Analysis of Geo-Hazards Caused by Climate Changes
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Landslides and Engineered Slopes – Chen et al. (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-41196-7 Analysis of geo-hazards caused by climate changes L.M. Zhang Department of Civil Engineering, The Hong Kong University of Science and Technology, Hong Kong, China ABSTRACT: This paper analyzes the effect of climate on the generation of possible geohazards. The rainfall and evaporation changes in Hong Kong in the past four decades are first reviewed based on records from the Hong Kong Observatory. Then the effect of climates on the generation of emerging geohazrzds is analyzed through a series of transient infiltration analyses taking the climate conditions as initial conditions. Three climate conditions; namely, extreme drought condition, extreme wet condition, and steady-state condition, are studied. Extreme yearly weather variations are shown to be the key to the generation of interchanging extreme hazards such as landslides and floods. The analysis results demonstrate that, in a prior extreme drought condition, an intermediate rainfall process can result in large surface runoff and thus surprising floods. In addition, dissipation of suction only occurs in the shallow soils. Hence, storm water infiltration into a dry ground is likely to cause shallow-seated landslides or debris flows under the combined effect of shallow perched ground water and surface erosion from increased runoff. On the other hand, in extremely wet conditions, the ground water table can rise substantially and failure of some slopes that have been stable for a long time can be triggered even by a moderate rainfall event. 1 INTRODUCTION value between 1964 and 2002 being 1405 mm (Figure 1a). In the past decades, climate phenomena became more 2. In the 56-year period between 1947 and 2002, the and more abnormal. Such abnormal climate phe- annual total rainfall at HKO increased from 2265 nomena as extreme droughts and extreme storms can mm in the 1950s to 2518 mm in the 1990s. It rep- result in aggravated landslide-flood cycles. However, resents an increasing trend of about 65 mm per the mechanisms behind these emerging geo-hazards decade, though not statistically significant at 5% caused by abnormal climate conditions have not been level (Figure 1b). well understood. In this paper, analyses on the geo- 3. The annual number of heavy rain days (i.e. the days hazards caused by climate changes are studied through with hourly rainfall greater than 30 mm, which characterizing the initial conditions created by extreme is the criterion for issuing Amber Rainfall Warn- drought and wet conditions and then studying the trig- ing) has been increasing from about 4.5 days a gering of landslide or the generation of flood by a new year in 1947 to about 7 days in 2002, though not rain event. Analysis of a normal rainfall condition is statistically significant at 5% level (Figure 1c). also undertaken for comparison purposes. While the HKO findings appear to suggest only a gradual minor change in average annual rainfall and 2 EMERGING GEO-HAZARDS FROM evaporation, the yearly variations have become more CLIMATE CHANGES extreme globally. Abnormal climate phenomena such as extreme droughts, storms, typhoons and tides occur 2.1 Phenomena more frequently than ever in the last decade. In the first Based on statistics of the Hong Kong Observatory half of 2004, Guangdong experienced five extreme (HKO) (Leung et al. 2004), there have been notice- climate events: one extreme drought, one 20-day cold able changes in several climate elements in Hong Kong front, one extreme heat wave, eight major storms, and since 1947: seven major tidal events. This was unusual. In year 2005, there were severe droughts in parts of Africa, 1. The annual total evaporation, measured using evap- Western Europe, and Australia; record-breaking heavy oration pans with evaporation surface 0.18 m above rain in India; and an extreme active hurricane season ground, decreased by 40% from the 1960s to 2003, in the north Atlantic. According to the HKO, in Hong at a rate of 184 mm per decade, with the mean Kong, year 2005 was the third wettest year on record, 1703 In extreme drought conditions, decrease in soil moisture content results in a substantial reduction in water permeability of the soil. After a long period of drought, little rainwater can infiltrate into the ground during a rainfall event due to the initially very small permeability of the soil. As a result, floods can be (a) Annual total evaporation generated more easily than in the normal climate con- at King’s Park (1964–2002) ditions. The flood in early July 2004 in Beijing and Shanghai indeed occurred under only about 40 mm of precipitation. The study of hydro-geological con- ditions at extreme drought conditions is essential to understand the formation of the ‘‘unexpected’’ floods. Storm water infiltration into a dry ground is likely to cause debris flow and shallow-seated landslides because of the combined effect of shallow perched ground water and surface erosion by increased runoff. The possible occurrence of such debris flow and landslides due to erratic pattern of extreme climate conditions needs to be addressed as they pose a threat (b) Annual rainfall at HKO Headquarters (1947–2002) to the safety of the public and the environment. On the other hand, in extremely wet conditions, the ground water table can rise substantially and a not-so-heavy rain event can trigger deep-seated slope failures. Fail- ure of some slopes that have been stable for a long time (c) Number of days with can also be triggered. hourly rainfall greater than 30 mm at HKO Headquar- ters (1947-2002) 3 ANALYSIS METHODOLOGY In this paper, the effect of climate on emerging geo- hazards is considered in two aspects: 1. The extreme weather creates extreme initial con- ditions in the ground, either extremely dry after a period of drought or extremely wet after a period of sustained rainfall. These conditions will affect the infiltration of water into the ground and the Figure 1. Changes in rainfall and evaporation over time in Hong Kong (After Leung et al. 2004). generation of surface runoff during a subsequent storm. 2. Depending on the initial conditions and the erodi- mostly due to a very active southwest monsoon in June bility and shear strength of soils, a subsequent and August. The total rainfall of 3214 mm was 45.2% storm can cause various hazards such as slope above normal years. June 2005 was the fourth wettest instability, debris flow, and flooding. since 1884, and August 2005 the second wettest. The In this paper, the pore water pressures in a slope rainfall in these two months alone amounted to 1865 and the surface runoff on the slope will be analyzed mm, about 84% of the normal annual rainfall. at a benchmark condition (normal initial moisture content), after a drought, and after a very wet period. Figure 2 shows the profile of a soil slope consid- 2.2 Mechanisms of geohazards triggering ered. It is 30 m high, with a slope angle of 32 degrees. Climate conditions affect engineering behavior of soils The slope consists of two soil layers; the lower layer is and the geological environment. With the occurrence the natural soil and the upper layer is a loose fill. The of extreme droughts, storms and tides, many unprece- loose fill is assumed to be a loose completely decom- dented ‘‘surprising’’ geohazards have been induced. posed granite (a silty sand), with a porosity of 0.41 and These unusual geohazards are emerging challenges a saturated permeability of 4.79×10−6 m/s. The lower that have started to affect the environment and socioe- layer is of less concern in this paper, with a porosity of conomic development of Hong Kong and the Pearl 0.28 and a saturated permeability of 8.36×10−9 m/s. River Delta region. These parameters are similar to the mean values found 1704 45 The initial conditions at shallow depths of a slope A 40 vary throughout the year. McFarlane (1981) measured 35 changes of suction with time in 1980 at five sites in 30 ) Hong Kong. Figure 4 shows the variations of suction 25 on (m i at two vegetated cut slopes in completely decomposed 20 Elevat granite (CDG); one at King’sPark and another at Lung 15 Cheung Road. The largest and most rapid changes in 10 suction occurred at shallow depths. The suction level 5 was high during the dry season, particularly in April, 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 A Distance (m) but low during the wet season. Particularly in July and September, the suction maintained was less than 10 Figure 2. Profile of the soil slope considered. kPa. In order to study the effect of climate conditions on rainfall infiltration and generation of runoff, three initial conditions are generated in this study: 1. Steady state condition. The pore water pressures are obtained by subjecting the slope to a constant rain intensity equal to the annual average rainfall intensity, 2518 mm/year or 7.98 × 10–5 mm/s. 2. Extremely wet condition. The pore water pressures above the initial ground water table are assumed to be zero, representing the scenario when the suc- tion in soil is completely destroyed due to sustained rainfall prior to a new rainfall event. 3. Extreme drought condition. The pore water pres- sures are obtained by subjecting the slope to a constant rate of evaporation/transpiration equal to one-half of the annual average rate of evaporation, 1405 mm/year or 4.46 × 10–5 mm/s.