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7/25/2019 NRECA Water Balance
1/54
Hydrologic
estimates
forsmall hydroelectric
projects
NRECA Small
Decentralized
Hydropower
(SDH) Program
7/25/2019 NRECA Water Balance
2/54
Hydrologic
estimates
for small
hydroelectric
projects
byDr.
Norman
H.Crawford
and
StevenM.
Thurin
Hydrocomp,
Inc.
September
1981
Prepared
for
NRECA
undera
Cooperative
Agreement
with
the
U.S.
Agency
for
International
Development
Small
Decentralized
Hydropower
Program
International
Programs
Division
NationalRural
Electric
Cooperative
Association
1800Massachusetts
Avenue
NW,
Washington,
DC 20036
7/25/2019 NRECA Water Balance
3/54
Small
decentralized
hydropower
program
Thispublicationis
oneof aseries that
fosterstheeffective
use
ofsmall
decentralized
hydroelectricpowersystems.
Theseries is
published
by
the Small
DecentralizedHydropower
(SDH)
Program,
International
Programs Division,
NationalRural
Electric
CooperativeAssociation(NRECA).
NRECAoperates
the
SDt,Program
underthe termsof
Cooperative
AgreementAID/DSAN-CA-0226
with
the
Officeof
Energy,Science and
Technology
Bureau,
U.S.
Agencyfor
International
Development.
Under theagreement,
begun inMay 1980,
NRECAprovides a
broad
range
of technical assistance
to developing countries. NRECA
provides such
technical assistanceby--
Designing
and implementingregional
workshops in
Africa,Asia,
andLatinAmerica
Scopingand
managing
in-countryresourcesurveys
andsite
assessments
Providing engineering,design,
supervision,
andspecialized
assistance
Developing specialized
publications,such
as state-of-the-art
reports, inventoriesof
manufacturers, and
assessment
methodologies'
Conducting specialstudies
intosubjects
of
finance,
management,
and
evaluation
Providing training services
in such
topicsas
operation
and
maintenance,
resourceassessment,
managemert, and fabrication
Carrying
out
specializedservices, such
as
tours
ofU.S.
manufacturing
plants
Creatingspecialized
products, suchas
productive-useplans
for
energy
from
small decentralized
hydropower.
Formoreinformationon
the SDHProgram,
pleasecontact:
Information Specialist
SmallDecentralizedHydropower
Program
International
ProgramsDivision
NRECA
1800
Massachusetts
Avenue
NW
Washington,DC 20036
Telephone: 202-857-9622
Telex:
64260
Cable:
NATRECA
7/25/2019 NRECA Water Balance
4/54
Contents
Section
1.
Introduction,
1
A.
The
purpose
of
this
manual,
1
B.Thebasis
of
hydrologic
estimates,
2
C.
Limitations
of the
hydrologic
methods
presented
in
this
report,
3
2.
Hydrologic
processes,
4
D.
Peakdischarges,
4
E.
Minimum
discharge,
7
F.
Frequency
of
highand
low
flows,8
3.
Estimating
peak
flow
at
asite,
13
G.
Field study,
13
1.
Physical
signs
of
high
water,
13
2.
Interviews
with
residents
andhistorical
accounts,
14
3.Calculating
discharge
fromwater
level, 16
H.
Calculation
of peak
flows
fromwatershed
data,
17
1.
Calculating
the
flowtime for
water
tomove
through
the
watershed,
19
2.Estimating
rainintensity,
20
3.
Estimating
final
peak
flow
value,
21
I.Determining
final
peakflow
value,
22
4. Estimating
the
flow-duration
curveat
a
site,
24
J.
Field
investigation
for
estimation
of
flow-duration
curves
at
a
project
site,
24
K.
Calculations
of
a
flow-duration
curve
frommeteorological
data,
27
1.
Rainfall
and potential
evapotranspiration
(PET)
data, 29
2.
Estimating
watershed
characteristics,
30
3.
Calculations
of
monthly
runoff,32
L.Determination
of
the
flow-duration
curve,
36
5.
Determining
the
final
answer, 37
Appendix
A.
Anexample
of
peak-flow
calculation,
41
B.A
Fortran
program
for
calculatingmonthly
runoff
volumes,
43
C.
NRECA
flow-duration
model,
47
Glossary,
49
iii
Contents
'6
7/25/2019 NRECA Water Balance
5/54
Tables
1
Monthly
runoffvolumes,
10
2 Monthly
runoff
volumee
inorder of
magnitude,
10
3 Watershed
loss
ratesand correction
factor for
predominant
vegetation,
22
4 NRECA
flow-duration
model:
Tabular calculation
of
monthly
runoffvolumes,
31
5 Example
of
Rio
Targaestimates,
37
6
Peak-flow
sensitivity,
39
7
Duration-curve
sensitivity,
40
Figures
1
The
hydrologic
cycle,
2
2
Flow
routing through
eorage
inchannels
or reservoirs,
6
3 Monthly
flow
volume
in
astream,
7
4 Flood
frequency:
Peak
flowvs.return
period,
9
5
Flow-duration
curve
for
monthly
flow
volume,
11
6
An
illustration
ofpeak-flow
field observations,
15
7
Stream
cross
scction
at flood
stage,
16
8 Finding
flood velocity,
18
9
An
example
of
rainfall
intensity
vs. duration
of rainfall
for
a
50-year
return
period,
20
10
Flowtime
forwater
tomove through
thewatershed
from
watershed
characteristics,
21
11 Flow-duration
curve for
monthly
flow
volumes,
25
12 Stage-discharge
rating
curve,
26
13
Asketch
ofmonthly
runoff
calculations
from
rainfall
and
potential
evapotranspiration
data,
28
14 AET/PET
ratio
as afunction
ofPRECIP/PET
and
soil-moisture
ratio,
34
15 Soil-moisture
storage
ratio,
35
16 Selecting
a
final answer
for
peak flow
from
sensitivity
analysis,
40
iv Contents
7/25/2019 NRECA Water Balance
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Section1
Introduction
A.
The purpose
of this
manual
Technologically sophisticatedprocedures
are
available
andwidely
used forthe selection,
planning,anddevelopment ofhydropower.
Most
of themarenot
useful
for
smallwatersheds
in
developing
countries,wherefewstreams
havebeengaged and there
is
little
long-term
data onstreamflowcharacteristics. Potentialinvestors
justifiablywantdetailed,
comparativehydrologic
analyses of
candidate sites,
but
the costof gaging
all possiblesites could
make the front-end costs
of mini-hydropowerdevelopment
excessive,
not to
mentiontime delays.
The
methods
presented
include
techniques
forvisualdetectionof
clues tohistorical flood levels,
guidelines
for
questioning
inhabitants in
the
neighborhoodof
the
stream,
and
equations
and
tabulardata for
calculating
peak
dischargefrom
the
scanty
data
likelytobeavailable.
It is hoped that
this
approachwillbe tested
on sites
inmany
countries
and that criticismand
constructivesuggestions
willbe
passedon toNRECAso that
these
procedures can
be further
adapted,
refined,and
improved. Commentsonthis document* are
welcome.
This
manualdescribes methodsand includes samplecalculations
for
(1)
Estimating the
peakstreamflow
at
asite,
and
(2) Estimating
the
flow-durationcurveat
a
site.
It describes
ways
to
collectfielddata
on
historicflows
forpeak
flow
and flow-durationestimates
and calculation techniques for
peakflowandflowduration
basedonrelatedmeteorologic dataand
watershedcharacteristics. The
methods described
canbe
applied
to
watersheds of
up
to 1000
square
kilometers in areaswheresnow
accumulation
and
melt is minimal
and
where streamflowsarenot
regulated
by large lakesor reservoirs.
These
methods
will
providereasonable
hydrologic
estimates when
usedby technicalor
professionalpersons.
The
hydrologic
analysis ofasite should
require less than5days of
technical or professional time.
Thegoalof thismanual
is
to
detail
selected,
simplemethods
for
obtaining hydrologic
estimates for
small
projectsthatuse field
*This
reportwas preparedforNRECA
by Dr.
Norman
H.Crawfordand
StevenM.ThurinofHydrocomp,Inc.
1
Introduction
7/25/2019 NRECA Water Balance
7/54
Figure
1
The
hydrologic
cycle
Precipitation
Interception
E
Evapotr
apiration
1701,i
Evaporation
C ndwa-.'
datausually
available in
developingcountries. Comprehensive
methods thatusemoreextensive
field
data canbe
usedfor larger
hydroelectric
and
waLer resourceprojects.
There
is
a
short glossary
at
the end
of
this
document.
B.Thebasis
of
hydrologic estimates
The hydrologiccycle (figure
1)
operates
continuously
in
all
watersheds.
The hydrologicinformation
needed
for
a
hydroelectric
project
are
apeak
flowand the flow
duration at the site.
The
basichydrologicprocesses
that
producepeakflowand flow
duration
at
a
siteare describedbelow.
Peakflowsare causedbyhigh intensity stormrainfall. Rainfall
that
moves as surface runoff into streamchannelsproduces
a
"floodhydrograph."
The
restof therainfall
infiltrates
into the
soil and
is
"lost."
It will later evaporate or transpire
orwill
providegroundwater flow
into the stream.
The
factors that must
be estimated
or
calculated to
estimatepeak
flowsare the-
(1)Rainfall intensity
and duration
on
the
watershed
(2)Amount
of
"losses"
or
infiltrationduring
the storm,
and
(3)
Flow
time
and storage
in
streamchannels.
2
Introduction
7/25/2019 NRECA Water Balance
8/54
Flow
duration for
awatershed
is
the percent of time
that flows
exceedspecific
levels.
The factors
that
control
flow
duration
are
the-
(1)
Annual cycleof
precipitation,
potential
evapotranspiration,
and actual
evapotranspiration
in
the watershed.
(2)
Amount
of
rainfall
that
infiltrates
and
moves
on
subsurface
flowpaths into stream channels. Tnfiltrationrates dependon
the
permeability
and
depth of
thewatershed
soils.
(3)
Subsurface
flow
velocities
and
the storage
capacity
of
subsurface
aquifers.
C.
Limitations
of
the hyrrologic
methods
presented
in
this
report
These
methods
operate
with
limited
fielddata
and
produce
results
of
moderate accuracy.
The
most important
factor
i n the
accuracy
of
a
hydrologic
estimate
is
the
accuracy
of the
meteorologic
data
on
which
it
is based.
If
the
24-hour--100-year frequencyrainfall
on
awatershed
is
60 mm,
and
a
45
mm
amount
is
usedin
calculations,
the
calculation
for
peak
streamflow
will
give
anircorrect
result.
The
calculation
techniques
in
this
manual are
based
on
sound
hydrologic
principles
and
represent
the
key
processes
inflood
flows
and
continuous
monthly streamflows.
They
donot
include
processes like
snowmelt
or
detailed
hydraulic
routing
and should
not
beused
where
snowmelt
is
akey
process
or where
there
are
large
lakes or
reservoirs.
They
apply
tc
small
watersheds,
generallyof
less than
1000
square
kilometers.
3
Introduction
7/25/2019 NRECA Water Balance
9/54
Section
2
Hydrologic
processes
The
hydrologic
cycle
was
sketched
in
figure
1.
The
water
in
the
system
comes from
precipitation.
It
willeither infiltrate
through
the land
surface
into
the
soil or
move
toward stream
channels as surfacerunoff. Waterthat infiltratesmaymove as
subsurfaceflow ormay
be
evaporated or transpiredby
vegetation.
In
mostwatersheds,
the
amount ofwater
lostby
evapotranspiration
islarger than theamount of
water
that
becomes
runoff. Runoffwatermoves over the surfaceas directflow
to
streams ormoves subsurface
as
delayed
or
groundwater
flow.
Directrunoff
maycause
floods,
whilethe delayedsubsurface flows
provide
continuous
or low
flows to the
rivers.
Two types
of
informationare neededfor
a
hydroelectric
site.
First, the flood flowor
expected
maximumwater level is
needed
to
size
a
spillway
(if any),
to
locate;
turbines-and
generators
above
thehighest
expectedwater
level,
and to design
diversion
structures
or canals.
Second,
the'statisticaldistribution
of
monthly
streamflow
volumes
isneeded
to
estimate
the
reliability
of the site for theproduction of
a
givenamountofelectrical
power and to sizethe turbine.
D.Peakdischarge
Peak flows resultfromacombination
of
heavy
rainfall andhigh
soil-moisturelevels,
which
prevent
thewater frommoving
into
the
soil. Peak
flows
on
small
watersheds are
frequentlycausedby
thermal
or
thunderstormrainfall. Peak flows onlarger
watersheds
arecausedbyaseriesof rtorms
or
bysnowmelt.
Flood
flows
are
plotted
on specialsemilog scales. If flood flows
aremeasuredfor
aperiodof years, this
historicdata
canbeused
to
estimate themagnitudeof floods statistically. Hydrologists
refer to50-year
or
1-year
floods,
whichare theflood levels that
wouldprobablybeequaled
or
exceeded
onlyonce
in
50 or
100
years. Toestimatepeakdischarges,
a
hydrologistmust estimate
themaximumrainfall rates thatwill occurinawatershedfor the
duration
or
length
of
storms
expected to
cause
amaximum
flow.
Therunoff
from thisrainfallmust thenbeestimated.
The
lifferencebetween the rainfall andtherunoff
will
be the
infiltrationthrough
thesoil
surfaceduring
the
storm
event.
Whenrainfall
exceeds thecapacity
of the soil to absorb
water,
surfacerunoff
occurs andenters theriverchannels, duringand
immediatelyafter therainstorm. Itflows through the river
channels
andis
measuredat
a
streamgage as
aflood
hydrograph and
apeak flaw. When
rainfall
onawatershedisplotted together
with
streamflow fromthe
watershed,the
close
relationshipbetween
rainfall
andpeak flowis
clear.
4
Hydrologic
processes
7/25/2019 NRECA Water Balance
10/54
The
factors
that determine
the
peak
flow or
maximum
flow
in a
flood
hydrograph
are the intensity
of the rainfall
that causes
the
runoff;
the
amount
ofwater
that
infiltrates
and follows
the
slower,
subsurface
paths
to the
stream
channel;
and the amount
of
attenuation
or subsidence
of
the peak
flow as
it moves
through
the
river
channels
to the
streamgage.
The
first
factor,
the intensity
of
rainfall
over the
watershed,
is acharacteristic
of
the climate
of the region.
In
the
United
States,
for
example,
a
rainfall
of
30
umi per hour
is
a
"heavyintense
storm"
in
Seattle,
Washington,
but it is
a
"light
rainstorm"
inHouston,
Texas.
Data
on
the
intensity
of rain
that
is
to be
expected
in
thewatershed
is
needed.
The
second
factor
that determines
flood
magnitude
is the
amount
of
water
that
is
absorbed
through
the soil
to
follow
the
slower
subsurface
flow
paths.
Infiltration
rates or
losses
depend
on
soil properties
and
on soil
moistures.
Soils may
range
from
tight,
lowpermeability
clays,
tohigh
permeability
silt
and
sand.
Soils
with
high
permeability,
like
sandy loam
soils and
forest
soils that
have
thick organic
layers
of decaying
vegetation,
absorbwater
quickly.
In
watersheds that havehigh
infiltration
capacities,
surface
runoff
may
be
unknown.
Soils
of
low
permeability,
such
as
clay,will
absorb
very
littlewater
and
there
will
be surface
runoff.
When
arainstorm
breaks a
long dry
spell,
soil
moistures
will
be
low
and the amount
of water
that infiltrates
or
is absorbed
by the
soil willbe
high.
If a
rainstorm
is
the
latest
ofa
series
of
storms,
soilmoistures
will
behigh
and the
amount
ofwater
absorbed
by the
soil will
be
low.
The
soilmoistures
that can
be
expected
at different
times
of
the year in
a
watershed
arealso
a
function
of the
climate.
Inhumid
climates
where
the rainfall
is
alwayswell
in
excess
of
the potential
evapotranspiration,
soil
moistures
remain
high.
In
arid
climates
where
the
potential
evapotranspiration
exceeds
the
rainfall,
soil
moistures
are
usually
low.
The
runoff
that
enters
stream
channels
from the
land
surface
may
bemodified
substantially
as
watermoves
through
stream
channels
towarda
gaging
site.
Flood
waters that
enter a
natural
lakeor
reservoir
are "routed"
or
attenuated
as
theymove through
the
reservoir.
The peak
flow
leaving
the
storage
may
bemuch
less
than
the peak
flow
entering
the storage.
This
is illustrated
in
figure
2.
Flow
along
natural channels
in
a
river
basin
also tends
to
attenuate.
Themaximum
rate
of discharge,
expressed
in
units of
flow
per unit
ofwatershed
area,
tends to decrease
as
the
flood
moves
downstream.
Hydrologists
account
for this
attenuation
process
by
using
"flood
routing"
or
"flow routing"
procedures.
The
amount
of
attenuation
of aflood
inawatershed
depends
on the
length,
shape,
androughness
of the
channels.
If the channels
have
broad
vegetated
floodplains
and if
the flood
moves
out
of
the
5
Hydrologic
processes
7/25/2019 NRECA Water Balance
11/54
Figure
2
Flow
routing throughstorage
Discharge(cubicmters persecond)
50
Inflow
40
30
IntflC
W
20 .* - _ _ _
30 t c
10
___________
0 1 2
3 4
5 6
7
Time
hours)
incised
channels
onto the floodplains, the
attenuation
of peak
flowswillbe dramatic. If
the channels
in
the
watershed
are
narrow and
steep, and flowvelocities
are
high,
the
attenuation
of
flood
peakswill
be
much
less.
If
flowsmove
into
natural
lakes,
swamps, marshes, or
manmade
reservoirs, reduction
in
peakflow
due
to
"reservoirrouting"
like that
shown
in
figure
2
can
be
expected.
In
sumary,
the
problem
of
estimatingpeak
flowon
a
natural
watershedrequires
(1)
Data
on
rain
storms thatwilloccur in
the
region,
andonthe
maximumrainfall intensities
that will occur.
(2)
Estimates ofthe
infiltration
losses that
willoccur
during
the
stormevent. This infiltrationwaill depend
on
typical soil
moisture
levels and
on
the
characteristics of
soils in the
watershed.
(3)The
attenuation
of peakflows
as
theymovefrom theheadwaters
of the
watershed
through
the
basin.
6
Hydrologic
processes
7/25/2019 NRECA Water Balance
12/54
Figure 3
Monthly flowvolumeinastream
Volumeof
discharge
(cubic
meters)
300,000.
250,000
200,000
.
150,000
100,000
50,000-
A
J J IF 1
IA IOIFI AIMJ J ASIOIN D
3 IFIIIA
IM
tJIA
IS1OIN ID
JIFNA
HIJJJJAISIOINID
1976
1977
1978
1979
E.
Minimum
discharge
Low flowperiods in
streamflow
canbe seen
infigure 3.
The
lowest flows
arereached
duringdroughts
when
very
little
rainfall
has
occurredfor an extended
periodof
time. Justas
the
peak
flows
ormaximum
flowsare reduced
by
water that isabsorbed
through the
soil surface,
the
low
flows
are increased
by
this
infiltrating
water
as
waterissupplied
to
the stream
channelsby
thesubsurface
or groundwater
flowpaths
that
wereshown in
figure 1.
Thus, the factors
that control the
lowflow inastream
are the length
of
theminimum
rainfall
periods, the
amount of
water that is
absorbed through
the
soilsurface
during rainstorms,
andthe
time needed
forwater to
flowalong
the subsurface
or
groundwater
flowpaths.
The climatein
the watershed
controls the
length
of drought
periods. In areas that
have seasonalrainfall,
there
may
be
a
drought
for
several
months
of
each year.
Inthese
climates, the
lowest
flow
willoccur toward the
end ofthe annual
dry
seasonand
7
Hydrologicprocesses
7/25/2019 NRECA Water Balance
13/54
will
be
much
lower than the
minimum
flow in anareawhererainfall
occurs throughout
the
year.
In
arid
climates
where the potential
evapotranspiration
exceeds the rainfall,
minimumflows
willbe
less
than
in
humid
climateswhere
the
rainfall
is greater
than
the
potentialevapotranspiration.
The
subsurface
flowpaths in awatershed
are stable
and
predictable.
If
awatershed
is
known to have
aminimum
flow over
a
10-year
period
of 10
cubic
meters/second,
it
is
unlikely
that
the
minimum flowwill suddenly
drop
to 2cubic meters/second.
Low
flows
respond to cumulative climate
changes.
One or two weeks of
dryweather
will
notcause
unusually
low
flows inawatershed,
but
lowrainfall
foraperiod
of6
to
36
monthsmay
result
in
the
minimum observed
lowflow. Alarge subsurface
groundwater
storage
will takemonths
rather thanweeks
to react to a
change
in
climate. Even
limited observations
of low
flows areveryhelpful
in
documenting
the low-flow
regime inawatershed.
The hydrologicprocesses
important
to lowflow
in streams occurat
and
below
the land
surface.
Water that is infiltrated at
the
land
surfacemaymove
along the subsurface flowpaths
into the
stream
channelwith
time delays
ranging
from
weeks
to
years.
These
time
delays
aremuch longer
than
the time
delays
in
channel
flow. When
water
enters a
river channel, the
typical time
delay
to
move to a
downstream
measurement
point
is
hours to days.
But
an
important
exception
to this rule
is
a
channel system
that
contains
major lakes or
marshes.
They
may
store waterduring
floodperiods
andrelease it slowly
over aperiodof
months,
causing
lowflows
to increase. Alteration
of
natural channel
storage to increase
low
flows
in
astream is
the
goal
of
astorage
reservoir.
Insumary, to
estimate the
minimum discharge
in
a
stream, the
important
factors
are
the
(1)Climate
of the
watershedand the
durationof
drought periods
(2)
Infiltration
of
rainfall
through thesoil surface
and
water
movement
into
the
subsurface
groundwater flow
paths
(3)
Typical
ti'e
delays
or
the subsurface
flowpaths,
and
(4)
Marshes,
lakes, or reservoirs (if
any) that
are present
in
the
watershed.
F.Frequencyof
high
and
low
flows
The continuous
measurements
of streamflowplottedin
figure 3
showedboth
the highand the
lowflows
from
a
watershed. It
is
sometimes convenient
to reorganize
these data
to show the
frequency
of high
or lowflows
separaLely on
semiloggraphs as in
figure
4.
For
example,
for
high flows, the
peak flow
measured
8 Hydrologicprocesses
7/25/2019 NRECA Water Balance
14/54
- -
Figure
4
Floodfrequency: Peak
flowvs.
return
period
Peak
flow (cubic
meters
per
second)
700 - - - - -: -
* *1" * , .'
. .....
. I... .
.....
...
50
r-tt f ::fl
- -
....
H--4ILL... ... ..
400 ,-7- - - K
.,.. . .. .~ . . . . ....
. ,.. . .,......
I
i 11
I:2
300
S... ....... . ...
.. ....
........
J........................
...
...... ...
.... ......... ....
...... ...... .....
-
..
".
,+-
............
....
. ... .
.... - -. i ii- .
I.... ...
..
.
n
. .
...........- *
.
|...
..
-
-
...
. .,
......
...
..
....
1
2
3
5
10
20
30
50
100
Returnperiod (years)
eachyear for
aseries ofyears
canbe plotted.
If 20
yearsof
data
are
available, the
flood flows are listed
in
order
of
magnitude
and the
highest
flow is
assumed
to recur oncein 20
years, the second
highest
flow, once
in
10 years,
the third
highest
flow once
in
6.67
years,
and
so on. This
allows the
points to
be
plotted
on
the
graph, as
shown
in figure4.
Peak
flows formanywatersheds produce
linear
or
near linear
curveswhenplotted
on semilogpaper.
These flood frequency
curves areused
to
estimate
floods for
project
design.
A
designer
9 Hydrologic processes
7/25/2019 NRECA Water Balance
15/54
might
choose
to
design
a
structure
for
aonce-in-100-year
flood,
a
flood
that
would
be
equalled
or
exceeded
an average
of once
every
100
years.
Flood
frequency
curves
like
figure
4are
strongly
influenced
by
themaximum
historic
flood
that
has
been
observed.
When
historic
flows
in
a
watershed
are
not
directly
measured,
newspaper
accounts
of
the
floodmarks
or
memories
of
local
residents
may
be
sufficient
to
estimate
the maximum
discharge
for
historic
floods.
Minimum
flows
can
be treated
like
maximum
flows.
A
frequency
curve
of
theminimum
discharge
each
year
could
be
prepared
and
plotted
using
the
same
procedure
that
was
described
for
peak
flows.
Avery
useful
technique
for
graphically
representing
the
continuous
flow
measured
in
a
watershed
is
the
flow-duration
curve.
To
construct
aflow-duration
curve,
assume
that
table
1
is
the
monthly
volume
of
runoff
for
ayear
in
awatershed.
These
runoff
volumes
are
rearranged
in
order
of
magnitude
in
table
2.
The data in table
2
are
plotted
in
figure
5.
It
canbe seen
in
figure
5
that
the
runoff
equals
or
exceeds
100
million
cubic
meters
40
percent
of the
time.
The
runoff
exceeds
175
million
cubic
meters
20
percent
of
the time.
Table
1
Table
2
Monthly
runoff
volumes
Monthly
runoff
volumes
in
order
of
magnitude
Volume
of
runoff
Volume
of
runoff
Month
(cubic
meters)
Rank
(cubic
meters)
January
102,300
1
291,900
February
189,000
2
211,600
March
291,900
3
189,000
April
211,600"
4
113,400
May
98,700
5
102,300
June
32,000
6
98,700
July
12,600
7
96,100
August
8,700
8
68,700
September
14,500
9
32,000
10
14,500
November
96,100
11
12,600
December
113,400
12
8,700
October
68,000
10
Hydrologic
processes
7/25/2019 NRECA Water Balance
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Figure 5
Flow-duration
curve
for
monthly
flowvolume
Monthly
flow
volume
(cubic
meters)
350, 000
300,000
250,000
200,000
0
150,000
100,000
50,000
0
20
40
60
90
100
Percent
of time
monthly
flow
volumes
are
equalled
or
exceeded
The
flow-duration
curve is
veryuseful
for
study
of
projects
that
divert
water
from
ariver
for
water
supply,
irrigation,
or
hydroelectric
power
production.
Flow-duration
curves
can
be
constructed
from any
period
of
streamflow
data
but are
more
reliable
if
they
are
constructed
for
several years
of
streamflow
data. Flow-duration
curves
use
ill of
the
streamflow
information
that
is
available
at
a
site,while
peak-flow
and
low-flow
frequency
studies
selectout
maximumor
minimum
flows
for
analysis. In
locations
where
very
little
streamflow
data
is
11 Hydrologic
processes
7/25/2019 NRECA Water Balance
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available,
it maybe feasible
to construct the flow-durationplot
butnot
peak-flow
and
low-flow
frequency curves. Ofcourse,both
peak-flow
and low-flow
frequency curves
and flow-duration
curves
increase
in
accuracy
as the
lengthof the
basic
data observations
increases.
12 Hydrologicprocesses
7/25/2019 NRECA Water Balance
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Section
3
Estimating
peak
flow
at asite
To
estimate
the
peak
flowthat
could occur
at
a
potential
hydroelectric
site,one should
(1)study thewatershed
to findout
what
high flows have
occurred in
the past,
and
(2)
study
the data
on thewatershed
to
calculatewhat
high flowsmight
occur in
the
future. Section
Gdescribes
how
to perform
field studies
at
a
siteto findout about
pasthigh flows.
Section
Hshowshowto
calculate future
peak flowsusing
the available information
about
thewatershed.
Section
I
tells
how tocombinethe
answers from
sections
GandHtoproduce
afinal
result.
G. Fieldstudy
Thefieldstudy
of
the
watershedwillanswer
the
question,
"How
largehave past
floods
been?"
Twomethodswill
beused to find
outhowhigh
thewaterhasbeen.
Themethods
are to-
(1) Study
the
physical
features near the
streamto findsigns
of
high water, and
(2) Talkto peopleliving
near
the stream
and
to
officials in
the
areaabout
past floods.
Once the
high
watermark
has beendetermined,
youcanusesome
calculations
to
find
the flowrate for
that
water
level.
These are explainedbelow:
1. Physical signs of
highwater
Thepurpose
of this section is
togainan
understanding
ofhow
the
water
level inthe streamvaries.
Writedown
the important
observations that
you
make
as shown
in
figure
6.
Flowingwater
moves
many
objects thatit
touches. Grass,
branches, and other
light objects
float alongnear thewater
surface.
Earthand sand areeroded
by
fast-moving
water
and
carried
downstream, orare
left onbeaches
or inpools.
To
begin
yoursurveyof
the stream
channel, stand on the
bankof the stream
andwatch
the edge of the
streamas
itflowsagainst
plants
and
rocks
or
sand.
Imagine
that
the
water
level
is
higher
by
1
or
2
meters,and
seewhatwould
beunder water. Look
for signs,
like
eroded
tree roots
or
sand
or
gravelbeaches,which
indicate
that
the
water
has been
at that level.
If
water
is
not movingvery
fast or is
flowing
through
bushes and
trees,
light
objects
which
are floating
willoftenbecome
caught
inthebranches
or lefton
floodplain
landswhenthe water
level
goesdown.
Lookforclumps
ofgrass and
sticks
caught
in
the
trees
oronbeaches
asyoumove away from
the stream.
13
Estimating
peakflowat asite
7/25/2019 NRECA Water Balance
19/54
Lookat the
general slope
of
the
land towards
the
stream.
Water
will
always
form
a
flat surface.
If
you find
ahigh
deposit
of
oldfloatedmaterial,
imagine
that
the water
isat
that level
everywhere,
and
determine
what
areawould
be
under
water.
Look
at
the
ground
at the same
elevation
asyour
high-water
level.
Are
there signs
that
the
ground
has
beenunder
water?
Look
for
large
branches
and logs
whichmight
have floated
to
the
edge of
the
stream
during
a
flood.
Once
you
have
foundwhat
appears
to
be the
highest
deposit
of flotsam,
look
forother
signs that
the
water
was
once
at
or
near
that
level.
Look
for
markson
trees
or
buildings
or
more
branches
or
logs at
the
sameheight.
Look
at
the
deposited
material
and
try
to determine
how
long
it
has
been
there.
Is
the
organic
material
totally
decayed?
If
so,
itis
probably
more
than2years
old. Look
for
growth
of
vegetation
which
has
occurred
since the floating
material
was
laid
down. Wave
action
athigh
water
levels
willvery
quickly form
a
beach,
eroding
finematerials
and leaving
adeposit
of sand
and
gravel
along
the
streambank.
Signs
of beach
erosion
can
remain
visible
for
severalyears after
a
major
flood.
How
old are
the
plants
which
are
growing
ovdr
the
sanddeposits
and eroded
areas?
After
getting
some
idea
of
how
high
the
recent
water
levels
have
teen,
talkwith
local
residents
abouthigh
flows,
as
outlined
in
aection
G2.
Ifyou
have
some
information
from
the plants
and
soil
deposits
near
the stream,
it
will
be
easier
to evaluate
the
recollections
ofresidents
about
high flows. Keep
in
mind,
however,
that
youmay
not
have
seen signs
of
any
flood
which
occurred
more
than
2or3
years ago,
while
longtime
residents
mc"
remember
floods
from
over50
years ago.
You
may
learn
of
floods
which
were
considerably
larger
thanwould
be indicated
by the
evidence
youhave seen
near the
stream.
2. Interviews
with
residents
andhistorical
accounts
The
next
step
isto
use the
knowledge
ofthe stream
obtained
from
your
field
survey to
interview
local
residents
and to
draw
conclusions
about
past
flood
levels.
People
who
have lived
for
a
long
time
neara
stream
mayremember
large
floods
quite
well.
If
ahistoric
flood
was
large
and
if watermarks
remained
visible
for
some
time,
residents
will
know
thehighest
water
level
andwhen
the
flood occurred.
Asample
of
the interview
notes
youmight
take
isshown
in
figure
6.
By
talking
with
severalpeople
and
by
comparing
their
recollections,
you
may
get
accurate
information
on
the largest
floods
in the
past
50
years.
Start
an interview
with
ageneral
discussion
of the
stream
and
ofhow
it
behaves
throughout
the
year, and
evaluate
whether
or
not
the person
being
interviewed
is
familiar
with
the river.
Next,
mention
physical
evidence
of past
floods
near
thestream.
Finally,
ask
the person
being
interviewed
to point
out
marks
where
the
highwater
has
been
recorded
orwhere
he
remembers
high
water.
14 Estimating
peak
flow
at a
site
7/25/2019 NRECA Water Balance
20/54
Figure6
An
illustration
of
peak-flow
field
observations
10/27/81-Started
field
study,
looking for
high-water
marks
on
east
bank, 500
metersupstream
fromroad
crossing.
Found
large
mass of
decayed
drygrass
and
branches
5mabove
waterline
in
atree.
Debris
appeared
to
be several years old.
Observed
other
smaller
bunches of
debris
at
slightly
lower
elevations.
Found
sand
deposit
25
meters
from
bank.
Significant
plant
growth
indicates
ithas
been
there
for
at least
5years.
Elevation
is 4.5
mabove
current
water
level.
10/28/81--Talked
to
Paul
Jones,
oldest
resident
(75)
living
near
the
stream.
He
showed
me
a'mark
on
his
door
where
he said
thewater level
was
in storm
of
December
1953.
He rememberedhis father tellinghim
about
a
storm
around
1890
which
had
beenabout
3meters
higher.
Talked toJohn
Smith
(62).
He
didn't
know
anything
about
flood in
1890.
Highest
waterhe
remembered
was
marked on
alarge tree
nearhis
house.
There
v: sa
date
(December
16, 1953)
written
on
the
tree.
Level
agrees
well with
flood
reportedby Paul
Jones.
Smith
also said
he
only
remembers
2
times
in
the
last
50
yearswhen
stream
has
driedup.
It
is important
to
tell
the
people
being
interviewed
that
accurate
data
will
help
you
design
asuccessful
and
reliable
project and
that
manydata
sources are
being
obtained for
comparison.
Be
careful
not
to lead
the
person
beinginterviewed.
If
aperson
thinks
that
describing
high flood levels
will
help
get
a
hydroelectric
powerplant, he
may tend
to
exaggerate
the level
of
flooding
he remembers.
Or
if the
person
being
interviewed
thinks
that
you do not
want to
know
aboutvery
high past floods,
he
may
describe
flooding
as having
been
less than
he actually
remembers
it.
To
avoid
problems
ofbiased
answers,
talk
to several
people
and
get
them
to
pointout physical evidenceof flood levels,
like
high-water
marks
on
buildings.
Ask
for any
available
written
documentation
or photographs
to confirm
their
reports.
Dateswhen
the
floods
occurred
and
asearch
forhigh-water
levels
at a
number
, nearby
pointsmay
alsohelp
confirm
reports of floods.
Other
ys of
verifying
residents'
reports
of
high-water
levels
include
searching
old local
newspapers,
church
records,
or
local
government
records of
road or
bridge
repairs.
15 Estimating
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at asite
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21/54
Figure
7
Stream
cross section
at flood
stage
Mrk on
tree
Wterline
reported
by
John
Smith
with date
Current
waterline
Horizontal
scale
I
cm 4
meters
Vertical
scale
I
cm
2
meters
2
13.6
boxes
x
4
meters/box
x
2meters/box
-
108.8
meters
Area
-
Width
-
72meters
Average
depth
-
area/width
- 1.5
meters
Current
depth
-
0.8
meters
Measured velocity-
1.7
meters/second
Flood
velocity
-
mas.
velocity
x averae
depth
-
2.6
meters/second
~curren
aptH)
Once
you
have
finished
your
field
inspection
and
your
interviews,
examine
the
results
as
recorded
in
your notes.
Askyourself
the
following
questions:
(1) Have
youfound
a
reliable
water
level
for
the largest
flood
which
anyone
remembers?
(2)
Do you
know
when
the flood
occurred?
(3)
Is
there
physical
proof
of thehigh-water
level?
If
the
answer
to
all three
ofthese
questions
is
yes,
you
are
ready,
to
goon
to the
next
step
of
calculating
the
flood
discharge
from
tie water
level.
If
youhave
not been
ableto
accurately
document
the
largest
flood,
it may
be
necessary
touse
aslightly
smaller,
more
recent
high-water
level
that
isbetter
remembered
and
documented.
3. Calculating
discharge
fromwater
level
The
final
step
in afield
study
for
peak
flow
is
to
transform
the
flocd
level
you
have
determined
into
an estimated
peak
discharge.
To
calculate
the
flood
discharge,
you
need
to know
the
cross
sectional
area
and thevelocity
of
the
stream
at the
peak
water
surface
elevation.
Thecross
sectional
areaof
thestream
may
be
calculated
by
first
plotting
the
height
above the
ground
of
the
peak
water
surface
along
alineperpendicular
tothe
stream
channel.
Plot
peakwater
depth measurements
at regular
intervals
from
the point
where
the
high-water
line
touches
the
ground
onone
side
of the stream,
all
the
way
over
tothe
same
point
on the
other
side.
Figure
7shows
an
example
of
astream
cross
section
plot
at flood
stage.
16
Estimating
peak
flow
at
a
site
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22/54
The
next
stepis
to
count thenumber
of
boxes between
the ground
surfaceprofile and thehigh-water
line.
This numberisthen
multiplied
by the scale factorsonyour plot to yieldthe
peak
flood's crosssectional
area.
Figure
7shows
how
this
calculation
isdone.
After
youhave
determined
the
areaof
thepeak
flood,youmust
determine
the
velocity
of
the
flow. Thiscanbest
be doneby
measuring
offastretchof 50meters
along the channel,
then
timinga
floating
object as the streamcarries
it
through
the
50-meter stretch. The
velocityof the
flowisthen--
Velocity50
(Eqn
)
average
time to float
through
(stretch
seconds)
Throw
theobject
into
the
center
of the
streamandmake
sureit
doesn't
strike anything
as it
moves
through the reach. Time
f"e
object
through
the
reachat leastfive times.
Thisvelocity
needs to be adjusted
to
take
into account thehigher
velocities
of
flood
flows. To do this, find
or estimate thewater
depthnear thecenter
of
the
channel
and the average
waterdepth
at floodlevels from
yourcross
section
plot. Use these numbers
as
shown
infigure
8to
find
the factor
by
which
you
mustmultiply
yourmeasured
velocityinorder
to getflood
velocity.
Onceyou
know
the flood
velocity
and the
cross
sectional
area,multiplythe
twonumbers togetherto
obtain
thepeak
flood discharge.
Q- VxA (Eqn
2)
H.Calculation
of
peak
flows from
watershed da'a
This section
describes
how
to calculate
the
peak
floodflow ata
possiblehydroelectric
siteusingthe
available
data
on the
watershed
and the
tables
and
figures presentedhere. The
things
that
you
willneed
are
a
contourmapof
the watershed and
regional
data onrainfall
intensityfor different storm
or rainfall
durations.
Rainfall
intensityis
given
inunits
ofmillimeters
per
hour. If45 mm of rain
is
measuredin
2hours, therainfall
intensity
is22.5
mm per
hour. Rainfall intensity
willdecrease
as
storm
duration increases.
InBoston,Massachusetts, for
example,
arainfallintensity
of
150
mn per
hour
willoccur for
storms of 10 minutesduration,but
a
rainfall
intensityof
30
mm
perhour
will occur for storms
of 4hoursduration.
The
steps
involved
in
calculating
the peakdischarge area-
(1)
Using channel
and
watershed
characteristics, determine the
flow time
for waterto
move through
the watershed.
17 Estimatingpeakflow
ata
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Figure
8
Finding
floodvelocity
Flood
velocity
-
measured
velocity*
(depth
of
flood/measured
depth)
2
/3
Flood
velocity/measured
velocity
4.0
/
3.5
_
3.0 _
2.5 _
2.0 _
1.5
1.0 -
2.0
3.0 4.0
Depth
of
flood/measureddepth
5.0
6.0
7.0
8.0
(2)
From
regionalmeteorologicdata,
determinerain
intensity
for
the
design
storm.
The
durationof
thedesign storm
is
assumedto
be
equal tothe
flow timefor
water tomove
through thewatershed.
(3)
Determine
watershed losses,
the
"excessrains"
or theamount
of
rainfallthatbecomes
surface
runoff,
andthe
peakdischarge.
18 Estimatingpeakflow
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This calculation
method
assumes
that
apeak
discharge will
occur
when the
storm
duration
is equal
to
the flow
time
for
water to
move through
the watershed.
This
is
an
idealization
of the actual
interactions
between
stormraiifallduration
and peakflow,
but
it
gives
peak
flow estimates
of
reasonable
accuracy.
Each step
in the
calculation
is
explained
in
the
following
sections
and
easy to
follow examples
have
been included.
For each
step, yourcomputations
should
be
written
down like the
examples
so
that
you
may
checkyour
work.
1.
Calculating
the flowtime
for
water
to
move
through
the
watershed. To compute
the flowtime
for
water
to move
through the
watershed,
you
willneed todetermine--
L =
channel length
in
kilometers
ER
- changein elevation
between
the
highest
point
in
the
watershed
and
the site, inmeters
AREA
- watershed area, upstream
of
the
site
in
square
kilometers
These
data
canbe found from
acontourmap
of thewatershed.
and ER
are
used in Eqn
3
to calculate
the flowtime
(TF)
through
the
watershed.
This flowtime
(TF) is assumed
to equal the
duration
of the
storm
rainfall
that
will
cause apeakflow.
The
duration
of the storm rainfall
is used to find the
rainfall
intensity
during
the storm
(RI),
using
local data similar
to that
shown
in figure9. The storm
rainfall intensity
(RI)
less a
loss
rate
(LR)
is
used to
find the
rate
of
runoff
or
rain
excess
(XR)
during
the storm (Eqn
4). Finally,
theexcess rain
is converted
to flood peakdischarge
by
multiplying
by
a
necessaryconstant
and
the watershed
area
in
Eqn
5.
The
flowtime
in hours
(TF) canbecalculated
from
L
andERusing
Eqn
3,or it canbe
read
from
figure 10.
0.95
x
(L
3
/ER)
"385
TF
-
(Eqn
3)
In
Eqn
3,
L
is in
kilometers,
ER
is in
meters,
and
TF is in
hours.
19 Estimating
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flowat
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Figure
9
Anexample
of rainfall
intensity
vs.
duration
ofrainfall
for
a50-year
return
period
Rainfall
intensity
(millimeters
per
hour)
140
120
100
80
lot.
40
20
4
3
12
16
20 24
Duration
of rainfall (hour)
2.
Estimating
rain
intensity
Once
youhave
calculated
the
TF from
Eqn
3,use
dataon
duration
of rainfall
versus
rain
intensity
to
find the
rainfall
intensity
on
thewatershed.
The
relationship
between
rainfall
duration
and
rainfall
intensity
isusually
displayedgraphically,
as shownin
figure
9.
It
maybe
possible
to
make
agraph
similar
to
figure
9
with
your own
data.
Graphical
plots
ofrainfall
intensity
versus
duration
of
rain
areoften
made for
different
statistical
return
periods.
A
rainfall
intensity
of 60
,m/hour
for
4hours
duration
would
be
equalled
orexceeded
once
every50
years,
as
shown
on
figure
9.
To usefigure
9,find the
duration
of
rainfall
inhours
that
equals
TF
on the
bottom
scale,intersect
the line,
andread
the
rainfall
intensity
(RI),
from
thevertical
scale.
20 Estimating
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flow
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Figure10
Flowtime
forwater
to
move through
the
watershed
from
watershed
characteristics
Channel
length (kilometers)
TF- 2
0
200
400
600
800
1000
1200
Elevation
(meters)
3.
Estimating
excess
rain
anddetermining
peak
flow
Rainfall
will
be
lost during
a storm
due
to infiltration.
Table
3
describes
various
soil
types
and
gives
a
loss rate
for
each.
Losses
are
greater
in
watersheds
which
have
very
heavy
vegetation.
A
correction
factor
forvegetation
density
can
be
selected
that is
multiplied
by the
loss
rate
for
the
watershed
soils
to
give
the
total
loss rate
for the
watershed,
LR.
Now
that
you
have
determined
the
rate of
rainfall
loss,
youmay
calculate
excess
rain
by subtracting
your
losses
from
the
rainfall
intensity.
The total
excess
rainfall
rate
is
then--
XRR
I
-LR
(Eqn
4)
where
XR
RI,
and
LR are
all
in units
of
irm/hour.
21
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Table
3
Watershed
loss
rate
and
correction
factor
for
predominant
vegetation
Predominant
soil
type
Loss
rate
(mm/hr)
Impervious
rock
1
Tight
clay
1
Clay
and
silt
3
Silt
and
sand
5
Sand
and
gravel
10
Correction
to
loss
rate
Predominant
vegetation
(multiply
by)
Sparse-Little
vegetation,
bare
soil,
scrub
brush
0.5
Moderate--Grassland,
cropland,
mixed
forest
1.0
Heavy--Dense
forest,
tropical
forest
2.0
Example:
A
watershed
with
clay
silt
soils
anddense
forest
cover
would
have
aloss
rate
of
3.0-,/hour
anda
correction
factor
of
2.0 so
its
total
loss
rate
would
be 3.0
x
2.0
- 6.0
un/hour.
The
final
stepis
todetermine
the
peak
discharge
at
the
site from
Eqn
5.
Peak
flow (cubic
meters/second)
- 0.28
x
XR
x
watershed
area
(ki2) (Eqn
5)
where
XR
is
exccss
rain
in
un/hour
and
peak flow
is
in
cubic
meters/second.
An
example
of a
peak
flow calculation
is
given
in
appendix
A.
I.
Determining
final
peak
flow
value
If
youhave
beenable
to
follow
both
the field
survey
and
the
calculation
methods
for
determiningpeak
flow,
youwill have
two
values
for
peak
flow
which
may
bequite
different.
This
isto
be
expected.
The remaining
task
is
to evaluate
the
L
values
in
orderto
arrive
at
onebest
value.
To
do
this,
you
will need
to
review
theprocess
by
which
you
calculated
each
number
and
determine
wherethere
was
likely
tohave
beena
large
amount
of
uncertainty
in
your
study.
22
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Which
method
gave
youa
higher
value?
Wasone
value
more
than
twice
as
muchas
the
other? If
the
calculation
method
value
was
much
larger,what
was
the return
period
for the
rainfall
intensity
versus
duration
of
rainfall
data
that
youused?
If
itwas
more
than
50
years,
it
should
have
givenyou
alarger
peak flow,
because the field
study
interviews
will document
floods
which
have
occurredwithin
thepast
50
years.
If
the field
study
gave
youa
lower
peak-flow
value,
compare
the
velocity
figure
youused in
the field
study to
the
average
flood
velocity,
which is
channel
lengthdivided
by
theflowtime
through
the
watershed(TF).
Thetwo
velocities
should
be fairly
similar,
although
the
fieldstudy
velocityis
likely
to be lower.
Try
calculating
the
field
study
peak
flowusing
the Chezy-Manning
equation,
V
=
1/n
D
2
/
3
S
1
/
2
where
nis
resistance
to
flow,Dis
mean
depth in
meters
at
flood
stage,
andSis
the slope
along
thechannel.
In
natural
streams,
meandepth (D
in
meters)
can
be assumed
to
beequal
to the
hydraulic
radius and
isthe
cross
sectional
area
ofthe
flow
dividedby
its
wettedperimeter.
Slope (S)
isinunits
of
meters
of
elevation
loss per
meterof
length
along
the channel,
so itis
dimensionless.
Velocity
(V)is
inmeters
per
second.
For
Manning's
equation,
typical
values
ofn
follow:
Clean,straight
channel
withsand
bed 0.035
Winding
channel,
sand
orgravel
bed
0.045
Winding
channel, graveland
stone
bed,
some
streambank
vegetation
0.06
Rocky,
winding
channel
with
pools
and
obstructions,
streambank
vegetation
0.08
Do the
peak
flows agree
more
closely
now? Ifso,
use the
new
value for the
field
study
result.
In
general,
if the
field
study
and calculation
methodresults
for
peak flow
agree
within30
percent,
you
might
average
themand
use
this as your
final
result.
If
you
have reason
to
believe
that
one
method
was better
than
the
otheron
your
watershed,
your final
result could
fall
between
the two
results,
but
closer
to
theone
on
whichyou
place the
most confidence.
23
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flow
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Section4
Estimating the flow-durationcurve
at a
site
This
section gives methods
forestimating
the flow-duration
curve
at a
hydroelectric site.
SectionJisa
summary
of field
investigations
forestimating flow-duration
curves,
and
section
K
describes calculationmethods
for
flow-duration
curvesbased
on
meteorologic
information.
The flow-duration
curve
isamost
useful
tool
for evaluating
low
flows at ahydroelectric site. The
flow-durationcurve in
figure
5
is
repeated
as figure 11. These
data
canbeused
to
estimate
the flow
that
canbeusedfor
powerproduction90 or
95
percent of the time.
Thisgives an
immediate
indicationof the
reliability
ofpower
production
at asite. Forexample,
ifa
20-meterhead
wereavailable,
the
flow-duration
curve in
figure 11
would
indicate
that
as
119-kWpower
source could;be developed
at
85-percentreliability,
assuming an 80-percentoverall
efficiency
fortheproject.
J.
Field investigationfor estimation
of flow-duration
curves
at a
project site
Peoplewho
livenear
ariver
orwho divertwater
froma
river
are
likely
to remember the typical stageat
different timesof the
year. Highand low
stages aswell asnormalstages throughout
the
yearwillbeknown to local residents, even
though theywill not
know
thecorresponding
discharge. Knowledge
of
stage
canbe
converted
to
discharge using
astage-discharge rating
curve. A
stage-discharge
ratingcurve can
beconstructedusing
measured
cross
sections
at
two
ormore
stages
and calculating
flow
velocities
at
each
stage, as was
described
insectionG3.
Dischargeat
a
given
stage isthe flowvelocitytimes
the cross
sectional
area
of
the flow at that stage.
When stage-discharge
ratings
are
plotted on log
scales, linear or near-linearcurves
are
found. Figure 12 is
an
example of
astage-discharge
rating
curve.
To construct aflow-durationcurve,
the
following sources of
informationmight be used.
(1)
Are
there lakes or
reservoirs on the stream? Stagerecords
over
aperiod
of years
at a
lake or
reservoir
can
beused to
calculate
streamflow
volumes
entering the reservoir.
Whenmonthly
stage
records
exist at
areservoir, locateareservoir stage
versusreservoirvolume chart
for
the
reservoir.
The
streamflow
volume entering
the
reservoir eachmonthwillbe
the reservoir
volume at the end ofthemonth, less the
reservoir
volume
at the
first of themonth,plus
the volume of water
released
from
the
24 Estimating the
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curve
at
asite
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Figure
11
Flow-duration
curve formonthly
flowvolumes
Monthly
flowvolume
(million
cubic meters)
3.5
3.0
2.5
2.0
0
1.5
0
_
1.0
__ _
_
_0
0
0.5
0
20
40
60
80 100
Percent
of timemonthly flow
volume
are
equalled
or exceeded
reservoir during
the
month.
If
astage versus
reservoirvolume
chart
is
not
available,
one can
be
made from
maps of
the reservoir
area atdifferent
elevations,
sincethe
increase in
reservoir
volumebetween
two elevations
is
the
elevation
incrementtimes
the
reservoir surface
area.
(2)
Correlation
of
low flowson
aregional
basis ispossible.
Do
streamflow
recordsexist
on
nearby,
similar
streams? TheU.S.
Geological
Survey
has summarized
commonlyusedmethods
for
regional
correlation
offlows inWater
Supply
Paper
No.
1975,
titled
"Generalization
of
Streamflow
Characteristics from
Drainage-Basin
Characteristics,"
by
D.M.
Thomas
and
M.
A.
Benson
(55 p..).
25
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Figure 12
Stage-discharge
rating curve
Stage
(meters)
8.0
_
_
4.0
2.0
/
pInt of zoro flow
10
20 30
Discharge
(cubic
meters
per
second)
(3)
Review
any
existingdata
on
flow
durationcompiled in
the
region
in
previous
studies.
(4)Question
local
residents, particularly
people
who are
using
the
stream
in
someway: ferry
operators, irrigation
farmers,or
people
who divertwater
for
water
supplies.
What
stages
are
expected
by season
ofthe year?
If
rainfall
in the region
is
seasonal,
what stages
are typical
of
the
lowest flow
month
of the
year?
What stages
are
expected
during
high-flow
months? What
is
the lowest
stage
that residents
remember? How
does
the lowest
stage
that residents
remember
comparewith
the typical
low
stage
that isobserved
eachyear?
(5)
In
somewatersheds, low-flowconditions
are
associated
with
poor
water
quality.
Is there
sometimes
unacceptable
water
quality,such
as
veryhigh
salt content?
Whendoes
this
condition
occur?
(6)Are
therewater
intakes along
the river?
Have
thesewater
intakes ever
been relocated
because of
low stage
in the stream?
Where
monthlystreamflow
canbe calculated
from
lake
or
reservoir
26
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Wheremonthly
streamflow
can
be
calculated fromlakeor reservoir
levels,
as
in (1)above,
or if regional studies
to relate
flow
at
asite
to nearby
gaged streams are
done as in
(2)above,
the
data
can
be
used
to
construct aflow duration as was done
in
section
F.
If
regional studies
of
flow
duration
already
exist
from(3)
above,
flow-durationcurves
will be
available.
Wheninterviews are
used
to establish seasonal stages,
as in
(4)above,useManning's
equation described in
sectionIor
float
&!asurements to
establish
a
stage-dischargerelationship,
as was
described
insection
G3. Convert
stages
to discharge to estimate
monthly flows
foratypical
year. These
meanmonthly flows canbe
used to
construct aflow-duration
curve as
in
sectionF. If the
field
interviews, (5)and
(6)
abovi,
give clear
evidence of
occasional
low flows
that are subscantially less
than
the
minimum
expected annual
flow, amultiyear flow-durationplot
might be
made.
To make
amultiyear
flow-duration plot,
estimatemonthly
flows for
additional years and include
the
historic
low flows.
If
your
field information shows
theminimumflow in
an 8-year period,
8
full years of estimated
monthly flows should
be used to construct
the
flow-duration
curve. These
additional onthly flow
estimates
ensure that the
lowesthistoric
flow
correctly
influences the
flow-duration
curve:
the lowest
monthly
flow
in
8
years is
a
99-percent condition;
in otherwords, in
95months outof 96
this
minimumflowwouldbeequalled or exceeded.
K.Calculations
ofaflow-durationcurve
from
meteorologic
data
Streamflow results from
precipitation.
Flows ofwater from
the
land surfaceduring and immediately following
precipitation
create
floodhydrographs
and peak
flows. Water that
is absorbedby the
soil duringrainstorms
moves as subsurface
flow
into
stream
channels
and
provides
low flows
inperiodswhenrain does
not
occur.
Flow-durationcurves are
based
on
continuous
streamflow
data. On
ungaged streams where streamflowmeasurements
are not
available, precipitation and
potential evapotranspiration
records
canbe
used to calculate
continuous
flows.
Thecalculations
mimic
keyhydrologicprocesses: infiltration of
water
into the
soil
profile,
surface
runoff,
and
flow
along subsurfaceflowpaths into
the
stream.
The
calculation
method
described in
this
section uses
monthly
precipitation and
potential evapotranspiration data
to
calculate
monthly streamflow.
When
the
calculatedtrinthly streamflows are found, they
areused
like observed flows
to
calculate
the flow-durationcurve.
A
27
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Figure
13
A
sketch
of
monthly
runoff
calculations
from
rainfall
and
potential
evapotranspLation
data
PRECIP,
MOISTURE
STORAGE
EXCESS
MOISTURE
DIRECT
LL4W
RECHG
TO
FIGURE 4.3
A SKEWC1 OF
MONTHLY
RUNOFF
CALCULATIONS FROM
RAINFALL
AND
9DTENTIAL
EVAPOTRANSPIRATION
DATA
GROU
ATER
STORAGE
GROUNDWATER
FLOW
TOTAL
DISCHARGE
sketch
of
the
calculations
isshown
in figure
13.
The
time
of
flow
in
strem
channels
in small
watersheds
isusually
less
than
1day
and
this
time
of
flow
can
be
neglected
when
monthly
interval
runoff
volumes
are
calculated.
Calculations
of
monthly
flows
from
meteorologic
data
are
based
on
the
water
balance
in the
watershed.
The
water
balance
equation
is-
Precipitation
- actual
evapotranspiration
+ storage
- runoff.
The
waterbalanceequation
applies
to
the
watershed
over
any
time
interval.
Where
precipitation,
actual
evapotranspiration,
and
runoff
are
the
volumes
of
water
entering
and
leaving
the
watershed
in
the
time
interval,
and
storage
isthe
change
in
soil
moisture
and
groundwater
storage
in the
time
interval,
the
initial
storages
less
the
final
storages.
Water
is held
in
storage
in
the
soil,
in
groundwater
aquifers,
and
in
lakes
and
snowpacks.
Allwater
flows
28
Estimating
the
flow-duration
curve
at
asite
7/25/2019 NRECA Water Balance
34/54
into
orout
of
the
watershed
are
assumed
to
beincluded
in
the
runoff.
Thefollowing
steps
canbe
used
tocalculate
monthly
runoff
and
a
flow-duration
curve
from
meteorologic
data;
theyare
described
in
the following
sections.
(1)
Assemble
5ormore
years
ofconcurrent
rainfall
and
potential
evapotranspiration
data.
(2)
Estimate
the
watershed
characteristics
of thebasin.
(3)Calculate,
using
a
tabular
form,
themonthly
streamflows
for
5
ormore
years
based
on
these
rainfall
and
potential
evapotranspiration
data.
(4)
Calculate
the
flow-duration
curve
using
the
calculated
monthly
streamflows,
aswould
be
done
ifobserved
datawere
available
(section
F).
1. Rainfall
and
potential
evapotranspiration
(PET)data.
Streamflow
calculations
requiremonthly
rainfall
on
thewatershed
andmonthly
potential
evapotranspiration
data.
Rainfall
data that
is
observed
in
or
near the
watershed
must
be
found.
National
meteorologic
services