The measurement of water temperature during frazil ice formation

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The measurement of water temperature during frazil ice formation

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THE

OF

WATER DUP+IIK;

FEAZIL

IClL FOEHhTXOH

G.P. Williams

Mhenever supercooled water in reservoitrs, l a k e s ar rivers comes

fn

c o n t a c t w i t h hydro plant: or water supply in-takes, there is a danger of clogging from the formation o f frazil ice. Because of this problem, there

has been considerable i n t e r e s t in measuring water t emperatare during f s a z i l

I c e f omation and using these measurements t o make p r e d i c t i o n s . There are,

however, f e w water temperature measurements reported i n the l iteratuse on

frazil ice and most give m l y s p o t recordings at a p a r t i c u l a r site. There

is n o t any record of contknuous water temperature measurements over a p e r i o d

of s e v e r a l days or weeks iln rapids where f r a z l l ice forms.

This report presents some water temperature measurements made in t h e

rapids of a river while f razil i c e w a s forming. These measurements were

made to o b t a i n some field i n f o n u a t i o n on the duratfon and amount of

supercooling a t t a i n e d in open river water under severe w i n t e r conditions.

The a c t u a l problems encountered in measuring the water terrrperature were a l s o a s s e s s e d .

The water temperature measurements

were

made in the Calumet r a p i d s of

the Gatineau River about: 100 km n o r t h of O t t a w a (Figure 1).

Tn

e a r l y

w l n t e s , t h e rapids are abaut 60 metres w i d e and several hundred metres l o n g

( F i g u r e 2).

Water temperature was measured w i t h a thermistor calibrated

in

the laboratory. The themistor was fastened t o t h e t o p of one of t h e l e g s o f a

t r i p o d . Each s t e e l l e g was

75

cm l o n g and 2.5 cm in dfameter, encased in

l e a d at the bottom for stability in s w i f t flowing r a p i d s , Shielded, plastic-covered microphone c a b l e connected t h e thermistor to a m i l l i v o l r recorder i n s t a l l e d in a c o t t a g e abaut 100 m fzom the edge o f t h e r a p i d s ,

Figure 3 shows water temperatures recorded Jan. 8-20, 1965 and a i r temperatures observed at a standard meteorological s t a t i c m at: Maniwaki, a

few kilometres north of t h e s i t e .

During t h e first cold days (Jan. 9-12) there

were

short p e r i o d s of supercooling when t h e temperature fluctuated

from

s l i g h t l y above t o s l i g h t l y

below 0 % . A s it g o t colder (Jan. 1 2 - 1 4 ) these w a s a p e r i o d of almost continuous supercooling, with the average amount under lD.L°C, h t h e last days of t h e c o l d s p e l l the supercooling became less frequent, but a m a x f m m

(4)

DISCUSSION OF

REZULTS

In

discussing the s i g n i f icmce of these r e s u l t s several factors rmst

be

considered. The dependence of water temperature T x , at a site

in

rhe

r a p i d a , on t h e

following

v a r i a b l e s is shown

in

Figure 4 :

(1) Temperature of water coming out of upstream ice cover

(T

2).

(2)

Heat l o s s to the atmosphere from the open water in

the

rapids

( Q l and Q 2 ) *

( 3 )

Flow conditions

in

river-depth of r i v e r , speed of

f

lowlng water.

( 4 ) Heat gained from ground under the water

(Q3

-I- Q b ) .

(5)

L o c a t i o n

of

instrument

in

rapids. ( 6 ) Instrumnt errors.

If temperature

T2

i s high enough, the open water nil

1

n o t cool

s u f f i c f e n t l y t o supercool and produce f r a z i l ice. T 2 is a f f e c t e d by t h e

temperature of the water ( T ~ ) as it leaves a reservoir about 100 kflornetres upstream from t h e s i t e and by heat losses t o the atmosphere fn t h e upstream

(Q

and open water areas.

In the e a r l y stages

of

i c e formation much a£ the river is open and the heat loss t o t h e atmosphere

(QL)

is h i g h - As Eseezzng continues, the i c e

cover increases u n t i l the r i v e r upstream from t h e r a p i d s is completely

covered. Further heat loss te t h e atmosphere results in t h i c k e r i c e w i t h relatively l i t t l e change in water temperature (T2). Ihergy l o s s due t o

f r i c t i o n under the i c e c o v e r tends to increase the water temperature (T

The heat gained by t h e water from t h e f r i c t i a n of moving under 100 km of

upstream i c e theoretically increases its temperature by about

Q.14

6 .

The heat gained from bottom material

CQ3)

a l s o t e n d s to increase the water temperature I T p ) , but n o t as s i g n i f i c a n t l y as the e f f e c t of friction.

Ground heat only increases water temperature

(TZ)

by 0.01-0.02"C.

The field measurements show t h a t the combined upstream ice cover and

f r i c t i m losses are suf f

i c i m t

ta increase w a t e r temperature, eventually reducing supercooling and f r a z i l ice production. For example, there w a s a

d i s t i n c t warming t r e n d in water temperatures with

only

sporadic supercooling after January

lbth,

even rhough

sir

temperatures remained well below -2Q6c,

B e s t lass

Q2

Heat loss from t h e open section of rapids (Q

1

i s the second major

v a r i a b l e determining temperature Tx. The d e p t h of water in the r a p i d s is

(5)

heat loss required to produce a

given

amount of supercooling increases a s

the depth and volume of t h e water does. Heat loss f r o m the bottom

(Q J ,

a

very s m a l l q u a n t i t y in comparison afth

Q2,

is probably of minor importance

i n frazil ice f o m t i m in a section of r a p i d s .

The p o s i t i o n of the thermistor r e l a t l v e t o t h e upstream i c e cover also

d e t e d n e s the amount of supercooling recorded a t p o s i t i o n x. P l a c i n g the

instrument near t h e upstream i c e

wfll

n o t provi rle satisfactory results,

s i n c e t h e water w i l l n o t have s u f f i c i e n t time t o supercaof, even under maximum cooling c u n d i r i o n s . Placing the instrument n e a r t h e edge of t h e

down stream i c e cover will n o t be satisfactory either, because t h e

supercooling cycle wfll already be finished. The mixture of f r a z 5 l ice and

water will be at: O o G . Only when t h e instrument is at t h e p a s i t i o n a h m in

Fig. 4 ( d ) will i t be in a zone of

maximum

supercooling.

As cold weather continues, sheet i c e formatfon at both the downstream

and upstream edge of the i c e cover reduces t h e area of open water,

E v e n t u a l l y it becomes so small t h a t , even under severe c o o l i n g c o n d i t i o n s , t h e r e is n o t enough time for t h e water t o supercool, and the supercooling c y c l e is finfshed.

O t h e r Cansidcratims

The time taken for f r a z i l ice to grew is another variable that a f f e c t s

the amount of supercooltng recorded. S t u d i e s i n d i c a t e t h a t the rate o f f r a z i l ice growth v a r i e s with t h e amount of c o o l i n g . Assumlng an average supercooling of

0.05'C

and an average growth rate of 2.0 x m J s , I t takes about 100 s for a frazil ice p a r t t c l e t o reach a diameter of 2 mm. If

the speed of t h e water is assumed to b e 3.0 m / s , the d i s t a n c e f o r an i c e

particle to grow to t h i s diameter is a b u t 300

m.

Temperaturesensing instruments located anywhere in the f i r s t 300 m of r a p i d s are in a zone of

c o n t i n u o u s supercooling.

Measuring water temperature under these conditions poses many

difficulties.

If

ice c o l l e c t s on the sensing d e v i c e , t h e temperature

recorded will be O'C even though there might be c o n s i d e r a b l e supercooling in the surrounding water. Sunlight penetrating the water c o u l d a f f e c t the

temperature reading, Changes in water l e v e l s a l t e r the p o s i t i o n of the

s e n s i n g devf ce r e l a t i v e to the surface, which might result in s l i g h t l y d i f f e r e n t supercoo2%ng temperatures. F i n a l l y , it shoald

be

emphasized t h a t

one is a t t e m p t i n g t o measure small temperature differences

In

turbulent

water near the freezing point, a task r e q u i r i n g sensitive

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-

@f

i

' C A L U M E T R A P I D S I (SITE OF W A T E R D ,./- JEMPERATW R E I M E A S U R E M E N T S ) I I \ 1 8 O 1 2 3 4 5 c m

I

-

t

TO O T T A W A 8 0 k m Ir crn = i.26 k m FIGURE 1

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d 0 , 3 -

5 0 . 2 - WATER TEMPERATURE

-

ZONE OF CONTINUOUS -

-

ALTERmTING

- -

-

FRAZlL ICE PRODUCTION

+

2 - 0 , 4

-

SUPERCOOLING ICONTI NUOUS SUPERC00tlNG1 POST-SUPERCOOL1 NG

-

- 0 . 5 - SUPERCOOL1 NG PER lOD

-

- 0 . b I I I I i t I I I I I I I

8 P 10 11 12 13 14 15 16 17 18 1 9 20 2 1

JANUARY 1%5 F l G U R E 3

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UPSTREAM Q , (ATMOSPHERIC HEAT LOSS)

Q2 (ATMOSPHERIC HEAT LOSS)

SENSING lNSTRUMENT F I G U R E 4 0 . 1 0

u

"

- 0 . 1 - S C H E M A T I C D I A G R A M S H O W I N G D E P E N D E N C E OF T x O N S E V E R A L V A R I A B L E S

-

SLIGHT SUPERCOOL1 NG T, -------I,,

.

W c4 3 0 . 1 I- Q n= w - 0 . 1

-

MODERATE Tx 0 4 - - - -

-

- - -

4 ( = )

'2-t

/ -

-

--#d a

r

0 . 1 - 0 . 1 T2 MAXIMUM Tx - T 3 "---I---

-

, _,

-

4 _{( d l} T2 \ /

-

---

\

/

r

x T3

-,--

0 . 1 0 - 0 . 1 - - \ NO SUPERCOOLING

=-

L..---

-

4

lei