Geomorphic processes at a coastal hillside near Po Pin Chau

Abstract

Po Pin Chau (破邊洲) is located in High Island (糧船灣), which belongs to the eastern side of the Sai Kung Peninsula in the eastern New Territories. The area is part of the Hong Kong UNESCO Global Geo-park and is famous for its hexagonal columnar-jointed rock. It takes one-hour hike to the studied area from the south of the east dam of High Island Reservoir. It’s about 45-minutes hike from Po Pin Chau. Some interesting observations related to weathering and erosion are noted. This area is made of hexagonal columnar-jointed rock. Although they are the same type of rock (hexagonal columnar joints composed with rhyolitic tuff according to the geological map), they are subject to different rate of weathering and erosion. Geomorphic processes are natural mechanisms of weathering, erosion, and deposition that result in the alteration of the surficial materials and landforms at the surface. The exogenic (external) and endogenic (internal) forces cause physical and chemical effects on the surficial materials. This brings about changes in the shape of the surface of the earth which are known as geomorphic processes. In order to investigate the geomorphic processes of the selected area, a study has been carried out to understand the weathering processes including physical weathering, chemical weathering and biological weathering and also to understand the erosion processes including water erosion, wind erosion and erosion caused by gravitational force. Obviously, water erosion and mass wasting have occurred at the site which is supported by the fact that rainfall in Hong Kong is relatively high and the site is on the hilly area. Furthermore, since this site is at the coastal hillside which opens to the east wind from the Pacific Ocean, we believe that wind erosion cannot be ignored. To understand the impact of wind erosion, wind simulation has been carried out. The selected area is located on the hilly area just above the coastal area in which waves and tides cannot reach it. The outcrop refers to the surface of the studied area. Literature reviews about geomorphology, climate and geology of the site and soil mechanics have been covered. Photos were taken and in-situ field tests were done during field trips. Different types of laboratory tests to understand the nature of the surface materials were performed. Finally, wind simulation by software had been carried out for understanding the wind impact. It is found that the surface materials are prone to erosion due to its material properties.