Under the adverse impact of climate change, over the past 5 years, in Da Nang, there have been many extreme and prolonged rains with a very large amount of rain, especially concentrated rain with an intensity of more than 40mm/hour. Combined with high tide, the city's drainage system is overloaded, causing local flooding. This requires a combination of different measures in a synchronous manner for sustainable flood prevention and control.

Although the construction and operation of the anti-flood pumping station has been completed and operated in the Green Island area since the end of 2019, Nui Thanh Street’s sections in Hai Chau District is still flooded due to heavy rains. Photo: HOANG HIEP

Partial inundation in case of concentrated heavy rain

Director of the Da Nang Sewerage and Wastewater Treatment Company Ha Van Thanh said that his agency is reviewing all inundation points throughout the city to determine the cause and seek solutions to this matter in the coming rainy season this year.

Previously, on the evening of April 1, 2022, the total amount of rain measured in 1 hour in Cam Le District was up to 58.4mm, resulting in 13 heavily -flooded points.

Although there were workers in charge of operating rainwater collection systems on the road surface and the anti-flood pumping station at the end of Ong Ich Khiem Street, the Le Dinh Ly - Nguyen Van Linh - Ham Nghi intersection was deeply flooded in 25cm of rainwater for 90 minutes, whilst Le Duan and Quang Trung were submerged in floodwater of 20cm deep for 30 minutes.

Similarly, despite the opening of the rainwater inlet and operation of the Da Xanh (Green Island) anti-flood pumping station, Nui Thanh’s sections were still heavily flooded, especially at the Nui Thanh - Tieu La intersection in 25cm deep for 20 minutes.

In a similar troubling situation, a section of Nguyen Luong Bang in front of the gate of the Hoa Khanh Industrial Park was flooded to a depth of 70cm with slow recession of rainwater.

Some urban areas and residential areas have low elevations and the rain-permeable area of the urban surface decreases due to urbanisation. Still worse, the impact of climate change and the amount of rain soaring has caused flooding.

Synchronising multiple solutions

In a great effort to solve the above problems, according to Mr. Hoang Ngoc Tuan, the Director of Institute of Irrigation Science in Central and Central Highlands regions, the city needs to increase the aperture of the flood drain through National Highway 1A stretching from Hoa Phuoc Commune to the Do (Red) bridge and re-build the new Da Co Bridge to reduce flooding.

The city is also highly advised to build new axes and drainage canals, upgrade and expand the apertures of sewers, increase capacity and invest in anti-flood pumping stations, upgrade flood drainage axes for residential areas, villages and fields, ensure flood drainage corridors on the both banks and riverbanks of Yen, Qua Giang, Tuy Loan and Cu De rivers in urban development planning.

In their perspectives, experts Le Tung Lam and Tran Viet Dung from the Da Nang Construction Association underlined a must-do for the city to approach the concept of Sustainable Drainage Systems (SuD that mimic the natural cycle of water management, by retaining water where it lands (instead of shedding it quickly to drains and watercourses, which can lead to floods). SuDS reduce rainwater runoff from a given site, filter it to improve water quality, and slow its journey downstream.

Above all, it is necessary to use a hydraulic model to review and evaluate the drainage capacity of the current urban drainage system and propose strategic, long-term solutions, investment divergence, and resource mobilisation for step-by-step implementation.

In the planning work, it is necessary to pay more attention to expanding flood drainage corridors, strengthening regulation lakes to increase water storage capacity, enhancing urban green areas to increase water permeability, reducing flow rates and creating urban landscapes, calculating drainage systems, selecting foundation elevations with taking climate change and sea level rise factors into account.

In addition, it is also necessary to develop a process for the operation of regulating reservoirs on the basis of releasing the reservoirs early before the rain, make the most of the regulation ability, prioritise the work against urban flooding, step by step replace and repair degraded sewers that do not guarantee the drainage aperture.

For current residential areas with too low elevation and frequent flooding, it is highly recommended to clear off them completely and re-plan accordingly, conduct urban reconstruction to arrange on-site resettlement for dwellers.

Mr. Le Hoang Nghia, lecturer of the Institute of Social Sciences in the Central region, suggested that, for urban areas and residential areas that are about to be constructed and planned in the future, it is a need-to-fulfill task to apply low impact development (LID) that  refers to systems and practices that use or mimic natural processes that result in the infiltration, evapotranspiration or use of stormwater in order to protect water quality and associated aquatic habitat. This method is being deployed in many parts of the world in an effective manner.

Accordingly, sidewalks will use water-permeable paving systems and roofs will go green. Also, there will be water-filtering gardens, fields, ditches, lakes and wetlands. Rainwater from the impermeable area will flow into the above-mentioned areas to promote water storage, weaken the flow, increase infiltration and reduce the volume of stormwater runoff, thereby limiting the risk of inundation and flooding.

According to Architect To Van Hung, the Director of the Da Nang Department of Natural Resources and the Environment, climate change and unusual weather phenomena are directly affecting our country, and Da Nang in particular, such as dramatic changes in rainfall regime and rainfall, floods, super storms and sea level rise.

Last, but not least, the city needs to soon synchronously deploy urban infrastructure and works, focus on urban design, organise modern urban space in harmony with nature and take into account factors of flood prevention and adaptation to climate change; effectively exploit the functions of parks, riverside areas and lawns to ensure the capacity of adaptation to floods and urban flooding, increase the area of green trees in urban areas and increase the rate of economic plantation forests and forest coverage.

Reporting by HOANG HIEP – Translating by A.THU