called service reservoirs, are the storage reservoirs, which store the treated water for supplying water during emergencies (such as during fires, repairs, cleaning etc.) and also to help in absorbing the hourly fluctuations in the normal water demand . It is small storage reservoir constructed for the supply of water in a city during there emergencies need and satisfies the varying demand by the ultimate consumers during various parts of the day. 9

10. 10 Distribution reservoirs or service Reservoir

11. Multipurpose Reservoirs It is a reservoir that serves two or more purposes. The term multipurpose reservoir includes all reservoirs actually designed and operated to serve more than one function. The most common purposes of these reservoirs are to generate hydroelectric power, provide flood control, store water , irrigation, drinking purpose, industrial purpose and navigation. 11

12. 12 Bhakhra - Nangal Project  ( Punjab, Haryana, Rajasthan ) (Multipurpose Reservoir)

13. Investigations for Reservoir Engineering Surveys Geological Investigations Hydrological Investigations 13

14. Engineering Surveys Conducted for dam, reservoir and other associated work. Topographic survey of the area is carried out and the contour plan is prepared. Th e hori z o n t al c o n t r o l i s usu a l l y p r o vided b y tr i a ngul a t i o n survey and the vertical control by precise leveling. Contour map gives the following information. 1) Water spread 2) Capacity of the reservoir 3) Suitable dam site 4) Site for waste weir and outlets 5) Storage Elevation Curve 6 ) Arrangement of lines of communication 7) Map of t h e a r ea t o ind i ca t e the land p r o pert y t o be surveyed. 14

15. Geological investigation 15 Geological investigations is done in two stages. Reconnaissance Survey and surface mapping: In this survey inspection of region is done and data collected physically without using precise survey instrument. Sub surface exploration: The surface and subsurface investigation or site exploration is carried out to collect the information about physical properties and characteristics of the subsoil material as well as the details of other geological features of the site area.

16. Geological investigation Geological investigations of the dam and reservoir site are done for the following purposes. Suitability of foundation for the dam Water tightness of the reservoir basin or permeable pockets Faulty structure in the basin Type and depth of overburden to be excavated Ground water condition of region Location of the quarry sites for the construction materials and their quantities. 16

17. Hydrological investigations The hydrological investigations are conducted fo r the following purposes : i ) To study the runoff pattern of the river or stream and to compute the storage capacity ii) To determine the maximum discharge at the site iii) To determine the hydrograph of the worst flood 17

18. Site Selection for a Reservoir Large storage capacity Water tightness of reservoir River valley should be narrow, length of dam to constructed is less Good hydrological conditions Deep reservoir or water-spread area should be less Land under Submergence should be minimum Silt free water or Low silt inflow Q uality of water reasonably good or No objectionable minerals Low cost of real estate and Site easily accessible 18

19. Zones in the Reservoir Storage Full Reservoir Level (FRL): It is the maximum normal level that can be conveniently stored in the reservoir under normal operating condition. It is the level corresponding to the storage which includes both inactive and active storages and also the flood storage , this is the highest reservoir level that c an be mai n t ained with o u t spillway discharge without passing water downstream to sluice ways. Minimum water Level ( Min. WL ): It is the lowest level in the reservoir to which water from the reservoir can be withdrawn. (Minimum pool level) 19

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21. Maximum Water Level ( Max. WL ): This is the maximum level in the reservoir to which the water will rise. This is the water level that is ever likely to be attained during the passage of the design flood. It depends upon the specified initial reservoir level and the spillway gate operation rule. This level is also called sometimes as the Highest Reservoir Level or the Maximum Pool Level. Zones in the Reservoir Storage 21

22. Minimum Drawdown Level (MDDL): It is the level below which the reservoir will not be drawn down so as to maintain a minimum head required in power projects . Outlet Surcharge or Flood S torage : This is required as a reserve between Full Reservoir Level and the Maximum Water level to contain the peaks of floods that might occur when there is insufficient storage capacity for them below Full Reservoir Level . Zones in the Reservoir Storage 22

23. Zones in the Reservoir Storage Live Storage or Useful Storage: This is the storage available for the intended purpose between normal pool level and minimum pool level of the reservoir. This may also be termed as the volume of water actually available at any time between the Dead Storage Level and Full Reservoir Level. Dead Storage Level (DSL): Below this level, there are no outlets to drain the water in the reservoir by gravity. The volume of water below the minimum pool level that cannot be utilized. 23

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25. Zones in the Reservoir Storage 25 Dead storage: It is the total storage below the invert level of the lowest discharge outlet from the reservoir. It may be available to contain sedimentation, provided the sediment does not adversely affect the lowest discharge. Buffer Storage: This is the space located just above the Dead Storage Level up to Minimum Drawdown Level. As the name implies, this zone is a buffer between the active and dead storage zones and releases from this zone are made in dry situations to cater for essential requirements only. Dead Storage and Buffer Storage together is called Interactive Storage.

26. Zones in the Reservoir Storage Bank Storage: it is the volume of water that is stored temporarily in the permeable reservoir banks. Valley storage: it is the volume of water that can be held by natural river channel on the soil above it. 26

27. Storage Capacity and Yield 27 Yield : It is the amount or quantity of water that can be supplied from the reservoir in a specified interval of time which is chosen for the design varies from a day for small distribution reservoirs to a year for large conservation reservoirs. Safe yield (firm): It is the maximum amount or quantity of water that can be guaranteed during a worst or critical dry period. Secondary Yield: It is the quantity of water available in excess of safe yield during periods of high flood flows. Average yield: It is the arithmetic average of the firm and secondary yield over a long period of time .

28. Fixing the capacity of the Reservoirs In general, storage capacities have to be designed based on certain specified considerations, which have been discussed separately in the flowing Bureau of Indian Standard codes: IS:5477 Fixing the capacities of reservoirs (part 1): 1999 General requirements (part 2): 1994 Dead Storage (part 3): 1969 Live Storage ( part 4): 1911 Flood Storage 28

29. Fixing the capacity of the Reservoirs Precipitation, Runoff and silt records Erosion of Catchment Losses in the reservoir Trap efficiency Water Demand Density and location of outlets Economic analysis Engineering and Geological Aspects 29

30. Mass Inflow Curve It is a plot of accumulated flow in a stream against time . 30

31. Demand Curve 31 It is a plot of accumulated demand against the time.

32. Computation of Reservoir capacity 32 Depends upon the inflow available and demand . I n flo w i n the ri v er i s al w a y s g r e a t er th a n the demand, there i s storage required . If the inflow in the river is small but the demand is high, a large reservoir capacity is required . The required capacity for a reservoir can be determined by the following methods : Graphical method, using mass curves Analytical method

33. Graphical M ethod (MASS CURVE) 33 Prepare a mass inflow curve from the flow hydrograph of the site for a number of consecutive years including the most critical years (or the driest years) when the discharge is low. P r ep a r e the mass dema n d cu r v e c or r espo n di n g t o t h e given rate of demand. If the rate of demand is constant, the mass demand curve is a straight line. The scale of the mass demand curve should be the same as that of the mass inflow curve. Draw the lines AB, FG, etc . Such that ( i ) they are parallel to the mass demand curve and (ii ) they are tangential to the crests A, F, etc. of the mass curve.

34. 34 Reservoir Capacity Determination

35. Determine the vertical intercepts CD, HJ etc. between the tangential lines and the mass inflow curve. These intercepts indicate the volumes by which the inflow volumes fall short of demand. Assuming that the reservoir is full at point A, the inflow volume during the period AE is equal to ordinate DE and the demand is equal to ordinate CE. Thus the storage required is equal to the volume indicated by the intercept CD. Determine the largest of the vertical intercepts found in step (4). The largest vertical intercept represents the storage capacity required. 35

36. Prepare the mass inflow curve. Draw lines from a common origin, representing demands at various rates, say ranging from to 5000 ha-m per year. From the apices A 1 , A 2, A 3 etc. of the mass curve draw tangents in such a way that their maximum departure from the mass curve does not exceed the specified reservoir capacity. 36 Determination of Safe Yield from a Reservoir of given Capacity

37. Thus in given fig ure the ordinates E1 D1, E2 D2, E3 D3 etc . are all equal to the reservoir capacity (say 1500 ha-m). Measure the slopes of each of these tangents this slopes indicate the yield which can be attained in each year from the reservoir of given capacity . The demand line if extended in forward direction should intersect the mass curve. 37 Determination of Safe Yield from a Reservoir of given Capacity

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39. RESERVOIR SEDIMENTATION Sediment is defined as the fragmental material either transported by suspended in or deposited by the water. All rivers carry silt during heavy rains due to erosion from catchments. Disintegration, erosion, transportation and sedimentation are the different stages leading to silting of reservoir. It is a difficult problem for which an economical solution has not yet been discovered, except by providing a “dead storage” to accommodate the deposits during the life of the dam . 39

40. 40 Reservoir Sedimentation

41. Factors Affecting S edimentation 41 The quality of sediment depends upon the extent of erosion in the in the catchment area, which depends upon following factors: Nature and type of soil in catchment area Topography of the catchment area Cultivation or vegetation cover in catchment area Intensity of rainfall in catchment area

42. 42 Sedimentation

43. Sediment Management 43 Maximum efforts should water should be released so that less sediments should retain in reservoir are as f ollow s : Soil conservation of drainage basin Flushing and de-silting of sediments or Mechanical stirring Catchment Vegetation cover will minimize impact of rain drops Retarding overland flow by terraces, contour bunding etc. Con s truction of c o f f er dams ( a w a t e rt ig h t e n closu r e pu m p e d d r y to permit construction work below the waterline ) low height barriers / Low level outlets / sediment sluicing(sliding gate)

44. 44 Methods of Reducing Sediments

45. 45 Methods of Reducing Sediments

46. 46 Methods of Reducing Sediments

47. 47 Wooden barriers Stepped watershed for sediment control Methods of Reducing Sediments

48. Procedure for calculation reservoir life 48 The useful life of reservoir is terminated when its capacity is reduced to 20% of the designed capacity. This will occur over a time period when the dead storage is reduced by siltation. Probable rate of siltation should be considered in reservoir planning. Knowing the inflow rate calculate the (capacity/inflow) ratio and obtain the trap efficiency from the curve. Divide the total capacity into any suitable interval, say 10 %. Assuming the 10% capacity has been reduced due to sediment deposit, find the trap efficiency for reduced capacity (i.e. 90% of the original) and the inflow ratio .

49. For this interval of 10% capacity, find the average trap efficiency by taking average if η found in step 2 and 3. Determine the sediment inflow rate water samples and drying the sediment. Multiply the total annual sediment transported by the trap efficiency found in step 3. Divide the volume interval by the sediment deposited to get the number of years to fill this volume interval of 10% capacity. Repeat the procedure for further intervals of the capacity. The total life of the reservoir will be equal to the total number of years required to fill each of the volume intervals. 49 Procedure for calculation reservoir life

50. 50 To be continue……