Here in Northeast Ohio construction activity doesn't stop just because a few snowflakes are falling. Projects have deadlines and extreme weather is just another logistical bump along the road to completion. As such, all typical erosion and sediment controls associated with the storm water pollution prevention plan (SWP3) need to be managed without interruption. Often times the weekly and storm event inspections and reports are overlooked because site contractors assume that the site is frozen and nothing significant is happening.
The reality is that monitoring and maintenance of erosion and sediment controls is just as important during these periods of snow and extreme cold in anticipation of the eventual snow melt. Northeast Ohio experiences temperature fluctuations throughout the winter months; often having numerous freeze and thaw cycles before the onset of warmer weather. These thaw events can produce a significant amount of water. Below are some estimates adapted from Meteorologist Jeff Haby (http://www.theweatherprediction.com/habyhints/346/)
Wet snow must have a partial liquid content of at least 50%. This means that there must be at least 50% frozen content for the precipitation that is falling to be considered snow. Anything greater than 50% and the precipitation would be classified as rain. This snow is sticky or slushy and packs well for making snowballs and snowmen. During freezing temperatures “wet snow” will has approximately a 5:1 to 10:1 ratio. As an example an extremely wet snow with a partial liquid content close to 50% will create a 5:1 ration. Thus, 5 inches of extremely wet snow will produce a 1 inch liquid equivalent.
Dry snow has very little liquid content. It creates a less dense, powdery snow that drifts easily but does not pack well. Dry snow has a snow to liquid ration of 10:1 up to 30:1 in extremely dry conditions. This means that it would take 10 to 30 inches of snow to equal 1 inch of liquid.
So what does this all mean?
Ohio EPA requires erosion and sediment controls associated with a project SWP3 to be inspected once every seven days and within 24 hours of a 0.5 inch rain event. If cold weather persists for a couple of snow events in a row the accumulate totals could add up very quickly. Given the numbers I’ve detailed above it would take as little as 2.5 to 5 inches of accumulated snow to equal 0.5 inches of liquid on the ground… requiring an inspection. 18, 24, even 36 inches of accumulated snow are not uncommon; especially when snow is stockpiled around the periphery of the project. Unfortunately, the edges of a project also happen to be the most vulnerable point of the SWP3.
Armed with this information construction site contractors and storm water inspectors should remain vigilant throughout the cold weather months. Routine inspections and proper maintenance of erosion and sediment control BMPs is critical to minimize sediment-laden discharges during a thaw period.
For more information or to ask a question contact Brent Eysenbach, Storm Water Program Coordinator.