c8.0.4 – How hydrogeomorphic processes are affected by forest management activities

Roy Sidle

Athens, Georgia – USA
Collocazione: c8.0.4 – Tipo Comunicazione: Presentazione orale
8° Congresso SISEF *
Sessione Plenaria *

Contatto: Roy Sidle (sidle.roy@epamail.epa.gov)

Abstract: The science of hydrogeomorphology includes not only the coupling of hydrologic and geomorphic processes in catchments, but also the spatial and temporal interactions of these processes. Hydrogeomorphic processes control stormflow generation, sediment production and transport, and fluxes of other materials in catchments, all of which can be modified by forest management activities. Because much of the natural forest cover is in steep terrain, it is necessary to understand the hydrology of these unique geomorphic environments in order to better more sustainably manage forests. A parsimonious model for stormflow generation in headwater forested catchments is presented that is based on a hydrogeomorphic paradigm supported by extensive field data from Japan. This model partitions the catchment into three hydrogeomorphic response units: (1) stream channel/riparian complex; (2) hillslope segments; and (3) zero-order basins (geomorphic hollows). To simulate storm runoff, a multi-tank model is used for the zero-order basins and hillslopes and a kinematic wave model is used to predict saturation overland runoff from the riparian zone and route inputs from the zero-order basins and riparian corridor through the channel. Preferential flow pathways and outflow from zero-order basins both exhibit nonlinear behavior during storms and are strongly affected by antecedent moisture. While forest harvesting affects hydrologic response through alteration of canopy interception and evapotranspiration, harvesting more strongly influences sediment production, particularly in steep terrain where landslides dominate the sediment budget. In steep catchments, zero-order basins typically represent old landslide scars that gradually fill in over time. Infilling of these ‘hollows’ is also affected by biogenic processes in forests. Timber harvesting reduces vegetation rooting strength in soil mantles, particularly during the ‘window’ of 3 to 15 years after tree removal, corresponding to minima of the combined root decay and regrowth curves. As such, reduced rainfall thresholds are required to trigger shallow landslides during this period following timber harvesting. This phenomenon is shown theoretically and with data from harvested catchments in Japan. Implementing leave areas in these vulnerable hollows is an effective way to reduce the incidence of logging-related landslides. Furthermore the linkage between hillslope landslides and channelized debris flows in steep forested terrain is elucidated for several catchments in Japan. Landslides can immediately mobilize into debris flows if they directly connect to steep channels. In contrast, landslides may deposit on hillslopes or in channels where the sediment may later be mobilized as either a debris flow or via another, less episodic, transport process. This latter process, where the landslide is disconnect in time from the debris flow, is more difficult to predict and is affected by a combination of channel attributes, vegetation cover and management, and the timing of peak flows.

Citazione: Roy Sidle (2011). How hydrogeomorphic processes are affected by forest management activities . 8° Congresso Nazionale SISEF, Rende (CS), 04 – 07 Ott 2011, Contributo no. #c8.0.4