Thursday, 31 October 2013

Sediment runoff from development in Hoi Ha will threaten the coral communities in Hoi Ha Wan Marine Park

Introduction

This is a comment on Draft Hoi Ha Outline Zoning (OZP) Plan No. S/NE-HH/1.

Background

When Hoi Ha Wan SSSI and Marine Park were being established, only two threats significant enough to cause irreparable damage the coral communities in Hoi Ha Wan were identified. These are pollution and sediment runoff from future development in the area, yet neither of these risks have been considered in the Draft OZP.

Geologic continuum

Hoi Ha Wan lies at the end of a geologic continuum, which starts in the hills above Hoi Ha and passes directly through Hoi Ha village into Hoi Ha Wan. This continuum comprises a simple cycle of uplift, erosion and deposition. In this continuum, Hoi Ha village is an area of deposition and the coral communities lie at its end.

Disruption of geologic continuum and threats of sedimentation

Development of village houses in the area will require excavation and exposure of the topsoil and underlying sediments, causing a disruption to this continuum. Because of the underlying geology, a landslide hazard has been identified as a Geotechnical Constraint in the area, but the Geological Constraints associated with this have not been considered.

Because Hong Kong is naturally subject to heavy downpours of rain and extreme weather events, the excavation and exposure of topsoil and sediment in the area could lead to a massive single discharge of sediment or prolonged discharges of sediment into Hoi Ha Wan, smothering the coral communities, leading to their degradation or in a worst case scenario, resulting in their total destruction.

Hoi Ha Wan has a history of this. In the 1980’s, the loss of a coral community was recorded on the eastern shore of Hoi Ha Wan because of soil and sand runoff,  when trees and rocks were removed from the surrounding hills for reclamation of land at the Ma On Shan new town1. In fact, this incident represents a classic case study to show what can go wrong if attention is not paid to the risks associated with disrupting a geologic continuum.

Comment

If, as stated in the Draft Hoi Ha OZP there is a need to conserve the areas of conservation value and protect the ecological significance of the area, no discussion can take place without making reference to the risks associated with disrupting the geological continuum in the area, meaning that the threats posed to the coral communities from sediment inundation have not been considered or addressed.

Reference

1 LAM, K YSHIN, P K SBRADBEER, R SKU, K K K HODGSON, P A and CHEUNG, S G, "Baseline Data of Subtropical Coral Communities in Hoi Ha Wan Marine Park, Hong Kong, Obtained by an Underwater Remote Operated Vehicle (ROV)", Marine Pollution Bulletin, Vol 54, January 2007, pp 107-112. 


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