Throughout the centuries this has lead to the development of an extensive network of canals and dykes which are today indispensable in the delta’s water management. Floods in the Red river delta are a consequence of the flow collection from upstream tributaries Da and Lo river and Thao river ( upstream name of Red river ) [FIG 1.1.1], which gather at the confluence of Viet Tri and become a direct threat for the flood security of Hanoi lying beneath. That is why except for the extensive system of dykes wrapping the whole RRD and the developing network of upstream dams and reservoirs, there is a plan B for the protection of Hanoi from exceptional flood events. Generally the flow converging in Viet Tri is naturally diverted to other rivers - Duong, Nhue and Day river. Day river plays a very important role in the water management during both rainy and dry season. The lower lying areas in the delta get water supply during the dry part of the year trough the Day river, that is directing a part of the Red river flow to provide for the vast agricultural needs. However the Day river is also a strategical tributary for the water management of the Red river in rainy season when it helps control a safe level downstream by diverging a part of the flow thought its basin. However in case of a flow reaching alarm situation the Day river basin can be used as a flood retention area to protect Hanoi. [FIG 1.1.6] Created in 1972-1973 after the devastating flood of 1971 when the Day and Nhue rivers were already cut from the Red river by the construction of the Day Dam and the Lien Mac Sluice (Nhue river) and therefore the inflow from the Red river could be controlled. With a number of sluices and dams today the Day river flood retention area is still considered as an evacuation corridor in exceptional cases, however in a conversation with arch. Tuan Pham Anh he explained that in the 2 times that the retention area was used it didn’t prove to be efficient enough as the flow could not discharge well through the sluices and
dams as they weren’t properly located and there were large quantities of sediment layering around them blocking the flow. However the areas designed to be the flood retention area are actually covered with spread urbanisation of villages and vast agricultural fields. Saving Hanoi from eventual floods by using the day retention basion means flooding thousands of homes and hectares of agricultural land.. Recently as a part of the upgrade of the dyke system project in the RRD of the Northern Hydraulic Institute of Vietnam, started implementation in 2010, the Day river basin is to be reduced of its current capacity to flood. That makes the flood retention area insufficient as such, due to the narrow space left for water. [FIG 1.1.5]
FIG 1.1.5 Red river evacuation corridor in case of extreme flood events to be reduced
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Source: Tuan Pham Anh 2011 , Water Urbanism to Respond to Climate Change, Belspo-MoST Research Project 2010-2012, 3rd workshop