reservoirs 1
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WATER RESOURCES ENGINEERINGWATER RESOURCES ENGINEERING
RESERVOIRSRESERVOIRS
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Introduction
Reservoirs are structures that store water.
In general, we observe high flow in winter and low flow
summer, and very high values in spring months or
snowmelt seasons in Northern Hemisphere.
On the other hand, the water demand is high in summand low in winter.
Therefore, the regulation of the streamflow is required
meet the demands. This regulation is possible by constructing reservoirs i
the stream.
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Introduction(conti
S, D
S
D1
t
Required Storage
Variation of Supply and Demand
S>DS
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Introduction
Reservoirs can be divided into two main categories
according to their storage capacities:
Storage (conservation) reservoirs
Distribution (service) reservoirs
Formation of a big reservoir behind a dam may have
various environmental aspects.
For example: For the construction of Keban Dam in Turkey,
30,000 people were dislodged, and
300 km road, 48 km railway, and
A large area were expropriated.
(cont
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Overview
Introduction
Characteristics of Reservoirs
Determination of Reservoir Capacity Mass Curve Analysis
Determination of reservoir capacity for a known yield Determination of yield from a known reservoir capacity
Sequent Peak Analysis
Operation Study Other Approaches in Capacity Determination
Reservoir Sedimentation
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Characteristics of Reservoirs
Potential for a Hydropower Reservoir
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Characteristics of Reservoirs(conti
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Characteristics of Reservoirs(cont
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Characteristics of Reservoirs
The capacity of the reservoir is very important since th
main function of the reservoir is the storage of the wat
The desired yield, which is the amount of water delive
from a reservoir to meet downstream requirements, mbe available most of the time.
For an ideal reservoir site: Sufficient impervious and sound formation (bed and sid
Deep and narrow valley (less evaporation, low cost of
expropriation)
(cont
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Characteristics of Reservoirs
After the determination of location of a reservoir, a
special topographic map (1:5000 scaled) of the
reservoir area is obtained with suitable contour intervaand
Elevation - Area Curve, and
Elevation Storage Curve are obtained.
(cont
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Characteristics of Reservoirs(conti
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Characteristics of Reservoirs(cont
Elevation Area Volume Curves for a reservoir
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Characteristics of Reservoirs(cont
Storage characteristics of a reservoir
Overview
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Overview
Introduction
Characteristics of Reservoirs
Determination of Reservoir Capacity Mass Curve Analysis
Determination of reservoir capacity for a known yield
Determination of yield from a known reservoir capacity
Sequent Peak Analysis
Operation Study Other Approaches in Capacity Determination
Reservoir Sedimentation
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DeterminationCapacity
The yield of a reservoir is defined as the quantity of
water, which is supplied for a certain duration.
The duration may change with the purpose of thereservoir.
A few years Large reservoirs
Week Day
Hour
The yield is a function of
the inflow, and
the capacity of the reservoir.
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Capacity
Safe Yield (Firm Yield): The amount of water that is
supplied for a critical period. It is a guarantied amount
during this critical period. Critical Period: The duration of lowest flow observed in
the records of the stream.
Depending upon the length of the flow critical period m
change so the safe yield.
Yield is not calculated certainly. The probability must bused.
During the periods of high flow there will be extra
available water, more than the safe yield which is callesecondary yield.
(conti
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Capacity
Average Yield: The arithmetic average of the safe and
secondary yields over a long period.
There is a risk involved for a reservoir.
The amount of risk depends on the purpose of the
reservoir.
Target Yield: The yield determined based on the
estimated demands for a reservoir.
(conti
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Capacity
For the determination of reservoir capacity, the critical
period must be determined first.
A long period of observed flow is required.
When short period of observed flows or no observatio
area available stochastic methods are used to genera
synthetic flows that has the same statistical propertiessuch as mean, variance, correlations etc.
(conti
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Capacity
There are four different methods to determine the
capacity if a reservoir.
Mass Curve (Ripple diagram) Method,
Sequent Peak Algorithm,
Operation Study, and
Other Approaches (Stochastic Methods and Optimizatio
Analysis etc).
(cont
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Capacity
1. Mass Curve Analysis
(Ripple Diagram
Method, 1883) One of the most widely
used methods.
Assumptions:
Demand is constant,
and,
the year repeats itself
continuously.
(conti
S,D
DS
VOLUME
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Capacity
1. Mass Curve Analysis (Ripple Diagram Method, 18
(cont
If the flow is the da
monthly discharge
the area under the
up to a certain tim
be the volume of rfor that period.
The slope of the mcurve at a certain
gives the discharg
that time on the
hydrograph
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Capacity
1. Mass Curve Analysis (Ripple Diagram Method, 18
(cont
Critical PeriodRequired storage cap
the reservoir is the vedifference a+b
C
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Capacity
1. Mass Curve Analysis (Ripple Diagram Method, 188a. Determination of capacity for a known yield
(conti
1. The tangents, whic
parallel to the demandare plotted at the high
(D and E).
2. The maximum depfrom the tangents to t
following low points o
mass curve ( F and G
determine the necessstorage amounts V1 a
3. The largest one of
volumes will give therequired capacity of th
reservoir.
The reservoir would be full at points D, D, E, and E.
The reservoir would be empty at points F and G.
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Capacity
1. Mass Curve Analysis (Ripple Diagram Method, 188
b. Determination of yield for a known capacity
(conti
1. The value V of knownreservoir capacity is plac
vertically in all the low po
the mass curve and tang
are drawn to the previoupoints.
2. The slope of these tan
(D1 and D2) indicate the ythat can be supplied for t
critical periods with this g
capacity.
3. The smallest one of th
can be supplied all the tim
The plotted tangents must cut the mass curve whenextended forward, as it is the case here with points
C and E. Otherwise, the reservoir will not refill.
C it
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Capacity
The mass curve gives results if D < S during the pof record.
The graphical approach is quite satisfactory if thereservoir releases are constant during the period of
analysis.
When reservoir releases vary, the sequent-peak analyis recommended.
(cont
C it
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Capacity
2. Sequent Peak AnalysisSequent Peak Analysis is more suitable when the data of long observatperiods or long generated data are used, or when the demand is not co
(conti
1. Differences between inflows demands (D) are calculated and
summations obtained.
2. (S-D) values are plotted agatime as shown in the figure.
3. On this plot the first peak valu
next larger peak (sequent peak)determined.
5. This process is repeated for a
peaks in the record period as shthe figure also. The maximum o
stora e values is the re uired c
4. The storage required between these two
points is the difference between the first peak
and the lowest point in this period.
C it
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Capacity
2. Sequent Peak Analysis
If the record period or generated data sequence is very long, the graphi
solution may be time consuming.
In that case and analytical solution procedure may be applied for the
analysis and it can be solved easily using a computer.
In this way, the required storage Vtat the end of a period tcan be expreas:
At the beginning of the analysis, initially Vt-1 is set to zero and calculatio
continue to find Vt values forup to twice the length of the record period
The maximum of all the calculated values of Vt is the required storagecapacity.
(cont
+
=
otherwise0
positiveif1tttt
VSDV