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SUSITllA HYDROELECTRIC PROJECT
SLOUGH HYDROGEOLOGY REPORT
l lar ch 1983
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SUSITNA HYDROELECTRIC PROJECT
SLOUGH HYDROGEOLOGY REPORT
MARCH 1983
TABL E OF CONTENTS
1 -OBJECT!YES AND APPROACH 1
2 -FIELD DATA 2
2.1 -Collect ion 2
2.2 -I nt erpret at ion •••••.•••••••.••••••••••••••••••••2
2 .3 -Determination of Material Properties ••••••••••••4
2.3 .1 -Hydraulic COnductivity ..... .............4
2.3.2 -Transmissiv ity ••••••••••••••.•.•••••••••4
2.3.3 - Thermal Propert ies •••••• ••••••••••••••••5
3 -ANALySIS.......................... ..............................................7
3.1 -Groundwater Flow...••••..••••.7
3.1.1 -Introduct ion .•••.•.•....................7
3.1.2 -Method...... ............................7
3.1.3 -Geometry......7
3 .1 .4 - Boundary Con d it ion s .••••.••••.•••..•••••8
3 .1.5 -Materi al Properties ••••••••••••••••. •••. 9
3.1 .6 -Resu l ts 9
3.1.7 -Discussion and Conclusions •.••.•••••••••10
3.2 -ThermaI Analyses ••..•..••. •• . . . ••• . . •• . . . ••• . . . • 11
3.2.1 -Gene ra l 11
3.2.2 - Method ...•..•. . •• ..•. ..•.••.•••. . ••••. . . 11
3 .2.3 - Geometry 11
3 . 2. 4 -Bo und ary Co nd it i ons and Loads 11
3 .2 .5 -Prop erties •. ••.••• ..••.•••.••••••••••.•. 12
3.2 .6 - Resul ts and Conclus ions 12
3.3 -Coupled Therma l and Groundwater 'Flow .••••• •. .•.. 13
3 .3. 1 -Met hod ...•.•.•.••..•...•••..•.....•.•.••13
3.3 .2 - Geomet ry 17
3 .3 . 3 - Boundary Condit ions 17
3.3.4 -Results and Conclusions .........•.••••.•18
3.4 -Dis cuss ions •. . ..••. ..•••. . ...•. . . ....... . . •...• 19
TA BLE OF CONTENT S
4 -POST -PROJEC T IMPACTS ••••••••••••••.•••••••••••••••••••••21
4.1 - Types of Changes 21
4.2 -Description of Impact •••••••••••••••••••••••••••21
4.2.1 - Watana Construction 21
4.2.2 - Watana I mpo undment 22
4.2.3 - Watana Operat ion 23
4 .2 .4 -Devil Canyon Construction •••••••••••••••25
4.2.5 -Devil Cayon Impoundment 25
4.2.6 - Devi l Canyo n Operation ••••••••••••••• •••25
References .....•....•••••.•..............................••.27
Li st of Tab les ..
List of Figures ii
LIST OF TABLES
1.Latent Heat Distribut ions for :
(a)Silt
(b)Sand
(c)Gravel
2. Ccmputed Seasonal Groundwater Tenperature
F luctuations
LI ST OF FIGURES
Fluxes
Fl ux es
Fluxes
(b)
(b)
(b)
Slough 8A Groundwater Contours
Slough 9 Groundwater Contours
Slough SA Groundwater Temperatures
Slough 9 Groundwater Temperatures
Grain Size Analysis :Alluvium from Slough 9
Slough 9 Flow Net Sketch
Hydrograph Of The Susitna River
Aquife.r Diffusivity From Aquifer Response To
Fluctuations In Susitna River At Slough SA
Aquifer Diffusivity From Aquifer Response To
Fluctuations In Susitna River At Slough SA
Slough 9 Area Modelled Using Finite Element
Method
Slough 9 Finite Element Mesh
S lough 9 Run 4: (a) Contours
Slough 9 Run 5 : (a) Contours
Slough 9 Run 6 :(a) Contours
Ground Surface Temperature
Temperatu re Variation with Depth
Deve lopm ent of Square Wave
Sus itna River Valley Tem per ature vs Distance
Susitna River Valley Temperat ure vs Distance
Susitna River Valley Temperature vs Distan ce
l.
2.
3.
4.
5.
6.
7a
7b
7c
8.
g.
lOa .b ,
lIa,b.
12a,b.
13 .
14.
15.
16.
17.
18.
i i
SUSIT NA HYDROEL ECT RIC PROJECT
SLOUGH HYDROGEOLO GY STU DIES REPORT
1 -OBJECTIV ES AND APPROACH
The objective of th is study task is to understand slough hy-
drogeo logy under exist ing.natura l conditions and thus pro-
vide a method ology by which post-project conditions can be
pred ict.ed ,
The s tudy is compr ised of fou r s t ages
- da ta coll~ction
-data int erpretation
- model ing of ex ist ing cond i tio ns to understand processes
-predi ct ion of post-project conditions.
2 - FIE LD DATA
2.1 -Collection
Field data collection has included:
(a)walk-overs and fly-overs of the various sloughs between
Talkeetna and Devi 1 Canyon to appreciate their morphology
(b)excavation of test pits and installation of shallow wells
in Sloughs 8A and 9 (spring 1982)
(c )measurement of slough profiles,cross sections and dis-
charges
(d)deep drill holes and i nst al lat ion of wa ter level and water
temperature measuring devices
(e)monitor ing of groundwater levels ,t emperatures ,river
stages and d ischarge .
On-going work includes completion of recen t deep dri lling in-
stru mentat ion and cont inuing monito ring of observation wells
and upwe ll ing tempe ratures.
Complete deta ils of all fie ld data are c ont i nued i n the Slough
Hyd rology In ter im Report (R8Jo1 Consu ltants 198 2b).
2.2 -Interpretation
The sloughs are form ed as s ide channel spi 11ways during ice
jam conditions at breakup or during ice front progression in
early winter.Apart from th ese occas ions and open water high
2
flow conditions,there is no direct connection between the
head end of the slough and the Susitna mainstem.
The groundwater prov ides two important funct ions with regard
to the fisheries habitat.Firstly,during the spawning sea-
son it provides flow within the slough to allow the sa lmon to
reach spawning areas in the upstream sections of the sloughs;
second ly,the groundwater upwe 11ing provides a near ly con-
stant temperature for incubation of the salmon eggs and pre-
vents freezing during the winter period.
The soil stratigraphy,determined by the drilling and test
pit excavation,consists of a thin layer of topsoil overlying
2 to 6 ft of sandy silt.Below this is a heterogeneous allu-
vium comprising sand,silt,gravel,cobbles and boulders.It
is probable that this alluvium has variable hydraulic conduc-
ti vities both vertically and laterally,reflecting the moving
stream bed location during deposition.
Observation well and piezometer installation ind ica te a gen-
eral groundwater flow in a downstream direction and locally
laterally toward the sloughs (Figures 1 and 2).
Temperature measurements in the main stem show a const ant tem-
perature of approximately O·C for the period of mid-October
to mid-Apri 1.The temperature rises to a dai ly maximum of
approximately l3°C in mid-July and then decreases to O·C by
mid-October.The slough temperatures show a si milar pattern.
The shallow groundwater temperatures vary between near O°C in
spring,up to BOC by late summer. In general,those closest
to the river show a faster respon se to river temperature than
do those more d istant (Figures 3 and 4).Up..telling -:
"-temperatures measured by intergravel probes show a near )
constant annual temperature of 2 to 4°C.
3
The upwell ings are visible as small "sand bo ils"at som e
d iscrete locat ions.However,suff ici ent measurements have
not yet been made to determine if upwelling is actually oc-
curr ing at a reduced rate in other areas where there is no
evident surface expression.Localized upwelling is not un-
expected due to the spatial variability of the alluvium.
Vis ible upwell ing probably occu rs in areas where there is
thin cover to a layer or lens with particularly high hy-
draulic conductivity.
2.3 -Determination of Material Properties
2 .3.1 -Hydraulic Conductivity
Hydrau li c conduct i vity has been es timated by the fo 1-
lowing methods.
(a)Grain Size Based on grain size analyses f rom a
bulk sample taken from the riverbank in slough g
(Figure 5),the hydraulic conductivity is esti-
mated at 170 ft/d from appl ication of the Hazen
formula (Terzaghi and Peck 1948).
(b)Similar Depos its -MeasJrements of the hydra ulic
conductivity of alluv ial gravels i n the city of
Fairbanks give a value o f 1,000 ftld (Nelson
1978).
Based on thes e data a value of 200 ft/d has been used
in analyses.
2.3.2 -Transmiss ivity
Transmissivity is defined as hydraulic conductivity
multipl ied by saturated thickness.In some techniques ,
the transm iss ivity is determined i ni ti a lly , and the hy-
draulic conductivity is calculated us ing an assumed
saturated thickness.Transmissivity has been
4
ca leu lated based on the fo llowing methods.
(a)Flow Net - From a flow net sketch (Figure 6 )and
ffieasurement of discharge into the upper reaches of
slough 9 of 1 cf s ,the transmissivity is estimated
at 9,000 ft 2/d.
(b)Wel l Response - From the response of shallow wells
to rapid changes in storage in the mainst~
(Figure 7),the method described by Pinder,
Bredehoef t and Cooper (1969),resu lts in es t imates
of transmissivity in the range of 1200 to 306,000
ft2/d.It appears that thi s method i s not
suitable for the particular site conditions.
Thes e results have therefore not been used.
2.3.3 -Thermal Properties
The thermal conductivity,specific heat and latent
heat of the soils is required for the thermal analy-
sis.No measurements were made in the field or lab-
oratory.However,published data (Kersten 1942)allows
the thermal ccnduct iv i ty and specif ic heat to be es t t-
mated with adequate accuracy from the natural moisture
content and dry dens i ty of the so i 1.The values used
in these analyses are surrmarized below.
Thermal Conductivity
(W/mK)
Unfrozen Frozen
Specif ic Heat
(Wy r /m 3K)
Unfrozen Frozen
S i 1t
Sand gravel,
cobb l es ,
boulders
1.42
2.70
1.42
3.70
5
0.068
0.083
0.056
0.064
Latent heat of the soi 1 is determined from natural
moisture conten t (Lunardini 1981).For the so ils in
the slough areas the following values have been used.
Latent Heat
(Wyr 1m 3)
Si It
Sand,gravel,cobbles,
boulders
6
1.525
3.002
3 -ANALYSES
3.1 -Groundwater Flow
3.1.1 -Introduction
The objective of the groundwater flow modeling is to
determine the flow patterns around the sloughs.Pre-
dicted head distribution$Jre compared with actual
water levels measured in the wells.Values of the
transmissivity are altered until a reasonable agreement
is reached and the model is then considered to be cali-
brated.It shou 1d be noted however that a part icu 1ar
head distribution can be obtained by a variety of
boundary conditions and material properties.That is,
a unique solution is not necessarily available.
3.1.2 - Method
The groundwater flow analyses have been undertaken
using a 2-D plan finite element method with the flow
integrated over depth,i.e.,equipotentia 1s are verti-
cal.The transmissivity at any point depends on the
depth of the bedrock (assumed impermeable)and the
groundwater surface elevation.Analy ses are therefore
nonlinear and require iterations to define the steady
state groundwater surface.Constant potentials and/or
def ined flu xes can be appl ied as boundary conditions.
3.1.3 - Geometry
Slough g was selected for model ing s.ince it was the lo-
cation of site investigation and drilling.The area
modeled is shown in Figure 8 and the finite element
mesh in Figur~g.
7
3.1.4 -Boundary Conditions
Fou r boundary cond ition s are requir ed
- va l1ey wa 11
- r iver boundary
-bedrock elevation
-streams and sloughs.
(a)Valley Wall
Thi s is assumed as an impermeab le ba rri er with
z ero fluxes.Initial analyses assumed the valley
side to be vertical.This was subsequently
changed to follow the approximate slope of the
e xposed valley wall.
(b)River Boundary
Water elevations at cross sections LRX-31 through
LRX-36 have been computed us ing HEC-2 program for
13 ,400cfs (RU'l 1982a).River water elevations
were ta ken as the f i xed bo undary potentials based
on interpolat ion betwe en the calculated values at
t he cro ss secti ons.
(cl Bedrock Elevat ions
Bedr ock el evat ion was as sumed t o be 100 ft below
rive r wat er e l evati on and constant in a direct ion
perpend i cular to the river flow.As noted above,
t he va l l r-y wall wa s in cluded alon g the lower mar-
gi n of t he mod e l.In addit ion,some a n al yses in-
cluded a postulated bedrock high in an attempt to
achieve model c a librat ion.
8
(d)Streams and Sloughs
Three -noded film elements were located along all
streams and sloughs to allow the fluxes into or
out of these surface waters to be computed.Mea-
sured elevations of the slough surface water were
applied as boundary conditions.
3.1.5 -Material Properties
Based on the analyses described in Section 2.3,a value
of 9,000 ft 2/d was used for the transmissivity,with
a value of 0.18 for the storage coefficient.
3.1.6 -Results
The anlayses undertaken are discussed below.
-Run 1 appl ied only river water level boundary condi-
tions and an impermeable valley wall with a fixed
saturated thickness.Flow paths were all in a down-
stream di rection with a gradient epprox i mat tnq the
rive r gradient .
For Run 2 the elements be low the slou ghs were given a
transm issivity higher by a f actor of 100.The objec-
t ive of this was to simulate the high conveyance of
the s ur face wat er in th e se areas.
-Run 3 used the s ame geometry as Run 2 with t.he incor-
porat ion of th e valley wall sl ope.Thi s was an at-
tempt to better match the elevat ions in th e area of
we Ils 9 -11 and 9-15 .
I n Run 4 the sloping vall ey wall g ~ocnetry is includ-
ed ,with the water elevati ons i n the sloughs applied
9
as fixed boundary conditions through 3-noded film
elements.The contours are shown in Figure 10 and
again indicate relatively poor agreement in the area
of piezometers 9-11.
- Runs 5 and 6 represent modifications to the bedrock
elevations in the vicinity of those piezometers.The
bedrock hi gh in this area was hypothes ized based on
the shape of the visible valley wall in this area.
However,neither of these runs were abIe to exact ly
reproduce the actual water well elevations in this
area (Figures II,12).
3.1.7 -Discussion and Conclusions
In general,the groundwater flow pattern as deduced
from the model compares reasonably well with that
measu red in the field.It indicates that flow is pri-
mari ly along the valley with local lateral flow toward
the sloughs.Typical flow path lengths between entry
and ex it are of the ord er of 2,000 to 4,000 ft.How-
ever,the model was not abIe to reproduce the ground-
water conditions in the area of well 9-11.This may be
due to a number of reasons.
-A surface stream exists in that area,probably due to
runoff from the up1and a reas.This could loca lly re-
charge the alluvial aquifer.
-Pond ing of surface water behind the rai lway embank-
ment has also been observed,and would lead to eleva-
ted groundwater levels .
-Soil stratigraphy adjacent to the vall ey wall mc:y be
much more vari able than in the center of the valley.
It may conta in si l ty layers which wou ld resu It in
perched water table conditions.The wells in this
area may therefore not be measuring the ma in alluvial
water surf ace.
10
3.2 -Thermal Analyses
3.2.1 -Genera 1
Seasonal fluctuations in air temperature cause fluctua-
tions in the soil temperature.The depth and magnitude
of these changes wi 11 indicate the relative importance
of air temperature compared with seasonal changes in
ri ver water temperature on the upwell ing groundwater
slough temperatures.
3.2.2 - Method
Analyses were made of a one-dimensional vertical ideal-
ization of the soil stratigraphy using a finite element
transient heat transfer progr~~which incorporates the
latent heat of freezing of the soil.
3.2.3 -Geometry
Since the finite element code uses 2-D elements.the
1-0 geometry was idealized as a 3.2-ft wide by 30-ft
deep (L-m by 9.1-m)vertical strip.Six-noded triangu-
lar isoparametric elements were used with material
properties for si It to a depth of 6.9 ft.Below this
properties for sand and gravel were used to the base of
the mode I.
3.2.4 -Boundary Conditions and Loads
The boundary at 3D ft is adiabatic,i.e.,no heat flows
across it.since the geothermal flow is considered neg-
ligible relative to surface temperature driven heat
flows.The temperature applied to the ground surface
was determined from the monthly average air temperature
measured at Talkeetna multiplied by the "n"factor.
The "n"factor for freezing is defined as tile ratio of
11
surf ace freezing index to ai r freezing index and for
thawing is defined as a ratio of surface th awing index
to air thawing index.Based on Lunardini (1981),"n
freezing "was taken as 0.Z9 representing a snow-covered
surface and "n thawing"was taken as 0.37 representing
a surface covered by trees,brush,etc.The monthly
average surface temperature was therefore determined by
mu It ip lying the monthly average ai r temperature by the
appropriate "n"factor depending on the season.This
ensures reasonab le surface freezing and thawing indexes
although it does not necessarily accurately reflect the
vari ation of surface temperature versus time nor the
vari ations which may occur due to different depths of
snow cover through the winter or from year to year.
3.Z.5 -Properties
Thermal conductivity and specific heat were determined
from published data for similar materials,and are de-
tai led in Section Z.3.3.Research has indicated that
unfrozen moisture exists in soi Is below O·C and thus
the latent heat is re leased over a range of tempera-
tures as indicated in Table 1.
3.Z.6 -Results and Conclusions
The resu Its of the thermal analyses are shown in Fig-
ures 13 and 14.The surface temperature follows an
approx imate s inusoida 1 shape reflect ing the seasona 1
temperature vari ations.The maximum depth of freezing
is approximate ly 6 ft and at a depth of 10 ft the an-
nual temperature range is less than Z·C.
An approximate analytical sol ution (Stefans equation)
gi ves resu lts wh ich are in agreement with the fin ite
lZ
element modeling in predicting a maximum freezing depth
of approximately 6 ft.
Since the depth of the groundwater table is typically
greater t han 6 ft,the impact of seasonal air tempera-
ture variations docs not appear to be sign if icant in
determining the groundwater temperatures.
3.3 - Coupled Thermal and
Groundwater Flow
3.3.1 - Method
Coupled thermal and groundwater flow has been analyzed
by cons idering conditions along a flow path,i ,e .1-0
solutions.Two processes are significant
-heat exchange between the flowing pore water and the
soil mineral skeleton
-longitudinal dispersion.
(a)Heat Exchange
The absorption of heat by the miner al skeleton
from t he water is analagous to the sorption pro-
cess whereby chemical species in solution are
sorbed onto soi 1 parti cles.Therefore the equa-
tions used for the former can be modified to
handle thermal considerations.The relative volu-
met r ic heat capacit ies of th e so il and water are
13
where
Vw•Vs =volumetric heat capacities of water
and soil skeleton respectively
Cwo Cs =volumetric specific heats of water
and soil respectively
n =poros ity
The ratio of heat capacities is therefore
Vs/Vw =Cs (l-n)
Cwn
The simi larity between the retardation factor or
contam inant and for thermal transport is illustra-
ted by the following.
For contaminant transpo rt
Rd =1 +f b Kd
n
Rd =retardation factor
14
(Freeze and Cherry 1979)
..
III
I b =bu lk mass dens ity
n =porosity
Kd =distribution coefficient
For thermal transport
Rd =1 + Vs/Vw
Also,for both
v =Rd
vr
where
v =average linear velocity of groundwater
vr =average retarded velocity of the mean con-
centration or temperature.
(b)Longitudinal Dispersion
The concentration of a dissolved species transpor-
ted by groundwater is described by the following
where
D =coefficient of hydrodynamic dispersion
v =average linear groundwat er velocity
c =concentration
x =coordinate direction
t =time
For transport in permeable media,the molecular
diffusion component in the coefficient of
15
hydrodynamic dispers ion can be neglect ed .
Therefore
D =d.v
where ~=dispersivity
For heat transport.the
to concentration.and
equation may be written
where T =temperature .
temperature is equ iva lent
therefore the governing
For a step function input boundary condition.at
I arge x or t ,the s oI ut i on is (F reeze and Cherry
1979)
TlTo =1/2 erfc(x-vt )
2!Dt
wh ere
T =
To =
er fc =
temperature at x, t
step temperature at x =o.t >O
compl ement a ry error function.
(c)Canbined Heat Exchange
a nd Dispersion
The combined effects of heat exchange and di sper-
sion can be approx imated by replacing v by Yr.
a nd t here fore
1 6
TITo =1/2 erfc(x-vrt \
\ifot )
where vr =average r et arded ve loci ty.
3.3.2 -Geometry
Analyses have EIIIployed 1-0 methods,i ,e,co nsider a ti on
of longitudinal dispers ion along a flow line.Typical
flow path lengths are i n the order of 1,000 to 4,000 ft
i n p lan .
3.3.3 - Bounda ry Conditions
The boundary cond it ions requir ing def inition are tem-
pe rature and average groundwater ve loc ity .
Ca) Temperat ure
The ri ver t empera t ure is DoC betw een mid-Octob er
and mid-Apri 1 , and ri ses to a da i Iy maximum o f ap -
prox imately 13°C in Ju ly.The mean ann ual t emper -
ature i s appro x imately 3°C.For t he purpose of
prelimi nary an alys i s,the ann ua l tempe ra tu re va r i-
a ti on ha s be en appro x imated by d sq uare wave with
6 months at DoC and 6 months at 6°C .
Cb)Av e rage Grou ndwate r Veloc ity
The aver age gr oun dwater veloc ity i s def in ed by
v =Ki ln
17
whe re
v =average groundwater ve loc i ty
K =hydraul ic c onduct iv ity
=hydraulic gradient
n =poros ity.
The r anges of these parameters resu It in a best
estimate average groundwater ve locity of 22. 2 ftld
(K =200 ft/d,i =2 x 10 -3,n =0.18).
(c)Retarded Groundwater Veloci ty
Using t he rela t ionshi ps in Sect ion 3.3.1(c)and a
poros i ty of 18 percent,the reta rdat ion facto r has
been c alc ula ted to be 3 .The a ve rage re tarded
groundwater veloc ity is therefore 0 .74 f tld
(270 ft/y r).
(d)Square Wa ve So lution
The annu al va riat ion i n river tempe rature can be
c o ars e ly ap proximat ed by a s qua re wave with 50
per cent duty cy c le ,represen t i ng aver age su mme r
and winter tempe ratures of 6°C and O°C r espec tive-
ly.This results in a mean annua l river t emper a-
t u re of 3°C.Th e so l ut ion for the propagat ion of
a squ are wave a long a fl ow lin e can be developed
by sup erpos it ion of a s er ies of pu l ses of 6°C f or
6 month s at i n ter va ls of 6 mo nt hs. Each pul se
comp rises t wo s tep in puts as sho wn i n Figu re 15 .
3.3.4 - Resu lts a nd Co nc:usi ons
Groundw at er t emp er atu res a long a flow lin e from t he
main stem ha ve been cal c ulated us ing the e qua ti on g iven
18
in 3.3.11c)abov e.Average retarded velocities of 270
ft/yr 1000 ft/yr and 2700 ft/yr ha ve been used.The
first value (270 ft/yr)i s based on the be st estimate
of properties:the other two values are included to
examine the sens itivity of the temperature r ange to the
retarded velocity.They r epresent for example ,an
increase in the hydraulic coroJuctivity by fa ctors 3.7
and 10.
Figu res 16,17,and 18 show the groundwater tempe ra-
tures along the flow line for summer and winter.The
annual temperature fluctuation at various d istances is
summarized i n Table 2. This shows that the temperature
is 3 .:1.5°C at d istances g reate r than 400 f eet from
the ma in stem,for average retarded velocit ies of 1000
ft /yr or less.For a retarded velocity of 2700 ft/yr
the temperatu re f luct uati on i s 3 .:1.5°C for distances
g reater t han 2400 f t.S ince t he f low li ne from main-
stem to sloug h is general ly greater than 500 feet and
typ i cally 1000 to 4000 ft dispersi on along the flow
1i ne and heat exch ange be tween t he wat er and so i 1 par-
tic l es ap pears t o b e a reasonab le me chani sm t o accoun t
fo r the ne ar ly cons tant slough u pwel l ing tempe rat ures.
3 .4 -Dis cus s ion
The 2-D groundw ater f low ana lys es sh ow t hat the f low d irec -
tion is pr in c ipally down st ream and th is acc ord s well with
f i eld observa ti on s.Loca l d et ai ls of the g ro undw at e r eleva -
tion qre no t reproduced by the model and this may be due t o a
var iety of f actors as d isc ussed i n Sect ion 3 .1.The th er ma l
modelin g indi ca tes th at th e atmo sph eric c ond it io ns do not
pen etra te d eepe r than a f ew feet in to th e s ubsoi 1 .Th ey are
t her efore not considered to b e a dominan t fac tor i n det ermi -
19
ning groundwater temperature conditions.The coupled thermal
and groundwater flow analyses show that the temperatures in
the mainstem Susitna can be transferred into the groundwater.
However,dispersion and interchange of thermal energy between
the water and soil skeleton along the long flow paths dampens
the seasonal fluctuations.As a result,the exit tempera-
tures measured in slough upwellings are close to the mean an-
nual average temperature of the mainstem Susitna.
20
4 -POST-PROJECT IMPACTS
4.1 -Types of Changes
Operation of the power plants wi 11 resu 1t il'modification of
the seasonal discharge pattern compared to the existing
natural flow regime. In particu lar,winter flows wi 11 be
higher (approximately 10,000 cfs,compared with 1,000
2,000 cfs at present),and the spring snowmelt flood peak
wi 11 be substanti ally reduced in order to store water in the
Watana reservoir.Summer and fall variations in discharge
due to rainfall events will also be reduced in magnitude due
to the routing of the flow through the reservoir.Because of
the large storage volume of the Watana reservoir,outlet
temperatures wi 11 be cooler in the early sumner and warmer in
the fall and winter.However,the mean annual river
temperature post-project wi 11 be close to the natural mean
annual river temperature (Acres 1983).
Scour and deposition will take
ject as the river attains
post-project flow condit ions.
ial properties of the alluvium
ified.
4.2 -Description of Impact
place downstream from the pro-
a new equilibrium under
However.the pr incipal mater-
are not anticipated t o be mod-
4.2.1 -Watana Construction
Since there wi 11 be no change in ma ins tem discharge and
hence no change in water lev el,there wi 11 be no change
in groundwater condi tion s in the vic inity of the
slouqhs downstream from Watana.Additionally,water
temperatures wi 11 also be unchanged.
21
4.2.2 - Watana Impoundment
(a)Mainstem
As a resu It of the decreased summer flows dur 'ng
filling,water levels in the main stem of the
river wi 11 be reduced between Watana and
Talkeetna.This will in turn cause a reduction in
adjacent groundwater levels.However,the ground-
water level changes wi 11 be confi ned to the river
floodplain area.The groundwater level will be
reduced by about 2 to 4 feet (0.6 to 1.2 m)during
the summer near the streambank with less change
occurring with distance away from the river.
(b)Sloughs
The reduced mainstem flows and associated lower
Susitna River water levels wi 11 slightly modify
the groundwater relationship between the mainstem
and the sloughs.The mainstem water levels
upstream and downstream of a slough control the
groundwater gradient in the slough and s ince both
levels change by approximately the same amount for
different flows,the gradient wi 11 remain the
same.
Because the sloughs are adjacent to the mainstem
of the river,the groundwater level in the sloughs
wi 11 be lowered by the same amoun t as t he stage
change within the mainstem. This will have the
ef f e c t of dewatering the areas in the sloughs
between where the groundwdter table currently
intersects the slough and where the lowered ground-
water tab 1e wi 11 intersect the s 1ough.
22
Data to confinn the areal extent of upwelling at
various flows are unavai lable at this time.How-
ever,it is be 1i eved that slough upwe11ing extends
from the slough mouths well upstream to the
steeper reaches of the sloughs near the ups tream
berms.Therefore,the areas that wi11 be
dewatered wi11 genera lly be the steep ups tream
ends of the sloughs.If both mainstem stage and
groundwater leve 1 change by approximate ly 2 feet
(0.6 ml ,the potent i a1 loss in groundwater
upwelling length will be the stage change (2 feet,
or 0.6 m)multiplied by the slough gradient.
Using the 18.6 foot per mile (3.5 m per km)
gradient measured in Slough 9,the dewatered
length would be approximate ly 570 feet 1171 m).
This is 10 percent of the slough length and,if a
uniform upwelling rate is assumed over the enti re
length of the slough,the decrease in slough
discharge at the mouth will also be 10 percent.
4.2.3 -Watana Operat ion
(a)Mainstem
Groundwater impacts between Dev i 1 Canyon and
Talkeetna during sunmer will be similar to those
descr ibed in Section 4.2.2 and will be confined to
the river area.Since powerhouse flows will
genera lly be greater than fi 11 ing flows during
sunmer,the groundwater level change from natural
conditions will be slightly less than during
fi lling.During winter.increased ice staging
wi 11 occur during freeze-up a nd hence groundwater
level will be increased along ice-covered sections
of the mainstem.
23
(b)Sloug hs
During winter in the Devil Canyon to Talkeetn a
reach ,some of the sloughs (i .e.,those nearer
Tal keetna)will be ad jacent to an i ce-covered s e c-
tion of the Susitna River.In ice-covered
sections,t he Susitna River wi 11 have staged to
form an ice cover at project operation flows of
abou t ~~.OO O cfs.The associated water level will
be a few tt ~above normal winter water levels and
wi 11 cause an increase in the groundwater tab 1e.
This wi 11 in turn cause an increase in groundwater
flow in the sloughs adjacent to an ice covered
reach of the ri ver.
Sloughs upstream of Gold Creek,in the vicinity of
Portage Creek,may be adjacent to open water sec-
tions of the Susitna River .Because flews wi 11 \
average approximately 10 ,000 cfs in winter,the ?
associated wa ter level wi 11 be less than water
1eve I s occu rr ing under t he na tura 1 freeze-up
process.Hence .the groun dwater tab le wi11 be
lower.Slou ghs i n thi s area may e xperi ence a
de crease in groundwater flow in the winter.
Dur ing the summ e r,the mainst em-s10ugh ground wate r
i nteract ion wi 11 be s imi lar to t hat d isc ussed in
Sect ion 4.2 .2 .with t he excep t ion th at op erational
flows wi 11 be greater t han the downstream flows
during f ill in g,and thus,the groundwa ter table
wi 11 be clo ser to the natural elevat ion than dur-
ing fill ing .
24
Preliminary i nves t igat ions indicate that groundwa-
ter upwelling temperatures in sloughs reflect the
long-term average water temperature of the Susitna
River which is approximately 3°(37.4°)(Section
3.4).In the Devil Canyon to Talkeetna reach,the
long term average temperature will not change sig-
nificantly from pre-project conditions (Acres
1983).Hence,groundwater upwelling temperatures
will also not change significantly.
4.2.4 -Devil Canyon Construction
Since the construction at Devil Canyon wiII not
modify the discharge,the groundwater impacts dis-
cussed under Watana operation (Section 4.2.3)wi II
remain relevant during this period.
4.2.5 -Devil Canyon Impoundment
No major groundwater impacts are anticip ated dur-
ing .ne fi lIing of the Dev i I Canyon res ervoi r .
There may be a sl ight decrease in the groundwater
table caus ed by the reduced fi Iling flows .A
decrease in the groundwate r le ve I in t he same
proportion as the de crease in mainstem stage would
be exp ec ted.
4.2.6 -Dev il Canyon Operation
Dow ns tream flows and henc e gro undwat er i mpact s
wi 11 be simi Jar to those occurring with Watana
op e rat ing alone.
25
The ave rage annua l temper at ure at Sherman is ca l-
cu lated to be approximate ly 4°C (Acres 1983).
This is an increase of about 1°C above th e natural
long-term average temperatu re.Ther efore .based
on the groundwater stud ies described i n Section
3 .3 and the above preliminary analysi s . t he slough
upwell ing temperature i n the v ic inity of Sherman
may in crease approximately 1°C .
26
J
References
Acres American Inc orpor at ed.198 3.Susitna Hydroelectric
Project,Application for Li cense for Major Project .
Freeze,R.A. and J.A.Cherry.1979.Groundwater.Prentice
Hall.
Kersten,M.S. 1949 .Thermal Properties of Soils.University
of Minnesota,Engineering Experiment Station Bulletin
28 .
Lunardini,J.F.1981.Heat Transfer In Cold Climates.Van
Nostrand Reinhold.
Nelson,G.L.1978.Hydrologic Informat ion for Land-use
Planning,Fairbanks Vicinity,Alaska .U.S.Geological
Survey,Open File Report 78 -959,Anc horage,Alaska.
P inder,G. F.,J. O.Bredehoeft and H.H .Cooper .1969.
Oetermi nat ion o f Aqu ifer oi ffus i v ity Fr om Aquifer
Response to Fl uctuation s i n River ~.t i\g e.Water
Resources Resea r ch,Volume 5 ,No. 4.
R811 Consultants ,Inc .1982a.Susitna Hy droel l'::t ric Pr oject,
Hyd raulic and Ice Stud ies .Prep ared f or ACI "es Amer ica n
Incorporat ed.
1982b.Susitn a Hydroelectri c Pro j ect,Slough
Hydrology Inter im Report.Prep ared fo r Acr es Ameri can
I ncorpor d t ed .
Jer zaqh t ,R.and R.B .Peck . 1948 Soil Me chanic s In
Engineering Practice.Wi ley.
27
TABLE 1
LATENT HEAT DISTRIBUTION FO H SILT
T Latent Heat
(GCI (Wy r/m 31
- 0 .11 0.184
-0 .2 0 .388
- 0.5
0 . 320
-1.0 0 .20 9
- 2.0 0 .140
-3.0 G.076
- 4.0 0 .066
-6.0 0.060
-8.0 0.041
-10.0.029
-12.0 .012
Tota 1 1.525
LATENT HEAT OI STRIBU TION
FOR SANOAND GRAV EL
T
(GC I
- 0
- 0.1
- 0.2
Latent Heat
(Wyr/m31
1.734
0.951
0.317
TABLE 2
COMP UTE D SEASO~AL GR OUNDWATER TEM PERATURE FLU CTUATIONS
Distance Temperature Fluctuation (OC)
(ft)vr =270 f t /yr vr=100 0 ft/yr vr=2700 ft/yr
min max min max min max
0 0 6 0 6 0 6
200 2.24 3 .76 0.65 5 .35 0.02 5.98
500 2 .80 3 .20 1.62 4.38 0.15 5.85
1000 2.94 3.06 2.48 3.52 0.53 5.47
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