Snow and Ice Control: Guidelines for Materials and Methods (2004)

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6CHAPTER 3 SNOW AND ICE CONTROL OPERATIONAL CONSIDERATIONS The major snow and ice control operational considerations, exclusive of agency resources, are climate, weather, site con- ditions, and traffic. Each has a profound effect on some aspect of operations. CLIMATE CONDITIONS Climate can be defined as the weather that occurs averaged over a specified period of time, normally 30 years. The climate is also defined by statistics about the frequency of extreme events. Climate elements are the averaged meteorological elements. Table 2 provides a listing of major climate-related issues. Most of these have value in the planning phase of operations. LOS goals are to some extent climate driven in terms of what is and is not possible. Strategies and tactics that support LOS goals are similarly climate driven. Certain recurring site con- ditions (microclimates) are climate driven and require spe- cific recurring operational responses. These include cold spots, high humidity locations, persistent windy areas, etc. In any climate, the achievable LOS is limited by the rate of precipitation, cycle time capability, sustainability of the main- tenance effort, site conditions that may cause road closure, and materials options. Higher LOS for similar weather condi- tions can be provided with shorter cycle times and the use of straight ice control chemicals. WEATHER CONDITIONS Field winter maintenance personnel in a given area are mostly concerned with anticipated winter weather conditions and not with climate considerations. “Weather” usually refers to the measurable or identifiable meteorological events that occur at a given site or in a given area at a particular point in time. Weather can be characterized by describing the meteo- rological elements associated with those events (e.g., precip- itation type and amount, visibility, wind speed and direction, temperature, and relative humidity). Precipitation is arguably the most important weather con- dition. Having a working knowledge of precipitation defini- tions is essential when designing a snow and ice control treat- ment. Those definitions appear in Table 3 and are taken from the Federal Meteorological Handbook (FMH) No. 1 (6). Other important meteorological variables that have an impact on winter pavement conditions are sky, or cloud cover conditions (solar radiation effects); air temperature, to the extent it establishes the trend in pavement temperature; dew point temperature; condensation; pavement temperature; rel- ative humidity; wind speed and direction; and evaporation. Condensation occurs when the pavement, or bridge deck, temperature is above 32°F and below the dew point temper- ature. Frost, on the other hand, occurs when the pavement temperature is at or below 32°F and below the dew point tem- perature. It is common for bridge deck surfaces to develop frost when the adjacent highway surfaces do not. This typi- cally happens in the fall and spring when these temperature conditions are satisfied, the sky is free of cloud cover, and the wind speed is calm (0 to 3 mph). The early morning hours, just before sunrise, are ideal times for bridge deck icing/ frosting to occur. The prediction of these icing conditions is particularly difficult, especially for rural areas with elevation changes and varied roadside vegetation coverage. Location of Road Weather Information Systems (RWIS) sensors in these sensitive areas has been most helpful in detecting the onset of frost conditions. Crosswind speeds in excess of about 15 mph may cause local snow drifting and inhibit anti-icing operations. Also, liquid chemical sprayers should be set closer to the pavement during windy conditions to avoid spray loss. SITE CONDITIONS Site conditions are those local situations that affect how snow and ice control operations are conducted. They influ- ence type of equipment, materials choices, materials appli- cation rate, priority and sequence of treatment, and type of treatment. Table 4 is a listing of important site conditions. TRAFFIC CONDITIONS The influence of traffic was discussed in the Chapter 2 sec- tion on Snow and Ice Control Operational Considerations Relating to Level of Service. The traffic conditions of possible importance are listed in Table 5.

7SUMMARY OF SNOW AND ICE CONTROL OPERATIONAL CONSIDERATIONS Climate is an issue that is primarily of importance in the planning phase of snow and ice control operations. With minor exception, the various snow belt climate zones in North Amer- ica all experience similar types of winter weather events with mixtures of precipitation classes. The difference among the climate zones is in the distribution of expected precipitation class events. Mountain climates, for example, see very few freezing rain events; however, they may see more snow events with high accumulations. The bottom line is that all snow belt maintenance forces have to be prepared to deal with all types of precipitation events. The ability to forecast and recognize the various types of precipitation is extremely important. This influences to a large degree the type of treatment, material choice, and mate- rial application rate. Site conditions, especially pavement temperature, snow/ ice on the highway surface, and ice/pavement bond, are major factors when deciding on material type and application rate. Traffic characteristics appear to be less important than weather and site conditions; however, higher traffic volume and speed can displace snow and ice control materials from the high- way surface. • Frequency of Snow and Ice Events – Low – Moderate – High • Severity of Winter Pavement Exposure – Mild – Moderate – Severe • Wintertime Precipitation – Type – Rate • Urban Influence – Small – Medium – Large – Industrial • Water Influence – Minor – River – Lake – Ocean • Elevation/Large Scale Topography – Plain – Rolling – Mountainous TABLE 2 Climate-related items Light Rain. Small liquid droplets falling at a rate such that individual drops are easily detectable splashing from a wet surface. Include drizzle in this category. Moderate Rain. Liquid drops falling are not clearly identifiable and spray from the falling drops is observable just above pavement or other hard surfaces. Heavy Rain. Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray from falling rain can be observed several inches over hard surfaces. Freezing Rain. When rain freezes upon impact and forms a glaze on the pavement or other exposed surfaces. Sleet (Ice Pellets). Precipitation of transparent or translucent pellets of ice, which are round or irregular in shape. Light Intensity of Sleet: Scattered pellets that do not completely cover an exposed surface regardless of duration. Visibility is not affected. Moderate Intensity of Sleet: Slow accumulation on ground. Visibility reduced by ice pellets to less than 7 mi (13 km). Heavy Intensity of Sleet: Rapid accumulation on ground. Visibility reduced by ice pellets to less than 3 mi (5.6 km). Light Snow. Snow alone is falling and the visibility is greater than 1/2 mi (0.9 km). Moderate Snow. Snow alone is falling and the visibility is greater than 1/4 mi (1/2 km) but less than or equal to 1/2 mi (0.9 km). Heavy Snow. Snow alone is falling and the visibility is less than or equal to 1/4 mi (1/2 km). Blowing Snow. When fallen snow is raised by the wind to a height of 6 ft (1.8 m) or more and is transported across a road. None. No precipitation or blowing snow. NOTE: An estimate can be made of the moisture content of falling snow as follows: 1 = powder snow 2 = ordinary snow 3 = wet/heavy snow a Definitions taken from Federal Meteorological Handbook (FMH) No. 1 (6). TABLE 3 Precipitation definitions

8 • Traffic Volume – Very low – Low – Medium – High – Very high • Commercial Vehicle Mix – Low – Moderate – High • Vehicle Speeds – Low – Moderate – High TABLE 5 Traffic conditions • Area Type – Urban – Suburban – Rural • Special Highway Segment Areas – Hills – Curves – Grades – Intersections – Bridges – Sags – Ramps – Crosslopes – Weaving Areas – Narrowings – Roadway Widenings – Elevated Roadways – Pavement Surface Types – Tangents • Shadings from Solar Influence – Forest/vegetation – Buildings/structures – Cuts • Pavement Conditions – Temperature – Ice/pavement bond – Frost or thin ice – Slush, loose snow, packed snow, thick ice TABLE 4 Site conditions