The 13^th in The University of the West Indies Faculty of Science and Technology’s Forum Series was held on Wednesday, March 3, 2021.The Forum was chaired and moderated by Dr. Thera Edwards (Department of Geography & Geology) and co-hosted by Dr Andre Coy (Physics). The presenters were Professor Simon Mitchell, Professor of Sedimentary Geology, Department of Geography & Geology; Mr. Canute Ricketts, Director, Research and Mapping Unit, Mines & Geology Division of the Ministry of Transport & Mining, Ms Diana McPherson, Senior Planner at the National Environment & Planning Agency and Dr Wayne Reid, Civil Engineer at Jentech Consultants Ltd.

Professor Mitchell opened the discussion by explaining that heavy rainfall and felt earthquakes trigger landslides. He then described the general geology of Eastern Jamaica and the geological features of three sites in the Irish Town - Gordon Town area where landslides had resulted from the exceptionally heavy rains associated with hurricanes Zeta and Eta in late October and early November 2020. Bedding planes, joints and faults in the shales, sandstones and conglomerates of the district form planes of weakness in the rocks along which slippage can occur under stress. The two main factors that led to the slope failures that blocked and broke away the roads were the heavy antecedent rains in October that saturated the ground, followed by the heavy, 100 mm rain event associated with hurricane Eta when the extra weight of water and its lubricating effect triggered the landslides. In one case the rainfall remobilized old landslide material. The second case resulted from slippage along bedding planes sloping towards the road exacerbated by a retaining wall that had undercut the bedding. The breakaway (‘Brukweh’) along the Gordon Town road resulted in a wedge failure due to slippage and gouging along a dense set of joint planes.

Mr Ricketts introduced the Hillside Development Manual a free online manual created in 2014, using the Jacks Hill and surrounding area as an example. The challenge of colluvium from nearby Granodiorite swelling in wet conditions leading to slope failure was explained with the use of diagrams and photos. Growing demand for upper income residential buildings and the gradual shift from single family houses to multi-family developments has led to large buildings on very steep slopes deeply dissected by gullies. Complex geology and extensive faulting predisposes the area to landslide occurrences. In particular the lobe-shaped deposits derived from weathered granodiorite are old landslide deposits or colluvium containing loose sand and gravel and plastic clays. Wedge failures along joints and faults in solid rocks and rotational failures in soft rocks have destroyed housing. Debris flows and flows also cause inconvenience. The manual promotes the safe and sustainable development of hillsides, aims to minimize risks of landslides, erosion and flood events and reduce disturbances in geologically sensitive areas and provides guidelines for the Development Approval Process. The manual establishes a maximum hillside slope of 30 degrees for development. Size limitations of cut and fill operations, grading of sites (regulated) and reasonable retaining wall heights as excessive heights can lead to failure and fatalities have occurred. Sedimentation must not affect neighbouring properties. Lot sizes for hillside areas should be larger than normal to accommodate construction. Colluvial slopes are not recommended for development. Compliance needed for developers and stakeholders.

Ms McPherson discussed the impact of landslides on development and development potential, landslide susceptibility and land use zoning and density allocation. The preparation of development orders for the Town and Country Planning Agenc. TCPA, and processing of applications for development permits and subdivision applicants, particularly the maximum number of units that may be constructed on a specific lot. Development for highest use of the land. Maximum number of units that may be constructed on a specific lot. Highest is normally in urban areas. Lower in rural, lowest in environmentally sensitive areas, also guided by the hillside development policy. Site specific assessments are required in geologically sensitive areas. Lower residential densities and less intensive use of the land applied to areas highly susceptible to landslides. Lower development potential can lead to potential conflicts with users of the land. Objective is to create a balance between human environment protection and economic growth.

Dr. Reid provided a brief historical background to road development. Ex-slaves went to settle in the hills. Their tracks gradually became roads. Land development occurred on either side of roads. The need for better access created need for road widening and improvement. Landslides were triggered by human intervention. When maintenance or repairs or widening is needed there are budget limitations and time constraints, usually the quicker the better for road repairs. Alternate routes are possible but in turn may require funds for initial improvement. Land acquisition presents another problem. Road widening may happen naturally as on the southern part of Junction Road. Drainage needs are often ignored or minimized but adequate drainage is mandatory for successful mitigation. The alignment of gully courses is not always taken into account. Additionally there may be water supply and sewage treatment concerns. Adequate foundation for retaining walls during mitigation is often not implemented. Positioning of walls is not always correct/appropriate. Improper location is another important factor. Construction of dwellings may increase the weight on a slope. Many persons who need housing solutions (usually informal dwellings) may not bother with permits and utilise local contractors who do not take into account planning and geological guidelines.

Questions from the audience revealed that the Hillside Development Manual is freely available on the Mines and Geology website. The application of typology in addressing current solutions for landslides was also questioned by another participant. It was explained that some areas can be easily assessed for sandstones, shales and bedding orientation and hence these can be mitigated for. However, the joint systems are more difficult to work with. The ‘outlook’ for landslide occurrence in light of climate change was queried and the response suggested that forest clearance and more severe/frequent hydrometrological events would lead to more frequent landslides. The use of physical methods other than retaining walls or vegetation was another question posed from the audience. Calliandra and Vetiver were recommended as vegetative controls while Maccaferri mats and tyresoil retaining structures were suggested as physical controls. The calculation of density requirements inland use planning also garnered wide interest.

The Forum presented information starting with the geology and geological issues, showcased development and regulatory responses and how these along with time and cost are taken into account by engineers in designing solutions. The presentations were highly complementary and packed with information.

To view the forum: https://www.youtube.com/watch?v=v4SNwySlAHk