People struggle to move through a flooded area in Da Nang City on October 14, 2022. VNA/VNS Photo

A historic and unprecedented downpour last week in Đà Nẵng inundated the entire city by up to 1.5m. Many said it was the worst flooding in 30 years that they had ever seen. Huỳnh Vạn Thắng, former deputy head of Đà Nẵng City’s Steering Committee for Flood and Storm Prevention - Search and Rescue, said a rethink is necessary to adapt and prevent natural disasters with extreme weather.

You said that the recent rain was an unprecedented historical rain in Đà Nẵng. Can you elaborate on this?

The "historical rain" that I am talking about here is the amount of rain calculated according to the criteria within three hours. In the forecast, we measure the rain according to the time criterion which is a 24-hours of rain. We have other measurements, according to the period criteria, like the amount of rain falls in 12 hours, six hours, three hours and one hour.

In terms of the amount of rain, the total rainfall of this period is not a record. But if you consider the period of rain, especially in three hours, then even in the historic flood in 1999, the period of rain in three hours in Đà Nẵng is not as much as the evening of October 14.

The rain amount reached a high level in a short time, and the urban area cannot drain water in time.

Many people blame the sewer system for the problem. What are you thoughts? 

I firmly believe that with that amount of rainwater pouring down, even if the drainage system can reach 100 per cent of its design capacity, flooding still happens.

We should take into account that, in addition to a large amount of rainwater pouring into the urban area of Đà Nẵng, it is also accompanied by many factors that make the drainage system not work to the maximum. In many areas of land and lakes were "full" of water because in the previous 15 days the city had a storm and heavy rain.

High tides and garbage also make the sewer system less effective.

Most of our urban areas are not designed to withstand extreme rainfalls of 100-200mm falling in an hour, so historical flooding is obvious.

Just a few hours after the rain stopped, the water immediately receded. This proves that this is not really flooding, but flooding due to excessive rainfall.

The city of Đà Nẵng suffered great damage after this downpour. What lessons can be learned so the same incident won't repeat itself?

Being caught by surprise, the damage is clearly visible. There are areas in Đà Nẵng that have never been flooded, but this time water inundated many houses, so most people did not respond in time.

The weather is extreme, no one thinks such flooding could happen in Đà Nẵng, so I think from now on we must have a plan to face this type of disaster.

With many years of fighting storms, I have realised that my people are more afraid of storms than they are of rain. When people are warned about a storm, they will be worried and make every necessary preparation like supporting the house, storing food and charging batteries. But people are not afraid of rain.

Even on the evening of the heavy rain on October 14, I saw many people driving around and then stuck outside all night.

In general, the people and the city's government did not react very proactively. 

If the preparation is more drastic, and proactive, and had the city authorities give prompt warnings, perhaps people could have prepared early to respond more properly.

Most of the central provinces have scenarios for dealing with storms and floods. Are we missing a detailed flood plan?

Except for the rural area of Hoà Vang District, the urban area of Đà Nẵng has almost no type of flooding. It's not like the water can't drain for a few days like in other localities.

Of course in the urban areas, there are still a number of small local flood hotspots that have been identified, and the solution is to build a sewer and a pumping station.

To build the flood map, the scenario of flooding due to heavy rain like in the past is not very feasible, because it depends on the capacity of drainage and construction density. In particular, urban flooding depends largely on the heavy rainfall falling in that area but is not greatly affected by the rainfall from upstream.

The characteristics of urban drainage are not the same as in the countryside, because the city centre is mainly box-shaped houses and tube-shaped houses, so the drainage depends on the sewer system and drainage canals. In urban areas, the larger sluice gates can be opened, the faster the drainage will be.

This has been discussed for a long time. But investing in infrastructure to face natural disasters that appear only once every 50 years is difficult, because the cost is very large, and must be done in separate stages. VNS