Analysis of Geo-Hazards Caused by Climate Changes

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 conditions 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 geohazards is considered in two aspects: 1. The extreme weather creates extreme initial conditions 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 suction in soil is completely destroyed due to sustained rainfall prior to a new rainfall event. 3. Extreme drought condition. The pore water pressures 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.

The initial pore water pressure distributions along section A-A (in Figure 2) are shown in Figure 5. After considering the various initial conditions created by the aforementioned climate conditions, three rainfall processes, with intensities of 70, 30, and 15 mm/hour are imposed onto the slope. Possible geohazards that can be generated by the rain events and their relation to the respective climate condition are then analysed.

4 RESULTS AND ANALYSIS Figure 3. The soil-water characteristic curves and permeability functions for the slope soils. 4.1 Influence of climate conditions on landslide triggering Rainfall infiltration is a well-known landslide trigger at the Sau Mau Ping slope (Hong Kong Government as it causes reduced soil suction (thus reduced shear 1976). The soil-water characteristic curves and perme- strength of soil) and added hydrodynamic loading. ability functions for the two soils, shown in Figure 3, The climate conditions prior to a particular rain event are generated based on the grain-size distributions and have a significant effect on the pore water pressures the porosity and saturated permeability values follow- in the slope. Figure 6 shows the effect of prior-rain ing the methods developed by Fredlund & Xing (1994) climate conditions on distributions of pore water pres- and Fredlund et al. (2004). sure in the slope along section A-A (see Figure 2). The boundary conditions are shown in Figure 2. The The rainfall intensities in Figures 6a, b and c are 70, bottom boundary is impervious but the ground surface 30 and 15 mm/hour, respectively, but the instants the is subject to either evaporation or rainfall infiltration. pore water pressures are plotted are so taken that the An initial ground water table is assumed. total rainfall amount is all 58.4 mm in the three cases.

1705 40 Suction (kPa) 35

30 Vegetated cut slope – King’s Park 25 20

15 Elevation (m) 10 Hydrostatic condition Extreme evaporation 5 Steady rain condition Extreme wet condition 0 -200 -160 -120 -80- 40 0 40 80 120 160 200 Initial pore pressure (kPa)

Depth normal to slope (m) 40

25 Suction (kPa) 20

Vegetated cut slope – Elevation (m) Lung Cheung Road 10

5 Extreme evaporation 0 -40000 -30000 -20000 -10000 0 10000 Initial pore pressure (kPa)

Figure 5. Initial conditions along section A-A possibly caused by extreme climate conditions. Depth normal to slope (m) the changes of pore water pressure during the new rain event are limited; hence the safety of the slope is less affected by the new rain. Given the same rainfall amount, the rainfall intensity does not appear Figure 4. Variations of suction at two vegetated cut slopes in completely decomposed granite (After McFarlane 1981). to affect the pore water pressures significantly. 3. After a period of extreme drought, the suctions in the ground become very high and may not be Given a rain event of a limited duration, the analysis destroyed in a new rainfall event of limited duration. results show that Deep-seated failure of the slope is unlikely caused by the new rain event although shallow seated fail- 1. If the condition prior to the rain event is very ures may be triggered due to the loss of suction at wet, say a new rain after a sustained heavy rain, shallow depths. The rainfall intensity plays a minor the ground water table in the slope will be high role in this special case because most of the rainwa- and the pore water pressures in the slope will be ter does not infiltrate into the slope. This important positive. The high positive pore pressures will point will be detailed in the next section. result in reduced shear strength of the slope soil and added seepage forces. All these will lead to Although the rainfall characteristics (e.g., intensity, reduced safety factor of the slope, and possibly duration, and pattern) are known to be key to landslides deep-seated failure of the slope. Given the same triggering, the above analysis results show that the cli- rainfall amount, a smaller but longer rain process mate conditions play an equally important role. Note will cause larger pore water pressures in the slope that the initial conditions may have a lessened effect and hence decrease the stability of the slope more in the case of a sustained new rain event. significantly. Therefore, shortly after a sustained major rain event, a moderate new rain event may 4.2 Extreme drought-flood cycle caused by climate be able to trigger a landslide that has not occurred conditions in a previous, much heavier rain event. 2. If the condition prior to the rain event is relatively Slope stability is only one special type of geohazard. wet, say a new rain after a sustained but light rain- A significant feature of recent disasters is the occur- ing period, small suctions can be maintained in the rence of repeated extreme droughts and floods within slope even during the new rain event. In addition, a particular year. This tendency can now be analyzed

1706 40 1.2 (a) Rainfall intensity = 70 mm/hour (a) Rainfall intensity = 70 mm/hour 35 1.0 30 0.8 25

20 0.6 15 Runoff rate Elevation (kPa) 0.4 10 Rainfall at a steady-state initial condition Rainfall at a steady-state initial condition 5 Rainfall after an extreme wet period 0.2 Rainfall after an extreme wet period Rainfall after an extreme drought period Rainfall after an extreme drought period 0 0.0 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 0.00.5 1.0 1.5 2.0 2.5 Pore water presure (kPa) Rain time (hour)

40 1.2 (b) Rainfall intensity = 30 mm/hour (b) Rainfall intensity = 30 mm/hour 35 1.0 30

25 0.8

20 0.6 15 Elevation (kPa) Runoff rate 0.4 10 Rainfall at a steady-state initial condition Rainfall at a steady-state initial condition 5 Rainfall after an extreme wet period 0.2 Rainfall after an extreme drought period Rainfall after an extreme wet period 0 Rainfall after an extreme drought period -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 0.0 Pore water presure (kPa) 0.0 0.5 1.0 1.5 2.0 2.5 Rain time (hour) 40 (c) Rainfall intensity = 15 mm/hour 1.2 35 (c) Rainfall intensity = 15 mm/hour 1.0 30

25 0.8

20 0.6 15 Elevation (kPa) Runoff rate 10 0.4 Rainfall at a steady-state initial condition 5 Rainfall after an extreme wet period Rainfall at a steady-state initial condition 0.2 Rainfall after an extreme drought period Rainfall after an extreme wet period 0 Rainfall after an extreme drought period -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 0.0 Pore water presure (kPa) 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 Rain time (hour) Figure 6. Effect of prior-rain climate conditions on distributions of pore water pressure in the slope after a rainfall Figure 7. Effect of prior-rain climate conditions on surface amount of 58.4 mm. runoff rate.

Figures 7 and 8 clearly demonstrate that through the effects of climate conditions on surface runoff generation. During a rain event, the amount of 1. The surface runoff rate strongly depends on the pre- water infiltration can be obtained by de- fining a flux rain climate conditions. It increases as the pre-rain section along the ground surface and recording the flux ground condition becomes drier. This is reasonable across the section over time. The water infiltration rate since the permeability of soil becomes very small obtained can be further used to calculate the surface when the soil is desaturated (see Figure 3b). The runoff rate, defined as the ratio of surface runoff to effects of prior climate conditions are particularly the total rainfall. The runoff rate values for the three significant when the rain intensity is relatively low rain events under the aforementioned climate condi- (e.g. 15 mm/hour in Figure 7c). When the prior tions are calculated and shown in Figure 7. Values of climate condition is very wet, the runoff rate from the cumulative average surface runoff per unit area are the new rain event is as low as 0.4. In contrary, after shown in Figure 8. Note the analysis does not consider a period of extreme drought, the permeability of any surface ponding and thus may overestimate the soil becomes so low that little rainwater infiltrates surface runoff. and the runoff rate reaches over 0.95.

1707 0.16 of precipitation. When the rainfall intensity is much (a) Rainfall intensity = 70 mm/hour 0.14 larger than the saturated permeability of soil, the prior climate conditions have a minor effect on )

2 0.12 /m

3 flood generation (Figure 8a). 0.10 3. This analysis reveals the mechanisms behind 0.08 increasing, interchanging geohazards caused by

0.06 climate changes; namely, extreme droughts followed by floods or debris flows, or extremely wet 0.04 Rainfall at a steady-state initial condition

Runoff per unit area (m conditions followed by deep-seated landslides. 0.02 Rainfall after an extreme wet period Rainfall after an extreme drought period 0.00 0.0 0.5 1.01.5 2.0 2.5 Rain time (hour) 5 CONCLUSIONS

0.08 (b) Rainfall intensity = 30 mm/hour`aa Hong Kong and the vicinity, as with other parts of the 0.07 world, are subject to climate changes. Clime changes ) 2 0.06

/m include the tendency of long-term changes and the ten- 3 0.05 dency of more drastic yearly variations, with the latter

0.04 tendency causing more geohazards based on the results of analysis in this paper. 0.03 If the climate condition prior to a new rain event 0.02 is very wet, the ground water in the slope concerned

Runoff per unit area (m Rainfall at a steady-state initial condition 0.01 Rainfall after an extreme wet period will be high and the pore water pressures will be pos- Rainfall after an extreme drought period 0.00 itive. These will lead to reduced safety factor of the 0.0 0.5 1.0 1.5 2.0 2.5 slope, or possibly deep-seated failure of the slope. Rain time (hour) Particularly, a smaller but longer new rain event will 0.16 (c) Rainfall intensity = 15 mm/hour cause larger pore water pressures in the slope and 0.14 hence decrease the stability of the slope more sig- )

2 0.12 nificantly. After a period of extreme drought, the /m 3 suctions in the ground become very high and may 0.10 not be destroyed in a new rainfall event of limited 0.08 duration. Shallow-seated failures or debris flow may 0.06 be triggered although deep-seated failure are unlikely.

0.04 More importantly, the runoff amount generated by a Rainfall at a steady-state initial condition Runoff per unit area (m moderate rain event after a long period of drought can 0.02 Rainfall after an extreme wet period Rainfall after an extreme drought period be twice that generated after a sustained wet period, 0.00 causing ‘‘surprising’’ flood disasters that are not likely 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 Rain time (hour) under normal prior climate conditions.

Figure 8. Effect of prior-rain climate conditions on surface runoff generation. A rainfall event after an extremely drought will generate a flood approximately twice as much as that in ACKNOWLEDGEMENTS the normal condition, which explains why extreme geohazards (say extreme droughts followed by floods) become more This research is supported by the Emerging High often. Impact Areas (EHIA) Program 2004/05 of the HKUST (Project No. HIA04/05.EG02 ‘‘Emerging Geohazards in Hong Kong and Pearl River Delta due to Climate 2. In response to the drastic changes in runoff rate Changes’’). due to prior climate conditions, given the same moderate rain event (Figure 8c), the runoff amount generated after a long period of drought can be twice that generated after a sustained wet period. REFERENCES This explains many cases of ‘‘surprising’’ flood Leung, Y.K., Yeung, K.H., Ginn, E.W.L. & Leung W.M. disasters that were caused by moderate rains. This 2004. Climate Change in Hong Kong. Technical Note No. tendency may be aggravated by increasingly paved 107, Hong Kong Observatory, Hong Kong SAR. ground conditions in an urban area. For exam- McFarlane, J. 1981. Soil Suction and Its Relation to Rain- ple, the floods in early July 2004 in Beijing and fall. GCO Report No. 13/81, Geotechnical Control Office, Shanghai indeed occurred under only about 40 mm Hong Kong.

1708 Fredlund, D.G. & Xing, A.Q. 1994. Equations for the Government of Hong Kong. 1976. Report on the Slope Fail- soil-water characteristic curve. Canadian Geotechnical ures at Sau Mau Ping 25th August 1976. Vols. 1–3. Hong Journal 31: 521–532. Kong: Hong Kong Government Printer. Fredlund, D.G., Xing, A. & Huang, S. 1994. Predicting the permeability function for unsaturated soils using the soil-water characteristic curve. Canadian Geotechnical Journal 31: 533–546.

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