Analysis of unsaturated hydraulic properties for low impact developments and their performance under changing climate

Climate change has resulted in an increase in both intensity and frequency of extreme rainfall events leading to a higher probability of flooding. To counter the impacts of climate change and urbanization, engineers have developed ingenious solutions to reduce flooding through the use of Low Impact Developments (LIDs). The soil is generally considered to be completely saturated when designing for the LIDs. However, this may not always be an accurate or realistic approach, as the soil could be variably unsaturated leading to inaccurate designs. Laboratory and field measurements of unsaturated hydraulic properties are cumbersome, expensive and time-consuming. An alternative approach is to estimate unsaturated hydraulic properties using pedotransfer functions (PTFs). This first part of this research presents a comparison between the direct measurement obtained through experimental procedures and the use of PTFs to estimate soil hydraulic properties for green roof and bioretention soil medias. Comparison between the measured and estimated soil hydraulic properties was accomplished using two different approaches. Statistical analyses and visual comparisons were used to compare the measured and estimated soil hydraulic properties. Additionally, numerical modeling to predict the water balance at the ground surface was conducted using the measured and estimated soil hydraulic properties. Results indicate that there is a high level of uncertainty when using PTFs for LID materials.

It is ideal to analyze existing and future LID systems within the context of changing climate. Thus, the second part of this research will assist in the determination of whether the LID system design objectives can be met within the context of changing climate. In order to conduct this analysis, numerical modelling was completed. Material properties previously measured in the laboratory for bioretention and green roof substrates were used in the numerical modeling. Long-term and short-term climate data for ten locations in Ontario was used in this examination. The results of this research suggest that LID performance may decrease due to the increased quantity of water projected in the future due to climate change. Results indicate that the percent change in future long-term maximum infiltration values for green roofs can be as much as 100%, whereas for bioretention facilities it can be in the order of 50%. Moreover, a general decrease in both the peak reduction and peak time delay for the green roof facilities can be expected for the future. Some 100-year storms in Ontario can see as much as 50 cm increase in untreated stormwater run-off from bioretention facilities.