Unnatural Accumulation of Ice: An Engineer's Perspective
Investigations of slip and fall accidents involving ice accumulation can be performed without falling into the realm of junk science.
- By Steven Zebich, S.E., P.E.
- Jan 01, 2004
THIS is the time of year when we at Packer Engineering are often asked to investigate personal injury claims involving that old nemesis of the Great White North: "Unnatural Accumulation of Ice!" In the Midwest, we have to live with ice and snow as a regular part of our existence, and we know when traversing the slippery fields of frozen delights, one must traverse with a bit more care than in the middle of August.
However, we in Illinois must live with the fact that if it can be shown one party is responsible for an unnatural accumulation of ice, then that party (and others, depending on who's writing the complaint) could be held legally liable for injuries occurring to some poor misfortunate who slipped on the unnatural accumulation.
A clear definition of what exactly an unnatural accumulation of ice is, cannot be found--not in the statutes, not in case law, and certainly not in the annals of junk science. And boy, do we see a lot of junk science out there! In our experience at Packer Engineering, the majority of slip-and-fall accidents that involve an alleged unnatural accumulation of ice typically involve an identified source. This article presents a rational method for determining whether an accumulation of water or ice can be considered an unnatural accumulation, based on sound engineering principles.
This source of unnatural accumulation is often reported as some sort of physical, man-made device, such as a downspout or roof edge. In recent years, however, we are beginning to see claims that even potholes and sidewalk cracks are sources of unnatural accumulation. Yes, we know potholes and sidewalk cracks exist virtually everywhere in urban and suburban America, and that essentially means virtually any time someone slips on ice, someone might make the claim that the ice in question accumulated unnaturally.
This is the crux of the problem. There are ludicrous claims being made these days that ice accumulated unnaturally, simply because there was a man-made object somewhere in the vicinity. What's even worse, there are experts who will support such nonsense with opinions that have virtually no technical basis.
I could write an entire article discussing some really amazing examples of junk science we've run across. However, we choose to be constructive when dealing with technical issues. Engineering professionals must utilize their training and experience to shed light on that which is misunderstood. Virtually every aspect of engineering theory had a beginning somewhere, typically building upon what was already known, using fundamental engineering principles.
The Technical Solution
A rational and technical basis for determining whether or not an accumulation of ice can be considered an unnatural accumulation is founded upon the basic principles of drainage system design. In designing drainage systems, several salient concepts must be understood:
1. The purpose of any drainage system is to divert or channel water in such a manner as to effectively transport it to a location suitable for either storage or removal.
2. Suitable locations can be man-made buildings, structures, or other appurtenances may be employed, as well as common ground surfaces such as pavement, culverts and swales, roads and walks, or even lawns.
3. It is understood and accepted within the design community that minor or de minimis deviations in surface profile can occur, creating ponding and puddles in man-made structures or naturally occurring topographic features. Minor ponding and puddles occur frequently in nature and are not considered to be unnatural accumulations.
4. An appropriate drainage system is one in which water is channeled or diverted in such a manner as to prevent concentrating water from a large area onto a smaller one in which pedestrians must traverse. A classic example of concentrating drainage water, and thus creating an unnatural accumulation, would be to drain a large roof of a building via gutters and downspouts onto one or two locations adjacent to an entryway or walkway. Minor architectural projections such as cornices and mansards do not create unnatural accumulations of water and ice.
Based on the above concepts, it is possible to determine whether an accumulation of ice or water can be considered unnatural. These concepts allow for a rational and reasonable assessment, rather than one that assumes any accumulation on or near a man made object is unnatural. In our experience, such assumptions are unreasonable and rarely have a sound technical basis.
In order for ice to accumulate unnaturally, from an engineering perspective, conditions surrounding the accumulation must meet the following criteria:
1. There must be a potential man-made source that caused the accumulation. The source must be a man-made structure or appurtenance that either channels or diverts quantities of water exceeding the flow that ordinarily occurs on common ground surfaces.
2. There must be a potential for the discharge to have occurred from the source. Weather data or other factual information must confirm that conditions for water flow from the source actually could have occurred.
3. The local topography must have allowed water to flow from the source to the point of the alleged accumulation. There must be quantified data using precise measurement techniques that accurately characterize drainage patterns along the area in question.
If it can be shown that water accumulated from a source meeting the definition given above, water discharge actually occurred from the identified source, and drainage from the source to a point where water could accumulate was physically possible, then the accumulation may be considered unnatural. In some instances, the source may be collocated with the point of unnatural accumulation. As long as the three conditions above have been met, there is a sound technical basis for confirming the existence of unnatural accumulation.
In order to offer additional insight to aid in understanding the three conditions, each is discussed in further detail below.
Identifiable Source
Look at any common ground surface, such as streets and sidewalks, yards and fields, and even parking lots. You'll see these surfaces are in no way perfectly level, nor are they perfectly smooth. In fact, you'll see these surfaces contain many common and expected irregularities and imperfections resulting from a number of man-made and natural reasons. It is intuitively obvious that when rain or snow accumulates on these surfaces, it accumulates irregularly, and in accordance with basic physical principals.
In addition to what simply collects on the ground, the water or snow that accumulates on window ledges, tree limbs, light poles, walls and roofs, rocks and cars, even the tops of our heads, all comprise a natural accumulation. The problem arises when melt from this accumulation is concentrated in an amount exceeding quantities that would ordinarily occur on these surfaces.
A classic example of a man-made object that channels and diverts large quantities of water onto a concentrated spot is a downspout. As an example, visualize that drainage from the entire roof of the building is channeled back to a gutter serviced by a single, and rather serpentine, downspout. Clearly, the amount of water that discharges from the end of such downspouts exceeds what would ordinarily occur on common ground surfaces. Hundreds, perhaps even thousands of square feet of drainage can potentially be discharged onto a single location. This is a classic example of a source that could create an unnatural accumulation.
By contrast, a minor architectural projection, such as a cornice, a mansard, or small awning, do not create accumulations exceeding quantities that occur along adjacent areas. If we are to assume these minor appurtenances create unnatural accumulations, then we should quickly eliminate all objects that can possibly collect snow or water, and in the process destroy nearly 4,000 years of architectural history. What's more, we in the Midwest will have to live in complete denial of winter.
Discharge from the Source
Clearly, there are truly legitimate potential sources of unnatural ice accumulation. However, in reviewing the work of some experts, we have found junk-science most often rears its ugly head when it comes to determining whether or not discharge from the source actually occurred.
If the issue is one of snow melt creating an unnatural accumulation of ice, then a proper weather data analysis is critical. The most useful sources of determining what the weather was doing at the time of a slip and fall on ice is photographic documentation of the scene as soon as possible after the incident. Unfortunately, this is rarely done. We are often left with nothing more than sketchy witness testimony that may have been taken years after the alleged accident.
Lacking other reliable documentation, the only reliable means available to evaluate meteorological conditions at a particular location and time is to review weather data recorded for the time in question from a reliable weather reporting station. The National Climatic Data Center (NCDC) can provide a wealth of information from weather reporting station locations, typically airports. This article will not attempt to train the reader in weather data interpretation; however, because of our extensive knowledge and experience in aviation-related matters, we regularly review aviation METAR reports in our investigations. Our experience shows several key concerns must be evaluated when conducting investigations related to ice accumulation:
1. Measured precipitation data gives only part of the picture. Snow or rain may have, in fact, occurred before a reported incident. However, temperatures, available solar radiation, or some other source must be present to induce freezing and melting. In order for interpretations of precipitation data to have any technically based validity, solar angle, solar radiation, or heat loss computations may be required. Unfortunately, most junk science practitioners don't include such a technical analysis to formulate their opinions.
2. Ground surface icing can and does occur without precipitation. We have reviewed a number of cases in which ice was present on the ground, and yet, there was absolutely no precipitation for weeks prior to the incident! In aviation, ground surface icing, commonly referred to as "black ice," is a well-known phenomenon that occurs when the relative humidity is high (85-100 percent), fog or haze is present, and the temperature-dew point spread is low. Essentially, water droplets within moist air masses can freeze upon contact with cooled surfaces. This is common when nighttime temperatures dip well below freezing, and ground surfaces, buildings, and other objects remain cool while the morning temperature rises.
3. When snow melts, it does not normally create a cascading torrent. Have you ever wondered why on sunny, warm winter days, the roads seem to dry up and the snow piles seem to just shrink away? Contrary to the theories of junk science, just because conditions are right for a snow melt to occur doesn't mean water will always flow in quantities large enough to create puddling. Ice or snow must absorb a substantial amount of energy, known as latent heat, before melting occurs. Once a melt does occur, it occurs slowly. On very sunny days, pavements can warm sufficiently to dry out and allow water to be absorbed into the pavement through wicking. If the relative humidity is low, some of the melt actually can evaporate.
Obviously, from the above discussion, it takes a little effort to analyze the effects of weather beyond simply determining if it rained or snowed. If the discharge happens to come from a man-made source, such as a sump pump discharge, another analysis needs to be performed to see whether the discharge is possible.
One case reviewed at Packer Engineering involved a sump pump discharge pipe. It was alleged the discharge pipe created an unnatural accumulation of ice when snow melted into the ground, collected in the perimeter drain tile, then collected in the sump, where the pump then discharged the water onto a walkway. Our review of an expert's report revealed junk science of the highest order! The report failed to indicate that on the day of the accident, snow ground cover was just less than 1 inch. It failed to mention temperatures were below freezing at the time of the incident. It failed to consider the ability (or inability) of the partially frozen soil to absorb melted water, and it failed to calculate the amount of melt that could occur.
In fact, the potential for water even entering the sump pit was minuscule based on the amount of snow ground cover. Furthermore, based on the local topography, melting snow would have flowed away from the spot of alleged unnatural accumulation!
Topography and Drainage
Once it has been determined a viable source could have actually discharged water to create an unnatural accumulation of ice, the road to be traveled must comply with the dicta of Sir Isaac Newton, "Water flows down hill because gravity so compels it to!"
Evaluation of slope and topography is perhaps one of the easiest steps in the entire process, because the means of measurement are simple and accurate. Nothing more than a simple digital level may be used to provide slope information. In cases where the path water must travel is long and rather complex, surveying equipment can be used to obtain extremely accurate topographic elevation data. This data can then be input into computer software, which not only produces a fine topographic plan but can also trace the path of water from any point on the projection. This is equivalent to performing a water flow test on the computer.
There are times when it may be just as simple to pour water from the alleged source to see where it flows. In one case we tested, a junk science practitioner hypothesized that water flowed over a grass-covered swale and flowed over the sidewalk and onto the area located under the arrow. Aside from the fact this particular expert did not evaluate the climatic conditions and the ability of water to actually traverse the grass-covered swale, not one single slope measurement was taken.
As the test clearly demonstrated, any water flow would have gone nowhere near the alleged spot of unnatural accumulation. Furthermore, the swale, which was a minor depression in the grass, could in no way divert water anywhere in excess of that which would occur on any common ground surface.
Conclusions
Rational methods utilizing basic engineering principles can help investigators avoid the pitfalls of junk science in an area of accident investigation that has virtually no useful statutory guidance from a technical perspective. Though the methods and tools discussed are well known within the professional community, their application in considering the unnatural accumulation of ice are not as well appreciated. This article has attempted to enhance that appreciation.
This article originally appeared in the January 2004 issue of Occupational Health & Safety.