HomeMy WebLinkAboutSu Hydro - Simulation of Reservoir Operation 1986COLD REGIONS HYDROLOGY SYMPOSIUM
JULY AMERICAN WATER RESOURCES ASSOCIATION 1986
THE SUSITNA HYDROELECTRIC PROJECT
SIMULATION OF RESERVOIR OPERATION
Yaohuang Wu, Joel I. Feinstein, and Eugene J. Cemperlinel
ABSTRACT: This paper presents the general
concept and methodology used in the simu-
lation of reservoir operation, which
played an important role in the study of
the Susitna Hydroelectric Project. The
objective of the simulation was to find
optimum operation rules which would meet
projected energy requirements of the
Alaska Railbelt, while at the same time
satisfying flow regimes which would
maintain habitat for resident and
anadromous fish. Computer models were
used for the simulation of reservoir
operation on a monthly, weekly, and hourly
basis, using streamflow records of 34
years. The results of the simulation
allowed the selection of a preferred flow
regime which could meet the projected
energy requirements and also provide nc
net loss of habitat for the fish.
(KEY TERMS: reservoir operation
modeling; rule curve; operating guide.)
INTRODUCTION
The proposed Susitna Project consists
Of two tandem reservoirs on the Susitna
River. The two proposed dam sites are the
Watana site, a rockfill dam to be located
at river mile 184 of the Susitna River,
and the Devil Canyon site, a concrete arch
dam 32 miles downstream from the Watana
site as shown in Figure 1. The project
Would operate by storing the high natural
flows in the summer for release during low
flow periods in the winter when the energy
demand is high. This alteration of the
natural flow pattern would affect the
quantity and availability of spawning,
incubating, and rearing habitat for fish,
P
JF' DAMSITE NVON
Q� DMSITE
>
A LASKA
�-
�P
ZIF GOLD DAMSITE
V
`CREEK
�
J
v
TALKEETNA ml
9 �
y
V
9
• ANCHORAGE
VALDEZ
z
0
20
0 20 60
SCALE �.% V'LES
Figure 1. Susitna Project Location Map
1 Respectively, Principal Engineer, DeLeuw, Cather and Company, 525 Monroe
(formerly Senior Power Planning Engineer of Harza Engineering Company);
Engineer, Hama Engineering Company, 150 South Wacker Drive, Chicago
Manager of Hydrologic and Hydraulic Studies, Harza-Ebasco Susitna Joint
Street, Anchorage, AK 99501
Street, TL 60606
Power Planning
IL 60606; and
Venture, 711 H.
primarily in the middle reach of the
river. The impounding of water in the
reservoirs and the alteration in flow
patterns would also change water quality
parameters associated with mainstem flow
such as water temperature, turbidity, and
suspended sediment. The simulation of
water temperature, ice formation, and sus-
pended sediment were conducted in a series
of separate studies using the reservoir
water levels and discharges from the
reservoir operation study.
To mitigate the impacts on chum salmon
spawning and incubation in side sloughs
and Chinook salmon rearing in side
channels, eight different flow regimes
were developed for evaluation (Alaska
Power Authority, 1985). Each of the flow
regimes was designed to provide a given
amount of habitat for the fish species.
Each flow regime consists of a series of
weekly maximum and minimum flows through a
year keyed to churl and Chinook salmon life
cycles. The maximum or minimum flows were
specified at the Gold Creek station, which
is located 15 miles downstream of the
Devil Canyon site. Figure 2 shows the
E-VI flow regime which was selected as the
preferred alternative. Minimum summer
flow requirements would provide flow
stability and would maintain a minimum
watered area for salmon habitat. Maximum
winter requirements would provide flow
stability and would minimize the potential
for winter water levels to overtop side
slough habitats affecting incubating and
rearing chum salmon.
50
40
U
30
10
NOTE.
1. DISCHARGE IS AT
SUSITNA RIVER AT
}
i
GOLDCREEK
2. ONE -IN -TEN YEAR
LOW FLOW }
... .....
MAX'
-
I
FOR NORMAL
FOR LOW
MIN
YEARS
FLOW YEARS2
J F M A M J J A S O N D
MONTHS
Figure 2. Environmental Flow Requirement, Flow Regime E -V I
The evaluation of the flow regimes was
carried out by estimating the total cost
to meet the projected Railbelt energy
demand. These include capital and
operating costs of the Susitna Hydroelec-
tric Project, other generating facilities,
and any mitigation measures required to
meet the objective of no net loss of
habitat value. Among the mitigation
measures included in the evaluation of the
alternative project flow regime were
hatcheries and multi -level intakes for
temperature and sediment control. Since
the maximum and minimum flow constraints
of the alternative flow regimes would
restrict the seasonal distribution of
Susitna energy production, the construc-
tion and operation of additional power
plants to meet the system demand were also
considered.
RESERVOIR OPERATION SIMULATION
Reservoir operation models simulate the
reservoir storage, power generation,
turbine discharge, valve release, and
flood release as a function of time based
on reservoir and power plant characteris-
tics, power demand distribution, and envi-
ronmental constraints. Cone valves may
operate at each dam to satisfy an instream
flow requirement or to keep the water
surface elevation at the normal maximum
level without having to use the spillway.
These simulations are normally undertaken
in two parts; long-term simulation and
short-term simulation (Dondi and Schaffe,
1983). The long-term simulation for the
Susitna Project uses a monthly program and
a weekly program for simulating the
operation for 34 years of streamflow
record. The monthly program was used to
determine the overall trend, while the
weekly program was used for refinement of
operation rules and to understand the
behavior of the reservoirs and flows
during critical periods. The short-term
simulation used an hourly program to
simulate the operation over a week, using
the output from the weekly simulations as
input data. The hourly program was
designed for simulation of hourly
generation to meet the daily peak and off-
peak loads.
The monthly operation used a single
rule curve as an operation guide to esti-
mate the annual energy production and to
4
satisfy the monthly instream flow require-
ments. A rule curve indicates the desired
reservoir water level in different months.
The weekly operation program used an
operation guide for seasonal adjustment of
flows which produce a series of reservoir
outflows with gradual changes. Operation
guides consist of a series of rule curves
to control the reservoir outflow. The
hourly operation program used an hourly
load curve as the upper limit of possible
generation. The load curve was based on
actual hourly load data and a load fore-
cast. This program tested how well the
energy obtained from the long term analy-
ses could fit the hourly load curve, sub-
ject to environmental restrictions on the
daily and hourly flow changes.
Power and energy production from the
monthly simulation of the Susitna Project
was used in the Railbelt expansion plan-
ning studies, which in turn was used in
both the economic and financial. analyses.
Although monthly simulations were suffi-
cient for these studies, they were not
adequate to estimate environmental
effects. Therefore, a simulation with a
weekly time step was needed to generate
input data for subsequent computer models
used in the environmental impact studies.
Other environmental studies required hour-
ly discharge to estimate river stage fluc-
tuations.
The monthly program was originally
developed by Acres American for the
Susitna feasibility study (Alaska Power
Authority, 1982) and later improved by
Harza-Ebasco Susitna Joint Venture. The
weekly program was developed by Harza-
Ebasco by using parts of the monthly pro-
gram. The hourly operation program was
developed by Harza- Ebasco. All of the
programs were written in Fortran IV.
The Susitna project was scheduled to be
built in three stages. First an initial
Watana Project would be developed, fol-
lowed by construction of Devil Canyon
downstream. Finally, Watana would be
raised to its ultimate height. The full
reservoir areas for the low and high dams
at Watana would be 19,900 and 38,000 acres
respectively, and 7,800 acres at Devil
Canyon. Because of the large reservoir
surface area at Watana, release of a large
quantity of water would cause a relatively
small change in the project head. In
contrast, Devil Canyon would have a small
surface area, and would hence lose consid-
erably more head for the same volume of
release. Consequently, the reservoir
operation methodology attempted to keep
the Devil Canyon reservoir close to its
normal maximum operating level. while using
Watana`s storage to provide' the necessary
seasonal regulation. Therefore, the
modeling effort in both the single and
double reservoir operation in monthly and
weekly simulation was focused on the
Watana operation. The operation levels of
the reservoirs for the various stages are
shown on Table 1.
Table 1
OPERATION LEVELS
Environ-
Normal mental Nominal
Min- Maxi- Sur- Plant On-
imum mum charge Capac- line
Level Level Level ity_ Date
(fft) (ft)� (MW) t) (-
Watana
Low Dam 1850 2000 2014 440 1999
Watana
High Dam 2065 2185 2193 1110 2005
Devil
Canyon 1405 1455 1455 680 2012
MONTHLY OPERATION MODEL
Monthly reservoir simulations were
carried out to optimize project energy
production subject to environmental flow
requirements. The rule curve which would
optimize energy production was determiner)
by trial and error. The optimal energy
production is a function both of the firm
energy and total energy. Figure 3 shows a
sample rule curve of the Watana
reservoir.
During the simulation in each time
step, the reservoir release has to satisfy
the firm energy and environmental minimum
flow requirement. If the end -of -month
water surface elevation is lower than the
corresponding rule curve elevation, only
firm energy is produced and no additional
5
water is released. If the end -of -month
water surface elevation is higher than the
rule curve elevation, the water stored
between These two elevations is released
to generate secondary energy up to the
system energy requirement.
2050
z 2000
Q
195`)
e
w 1900
a
1850
DRAWDOWN FILLING PERIOD DRAWDOWN
PERIOD PERIOD
NORMAL MAX. EL 2000'
MIN_ EI. 1850'
i F M A M i i A S U N U
MONTHS
Figure 3. Example Rule Curve For Watana Operation, Year 2004
The dry season is from October to April
and the wet season from 'May to September,
as shown in Figure 4. The rule curve
elevation at the end of September is at
the normal maximum pool elevation because
the reservoir is expected to be full at
30
20
Q
CC 10
DRY SEASON WET SEASON ! DRY SEASON
34-YEAR AVERAGE
NATURAL FLOWS AT
r THE WATANA DAMSITE
i F M A M i i A S 0 N D
MONTHS
Figure 4. Natural Flows at Watana
the end of the wet s< i, LT car:_*ras?.
the rule curve elevation 1r the end ;)f
April is at i.ts rlini°,r11.1:n 'rE au<;e the draw -
down is required for f i r r prcxhi -
tion in the dry season. ,'}le
minimum rule curve eli2 vat ion (at the erld
of April), the gre;iter t',t -irm e:Li, r:£y
production. This is hecalis�? the reservoir
levels would be kept relativF� 1 y hi.hor alid
the storage availabLe for ri rm eIle.n;y
would be more in the dhperiod,
Alternatively, the lower the rule curve,
the greater the total energy orod+iction.
This is because there would be Wore active
storage for flow ruul,rtion anti less
amount of spills on a long terin basis -
Various sets of rule curvF, e1.,1vations with
various minimum elevations will give diff-
erent values of firrl crier;°y and total
energy. The acceptti.le
mi r,i mur; rill" curve
elevation for tire. Susitn<i P`-0,J)0ct was
selected based on an ,)pe.rat-ion in which
the increase of total t7!nerg ✓ is tho(rt one
percent when lowerins;-
tion by five feet.
Once the maximum and mi.,li,aurl curve
elevations were deter%&,�ed, titk, rest of
the rule curve rmi��>_d
by a trial and error procedure to obtain
an acceptable distrihutiolz of enemy
through the year. i'he )perational. st..rate-
;y was to capture additio11 economic
benefits through ad j1Ist menu. of the
Susitna generation
thermal generation durinkt o,ch of r:ao
periods, the summer ii11im,, porind and t!ie
winter drawdown period. As ;stated
the reservoir would be ,nl__nvst full at the
end of September and woaid be at the low-
est levels at the end of April. There-
fore, Susitna energy distriblrtion during
the filling period, t=-) `-ep temlher, and
during the drawdown r; od , �)c•.tohe.r to
April, could be vaci.ed as a function. of
reservoir water surface variation withrrut
reducing total. project energy product ion.
It was assured that it. wWild b, 1`0re
economical to provi.le t1 oner=,v by
running the leas t0• , t e, rma ! rn `. is
-
throughout a whole n_'ri--d ratt��>r th-=n
running them for_ part cif ,_r - pe;'i.�d .3iorg
with other less of fici er"t =,r i t. Alan the
system was assumed to be nl(-o gel i al, tr, if
the thermal requi remt,nt soul t be zrbout the
sane from month to m011LO. L 1t�3 w,�: i�; e-an
that investment
capacity could be
sible, Therefore, t}1�_, rs�;• istrl_hr�ton
6
was adjusted so that the Susitna energy
production would maintain constant thermal
generation in both the filling period and
the drawdown period as much as possible.
The analysis of the projectes benefits
was based on its ability to meet energy
requirements. These requirements were the
total projected Railbelt system energy
demand minus the energy production of
existing hydroelectric facilities.
Figure 5 shows the monthly distribution of
energy requirements, and also indicates
how the Susitna energy would be distri-
buted throughout the year,
700
600
500
0
400
c�
300
UJ
W
w 200
100
0
ENERGY REQUIREMENT
(4570 GWh)
SUSITNA GENERATION
THERMAL (2430 GWh)
GENERATION
(2140 GWh)
F M A M J J A S 0 N D
MONTHS
Figure 5. Monthly Energy Distribution, Watana, Year 2004
The rule curve approach is predictive
because it attempts to achieve an end -of -
month elevation which presumes some knowl-
edge of the expected reservoir inflow
during that period. The operation guide
approach, which will be discussed below in
conjunction with the weekly program, is
non -predictive because it specifies a
discharge rate through the powerhouse
based on the reservoir elevation at the
beginning of the period. The rule curve
approach is easy to apply for simulation
but can be operationally difficult to
achieve, because reservoir inflows are
difficult to accurately forecast. The
operation guide approach is more difficult
to model, but more closely approximates
how the project would actually operate.
The two approaches yielded similar power
and energy results in many trial runs, so
the monthly model (rule curve approach)
was used for the economic and financial
analyses in selection of the best scheme.
The rule curve approach could not be used
in the weekly simulations for the environ-
mental studies because the release of the
storage above the rule curve elevation for
the secondary energy could cause an unrea-
listic change of discharge between two
consecutive weeks. The operation guide in
the weekly simulation was designed to
prevent these large changes.
The weekly operation model was primari-
ly designed for simulation based on the
operation guide. An operation guide is
composed of a series of rule curves which
are used as a guide for determining the
turbine discharge in each week during the
simulation. An example operation guide is
shown on Figure 6. Each guide has two
families of rule curves; increasing curves
and decreasing curves. Each curve defines
the reservoir level at which the power-
house discharge should increase or
decrease to a specified percentage rate of
the expected powerhouse discharge. These
specified percentage rates are in 20%
intervals. The expected powerhouse dis-
charges are a set of weekly discharges
which would produce an expected distribu-
tion of energy production over a year.
The single rule curve operation is based
on the target firm energy; however, the
operation guide uses 60% of the expected
powerhouse discharge as the minimum dis-
charge and 140% as the maximum normal
discharge.
2050
LL 2000
z
0
a
w 1950
J
W
U
Q
¢ 1900
cc
W
1850
3
1800
J F M A M J J A S 0 N D
MONTHS
INCREASING CURVES
--'---' DECREASING CURVES
NOTE.
THE CURVES REFER TO A
I PERCENT OF EXPECTED DISCHARGE
Figure 6. Operation Guide For Watana Operation, Year 2004
7
At the beginning of each simulation
week, these curves are used to determine
whether the current discharge rate is kept
the same, increased to the next higher
rate, or decreased to the next lower rate.
The water surface elevation at the begin-
ning of the week is put on the operation
guide and compared with the elevations of
increasing and decreasing curves. T_f the
water surface elevation is higher than the
elevation of the increasing curve of the
next higher rate, the discharge is in-
creased to that rate. If the water sur-
face elevation is lower than the elevation
of the decreasing curve of the next lower
rate, the discharge is decreased to that
rate. Otherwise, the discharge is kept
the same. The change of rates in two
consecutive weeks is limited to 20%. For
example, if the discharge rate is at 100%
of the expected powerhouse discharge in
the preceding week, the rate may be
changed to either 80% or 120% or stay at
1000. Because of the difference in
elevation between the increasing curve of
the next higher rate and the decreasing
curve of the next lower rate, the rate
will generally be kept the same for a few
weeks. In contrast with the simulation
using a single rule curve, an operation
guide will give an outflow hydrograph with
relatively gradual changes in the
discharges.
A smooth curve giving the energy
requirements throughout the year is shown
on Figure 7. The average energy produc-
tion of. the Susitna Project in the draw -
down and filling periods was determined
from the monthly simulation. Similar to
the monthly modeling, efforts were made to
capture the additional economic benefits
by leaving the thermal generation constant
in both the drawdown period (October to
middle Play) and the filling period (middle
May to September). In weekly runs the
reservoir level is the lowest in the
middle of May and full in most years at
the end of September. The energy pro-
duction in these two periods are inex-
changeable, but redistributing energy pro-
duction within either period does not
cause additional valve and spillway
releases or .flow deficits. Adjustments of
energy production within either period by
leaving thermal requirements constant was
assumed to increase the economic value of
the project. Gradual changes of thermal
energy requirements at the boundaries of
the filling and drawdown periods were
considered for a smooth transition of the
operation. The resulting weekly distribu-
tion of energy production over a year was
used for computation of the weekly ex-
pected powerhouse discharge.
160
140
Y 120
w
100
80
60
z
40
20
ENERGY REQUIREMENT
(4570 GWh)
SUSITNA GENERATION
THERMAL (2430 GWh)
GENERATION
2140 GWh)
J F M A M J J A S O
MONTHS
Figure 7. Weekly Energy Distribution, Watana, Year 2004
Development of an operation guide is an
iterative process. An assumed set of rule
curves for the operation guide were put
into simulation initially to find flow
deficits resulting from not satisfying the
minimum flow constraints or from discharg-
ing less than the minimum powerhouse dis-
charge (60% of expected discharge in the
example). The curves were then grad wally
improved by satisfying these two require-
ments through the whole simulation period.
The curves were again adjusted to maximize
the average energy production and improve
the energy distribution through the year.
A good operation guide should provi3e: 1)
turbine discharges close to the expected
powerhouse discharge, 2) discharge rates
generally constant for a period of at
least several weeks, and 3) average energy
production maximized.
Figure 8 shows the historical inflow
hydrograph at Watana. Figure 9 shows the
simulated outflow hydrograph of Watana
operation for the load year 2004. The
comparison of duration curves between the
pre -project and post -project flows at Gold
Creek Station is shown on Figure 10. Note
that, through flow regulation by Watana,
the high flows in summer woul3 be subst<+n-
tially reduced and the low flows in the
winter would be increased for power
generation. The simulated outflows from
Watana were, in general, consistent with
8
the expected discharges and there were no
rapid changes of discharge except those
during floods.
J
U.
Z
0
N
Q
will■■■■I f � w
w ■ 11■E■■I■■ m
11 now i on
.i..w�I 1111h�
11■■I■11� II 1II1■I 11 I■
Is@
11■I■Ih■ril■I■��I■ i.■ ■w■u'u ■I ■oil I
I■I■� 11■I■I�I�IIIIII
Will 11�II11■I■ICloll 11 ■111■IIII■t ■■ I■■1■I■II,I■I■�■I=IIIIII■11=.
u■ul�uun■uuuuu■mun■■m■■muunuu i■uw nuuw ■
' n■unnuu■■uun■mu■uu■1■�unu■■■■uu�i■n■iuunlu■
uu�uuuWnln�lluuuu■111■11�■�IIII■III■1■11IR IN IIIIIIIIilm
�uuwnuuumulnuwunu■� �1�1111■11111■111111■IIIIUIIIIIHI■
IIIIIIIIII'li11U1111IIIIIIIIIl111111/1lIIu1111111111111111 111 11111■
20
Figure B. Watana Inflows
YEAR
o
1960 1955 1960 1965 1970 1975 1980 1985
YEAR
r Figure 9. Watana Outflows, Stage I, Load Year 2004
HOURLY OPERATION MODEL
... The hourly program modeled a reservoir
operation over a week, using an hourly
104d curve
!thin a (for the demand distribution
` week) as the upper limit of
►116ible generation and discharges from
50
40
U
0 30
0
F
LEGEND:
—'•—•— PRE PROJECT FLOWS AT
GOLD CREEK STATION
—X—X— POST PROJECT FLOWS AT
GOLD CREEK STATION
0 20 40 60 80 100
PERCENT, TIME
Figure 10. Duration Curves of Flows at Gold Creek
the results of weekly simulation as input
data. Outflow fluctuations were restrict-
ed by maximum hourly and daily variation
constraints. The model tests how energy
obtained from the long term analyses can
fit the hourly variation of demand with
environmental constraints. The output was
used in river stage fluctuation studies.
An example of the output is plotted as
shown in Figures 11 and 12 to aid in
understanding the results. Figure 11
contains a plot of generation showing the
system demand, the existing hydro genera-
tion, and Susitna Project generation.
Figure 12 shows the reservoir discharge
through the powerhouse and the valves, and
the minimum flow required to meet the
environmental requirements at Gold Creek.
The reservoir outflow constraints limit
how the plant can operate in the system.
For example, if the daily variation con-
straint is very small, then the plant is
essentially base loaded, but if both hour-
ly and daily variation constraints are
large, the hydro plant can load follow.
Load following operation means that the
plant can increase or decrease its genera-
tion by following the hourly fluctuation
of the system demand, and will leave the
1200
1000
800
Q
z 600
z
3 400
a
200
20
16
12
s g
a
0
0
EXISTING HYDRO PEAKING
NON SUSITNA GENERATION
SYSTEM DEMAND
^l1
�'; 1 r t r 'I �IJ
cz
z
w
a
z
N
+ EXISTING HYDRO ON BASE '
SUN MON TUE WED THU FRI SAT
Figure 11 . Hourly Power Generation
TOTAL OUTFLOW I
R
POWERHOUSE DISCHARGE I
I
MINIMUM ENVIRONMENTAL RELEASE
I
CONE VALUE DISCHARGE
SUN MON TILE WED THU FRI SAT
Flqure 12. Hourly Reservoir Discnare
energy from other sources at constant
capacity. Base loaded operation means
that the plant generation is constant in
principle, but a certain percentage
fluctuation may be allowed. Load
following operation would provide more
capacity value for ttie pro ;r Ct th-1n base i
loaded operation. Base io�=dI operation h
would p r o v i 1e sta',1c. ,.?ows ir! ti,e downstream channel. Project operation o
is 0
currently constrained to be 1)ase loaded D
with allowable variations of irl total f
project discharge wit111rl i week,
therefore, providing stable flows and 1
U
Chum and sockeye sallrion spr7wn in r
sloughs along the river. These slouplis c
collect sediment and or;;anic matter a
throughout the normal course of tile year, u
which make it difficult for the fish io w
spawn and may reduce egg surviva'. f
Naturally occurring floods clean out the t
sloughs and thus provide better habitat, u
The dams would tend to reduce the,,e a
naturally occurring floods which may m
decrease the natural fish Habitat. Sor.>e c
of the flow regimes incorporate spikes of f
flow to create artificial tl s to clean f
the sloughs. The ho>_Irl , p� r : r n can al I-) 1
model these, spikes ;1� iu,_ Cream a
requirements at (Told Creel— f
Input to the hourly r„o<h_i ,onsists ::f p
the initial. storol _Te at Elio Ix- Tinriins; � f t
the week a�ld trig am, 1i 1 !)I iclLC1' to ie e
released during the we(-1. (c;`, f ;tirle.d tr,un
the results of the weekly c.iim>ilation). n
The program gener.att=s a k•�1r%e called a f
template for use as a f,u i- 1 simulation t
of hourly power g�eneratlon_ fhe first t
template is equivalent tr) the ijutirty sy,-- w
tem load minus the exlstil,g ilyrllro produc- w
tion. Turbine release in each, time st�=p m
is determined from the energy requirement f
of the template. The turbine release Is s
then checked with the flow constraints ai!l i
the total release is adjusted to satisfy f
the constraints if there is any violation. f
After the first iterallic%n with the tem- n
plate, the template f.-; acoor 1i P
to the ratio of the amoiin'of %,latc:,r to v
released and thr> tzIt -1_1 Dirt ` ic,w i r
previous iteratio.?. ulatior1 t
iterated wi t h novi to ,i milt j l L1k," S
flow for the wr-el,. �s rj u ,, ] t , i
of water to br, re; r -seJ I ,r - wh
The iIOU:'i" }?l -., +i'
h
reservoir. Wrilen tl; ' Di-' i t C.eI t
voir is in oper_atiun, tlh�
will ioai;-toy low a?fin
plant wi 1 1 t=_ basf,.. 2 i>zl,i::i {
release from Devil Cans ..,,—i 1? �l hf stable
and the variation 0 ih'ire th o
downstream '.f.a??n. 1 c�u11:.i cu.1 t
trolled. B c_!lsra tat !k;w is r n7
10
in off-peak hours and high flow in peak
hours, Devil Canyon will draw down in
off-peak hours and fill in peak hours. The
maximum drawdown for daily fluctuation at
Devil Canyon was estimated at one-half
foot.
The hourly program as well as the week-
ly program have provisions for flood
operation. During a large flood when the
reservoir is full, the reservoir inflow
could be greater than the sum of turbine
and valve capacities. If the spillway is
used, nitrogen would be entrained in the
water and there would be the potential
for nitrogen concentration to exceed
tolerable levels. In order to minimize
use of the spillway, the reservoir is
allowed to surcharge above the normal
maximum level_ up to an environmental sur-
charge level. The environmental surcharge
for Watana low dam would be 14 ft and that
for Watana high dam would be 8 ft. These
levels were determined on the basis of
avoiding the use of the spillway in a
flood of less than a 50-year return peri-
od. The spillway would not be open unless
the water surface elevation reaches the
environmental surcharge level.
In non -flood operation the valves would
not release water unless it is necessary
for the instream flow requirements. When
the water surface elevation is at or above
the normal maximum level, the excess water
would be released from the valves. As the
water starts to surcharge above the normal
maximum level in a flood, the total out-
flow could be increased hourly at a
special flood rate, designed to minimize
impacts on the fishery from changes in
flow and temperature, until the valves are
fully open. However, the outflow would
never be allowed to be greater than the
peak discharge of inflow. As stated pre-
viously, if the water surface elevation
reaches the environmental surcharge level,
the spillway would be opened for release
so that the outflow would be equal to
inflow. The falling limb of the outflow
hydrograph would also be constrained by an
hourly decreasing rate for flood opera-
tion.
CONCLUSION
The monthly simulation with rule curve
Operation is simpler and less expensive
than the others. It was effectively used
in the economic analysis of the project.
The weekly simulation with the opera-
tion guide more closely simulates the dis-
charge variations for the studies of envi-
ronmental impacts. The operation guide
restricts the discharge variation in a
specified limit to secure the protection
of fishery habitat. The simulation with
the weekly model was successfully used for
the evaluation of the flow regimes.
The hourly simulation was used to test
how the energy obtained from the weekly
analysis could fit the hourly load curve.
It was also used for the study of peaking
capacity with respect to the allowable
fluctuation of discharge in the downstream
channel.
Monthly, weekly, and hourly operation
models are all indispensable in the study
of the Susitna Hydroelectric Project.
REFERRXES
Alaska Power Authority, 1985. Case E-VI
Alternative Flow Regime, Susitna
Hydroelectric Project, Volume 1 - Main
Report, Document No. 2600.
P.H. Dondi and G. Schaffe, 1983. Simula-
tion and Optimization of a Series of
Hydro Stations, Water Power and Dam
Construction, Nov. 1983.
Alaska Power Authority, 1982. Feasibility
Report, Susitna Hydroelectric Project,
Volume 1, Fngineering and Economic
Aspects.
11
1