Potential for Obstruction of the Curb Drain or Sump Pump Discharge Line due to Freezing

Investigation by OHM into freezing of curb drain lines concluded that freezing occurred in curb lines due to “bellies” (low spots) in the sump discharge lines that allowed some of the discharged water to pool and cool in these lines. This allowed the water to subsequently freeze when it moved through the curb line.  These low spots could reasonably increase the likelihood of freezing in the area of the low spot in the sump discharge line. However, I am skeptical that low spots in the sump discharge lines would be a significant contributor to freezing in the curb lines.

It is my view that freezing in the curb line results from the configuration of the curb line (i.e. how it slopes, absence or presence of dips, and its length), the temperature of the soil around the curb line (dependent on burial depth and soil conditions above the line), and the volume of water flowing through the line. While the temperature of the water discharging from the sump is a factor in whether ice will form, sump water will cool significantly as it travels through the sump discharge line regardless of the absence or presence of dips in the line. The water will continue to cool (faster due to larger contact area) once it enters the curb line. Ice will build up in the curb line if the ground temperature surrounding the pipe is significantly below freezing and the frequency and volume of flow is too low to warm the pipe. Curb lines that experience the lowest surrounding temperatures are vulnerable to icing, and, among those, longer pipes with lower flows are most vulnerable.

Section C.5 of the executive summary indicates that it is not always possible to bury the curb line below the frost line due to the shallow depth of the receiving storm drain inlet. While this may be true in some cases, where the receiving drain is deep enough, more effort could have been made to bury these lines deeper than the 2 feet minimum required by the specification.  This could have been accomplished by specifying a depth below the frost line, where practical, and requiring specific approval for lesser depth. This would have reduced the likelihood of freezing in these lines.