5.1 reservoir planning

Reservoir regulation, Flood routing- Graphical or I.S.D method, Trial and error method, Reservoir losses, Reservoir sedimentation- Phenomenon, Measures to control reservoir sedimentation, Density currents Significance of trap efficiency, Useful life of the reservoir, Costs of the reservoir, Apportionment of total cost, Use of facilities method, Equal apportionment method, Alternative justifiable expenditure method.

1. Presented By : Pradeep T. Kumawat BE CIVIL, ME Geo-Tech. Late G. N. Sapkal College of Engineering Department Of CIVIL Engineering Unit 5 (Part II) Reservoir Planning Hydrology & water resources engineering 1

2. 2 5.1 Reservoir Planning

3. Silting of reservoirs means the deposition of silt and clay. This mainly occurs when runoff occurs. Runoff contain smaller soil particles such as silt, clay , fine sand which may deposited at the bed of reservoir, which reduces the storage capacity of reservoir. When these particles reaches into reservoir tries to settle down due to action of gravity, coarser one settle down while fine remain suspension therefore it is very essential to check silting of dam and water stored in reservoir. Sediment transported by the river can be divided into two heads: The Suspended Load is kept in suspension because of the vertical component of the eddies formed due friction of flowing water against bed. The Bed Load is dragged along the bed of the stream and is about 10 to 15% of suspended load. 3 INTRODUCTION

4. Methods of Preserving Storage Capacity of Reservoir 4 Catchment vegetation

5. Mechanical desilting from R eservoir 5 Sediment sluicing

6. Measurement of S ediments from R eservoir 6 Measurement of Sediment by sampling method: Collect water sample from various depths of reservoir in bottles. Then filtered sample and measure dried silt in the units of ‘ppm ’ Sediment load in ppm = (Weight of dry sediment / weight of sediment and water sample ) × 10^6 The reservoir sedimentation is measured in terms of its “Trap Efficiency” (η ).

7. R eservoir T rap E fficiency C urve 7 The trap efficiency of a reservoir is the percentage of incoming sediment which is trapped by the reservoir.

8. Density Current 8 It is defined as a gravity flow of one fluid under another fluid having a slightly different density. T he  density current will separate from the sloping bed and form an intrusion or interflow . Rate of sedimentation The  normal recommended rate of sediment or silt is about 200 to 500 m3/sec per year per square km area.

9. Sedimentation of Reservoirs The following points need careful consideration as regards design of reservoir and dam. Assessment of deposition of the silt in the reservoir . Determination of the life of reservoir. The sediment concentration varies over 200 gram per litre to 1600 gram per litre . The yearly loss of storage due to sedimentation in reservoir. 9

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11. Selection of site for Dam Topography of Catchment Geology and foundation conditions Size of Reservoir and Catchment Area Availability of Construction materials Spillway size and location Length and Height of Dam Life of Dam 11

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14. Selection of site for Dam If sand, gravel and crushed stone is available, a concrete gravity dam is preferred. If coarse and fine grained soils are available , an earth dam is suitable . If foundation consists of sound rock with no fault and fissures, any type of can be constructed. Rocks like Granite, Gneiss and Schist are suitable for Gravity Dams. Poor rock or gravel foundations are suitable for Earth dams and Rock fill dams. Earth dam with a separate spillways are suitable for low rolling plains . 14

15. Selection of site for Dam An Arch dam is suitable for a low narrow V-shaped valley. If large discharge is required, concrete gravity dam is preferable. If no other site is available for spillway, earth dam with central overflow section of concrete. Concrete or masonry gravity dams have very long life. Earth and Rockfill dams have intermediate life . If the length of dam is small but height is more, gravity dam is preferred. If a roadway is to be passed over top of the dam, an earth or gravity dam is preferred . 15

16. Economic Height of A dam The economic height of the dam is that one for which the cost of the dam per unit volume of water stored is minimum. The procedure adopted for determining the economic height is the elevation storage curve. 16

17. Reservoir Losses F ollowing seven major causes of losses of water from reservoir: Infiltration Seepage Evaporation from water surface Watershed leakage Interception Transpiration Soil evaporation 17

18. Sp i ll w a y s P assa g es c on s truc t ed e i th e r w ith i n a d am or i n t h e periphery of the reservoir to safely pass the excess of the river during flood flows are called Spillways . The capacity of a spillway is seen to depend upon the following major factors : The inflow flood The volume of storage provided by the reservoir Crest height of the spillway Gated or ungated 18

19. Free Over - fall Spillway In this type of spillway, the water freely drops down from the crest, as for an arch dam. It can decked v ert i c al al s o b e p r ovi d e d f o r a o v er fl o w da m wi t h a or ad v e r s e i n c l ined downstream face. 19

20. Overflow Spillway: The overflow type spillway has a crest shaped in the form of an ogee or S-shape. The upper curve of the ogee is made to conform closely to the profile of the lower nappe of a ventilated sheet of water falling from a sharp crested weir. 20

21. Chute Spillway : It is one whose discharge is conveyed from the reservoir to the downstream l e v el th r oug h an channel, p lac e d ri v e r open either along a da m a butme n t or through a saddle. 21

22. Side channel Spillway : A side channel spillway is one in which the control weir is placed approximately parallel to the upper portion of the discharge channel. 22

23. Shaft Spillway : A Shaft Spillway is one where water enters over a horizontally positioned lip, drops through a vertical or sloping shaft, and then flows to the downstream river channel through a horizontal or nearly horizontal conduit or tunnel. 23

24. Tunnel Spillway : Where a closed channel is used to convey the discharge around a dam through the adjoining hill sides, the spillway is often called a tunnel or conduit spillway. The closed channel may take the form of a vertical or inclined shaft, a horizontal tunnel through earth or rock, or a conduit constructed in open cut and backfilled with E art h materials. 24

25. Siphon Spillway : A siphon spillway is a closed conduit system formed in the shape of an inverted U, positioned so that the inside of the bend of the upper passageway is at normal reservoir storage level. Siphon spillways comprise usually of five components, which include an inlet, an upper leg, a throat or control section, a lower leg and an outlet. 25

26. Fl ood Routing Procedure to determine the flow hydrograph at a point on a watershed from a known hydrograph upstream As the hydrograph travels, it Attenuates Gets delayed Q t Q t Q t Q t 26

27. Why to route flows? t Account for changes in flow hydrograph as a flood wave passes d o wn s t r eam. This helps to: Calculate storages Studying the attenuation of flood peaks Q 27

28. Method of flood routing It is the process of calculating water levels in reservoir, the storage quantities and outflow rates corresponding to a particular inflow hydrograph at various instants. Flood routing is carried out in a reservoir to determine what will be maximum rise in its water surface and what will be the discharge in the downstream channel when particular flood passes through it. Som e of the imp o r t a n t m e thods which h a v e a mo r e p r acti c al bearing are : Calculus method or mathematical method S t ep b y S t ep M e th o d or Trial and Error Method ISD (inflow storage discharge) HM Cheng’s Method Woodward Method 28

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30. Inflow Storage Discharge method The inflow storage discharge method was first developed by L.G puls of U.S army corps of engineers. According to this method In the above equation all the quantities on left hand side are known and hence the quantity is determined. 30

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32. Fr om this quantity the value of Q 2 can be found out from the storage-discharge relationship as explained below. From the available storage discharge curve, the curves of (S-Q/2 ∆T) verses Q and (S+Q/2 ∆T) verses Q are developed. Such curves are called routing curves . F r o m th e kn o wn v a l u e o f Q 1 , the v al u e o f (S 1 - Q 1 /2∆T) is read from Q verses (S-Q/2 ∆T) curve . 32

33. Thi s v alue i s add e d t o ( I 1 +I 2 /2)∆ t t o gi v e (S 2 +Q 2 /2∆T). Entering the graph with this value of (S 2 +Q 2 /2 ∆T) , the value of Q 2 is read out from Q verses (S+Q/2∆t) curve. The value of Q 2 thus determined becomes Q 1 for the next time interval . 33

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35. If the inflow and outflow hydrographs are now plotted on the same time scale, it will be observed that the peak flow of outflow hydrograph is less than the peak flow of the inflow hydrograph. Simil a rl y , the t i me t o pea k i n the ou t flo w h y d r og r aph is more than the time to peak in the inflow hydrograph. These are the effects of reservoir storage on the movement of flood wave through the reservoir. The reduction of peak is known as the attenuation and the difference in times to peak is known as the reservoir lag . 35

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38. 38 Benefit-Cost Ratio Analysis

39. 39 System Approach

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42. Alternative Cost Allocation Methods 42

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44. 44 Every drop of water matters!!!