| Popis: |
The goal of this investigation is to determine threshold levels of intensity and duration of storm rainfall necessary to initiate debris flows on the steep hillslopes of the Honolulu District. These rainfall thresholds could find a future application in a debris-flow warning system for Honolulu, similar to a prototype warning system developed several years ago and now operating in the San Francisco Bay region in northern California. Previous studies suggest that debris flows are triggered when heavy rainfall, infiltrating into the shallow subsurface, becomes impounded and creates elevated pore pressures that weaken the slope materials. Based on this idea, a simple numerical model was developed to study the interaction between heavy rainfall and pore-pressures on steep hillslopes. In order to calibrate this model to the ground conditions within the Honolulu District, detailed recordings of both rainfall and shallow pore pressures have been made at two remote sites on steep hillslopes. These recordings confirm the presence of temporarily elevated pore pressures in response to heavy rainfall and show a correlation between the peak porepressure response and the maximum amounts of rainfall recorded for periods of 3 to 6 hours. An extensive historical data base of rainfall amounts and debris flow occurrences has been compiled for 17 large storms in the Honolulu District over the past 33 years. Plots of these historical data display well defined, lower bound relationships between the number of debris flows reported during storms and the peak rainfalls recorded over periods of 1 to 6 hours. Two threshold levels were derived from these lower bound relations: a "safety" threshold below which the likelihood of damaging debris flows is very low, and an "abundant" threshold level above which rainfall is likely to cause many debris flows and thus pose a hazard to life and property. |
