GEOGRAPHICAL CHARACTERISTICS OF VIETNAM’S COASTAL REGIONS

Vietnam, a large maritime nation located along the western coast of the Vietnam Waters, is associated with two major gulfs: the Gulf of Tonkin in the Northwest, which covers an area of approximately 130,000 km², and the Gulf of Thailand in the Southwest, spanning about 293,000 km². The Vietnam Waters serve as the only maritime connection between the Indian Ocean and the Pacific Ocean. The waters surrounding Vietnam are heavily influenced by prevailing monsoon winds that blow from the northeast and southeast. As a result, Vietnam's coastline faces significant risks from natural disasters and marine environmental incidents, particularly oil spills and pollutants entering its coastal waters.

The climate of Vietnam’s maritime regions varies across three major zones: (i) The Northern climate zone, extending from the Hai Van Pass Northward, which is characterized by a tropical monsoon climate with cold winters; (ii) The Southern climate zone, from Da Nang to the Mekong Delta, which has a tropical monsoon climate near the equator, marked by distinct rainy and dry seasons and consistently high temperatures; (iii) The marine climate of the Vietnam Waters, which shares tropical monsoon characteristics. The Vietnam Waters are susceptible to natural disasters such as storms, tornadoes, climate change effects, and tsunamis. On average, about eight storms hit Vietnam annually, with tsunami warnings indicating potential waves from deep underwater trenches near the western Philippines (Palawan) that could reach the Vietnamese coast within just two hours.

Vietnam's coastal oceanographic regime also exhibits clear seasonal variations. The surface currents and waves fluctuate in direction and intensity in accordance with the monsoon winds throughout the year. These climatic and hydrological characteristics contribute to the development of distinct geographical and ecological zones, each with unique biological resources and economic potential.

Vietnam’s continental shelf, which reaches depths of up to 200 meters, constitutes the majority of the Gulf of Tonkin, the Gulf of Thailand, and the waters of the Mekong Delta, while narrowing along the central region of the country.

Vietnam's maritime area encompasses internal waters, territorial seas, contiguous zones, exclusive economic zones, and the continental shelf, in accordance with the United Nations Convention on the Law of the Sea (1982).

With a narrow landmass (no point is more than 500 km from the sea), Vietnam boasts a coastline extending over 3,260 km (excluding islands). The coastline runs along an east-west axis, from Mong Cai (Quang Ninh) in the northeast to Ha Tien (Kien Giang) in the southwest. It is characterized by an irregular shape, featuring numerous bays, coves, and inlets. Approximately every 20 km along the coastline, a major river mouth can be found, totaling about 114 river mouths primarily flowing from the Vietnamese mainland. Notably, Vietnam is home to two vast and fertile coastal deltas: the Red River Delta in the north and the Mekong Delta in the south. These river systems discharge the largest amounts of water and sediment into the Vietnam Waters each year. While they provide essential nutrients to Vietnam’s waters and the Vietnam Waters, they also contribute to environmental pollution from runoff, particularly near the river mouths.

Administratively, Vietnam is divided into 63 provinces and cities, 28 of which have coastal areas. These include 125 coastal districts and 12 island districts. These administrative units play crucial roles in the country’s economic development and in safeguarding its maritime security and sovereignty.

From North to South, the 28 provinces and cities with coastlines include: Quang Ninh, Hai Phong City, Thai Binh, Nam Dinh, Ninh Binh, Thanh Hoa, Nghe An, Ha Tinh, Quang Binh, Quang Tri, Thua Thien-Hue, Da Nang City, Quang Nam, Quang Ngai, Binh Dinh, Phu Yen, Khanh Hoa, Ninh Thuan, Binh Thuan, Ba Ria-Vung Tau, Ho Chi Minh City, Tien Giang, Ben Tre, Tra Vinh, Soc Trang, Bac Lieu, Ca Mau, and Kien Giang.

Vietnam’s maritime zone lies in the tropical monsoon region, where its weather and climate are influenced alternately by cold air masses from the north and warm, humid air from the south. This results in a climate that is both temperate and tropical in nature.

The prevailing wind regime over the South China Sea is dominated by Northeast monsoon winds from October to April, covering seven months. During the remaining months, the dominant wind pattern shifts to Southwest monsoons, with the Gulf of Tonkin experiencing mostly Southerly winds. Wind intensity at sea is markedly stronger than over land, with average speeds ranging from 5-6 m/s and peak speeds reaching up to 50 m/s, generating waves as high as 10 meters or more.

The wind field exerts a profound influence on the hydrodynamic regime of the South China Sea, which exhibits two distinct seasonal oceanographic patterns: northeast monsoon currents and southwest monsoon currents.

1. Hydrological Characteristics of Vietnam’s Maritime Areas

1.1. Tidal Regime in the Vietnam Waters

Vietnam’s coastal waters exhibit diverse tidal patterns. Among them, the diurnal tidal regime of the Gulf of Tonkin is considered one of the most typical globally, characterized by a single high tide and a single low tide per day with remarkable regularity.

Over the years, an extensive tidal observation network has been established along the Vietnam Waters coastline, including Vietnam’s shores. These stations continuously monitor sea level fluctuations over various timescales, from monthly to annual cycles. Based on long-term observational data, key tidal parameters such as mean sea level, extreme tidal levels, tidal rise and fall durations, and harmonic tidal constituents have been systematically analyzed. The harmonic tidal constants derived from coastal and island observation stations provide fundamental data for tidal research in the Vietnam Waters, evolving from traditional methods to modern analytical techniques.

1.2. Current Regime in the Vietnam Waters

The surface current regime and wave patterns in the East Sea exhibit seasonal variations corresponding to the monsoon system, affecting both flow direction and intensity.

a. Winter currents                           b. Summer currents

Picture 1: Currents Regime in the East Sea

In addition to major ocean currents, Vietnam Waters also experience drifting water masses and vertical water movements, including upwelling and downwelling. These phenomena significantly influence the distribution and migration of marine organisms by driving nutrient circulation and impacting biological productivity.