United States
Environmental Protection
Agency
.Environmental Research
Laboratory
Duluth MN 55804
Research and Development
EPA-600/S3-81-008 July 1981
Project Summary
Water Temperature
Dynamics in Experimental
Field Channels: Analysis and
Modeling
Heinz G. Stefan, John Gulliver, Alec Y. Fu, and Michael G. Hahn
This report focuses on water tem-
perature dynamics in the shallow field
channels of the USEPA Monticello
Ecological Research Station (MERS).
The hydraulic and temperature envi-
ronment in the MERS channels was
measured and simulated to provide
some background for several biologi-
cal studies, including one on the
effects of artificially high water tem-
peratures on fish and invertebrate
populations. An analysis of the tem-
perature measurement problem, the
channel temperature regime, micro-
habitat conditions, and temporal* and
spatial water temperature dynamics
was essential. The results of this study
are also applicable to the study of
temperature dynamics in thermally
polluted shallow streams, to the design
of canals for cooling water disposal,
and to the study of natural water
temperature regimes of small streams.
A dynamic stream temperature sim-
ulation model (MNSTREM) with a 3-
hour time resolution was developed
and verified to give predictions with a
standard error of only 0.2 to 0.3°C
between measurements and predic-
tions. A data bank of 3-hour water
temperature data was developed and
analyzed statistically. A wind speed
function, Wftn = 0.0096 (A0V)V3 +
0.0083 W2. essential for the correct
prediction of water temperature, was
developed from on-site meteorologi-
cal and water temperature data. In this
formula, W2 is the windspeed at 2 m
and A9V represents a virtual temp-
erature difference. Longitudinal dis-
persion in the channels was measured.
Heat transfer into the bed material
below the pools of the experimental
channels was measured. A computer
simulation program (TSOIL) to predict
the unsteady water temperature dis-
tribution in the three-layered system
was also developed.
This Project Summary was devel-
oped by EPA's Environmental Re-
search Laboratory. Duluth. MN, to
announce key findings of the research
project which is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The purposes of this study were to
validate water quality criteria data
produced in the laboratory under semi-
natural field (mesoscale) conditions and
to identify significant response by
aquatic organisms under field monitor-
ing conditions. The field channels in
which the experimental studies were
conducted were supplied with water
-------
heated artificially by waste heat from a
nuclear electric generating plant.
The MERS is northwest of Minneapo-
lis, near Monticello, Minnesota, on 34
acres adjacent to a nuclear electric
generating plant owned and operated by
Northern States Power Company. The
MERS has eight soil bottom experimen-
tal open channels of approximately 520
m (1700 ft) length each. The Mississippi
River serves as the main source of water
for the channels which operate in a
once-through mode with the discharge
returned to the river. Heat exchangers
using waste heat (i.e. maximum At
19°C in winter, 13°C in summer) from
the power plant can be used to artificial-
ly heat the water in any of the channels.
Flow rates in each of the channels as
well as water stages can be controlled.
To provide assistance in the ecological
studies, it was necessary to document,
analyze, and predict water temperature
characteristics in the field channels and
to identify sources of variation and
error. Water temperatures in small open
channels are highly dynamic due to the
time-dependent character of the con-
trolling processes: advective transport,
wind and flow induced mixing, and heat
exchange with the air and the channel
bed. In the MERS field channels the
unsteady character of water temperature
is especially acute since both the mixing
characteristics and the effect of advec-
tive transport in the channels are of a
magnitude that must be considered. The
channels are exposed to a wide range of
air temperatures, humidity, wind veloc-
ities, and solar radiation. The corre-
sponding heat transfer with the envi-
ronment is significant: a difference in
temperature between the upper and
lower end as high as 15°C has been
recorded and diurnal variations above
5°C have been observed at the down-
stream end. The experimental channels
provide an excellent medium for the
study of unsteady water temperatures
in open channel flow and their relation-
ship to meteorological parameters.
The work performed for the USEPA
Monticello Ecological Research Station
(MERS) under this grant had two basi-
cally different purposes. One was to
provide background material and gen-
eral information in support of the biolog-
ical work conducted by the MERS field
station staff. The second purpose was to
investigate surface heat transfer, longi-
tudinal dispersion, and water tempera-
ture predictions in streams under field
conditions. This second purpose is basic
and includes verification of the applica-
bility of relationships developed for
larger water bodies to smaller streams.
Accomplishments
Measurement of Hydraulic
and Thermal Parameters
The field channels of the MERS rep-
resent a mesoscale ecosystem whose
hydraulic characteristics have been
measured and estimated.
The investigators intermittently col-
lected data on flow velocity profiles,
surface water slope, flow cross sections,
bed material size, bed porosity and
permeability, and water temperature
stratification below the channel bed.
These data have been used to derive a
number of hydraulic parameters as
follows.
The hydraulic characteristics of the
experimental channels under study in
1975-1977 include:
Manning's roughness: n = 0.06 in the
clearchannels
n= 0.3 with ex-
tensive weed
growth
Mean flow velocities: U — 0.1 m/s in
riffles
U^ 0.02 m/s
in pools
depending on flow rate and downstream
channel control. (U is based on fixed
conditions of flow and channel dimen-
sion.) Velocity profiles are strongly
affected by growth of filamentous algae
in the rock sections and macrophytes in
the pools.
The soil profile below pool beds is
composed of four layers: a highly organic
surface layer, water saturated sandy
loam, water saturated bentonite clay
layer, and a dry sandy loam, in downward
order. The depth of the highly organic
surface layer is variable and increases
with time. The thermal diffusivity of
each layer was determined by fitting soil
temperature prediction to temperature
measurements.
0 = 0.0017 cmVs in the organic
surface layer
D = 0.013 cmVs in the wet sandy loam
below
D = 0.0013 cmVs in the dry sandy loam
below the bentonite seal
The bed of the riffle is composed of
coarse rock with mean size dso = 30 mm.
The rock porosity was estimated at c =
0.42 and its Darcy permeability coeffi-
cient at k = 0.18 m/s. The thermal
diffusivity of the riffle bed was estimated
to be D = 0.0031 cmVs in the rock/water|
system without natural convection. ™
Water Temperature
Instrumentation
Development and operation of instru-
mentation for continuous monitoring of
flow rates and water temperatures in
the channels from 1975 to 1977 was
done by the MERS staff. The investiga-
tors participated in the selection of the
temperature sampling sites.
Temperature sensors (thermocouples)
were installed at the upstream and
downstream ends and in the center
pools and riffles of each of the experi-
mental channels 1, 4, 5, and 8 analyzed
in 1975-1977. The sensors provided
continuous water temperature informa-
tion which was recorded on strip charts.
Three stations per channel were not
sufficient, however, for the analysis of
stratification, surface heat transfer, and
mixing. Channel 1 was selected for in-
depth study of temperature regimes,
and 21 continuous recording sensors
were installed in it. Ten of the sensors
measured water temperature 12.5 cm
(5 in.) to 25 cm (10 in.) below the water
surface. There were two vertical arrays
of four sensors each installed in Pools 6
and 12 which provided information on
water temperature stratification and
soil temperature. Three sensors were
within 12.5 cm (5 in.) from the bottom of
pools 4, 10, and 18, and one sensor 2 to
5 cm (1 to 2 in.) in the rocks of riffle 17.
All temperature sensors were connected
to Leads-Northrup multi-channel color-
coded temperature chart recorders.
Temperatures were recorded from No-
vember 1975 through December 1977,
Water Temperature Data
Processing
Water temperature data processing
had the following elements:
• Extraction of 3-hour data from strip
charts
• Review and estimation of missing
or faulty data
• Calibration
• Storage on magnetic tape
Water temperature data for Channel 1
(strongly heated). Channels 4 and E
(unheated), and Channel 8 (intermedi
ately heated) were processed for the
period from December 1975 through
September 1977.
-------
Water Temperature Data
Analysis and Results
The water temperature data analysis
included:
• Development of statistical analysis
computer programs WTEMP1 and
WTEMP2
• Computation of a number of statis-
tical parameters from 3-hour data
• Determination of the seasonal
water temperature cycle
• Determination of the diurnal water
temperature cycle
• Analysis of longitudinal water
temperature gradients
• Analysis of water temperature
stratification in the pools
Development of Computer
Programs
The computer program WTEMP1 is
designed to extract from the 3-hour
water temperature data statistical infor-
mation including:
• Water temperatures at three prob-
abilities of occurrence specified by
the user
• Maximum and minimum water
temperature in the period analyzed
• Mean, standard deviation, and
dimensionless skewness coeffi-
cient of the water temperature
WTEMP1 can handle a series containing
up to 19,200 individual water tempera-
tures, equivalent to 400 days of data
from six stations in a single channel.
WTEMP1 can also handle sliding statis-
tic calculations for sample periods of up
to seven days. The output is in tabular
form.
A second interactive program WTEMP2
for computing statistics on specified
date-time intervals, composite station
location, etc., from daily statistics was
also developed.
Computations were carried out for
individual stations and channel com-
posites using weekly sliding samples.
The complete output comprised 151
pages for Channel 1 and 126 pages
each for Channels 4, 5, and 8.
Seasonal Water Temperature
Cycles
The amplitude of the seasonal water
temperature was of the order of ±15°C.
The annual mean differed by channel
and station depending on artificial
heating. Channel 1 was artificially
heated, and Channel 5 was not.
Water temperature fluctuations su-
perimposed on the seasonal cycle were
either due to irregularities in the artifi-
cial heating of the inflowing water or to
weather.
Diurnal Water Temperature
Cycles
Inflowing water was taken from the
Mississippi River. Because the field
channels are shallower than the river,
diurnal water temperature amplitudes
at the downstream end of the channels
were larger than upstream. Amplitudes
of diurnal water temperature cycles
Table 1. Monthly Mean and Standard Deviation fa) of Daily Minimum and Daily
Maximum Longitudinal (Spatial) Temperature Difference, and of Diurnal
(Temporal) Temperature Difference at the Outflow from Heated
Channel 1. December 1975 - November 1976
Minimum Maximum Diurnal
Month Longitudinal AT" Longitudinal AT" AT" at Outflow
December (1975)
January (1976)
February
March
April
May
June
July
August
September
October
November (1976)
Mean
PC)
5.2
6.0
4.6
3.2
1.9
1.8
2.7
1.7
0.9
1.7
0.9
1.0
a
PC)
2.0
1.5
1.3
2.7
1.8
1.3
1.5
0.9
1.1
0.7
1.0
1.0
Mean
PC)
8.0
9.5
7.7
6.9
5.6
5.7
4.8
4.0
4.5
3.5
3.4
5.7
a
f°C)
2.5
1.8
1.7
3.1
1.6
1.4
1.6
0.9
2.0
0.9
2.6
3.9
Mean
PC)
1.9
2.3
2.5
3.4
5.3
4.4
4.0
3.5
3.7
2.9
2.9
2.5
a
PC)
0.9
1.0
1.3
1.6
1.9
2.2
0.0
0.8
1.5
1.2
1.2
2.4
varied strongly with the season. Winter
values were of the order of 1 °C upstream
and 2 to 3°C downstream. In the sum-
mer, a 2°C amplitude upstream and a
5°C amplitude downstream were not
uncommon.
Longitudinal Water
Temperature Gradients
Water temperature differentials be-
tween the upstream and the downstream
ends of the channel were of interest for
fish studies in the field channels. Such
longitudinal water temperature gradi-
ents were dependent on several other
hydrothermal parameters. The longitu-
dinal At can be expected to rise with the
addition of artificial heat. A greater flow
rate will decrease the longitudinal AT
Since the geometry of the MERS chan-
nels is fixed in terms of length and
width, the two most important control-
lable parameters were flow rate and
heat input. Actual values of longitudinal
AT's depended on the amount of heat
released to the atmosphere, which is a
weather-dependent process.
The daily minimum and maximum
temperature difference between the
inflow and outflow of the artificially
heated Channel 1 was recorded over a
one-year period from December 1975
through November 1976. Table 1 sum-
marizes the monthly mean and standard
deviation of the longitudinal temperature
differences of Channel 1. Flow rates
ranged from 0.015 to 0.039 mVs.
Daily diurnal water temperature vari-
ations at the outflow of Channel 1 were
recorded over a one-year period and are
also reported in Table 1. These charac-
teristics are for a heated channel with
inflow temperatures increased up to
15°C above the temperature of the
Mississippi River water. Longitudinal
temperature gradients and diurnal
variations are not as large in a channel
with ambient unheated inflow.
Temperature Stratification
in Pools
Vertical stratification of the pool
sections was studied in the artificially
heated Channel 1 and was found to
occur most frequently in August The
temperature gradient was found to
occur primarily in the lowermost 20cm
of the pool below a 60 cm isothermal
layer; the temperature difference was
up to 5°C strong. Analyses of some
-------
1975/76 data in pools 6 and 12 showed
that a temperature differential of more
than 1°C developed during 33 percent
of all hours in August and less frequently
during other months. The more down-
stream pool showed a stronger tendency
to stratify, presumably because the
cumulative effect of atmospheric heat-
ing of the water in it is stronger.
Dynamic Soil Temperature
Prediction Model, TSOIL
A dynamic soil temperature computer
model TSOIL for the pool sections was
developed, primarily to assist in inverte-
brate studies. The model solves the
unsteady heat conduction equation and
predicts the soil temperature profile for
a specified time variation of the water
temperature at the soil/water interface.
It was verified that strongly transient
soil temperatures may be accurately
predicted with this model, as shown in
Fig. 1.
Longitudinal Dispersion Study
The flow field in the MERS channels
is very non-uniform and mean flow
velocities are small. Water moves through
the MERS channels in a way far different
from the movement of a piston in a
cylinder (plug flow). Transverse and/or
vertical mixing takes place between the
faster and the slower water masses.
Vegetation is present in pools and riffles
in the summer. There is a weak jet effect
at the transition from the riffles to the
pools. In the pools, water masses may
be entrapped near the banks. As a result
of all these interacting processes, heat
or material contained in the water willJ
spread out longitudinally along the*
channel. The effect of longitudinal
spreading had to be incorporated in the
channel water temperature model.
Since detailed study of the very com-
plex low velocity channel hydrodynamics
was not part of the research objective,
the water temperature simulation was
based on a uniform channel flow model
with a longitudinal dispersion term.
"Longitudinal dispersion coefficient" is
used herei n to describe a bu Ik coefficient
which lumps the effects of all the
processes mentioned above. To use
separate coefficients for riffles and
pools was considered, but the frequent
transition and short lengths of these
elements made it advisable not to.
The use of a diffusion equation to
describe longitudinal mass transport in
a channel is only justified after the
20
15
Measured Soil Temperature:
O 6 cm depth
Q 16 cm depth
A 28 cm depth
• 41 cm depth
I
I
•s
10
0 0
" — " •
19 | 20 21
22 23 | 24 | 25
December
| 26 | 27 | 28 | 29 \ 30 \ 31 \
1977
7 | 2 | 3
January 1978
Figure 1. Comparison of measured and predicted (solid lines) soil temperatures in Pool 12 during heat wave test.
Notation for depth of predicted soil temperatures: (1) soil surface, (2) 2.5 cm, (3) 6 cm, (4) 16 cm. (5) 28 cm,
(6) 41 cm, 17) 60 cm, 18) 1 m.
-------
nitial corrective period. The MERS
channels begin with a pool into which
water is discharged from a weir produc-
ing a very well mixed upstream condi-
tion. The mean residence time of the
water in a pool was anywhere from 10
to 30 minutes depending on flow rate.
The convective length of the first pool
was estimated to be approximately 5 m,
far less than its total length of 30 m. It
was therefore considered that the con-
vective length did not reach beyond the
length of the first pool.
Longitudinal dispersion in the MERS
field channels was studied using tem-
perature fronts. The longitudinal disper-
sion coefficient was found to be best
approximated by:
DL = 7.47 * Q.
D
where Q is flow rate and B is mean
surface width.
In the range of very low mean flow
velocities investigated (0.04 mVs dis-
charge and U =* 0.02 m/s in pool to 0.1
m/s in riffle), the typical wind and
vegetation effects could not be distin-
guished separately. A typical value of DL
was 0.1 mVs.
Air- Water Interaction
Evaporative and convective surface
heat exchange were found to be depen-
dent on wind speeds and natural con-
vection potential. A wind speed function:
Wftn = 0.0096 (A©,)1'3 + 0.0083 W2
was found to fit the experimental field
data well. A0U represents a virtual
temperature differential and \Nz is the
wind speed at 2 meters above the
ground in m/s. The windspeed function
is used to calculate evaporative surface
heat transfer and convective surface
heat transfer.
Finite Difference, Implicit
Computer Model (MNSTREM)
A finite difference, implicit computer
model (MNSTREM) for the simulation of
the very dynamic channel water tem-
perature regime was developed. It was
shown capable of simulating 1 hr, 3 hr,
or 6 hr water temperatures over periods
up to one month with standard errors of
0.2 to 0.3°C between measurements
and predictions. Weather data, specifi-
cally solar radiation, air temperature,
dewpoint, and wind velocity, are required
as input at least every six hours. Three-
hour input data are standard for the
model. If intervals of more than six
hours are used, the accuracy of the
prediction suffers. MNSTREM has un-
usually high time resolution (3 hrs
normal At, 1 hr possible At), meaning
that it can predict rapid water tempera-
ture changes in very shallow water (Fig.
2).
42
40
38
36
-------
Conclusions
In addition to providing information
on some physical characteristics of the
MERS experimental channels, this
research produced material applicable
to the study of temperature dynamics in
thermally polluted streams in general,
to the design of canals for cooling water
disposal, and to the understanding and
analysis of natural water temperature
regimes of small streams. Some of the
information may also be of use in
studies of mass transport and ecosys-
tem dynamics in shallow channels.
Continuously recording flow metering
and channel stage metering devices at
each end of a channel should be installed
in the MERS channels since flow rates
were only recorded manually once a day
and stages only intermittently.
Recommendations
Water temperature variations in time
and space are a characteristic of any
aquatic environment and especially the
MERS field channels. Standard methods
for the characterization of water tem-
perature dynamics and incorporation of
these into water temperature criteria for
pollution control are needed. The MERS
field channels are ideal for dynamics
studies. They should be used for contin-
ued studies of fate and effect of various
pollutants in an ecosystem at the meso-
scale.
The focus of this study was on water
temperature as a water quality param-
eter. For the model formulation, a
dispersion parameter and a surface
exchange relationship appropriate for
water temperature modeling were de-
rived. Studies of pollutant (mass) trans-
port in the MERS channels require more
refined and detailed information on the
flow and dispersion in a bed of macro-
phytes since uptake and precipitation
are added factors not present in the
temperature model.
If further ecosystem studies in the
MERS channels are conducted, detailed
investigations of the hydrodynamic and
transport dynamics of a pool and of a
riffle, including the effects of vegetation,
must be conducted. For example, the
effect of vegetation on longitudinal
dispersion needs investigation. Flow
effects must be sorted out from the field
observations to derive the true biological
and chemical kinetics of the system.
The 3-hour water temperature data
base derived from continuous recordings
should be analyzed further by time
series (spectral, correlation) analysis. In
addition, weather data and flow data
should be digitized.
Heinz G. Stefan. John Gulliver, and Alec Y. Fu are with the University of
Minnesota. St. Anthony Falls Hydraulic Laboratory, Minneapolis. MN55414,
and Michael G. Hahn is with Mead & Hunt, Madison, Wl 537O5.
Kenneth E. F. Hokanson is the EPA Project Officer (see below).
The complete report, entitled "Water Temperature Dynamics in Experimental
Field Channels: Analysis and Modeling," (Order No. PB 81-218 349;
Cost: $15.50, subject to change) will be available from:*
National Technical Information Service
5285 Port Ftoyal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Monticello Ecological Research Station
U. S. Environmental Protection Agency
Box 500
Monticello, MN 55362
'The final report and six additional memoranda (Project Report 193, Cost. $12 00, subject to change) is
also available from
St Anthony Falls Hydraulic Laboratory
University of Minnesota
Mississippi River at 3rd A venue, S £
Minneapolis, MN 55414
« U.S. OOVCTNMENT PRINTING OFFICE 10«1 -757-012/723
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
r
L
PS 0000329
U S FN'VIR PROTECTIUN
HEG10N 5 LIBRARY
230 S DEARBORM STREET
CHICAGO IL 60604
------- |