Shore Erosion Studies of Pakistan Coast in the Vicinity of Karachi

Abstract

This research work was undertaken to study the process of shore erosion along the coast of Karachi. The area under investigation is the coastline from Manora Island to Cape Monze. Six different locations in this area were selected for taking observations and collections of samples. Due to the limitations regarding instruments and sea-going facilities, the study was carried out near the shore only. The observations from the six fixed reference points along the coast were taken after every fortnight, including the recording of visual wave data, wind data and samples of sediments and sea water. In order to track the offshore movement of suspended load, satellite imageries were visually interpretated. In the laboratory the sediment samples were dried and sieved for the determination of variations in grain size in surf zone at each station for diffferent seasons. From the visual wave data an attempt is made to work out the longshore energy flux in surf zone due to breaking waves. It is observed that during SW-monsoon the dissipation of wave energy causes considerable changes on the beaches and surf zone, and large amount of sediment are brought into suspension by the collapsing waves. During this season of high wave energy the waves predominantly approach the coast lineobliquely resulting transport of sediment along the coast and "on and Off" shore. It is observed that during the post-monsoon and pre-monsoon period the beaches and nearshore zone, are subjected to only moderate to low wave energy conditions and rebuilding of beaches takes place in these periods. However it is observed that the beach material removed due to erosion is greater than the beach material deposited back and thus there is a net loss of beach material in the area. At station No.2 near Paradise Point erosion is quite severe as the waves attack causes landslides. On the otherhand at sandspit the most crucial area is near station No.6 where sea water is observed to cross the sandspit on several occasions. Protective measure must be taken here immediately to protect the sandspit.