the ice. What will be the most effective charge
three 25-lb charges. Put these values in
size or sizes?
columns 1 and 7 for the upper jam part
Step 1. Since the ice is considerably thicker at
of the table.
the downstream end, larger charges
Step 7. Determine the blast crater diameter for
will probably be required, so compute
the 3-ft-thick ice.
separately for the lower 0.5 mile and
From Figure 24, the crater size is 21 ft for 10-
the upper 1.5 miles.
lb charges, 25 ft for 15-lb charges, 28 ft
Divide the chart into two areas for the lower
for 20-lb charges, and 31 ft for 25-lb
0.5 mile and upper 1.5 miles. Fill in
charges. Fill in column 2 with the ap-
column 3 for 0.5 mile and 1.5 miles in
propriate crater diameter.
the two separate groups. Fill in columns
Step 8. Complete the rest of Table 21.
9, 11 and 12 for round trip time, explo-
Compute the values for columns 4, 5, 6, 8,
sive cost and helicopter cost, using the
10, 13, 14 and 15 as before. As can be
same numbers for each group.
seen, the 10-lb charges result in the low-
Step 2. Determine the minimum charge weight
est cost. Even if the pilot limited the
required.
load to four charges per trip for safety
From Figure 23 the minimum charge size is
reasons due to the time it takes to light
about 18 lb for 10-ft-thick ice and about
more charges, the 10-lb charges would
12 lb for 3-ft-thick ice.
still be slightly less expensive.
Step 3. Determine the number of charges per
These examples illustrate how effective charge
trip for various sizes of charges to blast
sizes can quickly be determined. It is best if only a
the 10-ft-thick ice.
few charge sizes are considered to reduce compu-
Since the load is limited to 80 lb, you could
tation time, so it is important to have a good idea
have four 20-lb charges, three 25-lb
of what charge sizes may be effective. It is also
charges, two 40-lb charges or one 80-lb
important to remember that the cheapest alterna-
charge. Put these numbers in columns
tive to getting the job done is not necessarily the
1 and 7.
best alternative if it takes considerably longer to
Step 4. Determine the blast crater for the 10-ft-
accomplish.
thick ice.
From Figure 24, find that a 20-lb charge has an
8-ft-diameter crater, a 25-lb charge has a
CONCLUSIONS
14-ft crater, a 40-lb charge has a 22-ft
crater and an 80-lb charge has a 36-ft
The historical data-collection effort found much
crater. Fill in column 2 with the blast
available information, both published and oral. Some
craters for the appropriate size of charge.
information, such as ice-thickness measurements
Step 5. Fill in the remainder of the chart for
from USGS stream-gaging records, is very time con-
the lower 0.5 mile.
suming to compile, while other information, such
Compute columns 4, 5, 6, 8, 10, 13, 14 and 15
as USGS discharge records, is relatively easy to ac-
as in the previous example. As can be
cess. In general, information about the physical
seen, the 40-lb charges are the least ex-
characteristics of ice and ice jams is the most diffi-
pensive, but the 25-lb charges only cost
cult to obtain, simply because there has not been a
about more and take only about 30
concerted effort in the past to collect such informa-
minutes longer to blast the lower part
tion, the information available is widely scattered,
of the jam. The larger charge may be
and most people do not take particular notice of
better because they will clear a wider
the ice until it is causing problems. The data-col-
channel; the smaller charges may be
lection program initiated as a result of this study
more convenient for handling.
should begin to provide better ice information.
Step 6. Determine the number of charges per
The analysis of the collected data attempted to
trip for various sizes of charges to blast
find relationships or trends among the various
the 3-ft-thick ice.
hydrological, meteorological and ice records per-
Since the ice is much thinner in the upper
taining to ice events in the study area. The results
1.5 miles, smaller charges can be used.
of the analysis were used to develop an ice-jam
You could have eight 10-lb charges, five
predictive model based on hydrologic and meteo-
15-lb charges, four 20-lb charges or
rological data that are relatively easy to access.
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