This document discusses reservoir planning and design. It describes how reservoirs are created by constructing dams across rivers. Investigations including engineering surveys, geological studies, and hydrological analyses are conducted. Reservoirs have different levels like full reservoir level and minimum drawdown level. Storage zones include live, dead, and flood storage. Methods to determine reservoir capacity and yield using mass inflow and demand curves are presented. Factors affecting reservoir sedimentation and management techniques are outlined. Flow routing methods like graphical and trial and error are described to model flood waves passing through reservoirs. Spillway types including free overfall are also summarized.
21. Reservoir capacity:
Depends upon the inflow available and demand
Inflow in the river is always greater than the
demand, there is no storage required
If the inflow in the river is small but the demand
is high, a large reservoir capacity is required
22. The required capacity for a reservoir
can be determined by the following
methods:
1. Graphical method, using mass curves.
2. Analytical method
23. Graphical method-
1. 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.
2. Prepare the mass demand curve corresponding to the
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.
24.
25. 3. 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.
4. 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.
26. •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.
5. Determine the largest of the vertical intercepts found
in step (4).
• The largest vertical intercept represents the storage
capacity required.
30. Procedure for calculation reservoir life:
•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.
•For this interval of 10% capacity, find the average trap
efficiency by taking average if η found in step 2 and 3.
31. •Determine the sediment inflow rate by taking
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.
33. Reservoir Sedimentation:
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.
Disintegration, erosion, transportation, and
sedimentation, are the different stages leading to
silting of reservoir.
34.
35. Causes of sedimentation:
Nature of soil in catchment area
Topography of the catchment area
Cultivation in catchment area
Vegetation cover in catchment area
Intensity of rainfall in catchment area
36. 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(a watertight enclosure pumped dry to
permit construction work below the waterline)/low height barriers
• Flushing and desilting of sediments
• Low level outlets / sediment sluicing (sliding gate)
45. Flow 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
46. Why to route flows?
Account for changes in flow hydrograph as a flood wave passes
downstream.
This helps to:
Calculate storages
Studying the attenuation of flood peaks
Q
t
47. 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.
Some of the important methods which have a more practical bearing
are:
• Calculus method
• step by step method: (1) Graphical method (2) Trial and error
method
48. Graphical method / 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.
49.
50. •From this quantity the value of Q2 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.
51. •From the known value of Q1 , the value of (S1-
Q1/2∆T) is read from Q verses (S-Q/2 ∆T) curve.
•This value is added to (I1+I2/2)∆t to give
(S2+Q2/2∆T).
• Entering the graph with this value of (S2+Q2/2 ∆T)
, the value of Q2 is read out from Q verses
(S+Q/2∆t) curve.
• The value of Q2 thus determined becomes Q1 for
the next time interval.
52.
53. •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.
•Similarly, the time to peak in the outflow hydrograph 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.