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January 17, 2022, 8:28 am

Plastic greenhouses may exacerbate flooding

(16:57:06 PM 06/06/2015)
( - Flash floods last week in Viet Nam's horticultural capital, Da Lat, in the Central Highlands, left many homes roof-deep in water and killed one person. But many people in the region wonder why flooding has become such a problem in recent years. Residents, both officials and the general public, offer many reasons.


 Plastic greenhouses may exacerbate flooding

Farmers grow vegetables in a greenhouse in Da Lat City. A sudden quick rise in river water and a higher frequency of flash floods may be due to the spread of horticultural greenhouses, expert said. 


Oddly enough, much of the blame is centred around the endless hectares of plastic greenhouses that keep spreading over farmland.


Tuoi tre (Youth) newspaper interviewed Tran Quoc Nhan, living in commune 9, Da Lat City, about the flooding.


Nhan said his family read the water gauge on the Cam Ly River near his home to predict a flood after solid rain.


"Whenever we see the water quickly rise, my family starts to shift furniture to higher places because we know water will soon pour through our home," Nhan said. "The deepness of the flood depends, of course, on how long it rains."


Nhan said his home had been battered by eight floods in the last two years. "Once, water rushed in so fast our family quickly shut the doors and rushed to a high nearby road to wait for it to recede."


Nguyen Dinh Thien, a Da Lat resident living near the Chi Lang Church in the same neighbourhood, blamed frequent floods on garbage from agricultural production that gets stuck in the sluice gates of drainage systems.


He said residents in the neighbourhood sometimes urged each other to clear the gates, but the blockages quickly returned.


Phan Cong Ngon, director of the Irrigation Sub-department of Lam Dong Province said extremely high rainfall in a short period of time was the main reason for the flooding.


Lam Ngoc Tuan, head of the Da Lat University's Environment Faculty, agreed that heavy rain was behind the flooding, but said it was not the fundamental cause.


Tuan attributed the sudden, quick rises in river water and higher frequency of flash floods to the uncontrolled spread of horticultural greenhouses.


He said this was especially true in key horticultural areas along the 60km-long Cam Ly river, including the districts of Thai Phien, Chi Lang and Me Linh.


Of the 18,000 hectares of land under agriculture in Da Lat, greenhouses account for 1,320 hectares - or about one thirteenth, but most were in key production areas near the river.


Tuan said that the greenhouses were a wall-to-wall blanket of plastic that prevented rain, light or otherwise, from penetrating the soil.


This causes the rain to run off directly into rivers and streams, often leading to sudden rises and violent flows.


The long-term effect was even worse because lack of penetrating rain could lead to the water table drying up, meaning there was no groundwater for wells or other domestic use.


"This is not a story of the future but is happening right now," Tuan said.


And, according to the Lam Dong Department of Agriculture and Rural Development, sediment had greatly reduced the capacity of lakes throughout the city, such as Than Tho, Me Linh and Xuan Huong. This also contributes to flooding.


Tuan said greenhouses in cold countries were designed to raise temperatures inside to grow crops more easily.


However, he said, greenhouses in the tropics, including Da Lat, were used only to shield crops from rains.


Tuan suggested that tropical greenhouses have two roofs, an outer layer made of nylon, which can open and shut, and an inner layer made of netting.


When rain was needed, the outer layer could be left open and the fixed inner layer used simply to ease the pressure of torrential rain damaging crops.


If this was done, he said, crops could be watered, groundwater topped up and violent surface runoffs prevented from developing into flash floods.



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