Why Do We Measure River Flow in Ho Chi Minh City?

1. Where is Ho Chi Minh City?

Geographical Background

Ho Chi Minh City, Vietnam's most populated city, is located in the south of Vietnam. It is located near 10.7627°N latitude and 106.6602°E longitude. The city lies in the Mekong Delta, a low and flat area formed over centuries of sedimentation by the Mekong River. The low-lying land has favored the growth of the city but put the city at risk of floodings, especially during the monsoon season. The tropical weather with a very wet season from May to November and a dry season from December to April is characteristic of Ho Chi Minh City. The wet season is featured by heavy rainfall and significantly contributes to the city's river regimes.

Humanities and culture

Ho Chi Minh City, formerly known as Saigon, has a rich and diversified cultural heritage. It was an important trading port during French colonial times, and the French architectural legacy is felt in buildings such as the Saigon Central Post Office and the Notre - Dame Cathedral Basilica of Saigon. It is an active center of traditional Vietnamese culture combined with modern influences. It is an arts, music, and cuisine center. The markets, including the Ben Thanh Market, are lively hubs where all can be tasted, including traditional Vietnamese handicrafts, tried, such as pho and banh mi, and imbibed, energetic atmosphere. Tet, Lunar Vietnamese New Year, and other festivities are all celebrated with vigor, including traditional ceremonies, dragon dances, and fireworks.

Hydrology and River Overview

The Saigon River is the primary river system flowing within Ho Chi Minh City. The Saigon River has its source in the southern Annamite Range of southern Vietnam and discharges after a course of approximately 220 kilometers into the East Vietnam Sea. Ho Chi Minh City's river plays a significant role in the city's economy, transportation, and water supply. It has been a major trade and transport waterway in the past, and wharves and ports on the riverbanks enable the shipping of goods in and out of the city.

The river is also the source of water for industry and home use, but the water must be treated in great quantities due to pollution. The Saigon River also features a very rich ecological system, with plenty of fish as well as other aquatic animals. However, urbanization and industrialization have resulted in high levels of pollution of the river, threatening its ecological health as well as that of the people depending on it. There are also tributaries to the Saigon River such as the Dong Nai River, which feed it with water volume and also form part of the city's complex water system.

2. What is the River Flow Near Ho Chi Minh City?

Factors That Shape

Precipitation and Runoff

Ho Chi Minh City is in the tropical region with two distinct wet and dry seasons, whose pronounced effect on the Saigon River's flow cannot be underemphasized. The high rainfall rate of the wet season, typically extending between 1,500 - 2,000 millimeters of rainfall annually, creates a huge amount of runoff. This fills up the river, both in volume and flow rate. Conversely, the dry season experiences a sudden decline in rainfall, leading to declines in river flows and water levels. The frequency and intensity of rainfall may differ from year to year, leading to river flow regimes altering unpredictably and complicating water management.

Topography and River Morphology

The low gradient of the Saigon River owing to the flat terrain of the Mekong Delta where Ho Chi Minh City is located makes the river sluggish. This allows sediment to settle on the river bed and banks over time. The width of the river varies in the various parts of the city, with some sections being wider and suitable for large ships and other sections being narrower. The meandering of the river also influences the flow, with faster-moving water on the outer bends resulting in erosion and slower-moving water on the inner bends leading to deposition of sediment. Human-made constructions like dams, embankments, and bridges have altered the natural flow regimes of the river, sometimes inducing local changes in water direction and velocity.

Reservoir Operations

Hydroelectricity, water storage, and irrigation are used by dams and reservoirs constructed on the tributaries of the Saigon River above Ho Chi Minh City, for instance, the Dong Nai River. The regulated release of water from the upstream reservoirs impacts the flow rate of the Saigon River in the city significantly. Decreased water release from the higher reservoirs during the dry or high water usage seasons results in reduced river flow in Ho Chi Minh City, affecting water supply and navigation. On the other hand, proper regulation of water release during the rainy season is critical to prevent flood incidents downstream.

Historical Hydrological Events

Ho Chi Minh City has experienced a string of severe flood events over recent years. An unusual amount of rainfall during the monsoon season in 2020 caused catastrophic flooding in most of the city. The floods, Vietnam Disaster Management Authority reports say, inundated thousands of houses, damaged infrastructure, and brought transport to a standstill. The Saigon River overflowed and inundated the low-lying areas, causing extensive damage to property. It is vital to track such past occurrences in an attempt to formulate effective flood - mitigation policies, improve urban drainage systems, and render the city increasingly resistant to hydrological disasters in the future.

3. How River Flow in Ho Chi Minh City is Monitored?

Traditional Methods

Surface Drift Float Method

The surface drift float method is the simplest method employed to quantify the hydrologic flow of a river. A floating material, such as a piece of wood or a buoy, is placed at the surface of the Saigon River. The time spent by the float in covering a known distance is recorded, and dividing the distance by the time provides an estimate of the surface speed of the river. This will only provide data on surface flow and is considerably influenced by wind, which has the effect of distorting the reading and providing erroneous values of speed. It is also not capable of providing any data regarding flow at different depths of the river.

Anchored Boat Method (Staff Gauge / Current Meter)

Anchored boat method involves the use of an anchored boat in a set place on the river. A staff gauge measures the water level, while the current meter is located within the river at different depths to measure water velocity. A number of velocity measurements are taken across the cross-section of the river, and by multiplying the cross-sectional area by the mean velocity, the river discharge can be determined. This is more revealing but is time-consuming, labor-intensive, and dangerous, especially in a crowded city river like the Saigon River with heavy boat traffic.

ADCP Introduction: Acoustic Doppler Current Profiler (ADCP)

Acoustic Doppler Current Profiler (ADCP) is a vital breakthrough in river flow measurement technology. ADCP is a non-invasive measurement of water velocity at different depths using sound waves. ADCP can measure continuous profiles of the flow of the river from the surface to the bottom of the river in a comparatively short span of time, and it provides a broader picture of the nature of the flow of the river than that provided by the traditional approaches. It is a highly potent technology that enables one to obtain precise and accurate data, and this is what is required for there to be effective water resource management, flood prediction, and ecological research in Ho Chi Minh City.

4. How Does ADCP Work on the Doppler Principle?

An ADCP current profiler works based on the Doppler principle. It emits sound waves of a certain frequency into the water. Because sound waves are interacting with moving particles in the water, such as sediment, plankton, or water molecules, the frequency of the sounding waves that bounce back alters. When the particles are traveling towards the ADCP, the frequency of backscattered waves is increased, whereas when they are moving away, the frequency is reduced. By precisely measuring this frequency shift (the Doppler shift), the ADCP can calculate the velocity of the water at different depths. Velocities in different directions are measured by some of the transducers in the ADCP, and the three-dimensional river flow pattern can be calculated.

5. What is Needed to Make Quality River Flow Measurement in Ho Chi Minh City?

Equipment Needs

For acquiring a set of dependable ADCP profiler data in the Saigon River, some equipment characteristics are preferable. Because the river is dynamic and contaminated, material reliability matters. ADCPs built with materials that are resistant to corrosion like stainless steel or titanium are better equipped to withstand the corrosive river environment exposed to contaminants and varying water chemistry. A light and portable construction would also be preferable, so that it could be conveniently transported around and installed, especially in the Ho Chi Minh City urban environment. A low cost would also be important, as this would allow the technology to be used more widely for general river flow monitoring.

6. How to Choose the Right Current Measuring Equipment?

Deployment Method

• Ship - Borne (Moving Boat) ADCP: Ideal for conducting large - scale surveys of the Saigon River in Ho Chi Minh City. It can quickly measure the discharge of the river along long sections and map the flow patterns in different sections of the river. As the boat moves along the river, the ADCP can collect continuous data, providing a comprehensive overview of the flow characteristics.
• Bottom - Mounted (Fixed) ADCP: Suitable for continuous long - term monitoring. Can be fixed on the riverbed at a location in the Saigon River for measurement over extended periods, which helps in understanding the long - term trends in flow, seasonal variations, and the impact of various factors on the river flow.
• Cable - Way ADCP: In the presence of cable - way infrastructure along river reaches, a cable - way ADCP can be suspended and moved across the river to measure flow at multiple points. This is particularly convenient where boat - based measurements are likely to be difficult owing to heavy traffic or other constraints.

Working Frequency

• 600 kHz ADCP: A 600 kHz ADCP gives a good balance between range and resolution, having a horizontal range of as much as 70 meters. It can be utilized in the majority of river sections in Ho Chi Minh City, measuring the flow velocity at different depths with precision.
• 300 kHz ADCP: With a greater range of up to 110 meters, a 300 kHz ADCP would be more appropriate for deeper and wider sections of the Saigon River.

Brand Recommendations

Reputable and high-quality ADCP brands such as Teledyne RDI, Nortek, and SonTek are renowned globally. Chinese brands also include the ADCP supplier’s "China Sonar Panda ADCP," which is cost-effective. It is made of all-titanium alloy, which gives it high durability as well as corrosion resistance, and is capable of weathering the tough environment of Rajasthan's rivers. It is also very cheap and therefore accessible to use in the majority of the monitoring projects in the state. For further information, you can visit their website at https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and models.