Institute of Space and Earth Information Science & The Center for University and School Partnership, Faculty of Education, CUHK (Sponsored by QEF)

Transcription

1 1 Landslides and Slope Stability in Hong Kong Introduction: There are many landslides in Hong Kong throughout the years. Some of these are disastrous, such as those occurring on Po Shan Road and in the Sau Mau Ping Resettlement Area on 18th June, People and property were lost. Buildings collapsed and the whole resettlement area was buried in the rubble, causing the death of 67 and 71 people respectively. Other landslides are less damaging but can still be threats to nearby residents and passers-by. The major purpose of this topic is to help students to investigate the nature and causes of landslide in Hong Kong. PART 1: Find out the general causes of landslides in Hong Kong Complete the following actions before you start: (1) Copy the folder from S-share to the desktop. (S:\Subjects\Geography\Landslide\Student). (2) Open Google Earth by double-clicking the shortcut on your desktop. (3) Select Borders and Labels, Places and Roads in Layers > Primary Database. Let s start the activity. I. Rainfall and landslides: 1. On Google Earth, click File > Open, select Student > Isohyets > Year 2004.kmz. Then, open Student > Rainfall & Landslide > 2004.kmz.

2 2 (a) What is the highest rainfall? (b) Name 2 districts where you can find the highest rainfall: (c) In which part of Hong Kong are these districts located? (d) What is the lowest rainfall? (e) Name 2 districts where you can find the lowest rainfall: (f) In which part of Hong Kong are these districts located? 2. Hold the SHIFT key and move the scroll wheel of your mouse to rotate and tilt the terrain for 3D view. (a) Describe the distribution of rainfall in Hong Kong in relation to relief. (Note that most of the rain-bearing winds in Hong Kong are coming from the east and southeast therefore there is more rainfall on the eastern slopes. There is also higher rainfall in Lantau South because of the SW monsoon.) 3. Look at the red dots which represent minor and major landslides. (a) In which part of Hong Kong are most of the landslides found? (b) Does the location match the distribution of rainfall?

3 3 4. Uncheck 2004 and Year 2004 on Google Earth. Open Student > Isohyets > Year 2005.kmz. Then, open Student > Rainfall & Landslide > 2005.kmz. (a) What is the highest rainfall? (b) Name 2 districts where you can find the highest rainfall: (c) In which part of Hong Kong are these districts located? (d) What is the lowest rainfall? (e) Name 2 districts where you can find the lowest rainfall: (f) In which part of Hong Kong are these districts located? 5. Look at the black dots and symbols X which represent minor and major landslides respectively. (a) In which part of Hong Kong are most of the landslides found? (b) Does the location match the distribution of rainfall? 6. Check 2004 and Year 2004 on Google Earth, adjust the slider bar below the Places panel to show the gradual change for comparison. Pay special attention to Yuen Long, Tsuen Wan, Lantau Island and the northwest of Hong Kong. (Remember red dots represent landslides in 2004, black 2005) : (a) Outline the differences in rainfall values and landslide locations between 2004 and (b) What is the relationship between landslides and rainfall?

4 4 II. Slopes and landslides: 1. Uncheck all other layers except 2004 On Google Earth, click File > Open, select Student > example > Tsuen Wan & Kwai CHung.kmz and Yuen Long Plain.kmz. 2. Look at Tsuen Wan & Kwai Chung. (a) Count the number of landslide in the marked area: (b) Drag and drop the Street View icon to on a road with the blue border in the area to enter street view; navigate and rotate your view using the scroll wheel of the mouse. (c) Observe the characteristic of the slopes in this area. (Use the Search Panel to find the following places) 11 Shek Pai Street Ting Fung Street Lai Yiu Street (Wah Yiu Road)

5 (d) What are the common characteristics of the slopes? 5 (e) What has the government done to prevent reoccurrence of landslide at these locations? 3. Look at Yuen Long Plain. (a) Did any landslides occur in this area in 2004? How many? (b) Uncheck 2004 and select 2005 (c) Did any landslides occur in this area in 2005? How many? (d) Are these major or minor landslides? (e) Drag and drop the Street View icon to Long Ping Road and Tsui Sing Road. Compare the slopes with those you observed in Tsuen Wan & Kwai Chung. (f) Click Add Path to draw a line across any one landslide location in Yuen Long and click OK.

6 Right-click on the path and select Show Elevation Profile. 6 What is the maximum slope? (Note: The higher the absolute value in % the steeper the slope is)

7 7 4. Look at Tsuen Wan & Kwai Chung. (a) Did any landslides occur in this area in 2005? How many? (b) How many of these were major landslides? (c) Using the method outlined in (3) (f), find the maximum slope of one major landslide and one minor landslide in this area. (d) Compare the slopes in Yuen Long and Tsuen Wan, which one has steeper slopes in general? (e) Do the major landslide locations in Tsuen Wan have steeper slopes? (f) Can you explain? 5. Uncheck all layers except Zoom in northern part of HK Island, and tilt and rotate to view in 3D. (a) Describe the landslides location in relation to land use here: _ (b) Can you suggest one more cause of landslide in this area? _

8 8 III. Geology and landslides: 1. Uncheck all layers on Google Earth except Yuen Long Plain and Tsuen Wan & Kwai Chung. 2. Select Then, open Student> soilmap.kmz. (a) Write the relationship between landslide in 2004 and rock type:

9 3. Uncheck 2004 and select (a) Write the relationship between landslide in 2005 and rock type: (b) In which type of rock did major landslide mostly occurred in 2005? 4. Select (a) Use the slider bar below the Places panel to adjust transparency of 2004 & 2005 to compare the landslides occurred in 2004 and 2005 on i. Lantau East and Lantau West: ii. HK Island: (b) What is the importance of rainfall in affecting landslide?

10 IV. Vegetation and landslides: 1. Uncheck all layers. Open Student> Vegetation.kmz and then select Describe the relationship between vegetation and landslides Uncheck 2004 and select Describe the relationship between vegetation and landslides. Are there any differences from the year of 2004? Can you suggest reason for such differences? Pay attention to the different locations for major and minor landslides:

11 PART 2 Case Study: Landslide on Po Shan Road, Mid-Levels (18 th June, 1972) Summary: 11 A devastating landslide occurred on 18 June 1972 near No. 14 Po Shan Road, Mid-levels. Approximately m 3 of debris travelled some 270 m down slope and resulted in 67 fatalities, 20 injuries, 2 buildings destroyed and 1 building severely damaged. Urban development of the Mid-levels area extended progressively upslope into the natural terrain during the late 1800s and early 1900s; Conduit Road and Po Shan Road were constructed in 1910 and 1925 respectively. The slope gradient above Po Shan Road was about 35 o. Downslope between Po Shan Road and Conduit Road the slope gradient was more gentle. A construction site above Conduit Road, where the major part of the landslide was located, was being redeveloped at the time of the landslide. In late 1971 two landslides occurred on the construction site. Most of the failed area is underlain by weathered rock of volcanic origin blanketed with 5 m - 10 m thick colluvium and locally fill. Cracks were noted in Po Shan Road (in the area of the 1972 landslide site) following Typhoon Rose about 11 months prior to the June 1972 fatal incident. Inspection at Po Shan Road on 16 June 1972 reported cracks and that attempts to seal these had failed. On 17 June 1972, a landslip occurred the construction site above Conduit Road resulting in a 30 o rotation of steel piling, and settlement of the garden terrace upslope. On 18 June 1972, the deterioration continued; the previously failed slip surface showed signs of creep and the sheet piling was creaking. A major detachment occurred at about 8:50 pm on 18 June The landslide started from the hillside above Po Shan Road, landslide debris travelling some 270 m downslope to below Kotewall Road. On 19 and 20 June 1972, two minor failures were reported after the major landslide. A large relict joint was exposed on the western flank of the failure. Site formation work on the construction site above Conduit Road, together with the exceptional heavy rainfall in early 1972, caused the landslide. About 1400 mm of rainfall was recorded between May and June 1972 and in particular more than 650 mm of rainfall was recorded from 16 to 18 June Source: The information given in this simplified account is extracted from a report prepared by the Geotechnical Engineering Office (GEO) of the Government of the HKSAR following its detailed investigation into the cause of the landslide ( Some descriptions on the Po Shan Road 1972 landslide dimension: 1. Volume of materials (m 3 ) Width of displaced mass (m) Length of displaced mass (m) Width of surface rupture (m) Length of surface rupture (m) Total length (m) Length of center line (m) Toe / crown angle (degrees) Travel angle (degrees) 27

12 12 1. On Google Earth, click File > Open, select Student > Po Shan Road > landslide location.kmz. Expand the landslide location.kmz, right-click on slip face (red path), and select Show Elevation Profile. Find out the approximate extent of the slip face. (a) Altitude of point A = (b) Altitude of point B= 2. Find the difference in altitude between the starting and ending points. (a) The difference in altitude between A and B= (b) Maximum gradient = 3. Open Leoworks from Application on the desktop. On Leoworks, click File > Open or, select Student > Po Shan Road > location of landslide.jpg. Find the approximate area of the slip face. (a) Refer to the manual on Leoworks. Find the area of the landslide site in square metres. The area of the landslide site : 4. Find the direction of the landslide (i.e. from A to B) in degrees and reduced bearings by using the Ruler function in the toolbar at the top of Google Earth. (a) Angular (whole-circle) bearing : (b) Reduced (compass) bearing :

13 13 5. Overlay aerial photos before and after the Po Shan Road landslide, soil map, and vegetation layers in Google Earth. With reference to the graph of rainfall in June 1972 (in Po Shan Road folder) and results of previous tasks. Can you identify the causes of the 1972 Po Shan Road landslide?

14 14 Part 3: From the above studies, we can come to the following conclusion: 1. The causes of landslides in HK are: 2. Comment on the following statement, Natural environment is the major cause of landslides in HK.