People navigate through flooded areas around the Huu Gate following heavy rains and floods in early November.

Why is Hue prone to flooding?

In response to this question, Associate Professor Dr. Nghiem Tien Lam (Water Resources University) provided a detailed explanation. The main reason Hue City is frequently flooded is its geographical location and topography. Hue City is located in the Perfume River Delta, a narrow coastal plain of the Perfume River basin. The Perfume River basin covers an area of ​​over 4,000 km2 but is mainly mountainous, especially with the high mountains of the Truong Son and Bach Ma ranges shielding it from the west and south. Moisture from storms, tropical depressions, and the Northeast monsoon is blocked by these mountain ranges (the "foehn" effect), causing rain. The amount of rainfall in this area in a few days can equal or exceed the average annual rainfall nationwide (around 1,800 mm). The French once measured the annual rainfall in Bach Ma at 8,000 mm.

To put it simply, imagine if the Perfume River basin were flat (like a tray, for example), a single rainstorm would cause flooding of nearly 2 meters across the entire basin. In reality, the water still flows into the sea, so this is somewhat reduced. However, the Perfume River basin is more like a pan tilted towards the southeast. Because 3/4 of the basin's area is mountainous, rainwater flows down and concentrates in the remaining 1/4 of the plain area, which is below 10 meters in elevation, causing severe flooding.

Therefore, whenever there is heavy rain, the plains in Hue, where the ground elevation is below 10 meters, will flood, especially severely during years affected by La Nina. Flooding in Hue is not a recent phenomenon; it has been documented for hundreds of years in the historical records of the Nguyen Dynasty.

Challenges from reality

Before a storm or flood arrives, the reservoir must release some of its water to accommodate the flood. A portion of the floodwater will be stored in the reservoir to create a flood control volume. When the reservoir is full and floodwater continues to flow in, the amount of floodwater flowing into the reservoir will be released downstream. In theory, that's how it works. This means that reservoirs are only beneficial in flood control/reduction if the design and operation are correct. Large reservoirs in the North, such as Hoa Binh, Tuyen Quang, and Thac Ba, have kept the Red River Delta and Hanoi safe for many years in this way.

However, the reality may be different. Firstly, hydropower reservoir owners often want to maintain high water levels for power generation. If forecasts are inaccurate, they may only release water when there is a risk of the reservoir being full to ensure dam safety, which could lead to a "flood upon flood." Secondly, forecasts are often inaccurate due to insufficient investment. Thirdly, climate change is making rainfall and floods more extreme, rendering design calculations (e.g., dam height, flood control capacity, spillway capacity) outdated and unsuitable.

So, is the solution of using reservoirs to control floods, as in the North, appropriate? The answer is no. No matter how many reservoirs are built in the Huong River basin, they will not have enough capacity to contain all the floodwaters mentioned above.

Based on quantitative research, it can be said that the influence of tides (high tides) and the opening of the Thuan An estuary on flooding in the delta is small, if not negligible. Tides and the opening of the Thuan An estuary mainly affect the water level in the Tam Giang lagoon and Thuy Tu lagoon. Whether the water in the delta can drain quickly enough into these lagoons depends on the water transfer capacity of the Perfume River and the overflow from the delta.

So, what is the water carrying capacity of the Perfume River? According to author Nghiem Tien Lam, under normal conditions, the Perfume River receives water from most of the basin area through the Ta Trach, Huu Trach, and Bo River tributaries to flow into the lagoon with an average annual flow rate of only a few hundred m3/s. During major floods, the floodwater flow can be up to 10 times greater (approximately 12,500 m3/s), thus far exceeding the Perfume River's flood drainage capacity. This inevitably leads to the flooding of the delta.

Specifically for Hue, dredging the Bo River to flow directly into Tam Giang Lagoon or dredging the An Cuu River to drain floodwaters into Cau Hai Lagoon to reduce the burden on the Perfume River would only have limited effectiveness due to high costs and would not be a fundamental solution, as it would require dozens of canals the size of the Perfume River to effectively drain floodwaters from the basin.

Is building dikes an effective solution? Unlike dikes in the North, which are used to prevent major floods, Hue and some central provinces also have dikes, but they are used to prevent saltwater intrusion and small early-season floods to allow farmers to harvest their crops in time. Therefore, the dikes need to be not too high so that large floods can easily pass through and drain away as quickly as possible. In other words, dikes are not effective in preventing major floods.

Planning for long-term coexistence with floods.

Not only Hue City, but all provinces in Central Vietnam need to accept the long-term reality of living with floods and inundation, therefore, many solutions need to be implemented.

Regarding planning and construction solutions: It is recommended to reorganize residential areas and infrastructure related to disaster prevention and mitigation, based on updated hydrological and flood calculations reflecting climate change and extreme weather conditions. In addition, drainage standards for construction and transportation projects should be updated, as these structures in Central Vietnam are often built higher than the natural ground level, impacting flood drainage and inundation.

Regarding non-structural solutions: Applying modern technology in disaster forecasting and warning. This includes sharing basic data freely and online to promote research and application of new technologies for disaster forecasting and warning. Regularly updating and calculating flood maps based on various disaster scenarios will serve as a basis for response efforts. Enhancing public awareness by publishing online maps of past and predicted floods. Simultaneously, people need to proactively adapt to flooding, such as building flood-resistant houses.

Source: https://huengaynay.vn/chinh-polit-xa-hoi/hue-truoc-bai-toan-thoat-lu-161420.html