LAND DEGRADATION IN THE CARIBBEAN: QUATERNARY GEOLOGICAL PROCESSES. RAFI AHMAD

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1 EXTENDED TASK FORCE MEETING OF PARTICIPATING AGENCIES AND LATIN AMERICAN COUNTRIES FOR ENHANCING SOUTH-SOUTH COOPERATION BETWEEN LAC-CARIBBEAN SIDS PARTNERSHIP INITIATIVE ON LAND DEGRADATION AND SUSTAINABLE LAND MANAGEMENT MAY 30 TH JUNE 1 ST 2005 BRIDGETOWN, BARBADOS LAND DEGRADATION IN THE CARIBBEAN: QUATERNARY GEOLOGICAL PROCESSES RAFI AHMAD rafi.ahmad@uwimona.edu.jm UNIVERSITY OF THE WEST INDIES DEPARTMENT OF GEOGRAPHY & GEOLOGY UNIT FOR DISASTER STUDIES MONA, KINGSTON 7, JAMAICA

2 NATURAL PROCESSES operating at the land surface on human time scale include: erosion, water flow, deposition, and soil formation. Erosion includes: gravity induced mass wasting or landslides, fluvial erosion, raindrop, sheet, rill, gully, piping. LAND DEGRADATION is the loss of land productivity through various natural processes such as landslides, salinization, depletion of nutrients etc. ( Blaschke, 2000). LAND PRODUCTIVITY is defined as the amount of primary production per unit of land area ( Dudal, 1982; Blaschke, 2000). Landslides( debris flows) in Scotland District, Barbados

3 OUR AIM IS TO DEMONSTRATE THAT THE CHIEF CAUSE OF LAND DEGRADATION ARE LANDSLIDES

4 PROCESSES OF LAND DEGREDATION ARE MOSTLY CUMULATIVE AND IRREVERSIBLE ON HUMAN TIME SCALE. WHY LANDSLIDES? 1. Geological materials of low strength are common: deeply weathered bedrock; abundant discontinuities. 2. In many areas tectonic stresses have caused extensive bedrock fractures, tropical weather conditions accelerate weathering processes 3. A mantle of poorly sorted silt-sand-coarse rock debris( colluvium or regolith) overlies the bedrock. This is the most common medium for landslides. 4. Influence of water on slope hydrology is profoundpore pressure; surcharge 5. Vegetation conversion also results in landslides

5 MUCH OF THE LANDSCAPE IN THE SCOTLAND DISTRICT HAS EVOLVED IN THE HOLOCENE AS A RESULT OF LANDSLIDES IN CLAY SLOPES

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7 Landslides described in this presentation include shallow landslides that are commonly molded into landslides of flow type and include varieties known as soil slips, mud/earth flows, debris flows, debris slides, debris avalanches, and slumps (classifications after Varnes, 1978; Hungr et al., 2001). Photo: J. Tyndale-Biscoe Spectacular development of rainfall-induced shallow landslides (debris flows) on the southern slopes of the Blue Mountain Ridge, Mt. Macungo area, St. Thomas, triggered by rainfall from the hurricane Gilbert on the 12 th of September, 1988.

8 Shallow landslides, St. Andrew, Jamaica

9 Landslides on Kenmure Ridge, Blue Mountains, Jamaica ( Tyndale-Biscoe photo).

10 Landslides: Coffee plantations, roads ( Tyndale-Biscoe photo) Cinchona Road, Blue Mountains, Jamaica

11 Debris flows in limestone following June 1979 Flood rains, Blue Fields area, Jamaica( Tyndale-Biscoe photo).

12 Number, size and area disturbed by past landslides on Dominica, St.Lucia, St. Vincent, and Jamaica. (DeGraff et al., 1989; Ahmad & McCalpin, 1999). Island Number of Landslides Landslide size ( in ha) Average Largest Landslide density (per km 2 ) Terrain disturbed (in %) Dominica St. Lucia St. Vincent Jamaica Parish of Kingston & St. Andrew, Area= 2, m 2 381,609 m 2 Area covered by landslides and deposits= km km 2 RATE OF SEDIMENTATION Hermitage Reservoirwatershed area14.25km2. 3 tons/acre/yr RATE OF EROSION Rio Pedro watershed:110 tons/km 2 /yr; Yallahs River watershed: 40tons/acre/yr

13 Hermitage Reservoir, Jamaica. Designed storage capacity, 1927: 500 mg Massive siltation following 1963 hurricane Flora rainfall-induced landslides. Capacity in 1963: 290mg; Capacity in 1993: 360mg

14 It can be demonstrated in central Jamaica that accelerated soil erosion occurs on the sites of landslides ( Landslide Hazard mapping in Rio Minho watershed, BGS/UWI Project, 2000).

15 Although not fully appreciated, it is now increasingly being recognized worldwide that landslides are the principal cause of land degradation and land productivity in the tropics including SIDS LANDSLIDE SUSCEPTIBILITY IN CLARENDON AND ST. CATHERINE, CENTRAL JAMAICA, DRAPED ON A DEM (Unit for Disaster Studies, UWI & British Geological Survey Project completed in 2002). Red areas indicate high landslide susceptibility zones and also are sites of accelerated soil erosion. Efforts to correct land degradation and loss of land productivity should take into consideration rainfall-induced landslides. We think that would tremendously benefit if they were to incorporate landslide data.

16 Bullhead Mountains, Clarendon, Jamaica (Tyndale-Biscoe Photo). Pine trees were planted in 1960 s to arrest accelerated soil erosion. Landslides, however, could not be controlled.

17 Landslide isopleth map, Rio Minho watershed, Jamaica. (Earle,1990).

18 WATERSHED CONDITIONS IN JAMAICA

19 RAINFALL INTENSITY-DURATION THRESHOLD FOR SHALLOW LANDSLIDES IN EASTERN JAMAICA: Using data for 19 storms, , a threshold relation between rainfall intensity-duration and landsliding was established Threshold for rainfall-induced shallow landslides in Eastern Jamaica ( ) 100 Rainfall Intensity (mm/hour) Rainfall Duration (hours) Rainfall events producing landslides ( ) Storms that did not cause landslides ( ) Rainfalls that caused landslides ( )

20 RAINFALL THRESHOLDS FOR LANDSLIDES AND SEDIMENT-WATER FLOODS. THE THRESHOLD RELATIONS REPORTED HERE ARE REASONABLE FIRST APPROXIMATIONS. In Jamaica, rainfall threshold relation is defined for storms that had durations between hours and average rainfall intensities between 2-93 mm/h. The threshold relation indicates that for rainfall of short duration (about 1 h): Rainfall Intensities > 36 mm/h are required to trigger landslides. These storms trigger mostly shallow landslides by causing an excess pore pressure in shallow colluvial zones. Such landslides were typically associated with type storms. Low average intensities of about 3mm/h appear to be sufficient to cause landslilding as storm duration approaches approximately 100 h. These storms triggered the largest, deepest landslides in eastern Jamaica, e.g., Flora, Gilbert, 2001 rainfall.

21 Threshold for earthquake-induced landslides in Jamaica. The earthquakes of June 1692 and January 1907 and flood rains of June 1979 created hundreds of landslides which caused severe deforestation and erosion on the island.

22 Conclusions 1. Geological processes operative during Holocene ( from 10k years to present) and late Pleistocene ( 1.7 my to 10k) need to be investigated 2. External factors related to tectonic deformation and climate changes control the nature and rates of surface processes 3. Landslides are a common occurrence and recurring problem for much of the mountainous small island states of the Caribbean 4. Fundamental questions: How are the natural functions of the watershed are impaired by erosive processes? Is it possible to model processes and make forecasts to guide management of watersheds and water resources?

23 THANK YOU