INNOVATION May-June 2012

fea t ure s

Forests and Flood Risk: A New Paradigm Sheds Light on Age-old Controversies Piotr Kuras PEng RPF Dr Younes Alila PEng

W ater resource engineers and geoscientists are often retained to determine design peak flows for various engineering works and engineered structures involving riverine systems. In British Columbia, it is often the case that the watersheds for such projects are mostly forested with forest harvesting having occurred in the past, underway, or planned to occur in the future. In order to provide adequate estimates of design peak flows in such circumstances, one would naturally refer to relevant scientific literature to gain a better understanding of the potential impacts of such land-use change on the hydrology of a watershed. Unfortunately the most relevant science of forest hydrology has been described by a distinguished hydro-

BC Ministry of Transportation

geomorphologist as having “a sad history of being embroiled in controversies that never seem to get resolved” (Thomas Dunne, 1998, University of California). This is particularly true on the topic of the effects of forests, forest harvesting and deforestation on peak flows (floods), and controversies on this topic have led to an ever increasing schism between science, public perception and land use policies worldwide. This has prompted repeated calls by scientists to educate the public and policy makers, but calls for questioning the science have been rare, perhaps in part due to the political sensitivity of the topic. While there is still some disagreement among scientists on how forests affect floods, some have argued in a 2007 Nature comment

Figure 1: Conceptualization of forest harvesting effects on the flood frequency distribution caused by shifts in (a) mean, (b) variance, and (c) both mean and variance (the shape of the distribution can also change but is not depicted here).

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