Are you ready to manage storm water as it builds and carries all the material deposited on the surface for four to six months? How about that huge melt event in the spring? What have you done to plan for a climate scenario that might include more snow mixed with ice and rain? These are all questions that anyone dealing with winter—even a single snow event—needs to deal with.
A transition has occurred in the last decade wherein storm water managers in cold climates have acknowledged that winter conditions and snowmelt behavior cannot be adequately addressed with only warm-region approaches. In fact, some of the most significant events of the year are usually associated with snowmelt. Water in the form of snow accumulation gathers for four to six months in colder regions, and perhaps for only one day in areas touched by a single unusual snowfall. In either extreme, storm water management is challenged.
As snowcaps melt, accumulated litter is exposed.
In addition to the water component, surface pollution from human activities (i.e., salt, sand, combustion byproducts, animal waste, hydrocarbons and metals) accumulates in snowbanks and storage areas. It is released in a series of small, often salt-induced melts and a large purge in the spring.
Plan as we might, the rainfall best management practices (BMPs) relied on for years do not cut it as effective snowmelt treatment systems. The reasons are numerous, including: ice cover that leads to short-circuiting over or under the ice surface and depleted oxygen in the stored water; cold water conditions that impede settling and biological activity; frozen conduits and outlets; and high soluble pollutant content that evades treatment.
Making the Transition
So, how to minimize the impact of winter on receiving waters? There are three different periods that cold-climate storm water managers need to think about right now to prepare for this winter and those that follow:
Short-term. Plan a deicing/anti-icing strategy with a focus on less salt use, and a snow management plan to assure that meltwater does not flow unabated to local receiving waters. Prepare BMPs for winter operation by, for instance, accounting for lower water levels and drying out infiltration and filtration systems.
Mid-term. Monitor storm water systems this year in anticipation of making adaptations for cold-climate operation.
Long-term. Examine the impact that climate change can have on systems (i.e., more precipitation, more ice/snow/water mix and warmer temperatures for BMP operation).
If rainfall BMPs have not been adapted, they will not perform nearly as well as planned when receiving meltwater or winter’s cruel mix of rain, snow and ice. An increase in cold-climate research has begun to show that alternative storm water management using natural treatment systems can be effective for cold climates. Introducing bioretention, infiltration, pervious pavement and wetlands into a system will reduce both the volume and water quality impact of winter and spring runoff. Any measures taken to dry the facility out will reduce the formation of ice in the soil and enhance water movement through it.
An ice layer poses problems in a detention system.
Accounting for a thick layer of ice and the impact it has on water movement is essential. In detention systems, for example, this layer will drive water below the ice to disturb previously settled material and displace poor-quality pond water. Next, it will flow quickly over the ice with zero settling depth from inlet to outlet, eliminating any detention time. Both of these problems can be addressed, though not totally solved, by lowering the water level prior to its freezing. This increases the storage capacity available for meltwater and can be designed to minimize the flushing of poor-quality water.
A Changing Climate
There is certainty that something is happening with the climate to warm it up; because winter storm water management involves some degree of handling a frozen medium, this implies some impact that must be addressed. Among the impacts in cold regions will be greater precipitation, likely shifting from all snow to a mix of rain-on-snow, ice storms and larger snow events; warmer conditions during the winter, meaning an increase in ice formation on roadways and additional salt for deicing; warmer water in lakes and ponds, leading to increased algal productivity, longer stratification and depleted oxygen; and more numerous mid-winter melts with earlier spring snowmelt.
These changes will mean that practices applied today could be obsolete and in need of change well before their design life is approached. The new mantra of adaptive management will be essential, but it must be based on observations and new data collected as the climate seeks to adjust itself to changing atmospheric conditions.