Stormwater treatment in cold-climate regions involves considerable additional difficulties than with similar treatment systems in semi-arid and wet climate regions. Cold regions can be defined by Figures 1 through 3. Figure 1 presents air freezing index values, which are the cumulative degree-days below 32°F. It has been suggested that ice cover will occur on lakes in regions with values greater than 100. Figures 2 and 3 present annual snowfall depths and length of freeze free period. Figures 4 through 6 are relevant to the question of how deep treatment systems must be placed subsurface to avoid freezing—for examples, fine-media filters and wet vaults. The information provides perspective. More definitive data available by state or province should be consulted. This article present’s a general review of the author’s experience.
Table 1 lists issues facing treatment in climatic regions where substantial freezing occurs. The table expands on summaries provided by others. Caution is warranted with many of the suggestions given lack of field experience. Most of the adjustments in Table 1 are discussed more fully elsewhere (Minton 2011). These discussions should be consulted, as some adjustments may conflict, be inappropriate, or be of inconsequential value in certain situations. Some issues may be overstated depending on the particular locale.
Snowmelt has a unique dynamic. Pollutants accumulate in the snow over the winter. Accumulation is variable, affected by traffic volume, snow age, prevailing winds, curbs, the amounts of deicing salt or traction sand used in the drainage area, and the characteristics of the drainage system.
It has been suggested that depth be added to wet ponds to account for space taken by ice and snow. The adjustment is to maintain sufficient liquid volume for acceptable performance during winter and spring melts.
- Increase water depth beneath ice.
- Drain the wet pool prior to the freeze period and operate as an extended detention basin during winter and spring melts.
Wet ponds and vaults, as well as wetlands, may be preferred by default given inadequacies and risks of freezing attendant to other systems such as swales and fine-media filters. Although the performance of basins in cold climates may degrade in comparison to their counterparts in temperate and semi-tropical regions, their performance may be better than the alternatives available for cold climates.
High concentrations of chloride in the melt water cause saline stratification during the winter and spring melts. Metals in the soil under anaerobic conditions solubilize, are released into the water, and may be lost in the spring melt. Zinc desorbs from the surfaces of cattails in the winter, but this also occurs in other climate areas. Concentrations of TSS tend to be greater in the effluents from basins in cold regions (Figure 8). Settling rates will be lower. This may be due as much to higher influent concentrations (deicing sand) as to cold temperatures and stratification. Seasonal comparison of the removal efficiencies of a pond showed that removal of cadmium and copper was about the same for summer and winter-spring, but removal of lead, zinc, and TSS dropped from the summer to the winter.
The strength of stratification and its effect on processes within the wet pool are affected by whether the ice remains throughout the winter. Intermittent melting of the ice may result in the re-oxygenation of the basin, particularly with winds, and the re-precipitation or sorption of metals previously dissolved. Thus, the impact of ice cover depends upon the latitude and nearness to large bodies of water that tend to moderate atmospheric temperatures.
Swales and Strips
The issue with flow-through swales is dormancy of the grass during the spring melt. Several spring storms may occur before the grass is fully functional. As a consequence, grass swales may not be an appropriate as the main treatment device and are of use only where significant infiltration occurs are of particular benefit for spring melt. If present, significant infiltration helps performance. However, it has been found that strips bordering agricultural fields are effective in the winter, perhaps due to the grass being taller than in urban swales. I have taught a short course in cold-climate regions in the United States and Canada, and none of the attendees when asked has indicated any concern about swales performing during the spring melt. It is possible that even dormant grass reduces the velocity of the stormwater sufficiently to allow TSS and attached pollutants to settle. Regardless, the question has not been researched and therefore remains a concern.
Filters and Infiltration
These two types of treatment systems respond similarly and therefore are discussed together. Fine media (and by extension, course media), if not saturated, will not freeze, but the hydraulic conductivity will be reduced. Studies of porous pavement, and experiments by the author, show that wetted sand will freeze but that gaps remain between the sand particles. With each successive freeze, a new layer of frozen water forms on the surface of the sand. A study of porous pavement found that at least 11 successive freeze/thaw events occurred before the hydraulic conductivity approached that of the design value used in the sizing of sand filters. Hence, sand filters should operate should operate well throughout the year. The surface of the filter perhaps should be cleaned before the winter season as fine material collected by the filter on its surface will freeze. However, this layer is very thin. As such, it would be expected that the frozen layer will be easily broken up if not melted by the incoming water. Organic media is most susceptible to freezing as it retains more moisture. As pellets or in a similar form, the organic media will lose its structural integrity.
Many alternative strategies are presented in Table 1, related to each specific problem. Some these strategies are discussed below.
Bypassing the main treatment unit protects this unit from damage, whether structural or biotic. It avoids loss of previously removed pollutants by bottom scour or by solubilization. Using this concept, the pretreatment unit would be sized to effectively treat intermittent winter melts and the spring melt. It also would be sized and configured to deal effectively with ice. Lower performance might be accepted when considering the total annual performance goal inasmuch as the environmental effect of discharges on the receiving water is likely less during periods of cold-water temperatures.
Four Flow Types
Sizing methods should consider four flow event types: winter melts, spring melt, rain-on-snow events, and rain-generated runoff events during the remainder of the year. Rain-on-snow events will have the greatest volume and likely flow rate. Whether this flow type should be the basis for sizing depends on frequency. It is recommended this question be considered in coastal areas and the Great Lakes area, where the annual snowfall exceeds about 3 feet (1 meter).