Shallow landslides and vegetation at the catchment scale: A perspective

Shallow, rainfall-triggered landslides are an important catchment process that affect the rate and calibre of sediment within river networks and create a significant hazard, particularly when shallow landslides transform into rapidly moving debris flows. Forests and trees modify the magnitude and rate of shallow landsliding and have been used by land managers for centuries to mitigate their effects. We understand that at the tree and slope scale root reinforcement provides a significant role in stabilising slopes, but at the catchment scale root reinforcement models only partially explain where shallow landslides are likely to occur due to the complexity of subsurface material properties and hydrology. The challenge of scaling from slopes to catchments (from 1-D to 2-D) reflects the scale gap between geomorphic process understanding and modelling, and temporal evolution of material properties. Hence, our understanding does not, as yet, provide the necessary tools to allow vegetation to be targeted most effectively for landslide reduction. This paper aims to provide a perspective on the science underpinning the challenges land and catchment managers face in trying to reduce shallow landslide hazard, manage catchment sediment budgets, and develop tools for catchment targeting of vegetation. We use our understanding of rainfall-triggered shallow landslides in New Zealand and how vegetation has been used as a tool to reduce their incidence to demonstrate key points

Collaboration and modelling – tools for integration in the Motueka catchment, New Zealand

A conceptual model of integrated catchment management (ICM) is presented in which ICM is defined as a process to achieve both ecosystem resilience and community resilience. It requires not only biophysical knowledge developed by hydrologists and other environmental scientists, but an active partnership with catchment communities and stakeholders to break the ‘paradigm lock’ described by the UNESCO-HELP programme.This paper reports observations from ICM research in the Motueka HELP demonstration basin in the upper South Island of New Zealand. The Motueka occupies 2 170 km2 of land yet the river effects are felt on the seabed more than 50 km2 offshore, so the true ‘catchment’ is larger. A hydrologically temperate mountainous catchment with horticultural, agricultural, plantation forestry and conservation land uses, the Motueka also hosts an internationally recognised brown trout fishery. Land and water management issues driving ICM research include water allocation conflicts between instream and irrigation water uses, impacts on water quality of runoff from intensifying land uses, catchment impacts on coastal productivity and aquaculture, and how to manage catchment processes in an integrated way that addresses cumulative effects of development.Collaboration with catchment stakeholders can be viewed as having two primary purposes:• Building knowledge and commitment of resource users towards sustainable resource management (collaborative learning)• Stakeholder involvement in resource management itself (governance).Examples are presented of a Collaborative Learning Group on Sediment learning of their differing perspectives on fine sediment impacts, and a Catchment Landcare Group working with scientists to improve water quality in their river. Success factors for water user committees making decisions about water resource management include creating opportunities to communicate and build trust, share scientific knowledge on the issue, and willingness to compromise. Functioning catchment groups have potential to take on delegated governance responsibility for meeting agreed water quality and other community goals.Finally a scenario modelling framework IDEAS (Integrated Dynamic Environmental Assessment System) is presented, in which environmental indicators such as nutrient fluxes are simulated alongside socio-economic indicators such as job numbers and catchment GDP for a range of land and marine use options.Keywords: integrated catchment management (ICM), resilience, HELP, UNESCO, water governance, Landcare, scenario modelling, collaborative learning, water allocation, water user committees, catchment groups, watershed managemen

Natural hazards

Hazards associated with natural events in the hydrological domain broadly include river floods, landslides and droughts. Over the last decade there has been a focus on New Zealand research in the areas of flood, related to storm rainfalls, earthquakes and volcanic activity. Emerging research continues in flood forecasting, flood frequency, storm rainfalls, hydrological ‘riskscape’ (i.e., predicting the costs of flood inundation), and there remains little new research on drought. This chapter focuses largely on flood hazards and hazards associated with rainstorm events, including rain-triggered landslides and debris flows. The intersection between flooding and both seismic and volcanic hazards is considered with reference to the impacts of the 2010-2011 Christchurch earthquake sequence and the 2007 Ruapehu lahars, respectively