Sweden Does Research on Urban Runoff

New Research on Transient Effects of Frozen Soils on Urban Runoff Control by Green Infrastructure

Urban drainage systems in cold regions with temporary snowpacks experience high volumes of runoff from green areas during the winter/spring transition, as a result of rain/snowmelt over frozen soils with drastically reduced runoff infiltration.

Recognizing that: (a) the resulting runoff volumes and peaks are particularly harmful during extreme events and concurrent catchment conditions, and (b) such circumstances may be further exacerbated by progressing climate change, with higher winter precipitation, faster snowmelt, and deeper frost penetration (because of shallower snowpacks providing less insulation), the INXCES researchers at the LuleƄ University of Technology (LTU) undertook a laboratory study of infiltration into two types of frozen soils. The first phase results indicate that the soil water content at the start of freezing was the critical factor affecting the thaw process; the soil moisture converted into ice blocking soil pores and water infiltration, and ice melting required high amounts of energy supplied by infiltrating water. Even for relatively low soil water content (0.10) but deep frost penetration and diurnal freeze/thaw cycles, the restoration of full soil infiltration capacity could take several days, and during that period, the performance of green infrastructure in runoff volume control would be seriously impaired. Increased runoff from green areas and additional precipitation stored in melting snow then significantly increases runoff volume. Consequently, the designers need to recognize these phenomena and provide bypasses and storage of excessive runoff from green areas to prevent flooding and concomitant flood damages.

Fig. 1. Experimental soil columns used in testing infiltration into frozen soils