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.Introduction:
Introduction 1 Purpose and scope 3 Description of study area Acknowledgments 3 Hourly rainfall 3 Rain-gage selection 3 Storm-period selection 4 Data collection 4 Reported debris flows 5 Data collection 5 Limitations of the data 6 Summary 6 References cited 7 FIGURE 1. Map showing location of study area and selected rain-gage stations 2 TABLES 1. Selected rain-gage stations with hourly data 4 2. Selected storm periods, 1935-91 5 3. Hourly rainfall for selected storm periods, 1935-91 9 4. Reported debris flows for selected storm periods, 1935-91 57 CONVERSION FACTORS Multiply By To obtain inch (in) 25.4 millimeter inch per year (in/yr) 25.4 millimeter per year foot (ft)
Instrumentation was installed at the surface of the landslide and in borings in order to measure rainfall, displacement, depth of the landslide, and groundwater levels. Surface instrumentation (fig. 2) consisted of rain gauges and an extensometer (for measuring displacement). Borehole instrumentation consisted of anchored cables for measuring displacement and open-tube piezometers for measuring water levels. Three borings each were cased for inclinometer and neutron-probe measurements. Rainfall observations are compiled in Appendix D and summarized below. Measurements of displacement are compiled in Appendix E. Difficulties resulting from materials at the site and from installation of the casing made the neutron probe ineffective for monitoring soil-moisture conditions. Displacement of the landslide since installation of the anchored cables has been insufficient (less than the diameter of a boring) to pull the cables down the hole a measurable amount. Consequently, no data from neutron-probe measurements or measurement of the cables are included in this report. The data collected from our piezometer and inclinometer measurements are compiled in Appendix C.
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