Flood Protection by Forests in Alpine Watersheds: Lessons Learned from Austrian Case Studies

Abstract

This chapter highlights the influence of mountain forests on runoff patterns in alpine catchments. We discuss the forest impact at different spatial scales and bridge to the requirements for an integrated natural hazard risk management, which considers forest as an efficient protection measure against floods and other water-related natural hazards. We present results from a wide range of research studies from Austria, which all reveal the runoff-reducing effect of forest vegetation in small and medium-size catchments (< 100 km2). Forests also contribute to runoff reduction in heavy rainfall events in macro-scale catchments (> 100 km2), e.g., by reducing surface runoff and delaying interflow, but above all by stabilising slopes and therefore reducing bedload transport during major runoff events. To avoid that forests become a hazard due to enhanced driftwood release, managing of steep riparian slopes for a permanent forest cover (“Dauerbestockung”) is a basic prerequisite. Often protective effects of forests are impaired by man-made impacts like dense forest road networks, insensitive use (e.g., false design of skid roads, compacting machinery, forest operations during adverse weather on wet and saturated soils), and delayed or omitted reforestation and regeneration. Flood risk management in mountain regions should include Ecosystem-based Disaster Risk Reduction measures, with particular emphasis on sustainable and climate change-adapted management of protective forests. This will require integral and catchment-based approaches such as comprehensive management concepts coordinated with spatial planning, and verifiable, practicable and correspondingly adapted legal guidelines as well as appropriate funding of protective forest research to close the existing knowledge gaps.

The Concept of Risk and Natural Hazards

“…Supporting an ecosystem-based integrated risk management and the acknowledgment of the key role forests have for risk reduction in mountain areas, the findings of GR4A help identifying mitigation strategies and subsequently efficient risk reduction measures through an improved and participative risk governance system. How forests can act as a solution for Eco-DRR is the subject of the following three chapters of this book [72][73][74]. Moreover, the methodologies and decision support tools related to the risk concept that were developed and applied within GR4A are presented in [75,76], the book chapters [77-79], and are explained in detail in the GR4A project reports [20,65,80,81].…”

Protective Forests for Ecosystem-based Disaster Risk Reduction (Eco-DRR) in the Alpine Space

“…Object protective forests are forests, which are located in process areas of natural hazards that endanger objects below and can have 1) a direct protective function and, if applicable, protective effect against gravitational natural hazards such as rockfall, shallow landslides, or snow avalanches, allowing to directly link the Protective Forests for Ecosystem-based Disaster Risk Reduction (Eco-DRR) in the Alpine Space DOI: http://dx.doi.org/10.5772/intechopen.99505 precise locations of the hazard and the endangered objects; or 2) an indirect object protective function (and effect) for floods and other water-related natural hazards [51]. The latter relationship is defined as indirect since an entire forested catchment can contribute to flood protection (see chapter [56] of this book), and it is not straightforward to establish a direct connection between a precisely located forest area and a flooding scenario, especially when applying protective forest models (see chapter [57] of this book).…”

“…Perhaps the economic and technical possibilities for implementing gray infrastructures and their great success have contributed to the fact that forest research was limited to silvicultural questions and that hardly any research funding has been made available to study the protective effects of forest. However, by the end of the 1970s, the limited effects and capacities for disaster risk reduction by gray infrastructures only were increasingly recognized, especially in flood and torrent control, which can be observed in many regions worldwide and is called the paradigm shift in flood risk management (see also chapter [56] of this book). Disaster prevention and mitigation strategies were supplemented through spatial planning (hazard zoning), which indirectly led to an increasing need to assess the protective functions and effects of forest.…”

Influence of Canopy Disturbances on Runoff and Landslide Disposition after Heavy Rainfall Events

“…The semi distributed model is based on a two-layer concept with a surface and a subsurface runoff module. ZEMOKOST needs the portions of surface runoff classes (RC const ) and surface roughness (c) classes for each sub-catchment (see chapter [5] of this book). The main parameters, runoff coefficient (RC const ) and surface roughness (c), have been investigated, following the code of practice developed by [15].…”

“…Steep headwater catchments typically provide the setting for such natural hazardous events because they are, besides being a drainage area for precipitation (see chapter [5] of this book), also sediment source zones of river systems, delivering significant volumes of sediment to the valley floor in highly dissected and coupled landscapes. As stated by Gomi and Sidle [6], headwater catchments differ from down-stream reaches by their close coupling to hillslope processes, more temporal and spatial variation, and their need for different means of protection from land use.…”