9 Reservoir Sizing.pdf

1

Reservoir Sizing

Water stored in reservoir, lakes and stream are most important source of fresh water supply

If river discharges were constant in time, surface water resources is simple, no reservoir required

Unfortunate, river flow are stochastic in nature and variable with time

Two extreme condition occurs High flow caused flood Low flow caused water shortage (drought)

ERT 246 Hydrology and Water Resources

2

The practical problem A water supply,

irrigation or hydroelectric project drawing from river may unable to satisfy the demands during low flow

Therefore, the main function of a reservoir is to stabilize the flow of water especially during dry spell

Streamflow, Q(t)

Spill

Diverted (Demand Area) Reservoir

(with storage Capacity, C)


3

Reservoir defined Impoundment of

surface water across a river

Storage capacity estimated from the area-elevation curve (topography)

Various zones of storage in a reservoir

Spillway crest

U M N S

D Stream Bed

Sluiceway

D: dead storage M: minimum pool level U: Useful storage S: surcharge storage N: Normal pool level


Reservoir/Dam from the sky

4 ERT 246 Hydrology and Water Resources

5

SEMENYIH DAM, SELANGOR


6

PROPOSED DAM SITE

dh

The topo map Q


7

Storage capacity of a reservoir

Storage capacity of a reservoir is estimated using topographic map of the reservoir site

S = {(Ai+1 + A i)/2}* (dh) S i+1 = Si + S S i+1 = Si + (A i+1 + Ai) dh/2


8

0

200000

400000

600000

800000

1000000

1200000

525 530 535 540 545 550 555 560

Ketinggian Pugak (m)

K

e

l

u

a

s

a

n

P

e

r

m

u

k

a

a

n

T

a

k

u

n

g

a

n

(

m

2

)

0

1000000

2000000

3000000

4000000

5000000

6000000

7000000

0 200000 400000 600000 800000 1000000

1200000

Luas Permukaan Takungan (m2)

S

t

o

r

a

g

e

T

a

k

u

n

g

a

n

(

m

3

)

h (m)

A (m2)

A (m2)

S (m3)

dtt cSA baSA tt


9

What and why low flow Low flow defined: Streamflow of less than

average flow Flow of water in a stream during prolonged dry

period The basic question:

How much water (Yield) can a reservoir collect water during low flow condition?

Yield defined the amount of water that a reservoir can supply during a specified time interval

Thus knowledge in storage-yield relationship (S-Y) of a reservoir is required

ERT 246 Hydrology and Water

Resources

10

Further questions a. Can the given demand be met from natural river flow or is reservoir required?

b. How much water can be reliably pumped from a river in a given time?

c. What is the probability that streamflow Q(t) will be less that a specified amount during a given period?

d. If the storage is necessary, how large the reservoir capacity, C, need to be to provide for a given controlled release or draft D(t) with acceptable level of reliability?

e. Therefore, the relationship between Q(t), C, D(t) and reliability must be found.

a. Question (a-c) is solved using concept, yield of unregulated streams (natural flow condition)

b. Question (d-e) is solved using storage-yield relationship ERT 246 Hydrology and Water

Resources

11

Streamflow, Q(t)

Spill (Hydropower)

Diverted Controlled Release, D(t) (Demand Area)

Reservoir (with storage Capacity, C)

Q(t), D(t), C Optimization


12

Yield of unregulated streams Those without artificial storage. Give example of

an artificial storage Source of inflow to the artificial storage

(reservoir) Source of water: slow flow or baseflow of the

rainfall hydrograph Tool of analysis

Flow duration curves Low flow frequency curves


13 ERT 246 Hydrology and Water Resources

14

Flow duration curve Simplest and most informative mean of

showing the low flow characteristic of an unregulated stream

Defined: the percentage of time during which specified discharge were equalled or exceeded during the period of the record

Is a cumulative frequency distribution


15

(1) (2) (3) (4)

Q(m3/s) Frequency

Cumulative frequenc

y %Cumulative

frequency Over 475 3 3 0.21 420-475 5 8 0.55 365-420 5 13 0.89 315-365 8 21 1.44 260-315 25 46 3.15 210-260 36 82 5.61 155-210 71 153 10.47 120-155 82 235 16.08 105-120 52 287 19.64 95-105 42 329 22.52 85-95 50 379 25.94


16

(1) (2) (3) (4)

Q(m3/s) Frequency Cumulative frequency

%Cumulative frequency

65-75 83 520 35.59

50-65 105 625 42.78

47-50 72 697 47.71

42-47 75 772 52.84

37-42 73 845 57.84

32-37 84 929 63.59

26-32 103 1032 70.64

21-26 152 1184 81.04

16-21 128 1312 89.80

11-16 141 1453 99.45

Below 11 8 1461 100.00

Total days 1461


17

Flow duration curve: procedure a. Group all data into class intervals (Column 1) b. Count the number of occurrence (frequency) of each

class interval (column 2) c. Class frequencies are accumulated beginning with the

largest discharge (column 3) d. Each cumulated frequency is expressed as a percentage

(column 4) e. Discharge is plotted against cumulated percentage of

frequency on normal graph paper or normal-probability paper or log-normal-probability paper


18

Flow Duration Curve (Normal-normal graph)

050

100150200250300350400450500

0 20 40 60 80 100

%time equalled or exceeded

Q

(

t

)

m

3

/

s

.


19

Flow Duration Curve plotted on log-probability paper

Q(t)

%tage of time exceeded 0.1 2 50 90 99.5

10

100

1000

- Flow duration curve Based on 4 years (1461 days) period - 2% of the 4 years period, Flow exceeded 290 m3/s - 96% of the 4 years period, The flow is between 12-290 m3/s - 50% time point provides the median value (45 m3/s) - Is the flow normally distributed?


20

Uses of flow duration curves in reservoir design

90, 95, 96 and 99% - measures of a streams low flow 90% of time discharge exceeded

a measure of groundwater and river bank storage contribution to streamflow

Use to estimate hydropower potential If the slope of the curve in low flow portion is flat, groundwater

contribution is significant Steep curve poor baseflow or cease to flow condition Valuable tool for comparing basic characteristics of the catchment area

in particular the geology groundwater Related to water quality

Indicate the %tage of time that various levels of water pollution will occur following of the introduction of a pollutant of a given volume. Eg. Total Maximum Daily Load (TMDL).


Resources

21

050

100150200250300350400450500

0 20 40 60 80 100

%time equalled or exceeded

Q

(

t

)

m

3

/

s

.Flat slope

Steep slope


22

Assignment

A. You are required to obtain a topographic map of an area in Johor State. The lower reach of the river is the main outlet (intake) of the proposed reservoir. You are required to propose a reservoir to serve people at the downstream area of the river. Estimate the potential storage capacity of the proposed reservoir and plot graphs showing:

a). Water surface area-elevation relationship and b) Storage- surface area relationship of the reservoir c) Establish the reservoir equation

B. You are provided with a river flow data sets taken from the proposed project area

stated in Question A. Prepare the duration curve (for all data set given) using normal graph and log-

probability graph, of the selected river and discuss on the finding in relation to water resources. The discussion must be supported by at least two journal papers of the related subject.

baSA tt


23

LOW FLOW FREQUENCY CURVE

Flow duration curves: use all kind of data Low flow duration curve: based on data sequence

(in order or in series) that are independent and homogenous

With independent and homogenous data, the probabilistic analysis of occurrence of a low flow can be determined

The data required Annual series (based on minimum flow event in each

year of record)


24

Assignment # 2

Use the same data you used in Assignment #1, develop a low flow frequency curve of the selected river. For each year, divide the data into quarterly and use minimum mean monthly flow as low flow record.


25

Storage-yield-performance (S-Y-P) relationship

Streamflow, Q(t)

Spill

D(t), Diverted (Demand Area) Reservoir

(with storage Capacity, S)


26

Estimation of the capacity-yield relationship for a reservoir on a stream

Study the relationship between storage capacity (S), Release or draft D(t) and Reliability

Question: Given active capacity S and streamflow Q(t), how much yield D(t) is available for a given reliability, or

Given Q(t) and D(t) for a given reliability, what is the size of S

Storage-yield-performance (S-Y-P) relationship


27

Flow conditions defined

Natural streamflow (virgin, unimpacted, unimpaired): flow in stream has not been affected by human influence like inter-basin transfer, diversions or land use change in the catchment or climate change

Unregulated stream; one does not have upstream diversions nor is regulated by an upsteram reservoir.


Resources

28

Reservoir outflow and yield defined Yield is the controlled release from a reservoir

system Expressed as a ratio or % of the mean annual

flow to the reservoir Eg: 70% yield means that during the period of

analysis the system will provide a regulated yield of 0.7 times mean annual flow

Other terms: release, draft and regulation Demand: the amount of water required by a

demand center (irrigation scheme or township) Required design demand: the demand at a given

level of security or reliability Reservoir yield is the water available for

distribution to a demand center for a given storage capacity and a given level of security

Streamflow, Q(t)

Spill

D(t), Diverted (Demand Area) Reservoir

(with storage Capacity, S)


29

Firm yield: the most important term in reservoir design

The yield that can be met over a particular planning period with a specified no-failure reliability

The largest quantity of flow that is dependable at the given site along the stream at all time

Usually based on historical record


30

Safe yield

The yield from a water supply system after a detailed storage-yield analysis

100% reliability mean the yield is safe, but it is never occur


31

Operational yield

To describe the yield of a system as obtained from simulation taking into account seasonal variations in demand and any restriction placed on supply

No knowledge of future inflows is assumed and decision are based only on the available water in storage


32

Type of storage

Spillway crest

U M

N S

D

Stream Bed

Sluiceway

D: dead storage M: minimum pool level U: Useful storage S: surcharge storage N: Normal pool level


33

Total storage, active storage, dead storage

a) Total storage: volume of reservoir at full supply equal the sum of active storage size and dead storage

b) Active storage: use for conservation purposes; water supply, navigation, irrigation

c) Full supply level is the level of the invert of fixed spillways

d) Dead storage is the volume of water held in the reservoir below lowest off-take (below this level, sediment may trapped)


34

Finite storage Using a reservoir Peclet dimensionless number

(P), measure the relative importance of the mean and variance of the net inflows, i.e. reservoir inflow less the outlow

P>+1, the stored contents almost never reaches the lower boundary and the reservoir can be regarded as bottomless

P


35

Date Q(in)m3/s Q(out)m3/s NetinflowJ 400 550 150F 550 496 54M 474 474 0A 591 626 35M 936 765 171J 1437 832 605J 1502 939 563A 1203 817 386S 662 905 243O 334 483 149N 251 466 215D 243 560 317

mean 56var 98039

d

d SP 22

36

Critical period and critical drawdown period

Refer to the period from a full reservoir condition to emptiness

The period from a full condition through to emptiness and to a full condition again

The period from full to empty is known as critical drawdown period (US)


37

Reservoir performance It is importance to characterize the likely future performance under the

wide range of possible demand and hydrologic conditions that are expected to occur during the reservoirs operating life

Performance criteria based on unsatisfactory operation (failure) during period of low reservoir inflow

failure is defined as the inability to provide the target demand during a given period

The main term to describe the performance of a reservoir system is reliability, i.e. the probability that the system can meet the target demand

Vulnerability quantifies the consequences of failure Resilience quantifies the ability of a reservoir to recover after a failure


38

Unsatisfactory region

Range of Satisfactory performace value System performance

Indicator, e.g. release

Unsatisfactory region

Hypothetical numbers indicate by how much the target is over-supplied: a desirable outcome for water supply situation

Hypothetical numbers indicate by how much the releases deviate from the target demand due to under-performance

3 3

4 4

2

6

4


39

Time-based reliability

NNsNF )(

NNsRt Rt = time-based reliability Ns = total number of intervals during which the demand was met

N = total number of time intervals in simulation

F = the shortage frequency Ns = total number of intervals during which the demand was met N = total number of time intervals in simulation

RtF 1


Time-based reliability


40

Q(m3/s)

month

41

Volumetric Reliability

Nj

j

fjjj

V D

DDR

)(1

'

1 if Dj is 100% satisfied, i.e Dj=Dj Rv = volumetric reliability f = no of failure periods (=N-Ns) Dj = actual supply from reservoir system during jth failure period Dj = target demand during jth period N = number of periods in the simulation


42

Resilience

It is necessary to know how readily a system will recover following failure

An indicator of the speed (probability) of recovery following failure

10;1

d

s

s

d ff

ff

is resilience, f s number of continuous sequences of failure period, fd is the total duration of the failure, i.e. N-Ns


43

Critical Period Method

Based on continuity equation in which the required storage equals the maximum difference between outflow (draft) and inflow during a critical period.

The reservoir is assumed to be at full supply level at the beginning of the worst critical period


44

Storage-yield relationship

Estimation of the capacity-yield relationship for a reservoir on a stream

Study the relationship between storage capacity (C), Release or draft D(t) and Reliability

Streamflow, Q(t)

Spill

Diverted (Demand Area) Reservoir

(with storage Capacity, C)


45

Definition of Terms Critical period: a period during which a reservoir goes

from a full to an empty, without spilling The start of a critical period is full condition, the end is

the first empties Thus, only one failure can occur during a critical period

1980 1981 1982 1983 1984 1985 1986 1987 1988 1999

Full Storage Critical period

Critical period

Empty Storage


Documents

9 Reservoir Sizing.pdf