10% of the urban area is used as a reservoir to reduce the flooding scene of Hanoi

Trường Lăng

June 9, 2022


10% of the urban area is used as a reservoir to reduce the flooding scene of Hanoi

Trường Lăng

June 9, 2022

Flood in the big cities is caused by the increasingly abnormal weather and the anti-flood and natural lake protection, and new multi-purpose reservoir construction investment uncared.

There are more historical rains 

The rain lasting for 2 hours occurred on the afternoon of May 29 flooded many streets in Hanoi, people and vehicles were submerged in water everywhere.

Reported the National Center for Hydro-meteorological Forecasting, the total rainfall accumulated for about 2 hours on May 29 in some places: 118mm in Van Dien; 138mm in Lang; 150mm in Tay Ho; 170mm in Cau Giay.

nga p nguye n tra i
Rain flooded on Nguyen Trai Street on the afternoon of May 29

Historical monitoring data at Lang meteorological station shows that the cumulative rainfall for 2 hours is 132.5mm recorded on June 18, 1986. Thus, the rainfall of 170mm in Cau Giay for 2 hours is the highest according to the data available so far, the value corresponding to a cycle of about 100 years occurring once. Rainfall of 150mm in Tay Ho corresponds to a cycle of about 50 years occurring once.

Notably, when the principle of drainage in the urban areas is to drain every hour of rain, when the rainfall exceeds the drainage capacity, Hanoi will lead to flooding.

There was no rain above 100mm/hour in Ho Chi Minh City in the 1950s. Until the 1960s, there was only one rain of 100mm/hour, there was one more rain over 100mm/hour in the 1970s. But the heavy rains gradually increased in the 1980s and now there are more than ten heavy rains over 100mm/hour a year.

One aspect should be noted: the previous drainage design for Ho Chi Minh City was over 90mm/hour only. With the heaviest rains measured, the rainfall increased by 40% compared to the drainage capacity, many other rains often increased 20-30%, so the drainage system capacity is not guaranteed.

Similar to the Mekong Delta, experts also calculated that heavy rains here have increased from 9 to 17.5%.

Urban inadequacies from the flooding rains

There are a few problems emerging from the flooding rains in the big cities and towns. Firstly, natural disasters have extremely and unusually increased under the impact of climate change. The reality has shown that there is an increase in heavy rains, occurring in a short time, exceeding the design of drainage, such as flooding used to occur in Phu Quoc or many other big cities such as the Mekong Delta, Hanoi, Ho Chi Minh City.

Secondly, sectoral plans are no longer suitable; construction designs, including the designs of sewerage systems, must be reviewed and re-judged. Even the highland areas where drainage is favorable. Whenever flooding occurrence shows that the designs of the previous drainage systems are no longer suitable, or the maintenance, dredging of sludge to clean manholes are not carried out regularly.

Thirdly, flooding in some urban areas and big cities is also caused by the construction planning of the “oil slick” style, that is, when the urban area is expanded, population increased but the drainage system is only prolonged. Specifically, it is possible to imagine that the pipeline is designed for a drainage area with an area of ​​about 20 hectares, until that area expands to 40 hectares, that means the area is doubled but the sewer remains the same, it is only prolonged without changing with a bigger sewer system. Then the drainage in the way of bringing all stormwater to the low-lying areas result in many areas flooded in the type of water “navel”.

Recalculating the drainage ability when climate changes

After floods in the big cities and towns, it is necessary to review and recalculate the drainage problem. This is mandatory when the impact of climate change, extreme heavy rain has caused the places where have never been flooded, now are flooded, and the places where were flooded are now at risk of deeper and heavier flooding. The calculation process should clarify what extent the previous drainage design can be met and the design of which area is no longer satisfied.

With the existing drainage system, it is impossible to expand the drainage system, it is necessary to take a solution to store a part of the stormwater when it rains heavily to reduce the drainage pressure immediately. It is to supplement small reservoirs, storing water from the household. Stated experts, flood will not occur when there is heavy rain in an area of 100m2, and 2m3 of stormwater can be stored for slow drainage.

10% of the urban area is used as a multi-purpose reservoir, great benefits

When talking about the solutions to limit flooding in the big cities like Hanoi, Ho Chi Minh City and the Mekong Delta, Prof. Dr. Dao Xuan Hoc, former Deputy Minister of Agriculture and Rural Development proposed that there should be the regulation for 10% of the area as a multi-purpose biological reservoir for the new urban areas.

10% of the urban area used as a reservoir does not cause economic losses to investors, even the environment and landscape in the urban area is more beautiful. Said he, this is a very important solution for the Red River Delta and the Mekong Delta.

10% of the urban area is used as a multi-purpose reservoir that is beneficial. Firstly, for each ecological lake with an area of ​​10% of the total land fund, if the depth is 5-6m, it will create a volume of leveling sandy soil, helping to raise the height of the foundation to about 60-70cm. In addition, if soil and sand dug from biological lakes is used to level the ground, the pressure on exploitation demand of sand, river sand is reduced, and riverbank and coastal erosion is reduced.

Secondly, said Professor Hoc, it is only necessary to have the lake area at 5% of the urban area to contain the increased rainfall, suitable for the existing infrastructure, ensuring anti-flooding. The reservoir area of 10% is very safe for drainage. This reservoir area can also be used for domestic water production in the urban areas.

Thirdly, by adding multi-purpose reservoirs in some places in the Mekong Delta and Ho Chi Minh City, this is also a solution to reduce groundwater exploitation and land subsidence. In addition, the entire lake surface area can be used to carry out the solar power projects to supply electricity to the urban areas… Therefore, it is necessary to aim at the framework regulation so that each urban area can reserve the land fund for this multi-purpose reservoir.

Source : VietNam.Net

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