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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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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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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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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Elevation Area Volume Curves for a reservoir


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Storage characteristics of a reservoir

Overview


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Overview

Introduction


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


(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


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

Reservoirs 1

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