Why are cities experiencing severe flooding?

" SUSTAINABLE DRAINAGE"

Flooding is a major concern for residents and local authorities in Da Nang City. Since the beginning of the rainy season (September), Da Nang City has experienced two severe flooding events, and the city government has held numerous meetings to analyze the causes.

Mr. Le Trung Chinh, Chairman of the Da Nang City People's Committee, assessed that despite launching three intensive campaigns, the work of clearing and dredging inlets and drainage pipes within the assigned management areas has not yet been completed; the units have mostly focused on prioritizing dredging and clearing in flood-prone areas.

To address flooding, Mr. Le Trung Chinh requested the People's Committees of districts and counties, the Drainage and Wastewater Treatment Company, and investors to improve the efficiency of clearing inlets and drainage ditches and dredging sewers.

Some people had to use inflatable rafts to get around in residential areas in Hoa Khanh Nam Ward (Da Nang).

According to Mr. Le Tung Lam, Secretary of the District Party Committee and Head of the Da Nang City People's Council Delegation for Thanh Khe District, the flooding situation is due to four main reasons: inadequate urban planning, renovation, and development; the slow pace of renovating the old drainage system and constructing new ones compared to urban development; the failure to adequately address changes in awareness and actions regarding drainage systems to respond to climate change and extreme natural disasters; and limited resources and capacity for urban management, especially in the operation of drainage systems.

To solve this problem, significant resources and time are needed. According to the urgent plan for the next 5 years, Da Nang City needs to invest 5,500 billion VND; however, in the short term, priority should be given to addressing localized flooding in the central urban area of ​​the city and in older residential areas with degraded drainage systems.

"To address urban drainage needs in a situation of limited resources, a sustainable drainage approach (slow drainage) is necessary. It is essential to construct reservoirs, underground tanks, sewers, and regulating wells in areas where there are no longer ponds or lakes to regulate water flow. Water storage spaces should utilize public and private land, underground and above-ground spaces should be allocated, sidewalks should be improved to increase permeability, and effective drainage management plans and hierarchical structures should be in place. Acceptable flooding in some areas should be tolerated, and simple response scenarios and flood maps should be provided for easy implementation by each household," said Mr. Le Tung Lam.

C. FLOOD CONTROL IN URBAN AREAS: A DIFFICULT PROBLEM

In Quang Nam province , a leader of the Tam Ky City People's Committee stated that from 2018 to the present, flooding in Tam Ky city has increased in frequency, severity, and affected area. The objective reason is the excessive rainfall, which causes a large volume of water from Thang Binh and Phu Ninh districts to flow into the Ban Thach, Ky Phu, Tam Ky, and Truong Giang rivers. The subjective reason is that the ongoing construction to upgrade and expand the drainage system downstream has not yet been completed.

At a recent workshop proposing solutions to address flooding in Tam Ky City and its surrounding areas, Associate Professor Dr. Nguyen Chi Cong (Da Nang University of Technology - Da Nang University) stated that Tam Ky City has experienced two types of flooding in recent years. Firstly, widespread flooding due to rising water levels in the Tam Ky and Ban Thach rivers, preventing urban water from draining into the rivers, and even causing reverse flow. Secondly, urban flooding occurs even when water levels in the Ban Thach and Tam Ky rivers are low, preventing urban water from draining into the rivers. Furthermore, the flood drainage capacity of the river system is also a contributing factor.

Associate Professor Cong believes that the main cause is the extremely heavy rainfall, coupled with an inadequate drainage system, resulting in slow drainage and flooding in the inner city. To improve flood drainage capacity, it is necessary to renovate and upgrade the inner-city drainage system, lower the water levels of the Ban Thach and Tam Ky rivers to allow for faster drainage. Furthermore, floodwaters should be diverted from the Ban Thach River to the Truong Giang River and then drained from the Truong Giang River to the sea. In addition, underground sewers and drainage systems in the inner city and along the Ban Thach River sub-basins need to be cleared.

In Thua Thien- Hue province, in recent years, when people in Hue City mention "low-lying areas," they immediately think of the urban areas in the southeast of the city. Previously, this was a rice paddy area, but in the last 5 years it has been planned and developed into an urban area with a series of social housing projects, high-end apartments, townhouses, and commercial centers…

According to observations by Thanh Nien reporters during recent floods (2022 and 2023), just one heavy rainstorm was enough to flood roads in the southeastern part of Hue City, such as Hoang Lanh, Vu Thang, Nguyen Lo Trach (Xuan Phu Ward), and An Cuu City (An Dong Ward), with water levels ranging from 0.5 to 0.8 meters, and in some places exceeding 1 meter.

According to Mr. Nguyen Tri Dam (a resident of Xuan Phu Ward), the upgrading, expansion, and increase in height of To Huu and Vo Nguyen Giap roads act like a "dike" blocking water, turning this new urban area into a low-lying region frequently prone to flooding.

Furthermore, Mr. Nguyen Van Hung, Director of the Hue Regional Meteorological and Hydrological Station, acknowledged that urban flooding is partly due to extreme weather events, particularly heavy and prolonged rainfall. Specifically, during the recent rain (October 12-14), the rainfall measured in Phu Oc (Huong Tra Town) reached up to 147 mm/hour. "This is a very large amount of rainfall; previously, the highest rainfall was only about 40-50 mm/hour," Mr. Hung said.

Does the elevation in Da Nang not match the elevation on the national topographic map?

It's not just this one rainstorm; even with moderate rainfall, Da Nang is often submerged in water. Therefore, Professor Vu Trong Hong, former Deputy Minister of Agriculture and Rural Development, believes Da Nang needs to be extremely cautious because rainfall patterns are very complex.

"I noticed that the elevation in Da Nang seems to be inconsistent with the elevation on the national topographic map. The elevation in this city needs to be re-checked because although the rain wasn't heavy, many areas were still flooded," Professor Hong said.

According to Professor Hong, Da Nang is very close to the river and the sea, and has a sparse population, but the water in the city cannot drain out. It is highly likely that the elevation here is different, lower than the river and the sea, which is why this situation occurs.

Dinh Huy

The cross-sectional area of ​​the sewer pipe is only one factor.

A representative from the Ministry of Construction stated that, in response to climate change, the Ministry has proactively issued amendments and updates to increase the cross-sectional area of ​​drainage pipes in urban areas.

Architect Pham Thanh Tung, an expert in urban planning and architecture, argues that flooding during heavy rains in many cities cannot be solely attributed to the small cross-sectional area of ​​sewer pipes. He suggests that the cross-sectional area of ​​sewer pipes should only be considered as one factor when studying urban flooding during heavy rains.

According to Mr. Tung, to understand the reasons why many cities flood during heavy rains, it is necessary to consider the characteristics of each urban area and each region affected by rising water levels due to insufficient drainage of rainfall. At the same time, the quality of urban planning needs to be reviewed, as recently even newly developed urban areas have experienced flooding during heavy rains, not just older areas.

"The practice of dumping waste oil and grease into sewers, littering, especially throwing plastic bags indiscriminately into canals, ditches, ponds, lakes, and drains... is still very common. Poor environmental hygiene awareness contributes greatly to heavy rains causing flooding," Mr. Tung said.

According to Mr. Tung, simply basing the criteria on the cross-sectional area of ​​drainage pipes in small urban areas to raise standards and regulations could easily lead to a waste of resources. "It's necessary to thoroughly investigate the causes of flooding in each city during heavy rains, and from there find appropriate solutions," Mr. Tung said.

Le Quan

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