HOW TO DETERMINE REQUIRED CAPACITY OF SNOW FENCE SYSTEMS
I.
ESTIMATE AVERAGE WINTER SNOW TRANSPORT (QT)
A. By field measurement
1. Measure cross-sectional area (A, ft2) of snow in cut or natural snow
accumulation area that is not completely full (not necessarily the same
location for which fences are being designed).
2. Measure “fetch” distance contributing snow to accumulation area (ft).
3. Calculate Q′T = 0.45A (A in ft2, Q′T in ft3 H20/ft). The prime (′) is used to
distinguish the measured accumulation from the estimate for the fence site.
This calculation assumes the snow water equivalent is 45%.
4. Enter curves (fig. 1) and determine relocated precipitation (Pr), in inches.
Adjust this value if measurement year was known to be above or below
“normal”.
5. Use this value to determine snow transport (QT) at fence sites having different
fetch distances, using fig. 1.
6. Example: Measure A = 1500 ft2 and fetch = 5,000 ft.
Calculate Q′T = 675 ft2 H2O/ft. From fig. 1, relocated precipitation = 2.7
inches. Interpolating 2.7 inch curve in fig. 1 estimated transport for 10,000 ft
fetch would be about 975 ft3/ft.
For 2,000 ft fetch, estimated transport (QT) = 400 ft3 H2O/ft.
7. Go to II.
B. From precipitation data
1.
Determine average winter precipitation (P, in inches water-equivalent) from
historical records (we use November 1 to April 1 for the winter season at
elevations between 7,000 and 9,000 ft in Wyoming).
2.
Estimate “relocated” winter precipitation (Pr, in inches water-equivalent)
from Pr = P – 0.35 (vegetation height).
Example: if P = 6.0 inches and height of vegetation over fetch distance is
8 inches, Pr = 6.0 - 0.35(8) = 3.2 inches.
3.
Determine fetch distance (R, ft) contributing snow to fence site.
4.
II.
Enter curves (fig.1) with R and Pr and read snow transport from Y-axis.
Example: if Pr = 3.2 inches and R = 10,000 ft, QT = 1180 ft3 H2O/ft.
___
CALCULATE REQUIRED FENCE HEIGHT (H, ft) from H = 1/3 √QT
(Derived from QS = 9 H2, where QS is the storage capacity (ft3 H2O/ft) of a snow
fence having height H (ft)).
III.
If H as determined in Step II is taller than the maximum standard-plan height
(12.4 ft for the Wyoming Highway Department fences) DETERMINE HOW
MANY ROWS OF DIFFERENT HEIGHTS WILL BE REQUIRED. Waterequivalent storage capacity (QS, ft3 H2O/ft) for a fence of height H (ft) is given by
QS = 9 H2:
H (ft)
5.0
6.0
8.0
10.4
12.4
QS (ft3 H2O/ft)
220
320
580
970
1380
Example: If estimated snow transport is 2000 ft3 H2O/ft, use one row of 12.4 ft
fence and one row of 8 ft fence.
IV.
REFERENCE
R. D. Tabler. 1975. Estimating the transport and evaporation of blowing
snow. Proc. Symp. on Snow Management on the Great Plains (Bismarck,
N. Dak., July, 1975), Great Plains Agricultural Council Publ. 73. p. 85104.
Figure 1. Curves for estimating snow transport from “fetch” distance and
relocated precipitation. Graphical solution of:
Q = 5000 Pr (1-(0.14)0.0001R)
From Tabler, R.O. 1975. Estimating the transport and evaporation of blowing snow.
Great Plains Agric. Council Publ. 73. pp. 85