Pakistan has a coastline that stretches to over 1050 km, (990 Km measured as a straight line) along the Arabian Sea. It comprises two distinct units, the passive margin of Sindh; and the active margin of the Balochistan coast (Figure 1). The coastal and offshore geology of Pakistan tectonically exhibits both active and passive margin features. The Balochistan coast is active whereas the Sindh coast and Indus deltaic area and offshore Indus basin is geologically passive. The Sindh and Balochistan coasts have different climatic conditions, geographical location and socio- economic factors. The Sindh coast can be further divided into two parts, namely the Indus deltaic coast and the Karachi coast. The coast in the vicinity of Karachi, which is approximately a 70 km stretch, is relatively well developed as compared to the rest of the coast.
The coast of Pakistan consists of sandy beaches located in Sindh and separated by rocky protruding points from each other. The rivers are the predominant source of sediment to the sandy beaches. The Balochistan coast is drained by seasonal rivers Hingol, Basol, Shadi Khor and Dasht. Flash floods are frequent and even during scanty rains, there is erosion of top soil from the uncovered hillsides and muddy banks. The eroded material is deposited along the coast at the mouth of the rivers. The Balochistan coastal region has cliffs, occasionally with rocky headlands, and a number of sandy beaches with shifting sand dunes. The coastal regions of Balochistan consist of coastal lagoons with scanty mangrove patches. The coastal Sindh consist of 17 major and minor creeks in the Indus Delta, which is dominated by mangrove forests.
The Sindh coastal region is located between the Indian borders along Sir Creek on the east to Hub River Bank on the west (320 km). The Indus River drains into the entire lower plain of Sindh. The Indus delta is the most prominent feature of the Sindh coast. The sediments are subjected to coastal dynamic processes, such as tides, winds, waves and currents, leading to accretion and erosion of the Indus deltaic coast. The coastal morphology is characterized by a network of tidal creeks and a number of small islands with sparse mangrove vegetation, mud banks, swamps and lagoons formed as a result of changes in river courses. The present delta covers an area of about 160,000 hectares and is characterized by 17 major creeks and innumerable minor creeks (Fig-7.1), mud flats and fringing mangroves. The delta supports wetlands rich in nature and culture, and also nurtures the largest area of arid climate mangroves receiving an average annual rainfall about 200 mm. Twenty seven percent of this land is under water in the form of creeks and watercourses. These watercourses intervene into the island, these are calm and protected water, and are flushed daily by tides up to 3 meters.
The coast of Karachi is situated between the Cape Monze, a high cliff projecting into the Arabian Sea and the Korangi creek. The coastline of Karachi metropolitan is about 70 km long. It is generally oriented NW-SE. On the western side it is bounded by the Hub River and on the east by the mangrove swamps and creeks of the Port Qasim area. The Layari and Malir rivers are the seasonal streams which flow during South West monsoon. The rain water from Karachi and its adjoining area drains in the Arabian Sea. The prominent feature of Karachi coast is shallow and raised beaches, marine terraces and dune fields. Four major inlets, Manora Channel (Karachi harbour), Korangi creek, Phitti creek, and Khuddi creek, invigorate the coastline. A small crescent shaped sand bar exists at the mouth of the Korangi creek. The shore terraces and sea cliffs are to the west of Hawks bay area. The Cape Monze beach is an example of raised beaches along the coast of Karachi. The eastern coast has tidal creeks with mangrove and mud flats. In the region the seabed is generally smooth. The bed slope has a low gradient and is in the order of 1/500 to 1/1000.
Figure-7.1: Coastal and offshore areas of Pakistan source: NIO
The coast west of Manora breakwater to Buleji consists of sand beaches, (Manora, Sands spit and Hawks bay) rocky protruding points separate these beaches from each other. From Buleji to Cape Monze the coast consists of hard conglomerate and shale cliffs. Beyond Hawks Bay towards west up to the Cape Monze, the unconsolidated sandy clays are exposed to coastal weathering and erosion. The rivers are the predominant sources of sediment to the sandy beaches during the rainy periods. The Clifton beach is largely composed of dark, grey silt material with minute flakes of mica. Further east of Clifton there are agglomerations of Ghizri hills. The coastal areas of Karachi are densely populated. The beaches of Karachi attracting large number of people are a source of recreation for the local inhabitants.
The present Indus delta is located at the head of the Arabian Sea, between Korangi Creek and the Runn of Kutch. The Indus River is the world’s sixth largest river; it drains into the north-eastern Arabian Sea forming a large delta. The river discharges nutrient rich sediment load that has a great influence on the marine life of the Indus Estuary and the near shore areas. During the prehistoric times i.e., 5000 6000 years B.C. the delta has protruded 150 km at the rate of 30 m/year to the present position. Fan shaped deltaic complex spread over an area of about 30,000 sq. km. The deltaic complex is comprised of abandoned, active and sub aqueous parts of the delta and river flood plains. With the changes in the river discharge, the present active delta has consequently shrunk to a small triangular area of about 120 sq km stretches in the vicinity of Keti Bandar. The whole area presents a peculiar geomorphology of mud banks, swamps, lagoons and sand dunes. The soil cover in the deltaic area is of drift type made up of altered material transported by rivers. The Indus delta has been found changing its fluvial characteristics due to damming upstream, which has reduced river borne sediments. This has resulted in drying up of the estuaries and has induced sea encroachment further inland.
The Balochistan coast extends from the mouth of the Hub River in the east to the middle of Gwadar Bay in the west and stretches over a distance of about 660 km. The Balochistan coast along Makran and Lasbella districts is an arid coast, owing to scanty rainfall and highly saline soil. The Balochistan coast has almost entirely desert like condition with only 150-mm/year of rainfall. This region has cliffs mingled with rocky headlands and a number of sandy beaches with shifting sand dunes. The region of creeks and coastal lagoon is marshy with scanty mangrove patches.
Principal geomorphic features of the Balochistan/Makran coast are cliffs, headlands, and mud volcanoes. Rocks exposed along the coast are the assemblages of sandstone, shale and mudstone. The mountains are composed of bare rocky limestone or conglomerate, with the exception of some upper highlands with a little or no vegetation. The coastline faces considerable erosion. Owing to shortage of promontories and sheltered areas, most of the littoral material is lost to the sea. Spectacular mud volcanoes are found in several locations along the Makran coast where gas-charged water escapes to the surface.
The prominent coastal areas of Pakistan can be classified as:
Tidal lagoons (a) Miani Hor (b) Kalmat Khor
Sheltered Bays a) Hawks Bay, b) Gadani Bay,
c) Sonmiani Bay, d) Ormara Bay (east and west),
e) Pasni Bay, f) Gwadar Bay (east and west),
g) Gwatar Bay
Seasonal Rivers a) Hub River b) Basol River c) Hingol River
d) Shadi Kaur, e) Dasht River, f) Porali River,
g) Windar River
Other coastal features of Sindh and Makran coast include small-inhabited and uninhabited islands
Geologically offshore the Balochistan coast has a sub-duction plate as a result the coast is reportedly rising. Due to seismic activity the Balochistan coast has been experiencing minor earthquakes. Epicentres of these earthquakes occur along the Balochistan offshore areas. This tectonic activity has produced many mud volcanoes along the coast as a result gas charged water escapes to the surface opening of the volcanoes. The Makran coastal belt extends 50-100 km inland from the coast. It is formed of thick moderately deformed mid-Miocene to mid-Pliocene basin plain, slope and shelf sediments. In addition, as a result of recent uplift, under-formed upper Pliocene and quaternary sediments occur in places near the coast.