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Field determination of free water content in wet snow
Williams, G. P.
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Ser
TH1
N21r2
no. 26
NATIONAL RESEARCH COUNCIL
CANADA
DIVISION O F BUILDING R E S E A R C H
A F I E L D DETERMINATION O F F R E E WATER C O N T E N T IN W E T SNOW
B Y
G. P. WILLIAMS
R E P R I N T E D F R O M 1956 PROCEEDINGS O F THE
WESTERN SNOW C O N F E R E N C E
NRC 4110
RESEARCH P A P E R NO. 26
O F THE
.NATIONAL RESEi2C!I C O U N C I LDIVISION O F BUILDING R E S E A R C H
OCTOBER 1956
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A FLELD DETERMINATION OF F R E E WATER CONTENT LN W E T SNOW by
G. P. wil1iamsl-l Summary
A field method of determining the f r e e w a t e r content of snow by m e a s u r i n g t h e density t o which wet snow can b e compacted w a s investigated. E x p e r i m e n t a l t e s t s show that t h e r e is a good relationship betweeq f r e e w a t e r content and compacted density e s p e c i a l l y f o r settled and c o a r s e spring snow and f o r water contents above 5 percent. It is considered that the main value of taking compacted densities will be t o give information on the physical s t r u c t u r e of wet snow during t e s t s .
T h e f r e e w a t e r content in wet snow i s a n important p r o p e r t y that must be d e t e r m i n e d in a n y r e a l i s t i c classification of snow. It is a p r o p e r t y that should b e recorded during any t e s t s under wet snow conditions. Unless t h i s b a s i c f a c t o r is known, such t e s t s cannot be c o m p a r e d o r t h e i r r e s u l t s properly analyzed. In addition a knowledge of the f r e e w a t e r content and w a t e r holding capacity of wet snow h a s long been of g r e a t i n t e r e s t to hydrologists.
T h e method most commonly used to d e t e r m i n e f r e e w a t e r content i s the c a l o r i m e t r i c method described i n detail by Halliday (1). In t h i s method a quantity of wet snow is placed in a c a l o r i m e t e r containing hot w a t e r . The weight-of snow. the weight of hot w a t e r , a n d the initial and final t e m p e r a t u r e s of t h e w a t e r a r e m e a s u r e d . Then by using a s i m p l e heat balance equation it i s possible t o calculate the percentage of f r e e w a t e r in the s a m p l e . Halliday finds quite a range in the a c c u r a c y claimed by s e v e r a l different o b s e n r e r s using t h i s method.
E v e n if reasonable a c c u r a c y can be obtained the c a l o r i m e t e r method is not r e a d i l y s a t i s f a c t o r y f o r field use. F o r good r e s u l t s a c c u r a t e t e m p e r a t u r e and weight readings a r e necessary. A s f r e e water content v a r i e s appreciably with t i m e in a single day, with snow depth, and with position a suitable field method should p e r m i t the taking of s e v e r a l representative s a m p l e s over the a r e a where the snow is being tested. In the c a l o r i m e t e r method t h e number of s a m p l e s that can be handled is limited by t h e number of c a l o r i m e t e r s available. T h e whole procedure is time-consuming and not suitable f o r field use, especially i n isolated a r e a s .
Other non-calorimetric methods have been suggested by B a d e r (2). G e r d e l ( 3 ) h a s developed a m'eter which m e a s u r e s t h e percentage of f r e e w a t e r by measui-ing the d i e l e c t r i c content of snow. Aura and Kinesita ( 4 ) investigated the m e a s u r e m e n t of f r e e w a t e r content in snow by centrifugal separation. T h e s e methods a r e promising but r e p o r t s on t h e i r u s e a r e limited.
In s o i l mechanics, P r o c t o r (5) found that the m o i s t u r e content had an effect upon the density t o which soil could be compacted. By using t h i s dependence he w a s able to m e a s u r e the m o i s t u r e content of a p a r t i c u l a r s o i l by the u s e of a plasticity needle. A s the s i z e v a r i a t i o n of individual g r a i n s is l e s s in snow than in soil a n d a s t h e specific gravity of i c e i s constant, the possibility a r i s e s that the relationship P r o c t o r established f o r soils might prove to be even m o r e applicable t o wet snow. It w a s t h e r e f o r e decided to investigate the compacted density of snow u n d e r v a r i o u s conditions t o s e e if a dependence with f r e e w a t e r content could be e s t a b l i s h e d .
Experimental P r o c e d u r e
T h e f i r s t step w a s to devise a method of compacting snow s a m p l e s that could be duplicated and could be r e l i e d upon to produce consistent r e s u l t s . Two methods of compacting wet snow s a m p l e s w e r e tried.
In the f i r s t method the wet snow s a m p l e s w e r e collected in a specially p r e p a r e d cylinder of 100 c c . , with p e r f o r a t e d s i d e s t o facilitate the e s c a p e of a i r . T h e wet snow was i n s e r t e d loosely into the sampling cylinder. A constant load of 1000 gm/crn2 w a s quickly applied by hand u s i ~ g a n N. R. C . snow h a r d n e s s gauge. When using this method it w a s n e c e s s a r y to m e a s u r e the height of the c o m p r e s s e d s a m p l e and weigh i t t o find the final density. Also f o r consistent r e s u l t s it w a s n e c e s s a r y to i n s e r t the s a m e weight of snow into the container for each compaction test.
In t h e second method the wet snow was i n s e r t e d loosely in one-inch l a y e r s i n a 250 cc. container. Each l a y e r w a s tamped with four blows of a 1000 g r a m weight dropping 10 centimeters. T h i s method, modeled a f t e r a m e a n s of obtaining r e l a t i v e density i n s o i l m e c h a n i c s (6), produced m o r e consistent r e s u l t s . The final compacted. density w a s obtained by adding the weights of four 250 cc. c o m p r e s s e d s a m p l e s .
F i g u r e 1 shows typical c u r v e s showing t h e relationship between compacted snow density a n d the number of t i m e s a 1000 g r a m weight i s dropped 10 c m . on a one-inch l a y e r i n the confined sample. T h e points w h e r e the c u r v e s intercept the Y a x i s w e r e obtained by i n s e r t i n g the snow loosely i n one-inch l a y e r s without tamping. F i g u r e 1 shows that f o r d r y snow the maximum density i n c r e a s e s only gradually a f t e r four blows of t h e 1000 gram weight.
-
. . .
l r ~ i v i s i o n of Building R e s e a r c h , National R e s e a r c h Council, Ottawa, Canada.
-
- 7 0
I
I
I
1
I
rCOARSE
GRANULAR
-6
0
-5
0
40
FINE FRESHLY DRIFTED
N.D.
-
NATURAL
DENSITY
1
NUMBER OF BLOWS
/
LAYER
FIGURE 1
TYPICAL CURVES SHOWING RELATIONSHIP
BETWEEN COMPACTED SNOW
DENSITY
&
NUMBER OF TIMES
A
1000
GRAM WEIGHT
IS DROPPED
10 CM
ON ONE INCH SNOW
LAYERS IN CONFINED SAMPLE
I
COARSE SNOW
-7
* 'F I N A L COMPACTED DENSITY
(COMPACTION
METHOD
NUMBER 2 )
FIGURE
2
TYPICAL CURVES SHOWING
FINAL
DENSITY OF COMPACTED
SAMPLE
V ,
PERCENT BY WEIGHT OF FREE
WATER
I N WET SNOW
FINAL DENSITY OF COMPRESSED SAMPLE
(COMPACTION
METHOD
NUMBERI )
LABORATORY
RESULTS
FINAL COMPACTED DENSITY
(COMPACTION
METHOD NUMBER
2 )
0
FINE GRANULAR SNOW 1.19 MM.
$LESS
COARSE
11 1 1 4.76MM. T O
2.30
MM.
e----.
MARCH
2/56 NATURAL DENSITY
=.a20
0-0
MARCH
21/56
NATURAL DENSITY =.260
FINAL DENSITY OF COMPRESSED SNOW
FIGURE
4
SAMPLE
(
LOAD
=
1000
GM/CM*
)
VERSUS
THE RELATIONSHIP
BETWEEN COMPACTED
PERCENT
BY WEIGHT OF FREE WATER
DENSITY 8 PERCENT FREE
WATER
FOR
The f r e e water content of wet snow was determined by the calorimetric method using essentially the pro- cedure recommended by SIPRE (7). To keep e r r o r s to a minimum the r e s u l t s f r o m tests in t h r e e thermos bottles were always averaged.
Whenever possible tests w e r e run on days when a i r temperature were below freezing at night and above freezing during the day. During the morning the dry snow was compressed and the compacted density measured. As the water content increased, further compacted density measurements were made and the f r e e water measured.
At ,first the snow in the test a r e a was checked by the method outlined hy Krumbein
(a),
t o see if it was homogeneous over the a r e a sampled. However experience under field conditions indicated that a few check samples of compacted density were sufficient to ensure that samples were being taken under essentially uniform conditions. Snow samples for compaction were taken f r o m four points immediately adjacent t o the points f r o m which t h e three samples were taken for f r e e water rontent determinations.Experimental Results
Figure 2 i s a plot of typical curves showing the relationship between final density of a compacted sample and the percent by weight of f r e e water in wet snow. The effect of f r e e water pontent on new snow i s to increase the density to which it can be compacted. Fine-grained settled snow and c o a r s e spring snow have a more gradual in- c r e a s e in compacted density with increasing f r e e water content. At f r e e water contents of 5 percent and g r e a t e r the r a t e at which compacted density increases with increasing f r e e water content s e e m s t o be comparable f o r dif- ferent snow types.
To check the effect of c r y s t a l s i z e on the density t o which wet snow can be compacted, measurements on f r e e water content and the compacted density were made on wet snow in the laboratory. The snow was screened into two different size ranges: 4.76 mm. t o 2 . 3 0 mm. and 1 . 1 9 mm. and less. The snow was compacted by method No. 1. Figure 3, showing the results of these experiments, indicates that the final density of the compressed sample for any particular water content above 5 percent i s about the s a m e f o r both s e t s of laboratory samples. , There i s some
tendency for the fine-grained type to compress to a higher density but the variation i s not a s great a s might be ex- pected.
Figure 4 shows how the experimental results can be duplicated. Observations on compacted density and f r e e water content were made on two different types of snow on different days under field conditions. The dependence between compacted density and f r e e water content is very nearly the same f o r both tests. Most of the variation i s probably due to e r r o r s in measurement of f r e e water content.
Discussion of Results
To estimate f r e e water content f r o m compacted density an observer would have t o know the compacted density of the sample when dry and the compacted density vs. f r e e water content curve f o r each snow type. F o r water contents above 5 percent, especially f o r settled and coarse spring snow, t h e r e i s a close relationship between f r e e water content and compacted density. Below 5 percent t h e r e i s more variation in this relationship, especially f o r new snow.
One objection to this method i s that it is sometimes difficult t o standardize the compaction procedure. How- ever t e s t s by two different observers instructed in the second compaction method have shown that the final compaction densities can be duplicated satisfactorily. An obsenrer using this method will have to be f a m i l i a r with the different snow types and have had some experience with the technique of compacting snow samples.
It i s considered that the main value of taking compacted densities will be to give infarmation on the physical structure of wet snow during tests. F o r example if ski equipment is being tested under wet snow conditions, an ob- s e r v e r could keep a record of compacted density during the experiment and thus would have a means of a s s e s s i n g the changes in amount of f r e e water which have taken place during the experiment. Compacted density would b e recorded in the s a m e manner a s undisturbed density; grain size and shape a r e recorded f o r tests of this kind.
Tests on the density to which wet snow can be compacted might be an objective way of classifying the physical structure of snow during the spring melt period. There has been considerable discussion about the t e r m "ripeness" a s applied t o snow cover during the melting period ( 9 ) . Church ( 1 0 ) has suggested that the relative density of the snow o r a study of its c r y s t a l structure will indicate its capacity to hold melt water. As most studies on the water holding capacity of a snow pack deal with c o a r s e settled snow with f r e e water content above 5 percent, this method of determining f r e e water content might be of value t o hydrologists and give the general term of "ripe- ness" mcre meaning.
Acknowledgments
I
I The author wishes t o express his appreciation t o L. W. Gold f o r his encouragement and to R. Armour for his help in taking field observations.
This paper is published with the approval of the D i r e c t o r of the Division of Building Research, National R e s e a r c h Council, Ottawa.
R e f e r e n c e s Cited
( 1 ) Halliday, I. G., 1950: The liquid water content of snow measurement in the field. J o u r n a l of Glaciology, 7:357-361.
(2) Bader, H., 1948: Theory of non-calorimetric methods f o r t h e determination of t h e liquid water content of wet snow, Schweizerische Mineralogische a n d Petrographische Mitteilungen. 28:355-361.
(3) Gerdel. R. W.. 1954: The t r a n s m i s s i o n of w a t e r through snow. T r a n s . American Geophysical Union. 35:3:475-485.
(4) Aura. H. and S. Kinesita. 1954: Measurement of f r e e w a t e r content in snow by centrifugal s e p a r a t o r , Low T e m p e r a t u r e Science. S e r i e s A12
-
S e r . CI L91.(5) P r o c t o r . R. R.. 1933: Design and construction of rolled e a r t h dams, Engineering News-Record. 111:245-248, 286-289.
(6) . B u r m i s t e r . D. M.. 1948: T h e importance and p r a c t i c a l u s e of r e l a t i v e density i n s o i l mechanics, R e p r i n t f r o m P r o c s . . A m e r i c a n Society f o r Testing Materials. Vol. 48. 20p.
(7) SIPRE, 1952: Instructions f o r the Measurement of t h e F r e e W a t e r Content in Snow, Snow. Ice and P e r m a f r o s t R e s e a r c h Establishment Operation Manual No. 2, C o r p s of Engineers, U. S. Army, Wilmette, Illinois. (8) Krumbein. W. C . . 1955: Experimental design in e a r t h sciences, T r a n s . American Geophysical Union,
36:l:l-11.
(9) Gerdel. R. W.. Discussion of the t r a n s m i s s i o n of w a t e r through snow, Trans. A m e r i c a n Geophysical Union. 35:475-485. 1954; 36:2:347, 1955.
(10) Church. J. E . , 1948: The evolution of snow-melt by dyes and drip-pan, Assoc. hydrologie sci. A s s e m b l 6 e A n h a l e dlOslo, 19-20 a o t t , 1948, 2:115-117.
