♦ PhD. Nguyen Quang Anh

(Continued from Part 3)

 

Water resources

 

Hydrological regime

 

The river and lake network of Hanoi has its own distinctive characteristics. The Red River is the main river of Hanoi, entering the city at Bat Bat Commune and leaving the city in the Dai Xuyen area. The section of the Red River flowing through Hanoi is 163 km long, accounting for about one-third of its total length within Vietnam. Hanoi is also influenced by the Da River, which joins the Red River in part of the Ba Vi  - Son Tay mountainous area in the northern part of the city. In addition, there are many other rivers within Hanoi’s territory, such as the Day River, Duong River, Cau River, and Ca Lo River. Smaller rivers flowing through the inner city, including the To Lich River and Kim Nguu River, serve as major drainage channels for Hanoi.

 

Diagram of the river and stream network of Hanoi City

 

         The Red River basin has a rainy season that usually begins from May to September or October, during which rainfall accounts for 75 - 85% of the annual total, while the dry season lasts 6 - 7 months, with rainfall accounting for only 15 - 25% of the annual total. As a result, the Red River flow regime shows strong temporal variability and is characterized by two distinct seasons. The flood season extends from May to October, during which flood - season discharge accounts for approximately 70 - 80% of the total annual runoff. The three consecutive months with the highest discharge usually occur in July, August, and September, contributing about 50 - 60% of the annual runoff. The maximum mean monthly discharge typically occurs in July or August, accounting for about 15 - 20% of the annual runoff. The low-flow season lasts from November to May of the following year, during which runoff accounts for only 20 - 30% of the annual total. The three consecutive months with the lowest discharge usually occur in January - March or February - April, contributing less than 10% of the annual runoff. After the Hoa Binh Reservoir came into operation in 1989, although its primary function has been flood control for the downstream area, the long - term average discharge did not change significantly. However, due to regulation through the Hoa Binh Hydropower Dam, flood peaks have been reduced, while flood duration has been prolonged as water is released gradually through the dam. Based on observed data since 1960, the mean annual discharge at the Hanoi hydrological station prior to the construction of the Hoa Binh Hydropower Project was 2,667 m³/s. During the construction period (1986 - 1989), due to river closure and reservoir impoundment, downstream discharge decreased. The average annual discharge during this period was 2,300 m³/s, representing a reduction of 14.6% compared to the pre-construction period. Low - flow discharge decreased by 13.2%, while flood discharge decreased by 19.4%. After the project became operational, the average discharge during 1990 - 2011 was 2,426 m³/s, a decrease of 3.7% compared to the pre - construction period. Because the Hoa Binh Reservoir performs annual flow regulation, low - season discharge increased by 17.1% compared to the 1960 - 1985 period, while flood discharge decreased by 2.1%. The duration of the low-flow season was extended by one month compared to the 1956 - 1985 period, but its severity was reduced. Thus, it can be concluded that the regulation regime of the Hoa Binh Hydropower Project has significantly influenced the downstream flow regime. At present, several additional large hydropower reservoirs, such as Son La and Tuyen Quang, have been constructed on the Red River and Da River, which will have considerable impacts on the flow regime and hydrological conditions of the Red River.

 

Lakes of Hanoi

 

Hanoi is a city renowned for its numerous lakes and ponds, with 154 natural and artificial lakes, ponds, and wetlands. The lakes of Hanoi play an important role in the activities and development of the capital. In addition, several new lakes have been and will continue to be excavated, such as Me Tri A, B, and C Lakes, while some small ponds have been filled for construction purposes. Most lakes are managed by the Parks Company, while others are managed by the Fisheries Company. These lakes serve multiple functions, including landscape enhancement, cultural and recreational activities, microclimate regulation, as well as stormwater regulation, wastewater treatment, and aquaculture. Ho Guom (Sword Lake) is located in the center of the traditional urban core and holds a special position in Hanoi. Ho Tay (West Lake), the largest lake with an area of approximately 500 ha, plays a significant role in the urban landscape. Within the inner city, other well - known lakes include Truc Bach, Thien Quang, and Thu Le, etc. In addition, there are many large lakes and wetlands within Hanoi’s territory, such as Kim Lien, Linh Dam, Ngai Son - Dong Mo, Suoi Hai, Meo Gu, Xuan Khanh, Tuy Lai, and Quan Son.

 

Me Tri Lake 

 

The lakes of Hanoi are not only scenic attractions and aquatic green ecological landscapes - an indispensable feature of urban architecture - but also contribute significantly to agricultural irrigation, stormwater regulation, and local microclimate moderation. In addition, some lakes also serve as reception and treatment areas for urban wastewater. The list of major lakes in Hanoi and the areas of several representative lakes is presented below:

 

 

 

Groundwater resources

 

Groundwater is also an important natural resource of Hanoi. However, at present, the exploitation of this resource has not been strictly managed and remains inefficient, with considerable waste. In the coming years, demand for this resource for socio - economic development is expected to increase significantly. Therefore, appropriate planning and effective management measures are required for the exploitation and use of construction materials and groundwater resources, in order to avoid waste and losses.

 

The Holocene aquifer (qh) is exposed at the surface and is distributed along the Red River and Duong River, extending southward. In the northern area, it occurs only as a narrow belt along the Cau River and Ca Lo River, with a relatively small thickness.

 

Based on lithological composition, the aquifer is commonly divided into two units. The upper unit, which is discontinuously distributed, consists of sandy clay belonging to the upper part of the Thai Binh Formation (Q₂³tb), with a thickness ranging from very small values up to 10 m. The lower unit is composed of sand of various grain sizes mixed with gravel. Its average thickness is 9.2 m in the area north of the Red River and Duong River, 10.1 m in Gia Lam, and 13.3 m south of the Red River, and it has good water-bearing capacity.

 

Recharge to this aquifer is derived from rainfall and irrigation water. In the riverbank zones, river water becomes the main recharge source during the flood season; however, in the dry season, groundwater from this aquifer discharges into rivers, is lost through evaporation, and also contributes to recharge of the underlying aquifers.

 

This aquifer has only limited water-supply significance. In suburban and rural areas, local residents commonly dig shallow wells or drill small-diameter wells to exploit groundwater from this aquifer. The aquifer is highly vulnerable to contamination, and in many areas it has already been polluted.

 

Pleistocene aquifer (qp): This aquifer is relatively thin and is exposed only in valley areas or along the margins of hilly and mountainous zones in Soc Son, Ba Vi, Thach That, Quoc Oai, Chuong My, and surrounding areas. The fully covered portion is continuously distributed southward from the Me Linh - Dong Anh - Soc Son area. It is encountered only in boreholes at depths of 2 - 10 m north of the Red River and Duong River, 5 - 22 m in Gia Lam, and 10 - 35 m south of the Red River.

 

The qp aquifer consists of two layers. The upper layer is composed of medium - to coarse - grained materials mixed with gravel and pebbles, with an average thickness of 10 - 15 m. The lower layer consists of cobbles mixed with sandy gravel, and locally includes sand and clay at the base. Its thickness is 12 - 22 m north of the Red River and Duong River, and 30–35 m in Gia Lam and south of the Red River.

 

Recharge to the qp aquifer is mainly from river water during the flood season, rainwater percolating through the overlying qh aquifer, while part of the groundwater discharges into rivers during the dry season, and the remainder contributes recharge to the underlying aquifers. Due to its large groundwater reserves, the qp aquifer is of great importance for domestic water supply for Hanoi.

 

 (To be continued...)