I. Douglas et al., The role of extreme events in the impacts of selective tropical forestry on erosion during harvesting and recovery phases at Danum Valley, Sabah, PHI T ROY B, 354(1391), 1999, pp. 1749-1761
Citations number
46
Categorie Soggetti
Multidisciplinary,"Experimental Biology
Journal title
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES B-BIOLOGICAL SCIENCES
Ten years' hydrological investigations at Danum have provided strong eviden
ce of the effects of extremes of drought, as in the April 1992 El Nino sout
hern oscillation event, and flood, as in January 1996. The 1.5 km(2) undist
urbed forest control catchment experienced a complete drying out of the str
eam for the whole 1.5 km of defined channel above the gauging station in 19
92, but concentrated surface flow along every declivity from within a few m
etres of the catchment divide after the exceptional rains of 19 January 199
6. Under these natural conditions, erosion is episodic. Sediment is dischar
ged in pulses caused by storm events, collapse of debris darns and occasion
al landslips. Disturbance by logging accentuates this irregular regime. In
the first few months following disturbance, a wave of sediment is moved by
each storm, but over subsequent years, rare events scour sediment from bare
areas, gullies and channel deposits. The spatial distribution of sediment
sources changes with time after logging, as bare areas on slopes are revege
tated and small gullies are filled with debris. Extreme storm events, as in
January 1996, cause logging roads to collapse, with landslides leading to
surges of sediment into channels, reactivating the pulsed sediment delivery
by every storm that happened immediately after logging. These effects are
not dampened out: with increasing catchment scale. Even the 721 km(2) Sunga
i Segama has a sediment yield regime dominated by extreme events, the sedim
ent yield in that single day on 19 January 1996 exceeding the annual sedime
nt load in several previous years. In a large disturbed catchment, such roa
d failures and logging-activity-induced mass movements increase the mud and
silt in flood waters affecting settlements downstream. Management systems
require long-term sediment reduction strategies. This implies careful road
design and good water movement regulation and erosion control throughout th
e logging process.