Holes In Ice
by Bob Dill - November 1990
There are two classes of open water features in ice. Holes are formed by either delayed freezing or remelting of part of an ice sheet. Cracks form from stresses caused by temperature change or wind. Holes constitute a significant hazard for iceboaters. A better understanding of their origin will make it easier to stay out of them. There are at least 9 fairly distinct types of holes. I have named them as follows (listed in rough order of commonness for ice boaters on northern Vermont ice).
* Thawed Pressure ridges
* Warm Water Holes
* Gas Holes
* Drain Holes
* Lake Shore Holes
* Reef Holes
* Man made Holes
* Current Holes
* Spring Holes
The following is the first in a series of articles about holes. This article covers warm water holes and gas holes. Both are common early ice holes.
Warm water holes:
When the ice first freezes it usually does so during a cold snap in calm conditions. In many cases the bulk of the water is not really as cold as it should be to sustain a frozen surface. In this case there will often be holes from a few feet to a few tens of feet in dimension that do not freeze over until several hours to a couple of weeks after the main ice sheet forms. These areas probably stay open because of convection currents set up in the water by the large difference between the surface temp of the water under the ice and the water exposed to the very cold air above the holes. They often stay open for days or weeks because early season cold snaps are often followed by a period above freezing weather. Characteristics of warm water holes:
1. They form on water that is "not ready to freeze happily".
2. Are usually roundish in shape.
3. They are associated with first ice.
4. Often seem to be in groups.
5. They often have a ring of white ice on some or all sides from water that splashed out and froze.
6. When these holes do skin over they are typically one to 4 inches thinner than the surrounding ice.
7. They generally do not all freeze at the same time.
8. The hole edge is usually nearly full plate thickness to within a few inches of the edge .
9. They are very easy to spot from shore (ideally at a little elevation above the lake) on a sunny day with some wind to make the open water dark.
10. They do not tend to form in the same spot year after year.
Warm water holes are probably the second most common form of Holes. They are particularly insidious when they first skin over. This makes them very hard to see and gives the sailor no way to know how thick the ice is. We have generally found the only way to be sure they won't be a problem is to carefully check out the area we will be sailing. Observing the ice as it freezes will give a good idea of where to expect problems.
At the second attempt to sail the 1986 NA regatta on Lake Honeye in western New York. Henry Bossett was sailing at speed to windward with several other sailors when he dropped a side runner into a gas hole. Henry was not hurt but his boat was destroyed. Luckily, it was not his front runner.
When we looked around there were holes and runner cracks all over the place in about a 100 yd by 200 yd area. The holes ranged from a couple to several feet in dimension with a circular shape. All of them had a stream of gas bubbles coming up from the bottom near their center. After the accident the sailors walked most of the lake and did not find holes other than this and another small area.
Gas holes are common in certain areas and unknown in most areas. They are caused by the steady release of marsh gas from the lake bottom. The rising gas pulls a plume of warmer water up from deeper in the lake that keeps the hole open. The gas comes from decomposition of vegetable matter on the lake bottom. They are almost always in a limited area. Once the area of concern is identified, it is easy to check to see if they will be a problem. Most (but not all) the ones I have encountered were associated with a stream that empties into the lake nearby.
They tend to be a problem in the early part of the season. By mid-season the cold weather often overpowers the ability of the gas to keep the hole open. Late in the season or during mid-winter thaws they are often the first places to open up. Last spring on Lake Dunmore, Dave Terwilliger sailed up to the starting line, stepped out of his boat and fell through a gas hole to his shoulders! They certainly were not easy to see. We had not encountered them in this area in the early winter when we usually sail there. Based on this, they may be more numerous in the spring than at first ice.
Hud Brush, who has several gas holes in front of his camp says they will freeze solid it you poke a hole in the skinned over holes to let the gas out. It this is done with the help of some matches the project can be quite entertaining. Be Careful!
Characteristics of Gas Holes
1. They have a stream of gas bubbles in their center. The bubble stream may be intermittent.
2. They are typically round with a dimension of 1 to 4 feet although they often join together to form larger, irregular holes.
3. They often skin over at night making them treacherous.
4. They are almost always in the same area or even the same exact spot from year to year. (When you are sailing on a new lake, ask the locals about the ice. On Lake Honeys it turned out that local fishermen had told some of the iceboaters about the gas holes there.)
5. If the ice is clear it will often have large white gas bubbles in it.
6. They are often (but not always) associated with stream or river entrance areas. They may be a considerable distance from the actual stream entrance.
7. They are more of a problem in the early and late season than in the cold part of the winter. In the spring they may be an especially white spot or lump on the ice. They are hard to recognize with old snow drifts and other features in spring ice.
8. They often skin over at night and reopen during the day.
The next article will cover holes that are typical of midwinter ice. It you have anticdotes, other perspectives or additional thoughts on this subject, please give me a call or drop me a line.
by Bob Dill - December 1990
This is the second article on ice holes. It covers holes that are common in midwinter ice.
Holes in Pressure Ridges
Pressure ridges are really cracks although they often have true holes in them after a period of warm weather. They are the most popular way to fall through the ice. On Lake Champlain I estimate that over 80% of the car-through-the- ice-accidents occur at pressure ridges.
They are a feature of all ice with dimensions larger than about a mile. They tend to run from point to point and along the shore. They are caused by buckling failure of the ice sheet as it is heated during the day. They tend to occur in about the same locations every year but can occur in unexpected places. They usually form when the ice is 2 to 4 inches thick. When they form in much thicker ice the event is similar to an earthquake. Pressure ridges will be covered in more detail in a future article on cracks.
Pressure ridges form "holes" in three ways:
1. The plates can push down to form a ice bottomed puddle that can be 15 feet deep and 50+ feet wide. This is common on at the ends of ridges that do not go to shore. Keep this in mind when you are sailing around the end of a ridge.
2. The ice in the active part of the ridge may be quite broken up with no solid sections.
3. There can be places where there the plate is missing. Usually this is because it melted during warm weather. In sunny thaw conditions, the underwater ice in a ridge melts more quickly than the ice on the surface.
These areas are often characterized by a lack of pushed up plates in this part of the ridge. In thaw conditions, a ridge can deteriorate into open water very quickly.
In general ridges are easy to see because of the pushed up ice blocks along the ridge. They can often be crossed safely if they are checked out carefully first. Some sort of a probe is helpful for this. The boat makes a good bridge to cross the wet side of the ridge. Crossing ridges always requires good judgment and experience.
On new black ice in the 3 to 4 inch range I have seen pieces of the plate break off under the leeward runner when we are trying to cross a ridge. So far we have been able to get the runner out with out getting wet. A wet suit, bear claws and a rescue throw rope are in order for crossing ridges in marginal conditions.
Sailing across pressure ridges is always a bad bet. The little time it takes to check the ridge and walk across greatly reduces the chances of getting in trouble.
In Trenton, Ontario at the 1988 North Americans, Joe Norton slowly sailed across a small ridge directly behind another boat. The first boat was lucky and went across with out a problem. Joe was lucky to, he only broke off the nose of his boat and hit his jaw hard on the tiller. It could have been worse.
Reef holes are similar to warm water holes except that they occur at the same spot each year. They tend to be about the same size as the reef and often have thinner ice right at their edge. They tend to skin over in cold spells and reopen in less cold weather. The reefs that run between islands and shore are often open or covered with much thinner ice than most of the ice sheet. Keep this in mind when sailing between islands.
We have a couple of reef holes in Lake Champlain that almost never freeze. These holes are typically about 75 by 150 ft. The water under the holes is about 15 feet deep in what is otherwise the deepest part of the lake.
Characteristics of Reef holes:
1. Occur over reefs.
2. Can be as small as to feet up to almost any size.
3. Tend to be round or oval in shape.
4. Often have well developed rim of splashed out water and frozen slush.
5. Are often populated by ducks as they often are the last available open water in the area. (Ducks are a reliable sign of open water).
6. Reef holes can freeze over but will generally remain thinner than deep water ice and will then reopen in thaw conditions.
Ducks, geese and other water fowl will congregate in larger holes of any sort. These holes are often the only open water left in the area. I suspect the birds are not usually the source of these holes although they help keep them open.
Characteristics of duck holes:
In rivers and lakes where rivers enter them there are often holes and thin spots associated with moving water. The ice gets thinner or opens with warmer weather and/or higher flow rates. Current holes are particularly dangerous because if you do go through one you can be swept down stream under the ice! When rescuing a boat in this situation be sure you have a belay rope on the rescuers. Sailing on large rivers (as the Montreal sailors do) requires special knowledge of the ice.
There is a widely held belief that underwater springs are responsible for most holes. In my ten years of looking at ice holes I have yet to find one that I thought was caused by a spring. Most of the supposed spring holes are gas, warm water or reef holes. There probably are real spring holes, but they are clearly much less common than many people believe.
Man Made Holes
Bubblers are probably the second most popular way to go swimming in the winter. Air bubblers or underwater propeller stirrers are used to keep the ice away from docks or boats. They typically keep the water ice free for about 5 to 25 feet away. They often are turned off at night to save energy cost. The open water generally skims over resulting in a difficult to see area of the thin ice. In general it is a good idea to approach unknown docks cautiously.
Fishermen certainly make the most plentiful holes in the ice. In most areas the fish are too small to warrant holes larger than about 8 inches in diameter. Unless these are eroded into larger drain holes they are too small to get a runner into. In some areas there are big fish under the ice and larger holes are cut, some with dimensions more than large enough to fit a runner. There is often a block of ice nearby. Bob Cummins reports that sturgeon fishermen make holes that are refrigerator sized! And they push their blocks under the ice.
Fishermen are well equipped to make holes of just about any size. This is one more reason to avoid conflicts with them.
Divers, ice cutters and others also cut holes in the ice. Local knowledge is probably the best protection from these types of holes.
If you have thoughts on this subject you would like to share with the class, write me a letter or give me a call.