3.1. Water surface function and role
Based on Fig. 4, There are two main water surface ecosystems in Hanoi: The Lotic ecosystem consisting of the Red River and small river systems, and the Lentic ecosystem of independent ponds and lakes.
In the Lotic ecosystem, the Red River is the largest natural river flowing through Hanoi. The river brings abundant alluvial soil for agriculture. Therefore, the two sides of the Red River are cultivated and become land for cultivation in this area.
Because Hanoi's topography slopes from North to South and from West to North, a system of dykes have been erected on both sides of this river area to prevent flooding from the river during the high season. In addition, the remaining rivers of Hanoi are pretty narrow in width, and in a state of complete concreting on both sides. Rivers are the main drainage channels for Hanoi. Water pumping stations from small rivers will pour into the Nhue River. The Nhue River located in the East is the main river to hold water and pump water to the Red River.
The rivers flowing deep into the city center (such as the To Lich River, Nhue River, and Set River...) have changed shape over the years. Many river sections are encroached upon and narrowed in width, obstructing the flow. Many sections of the river were eroded, and the embankment was damaged. To Lich River is a river extending from West Lake to Nhue River, passing through many districts in the city (Fig. 4). Due to leveling, many sections of To Lich River have a very small cross-section, only about 1 meter.
The occurrence of urban flooding in the rainy season, floods lead to higher river levels and larger discharges, so the flood discharge capacity of rivers is greatly weakened. The greater the density of the river network, the greater the probability of flooding (Shuqi Ma 2021). In the past, Hanoi was a delta with a dense network of rivers. The rivers are interconnected and connected with the Red River. Due to the process of urban development, many rivers were filled to form the rivers and lakes today. However, Hanoi is still one of the cities with a large water surface density, so Hanoi is very susceptible to flooding due to rising water in the rainy season. And residential areas located near rivers and lakes will be prone to flooding.
In the Lentic Ecosystem, independent lakes are divided into four main categories: natural lake, flood control, non-existent, and water treatment.
Group 1: These water surfaces are natural lakes, accounting for about 50% of the surveyed water surface.
These lakes are formed naturally from old ponds, lagoons, and partly filled rivers.
In addition, at present these water surfaces which scatter in residential areas for air-conditioning have not been affected too much by construction.
Group 2: Large water surfaces from natural or artificial lakes (accounting for nearly 40%) have been renovated and built to serve the purpose of temporary water storage and control urban water surges. In which, West Lake is the largest lake in Hanoi. West Lake was part of the Red River a long time ago.
Group 3 includes previously existing water bodies that have been leveled for construction or have dried up and disappeared; and water surfaces that are difficult to access due to private ownership.
Group 4: Specific purpose water surfaces such as water treatment for a group of uses. However, this surface water area accounts for a very small percentage. Some are unmanaged and abandoned, overgrown with wild plants; affect the surrounding residential areas
3.2. Water surface status and impacts
The Red River and the environment on both sides of the Red River play an indispensable part in agriculture and air-conditioning for Hanoi, and most of the river remains natural and unpolluted. Rivers that are polluted are a concentration of domestic waste. It is impossible to clean some rivers due to waste blockage, especially the To Lich River. Since being leveled down due to urbanization, To Lich river has become a drainage channel for residents' sewage. Many sections of the To Lich River are concentrated places for domestic wastewater. After the big flood in early November 2008, the water of the To Lich river became as "clear" as before. At that time, the To Lich river was by rainwater, rising for meters, billowing, and even fish swam. That flood made the To Lich River clean again within a few weeks (VNU Journal 2013).
In the late 1990s, people began to dredge, embank, clean the river and prevent encroachment. Nevertheless, the river is highly susceptible to re-pollution due to the difficulty of actively renewing the flow.
Table 1
Construction status of some surface water areas in the study area (survey by the author)
Name | Area (ha) | Under Built | Balance Built | Over Built |
West lake | 512,4 | | x | |
Hoan Kiem | 12 | | x | |
Thien Quang | 3.1 | | x | |
Ba Mui | 11.6 | x | | |
Linh Quang | 3 | | x | |
Giang Vo | 7.2 | | x | |
Thu Le | 14.4 | | | x |
Dong ba | 8.2 | | x | |
Lang Thuong | 2 | | x | |
Ngoc Khanh | 3.7 | | | x |
Thanh Cong | 5.5 | | x | |
Xa Dan | | | | x |
Thanh Nhan | 2.5 | | | x |
Dam Hong | 8.1 | x | | |
Dinh Cong | 18.3 | x | | |
Dam Doi | 2.5 | x | | |
Linh Dam | 78.1 | | x | |
Yen So | 172 | | | x |
Rue Lake | | | x | |
Than An | 2.4 | | | x |
Dia Ca | 4.5 | | | x |
Dong Mo | 10 | x | | |
Yen Duyen | | x | | |
Dong Rieng | | x | | |
Small water surfaces are mainly natural lakes located in residential areas. At the same time, these lakes scatter with no flow connection with rivers or other large bodies of water. Therefore, the water in these lakes is often unchanged, renewed, or inaccessible to the flow (Fig. 4). Water surfaces that can be judged well by the naked eye are considered in good condition, not contaminated, still accessible to people, and they can be habitats for organisms. And this percentage accounts for over 80% of the surveyed water surface (Fig. 5).
Due to urban space constraints, contaminated water bodies that are stagnant waters in residential areas have been used to store waste. Moreover, they usually are situated in densely populated areas. The water surfaces are highly contaminated because people construct them to contain water without proper management. As a result, they become a focus of wastewater.
Through the survey of 24 large lakes in Hanoi, most of the lakes are concreted (Table.1), and some of the water surfaces are artificially built-in combination with urban green landscapes or parks. However, there is no setback or little transition for biodiversity and water level rise. Although Hanoi has a large water surface area, most of them are small and discrete lakes and unable to self-regulate.